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Agency for Toxic Substances and Disease Registry

Division of Health Studies

THE USEFULNESS OF COMPUTED TOMOGRAPHY IN
DETECTING PULMONARY LESIONS NOT FOUND BY CHEST
RADIOGRAPH IN INDIVIDUALS EXPOSED TO ASBESTOS

Libby, Montana

PROTOCOL

April 30, 2001

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICE

Public Health Service

Agency for Toxic Substances and Disease Registry

Atlanta, Georgia 30033

The Usefulness of Computed Tomography in Detecting Pulmonary Lesions Not Found by Chest Radiograph in Individuals Exposed to Asbestos

CONTENTS

ATTACHMENTS

INTRODUCTION

Vermiculite mining in and near the city of Libby, Montana began in the 1920's and was continued by the W.R. Grace Company from 1963 until 1990. The operation at this site included strip mining for the ore, transporting it by truck to a sorting facility and to processing plants in downtown Libby, expanding the ore by heating, and shipping it by rail as a commercial product. Vermiculite is a mineral with chemical properties similar to asbestos, but it is not fibrous in nature. At this time, the toxicity of the vermiculite has not been completely studied, but to date no serious health effects have been associated with exposure to vermiculite, per se (1). However, the vermiculite ore taken from the Libby mining operation has been documented to be contaminated with asbestiform minerals, including tremolite, actinolite, and others. Previous studies by NIOSH (2,3,4,5) and McGill University (6,7) investigators found that former employees of the mine had substantial occupational exposure to these asbestiform minerals. These investigators also documented pulmonary abnormalities and disease (asbestosis and lung cancer) among employees. Recently, cases of asbestos-related pulmonary diseas have been reported among household contacts of former mine employees and others in the community with no connection to the mining operations.

In the fall of 1999, the U.S. Environmental Protection Agency (EPA), Region VIII, began an emergency response action to identify and control asbestos contamination in the Libby community. In support of this action, the EPA sought assistance from the Department of Health and Human Services (DHHS) Denver Regional Office, the Montana Department of Health and Human Services, and the Lincoln County Health Department to address the public health implications of human exposure to asbestiform minerals in Libby. Senator Max Baucus of Montana recently asked DHHS Secretary Shalala for Departmental assistance to Libby residents who need medical testing for possible asbestos-related health effects. This request was referred to ATSDR as the lead DHHS agency for providing this assistance.

The specific goal of this health study is to evaluate usefulness of chest radiograph in detecting pulmonary lesions in individuals exposed to asbestiform minerals associated with vermiculite in Libby, Montana using HRCT.

OBJECTIVES

1. To determine the prevalence rates of various asbestos-related abnormalities (e.g., pleural abnormalities and parenchymal abnormalities) detected by HRCT among three different risk groups of asbestos-exposed individuals who had indeterminate chest radiographs (only one of three physicians identified a possible abnormality) in the recent Medical Testing Program conducted in Libby, Montana.

2. To compare the HRCT findings among the three groups with different exposure characteristics to find out whether there is any significant difference.

BACKGROUND

Vermiculite was mined from Zonolite Mountain for more than 65 years (until 1990). The mine itself is located approximately 6 miles from the city of Libby. A transfer facility was located at the base of the mountain, approximately 3 miles from Libby. From the transfer facility, vermiculite was loaded onto trains for shipping or onto trucks going into Libby. Two expansion ("popping") facilities operated at different times within the town; these plants heated vermiculite to approximately 600 degrees Fahrenheit to expand "pop" the crystals. One of these facilities was next to a baseball field and was readily accessible to the community's children.

The mine was for vermiculite which was crushed and milled. The vermiculite ore body in Zonolite Mountain contains an asbestiform amphibole mineral (8). Asbestiform minerals are of concern because inhalation of asbestos fibers suspended in air can result in lung diseases, such as asbestosis, mesothelioma and lung cancer. The risk of developing any one of these diseases depends upon many factors including type of fiber, level and duration of exposure, and smoking history of the exposed individual. All types of asbestos fiber, the asbestiform types only, not the type of tremolite that results in short clearance fragments, are associated with the development of asbestos-related scarring (9) and malignancies (10,11).

LITERATURE REVIEW

A. Asbestos-Related Diseases of the Chest

NON-NEOPLASTIC PLEURAL DISEASES

Pleural changes can often be detected radiographically in persons who have been exposed to asbestos. These changes include pleural plaques (the most frequent lesions): discrete, elevated, opaque, shiny fibrosis lesions that are currently more common than asbestosis in exposed persons; they are found mostly on the parietal pleura and, characteristically, the posterolateral aspect of the lower parietal pleura or diaphragm and occasionally on the visceral pleura (12), diffuse pleural thickening: it is of lower frequency than pleural plaques and typically forms a continuos sheet at least 8 to 10 cm in a craniocaudal direction and 5 cm in a lateral direction (13), pleural effusion: may antecede diffuse pleural fibrosis, although a history of effusion is not always obtained from patients with diffuse pleural thickening.

ASBESTOSIS

Asbestosis is the term reserved for pulmonary fibrosis caused by inhalation of asbestos fibers. Radiographically, it is characterized by small irregular opacities predominating in the lower lung zones. The latency period for the onset of asbestosis is typically 10-20 years after the initial exposure. Clinically, asbestosis may present as shortness of breath with exertion and a dry non-productive cough. The disease can vary from asymptomatic to disabling and potentially fatal.

MASS LESIONS

Persons with significant exposure to asbestos are at risk for developing various mass lesions in the chest. These masses include pleural plaques, rounded atelectasis, pleural mesothelioma, and bronchogenic carcinoma (12). CT can differentiate between benign and malignant masses.

B. The Role of CT and HRCT in Identifying Asbestos-Related Chest Lesions

Over the past twenty years, several investigators have evaluated the potential of conventional CT scans for detecting abnormalities in asbestos-exposed workers. It has been found that CT offers several advantages over routine chest radiography. First, the use of cross-sectional images often makes it possible to distinguish between densities that would be superimposed on plain radiographs. Second, CT is far better than routine radiographic studies at characterizing tissue density, distinguishing subtle differences in density between adjacent structures, and providing accurate size assessment of lesions (14). Gamsu and colleagues (12) have shown that CT scans are more sensitive than chest radiography for the detection of asbestos-related pleural abnormalities. In their study on a large group of workers who had been occupationally exposed to asbestos, high-resolution CT (HRCT) was even more sensitive than conventional CT, even though images were obtained only at preselected sites and covered a small fraction of the total pleura and lung. At the same time, the study found that conventional CT is successfulin defecting these pleural abnormalities in over 80% of the cases.

Similar results were seen in the study by Neri and colleagues (15). Seventy asymptomatic shipyard workers whose chest radiographs had been judged "normal" by outside readers, were tested by HRCT. Among the 70 workers, 34 were found to have pleural plaques alone, 6 were found to have parenchymal abnormalities alone, and 13 others were found to have both parenchymal and pleural abnormalities; no pathological findings were shown in the remaining 17 workers. That study also found that pulmonary or pleural changes due to exposure to asbestos can be detected by HRCT before the onset of any clinical symptoms.

Zerhouni and colleagues (16) and Stein and colleagues (17) showed that the density resolution of conventional CT permits the visualization of abnormalities not found with chest radiography. Sargent and colleagues (18) showed that extrapleural fat can mimic pleural thickening on chest radiography in individuals exposed to asbestos, and that conventional CT or HRCT can distinguish between the two. Recently, low-radiation dose spiral CT scanning of the chest has been shown to detect lung cancer with a six-fold greater sensitivity than the standard chest radiograph (19,20). Katz and Kreel (21) showed parenchymal abnormalities consistent with asbestosis in 33% of their asbestos-exposed subjects studied by CT, whereas conventional chest radiographs were abnormal in only 16%. Sperber and colleagues (22) and Yoshimura and colleagues (23) found similar results.

Begin and colleagues (24) compared the ability of posteroanterior, four-view radiographs and conventional CT of the chest in detecting asbestos-related pleural and parenchymal fibrosis in 127 workers occupationally exposed to asbestos. They found that CT did not detect asbestosis in 19% of 53 subjects whose chest radiographs were abnormal. It should be mentioned that this study likely utilized an early generation CT scanner and protocols.

Aberle and colleagues investigated 100 workers occupationally exposed to asbestos using chest radiographs, pulmonary function tests (PFT), and HRCT (25). Among the 55 study participants with normal chest radiographs, parenchymal abnormalities on HRCT were present in 30 and highly suggestive of asbestosis in 20. They also found a highly-significant correlations between increasing abnormality on HRCT and reduced PFT measures, indicating a restrictive pattern of respiratory dysfunction. Similar HRCT findings were seen by Staples and colleagues (26) when they studied asbestos-exposed persons with normal chest radiographs: the lung parenchyma was suggestive of asbestosis in 57 of the 169 subjects scanned. Additionally, these 57 persons had significantly different mean PFT results (lower percent predicted vital capacity and diffusing capacity, consistent with restrictive changes) compared to a group of 76 individuals with normal HRCT scans. (Typically, asbestos-related disease causes a restrictive pattern on PFT (27)). A low forced vital capacity (FVC), a low total lung capacity (TLC), and/or a low DLCO (where "low" means less than the 95% confidence lower limit of the normal predicted value), are used to screen for the presence of an interstitial fibrotic process consistent with asbestosis (28).

Despite the fact that there is no developed widely-accepted system for the diagnosis and categorizing of asbestos-related disorders by HRCT scans (similar to standardized International Labour Organization (ILO) system of classifying chest radiographs for pneumoconioses (29)), HRCT is used for confirming findings of asbestosis by chest radiography and allows one to find early changes not seen on chest radiographs. However, the extent and severity of the asbestos-related abnormalities seen on HRCT are usually assessed subjectively. Gamsu and colleagues have undertaken a study (30) to: (a) compare sensitivity of subjective semiquantitative scoring method and a method using a cumulation of the different HRCT features of asbestosis in suggesting asbestosis in a group of patients with histopathologic confirmation of disease; and (b) compare the results of these two HRCT methods with chest radiographs in the same population. This study group consisted of 24 patients and six lungs obtained at autopsy. Histopathologic asbestosis was present in 25 of the 30 patients or lungs. The patients or lungs were imaged using selected HRCT. This study demonstrated that (a) HRCT predicted asbestosis with a higher frequency than chest radiographs using the ILO classification; (b) a subjective semiquantitative grading system of the extent and severity of asbestosis and a method using a cumulative addition of the different HRCT features of asbestosis give similar results in suggesting the presence of disease. Thus, the authors concluded that for the HRCT detection of asbestosis, a combination of the cumulative number of different findings and an assessment of the extent and severity of the abnormalities could be complementary. Another significant finding of the study was that asbestosis can be present histopathologically with a normal or near normal HRCT (the HRCT scans were normal or near normal in five (out of 25) instances of histologically proven asbestosis).

One of the concerns about the use of HRCT has been the high dose of radiation (31). Evans and colleagues (32) and Murphy and colleagues (33) showed that radiation doses at the skin of surface of the breast for patients undergoing conventional contiguous 10-mm collimation CT scanning of the chest are on the order of 20 mGy. These doses are approximately 100 times greater than those associated with posteroanterior chest radiographs, 40 times greater than those associated with lateral chest radiographs, and 10 times higher than those associated with mammographic examinations (32, 34). Mayo and colleagues (35) calculated radiation doses to the patient from HRCT scanning that used contiguous sections. They showed that radiation dose at 120 kVp and 300 mAs was 55 mGy for 10-mm-collimation scans and 61 mGy for HRCT with 1.5-mm-collimation scans. The latter dose represents the upper limit of the dose to the patient if contiguous HRCT sections are obtained and is approximately 300 times greater than those associated with posteroanterior chest radiograph and 30 times greater than those associated with mammographic examinations. However, Evans and colleagues (32) showed that HRCT may result in less then half the radiation dose of conventional CT when kilovoltage, milliampere seconds, and scanning intervals are kept constant. Lucaya and colleagues (36) conducted three controlled studies in different groups of children and young adults using HRCT of the chest at 180, 50, and 34 mAs. These studies have shown that low-dose HRCT provided a significant reduction in radiation dose (72% for 50 mAs and 80% for 34 mAs) and good-quality images of the lung when performed with 50 mAs in non-cooperative and 34 mAs in cooperative pediatric and young adult patients. Diederich and colleagues (37) have conducted a screening for asymptomatic early bronchogenic carcinoma with low-dose (50 mAs at 120 kVp) CT of the chest. Radiation exposure was equivalent to three (male) or five (female) posteroanterior and lateral chest radiographs. The authors have concluded that low-dose CT could detect pulmonary nodules > 5 mm reliably. It should be noted that neither of the two latter studies utilized low-dose CT for detection of asbestos-related pulmonary lesions.

The International Expert Meeting on Asbestos, Asbestosis, and Cancer, Helsinki 1997 (38) indicated a need for (a) development of a standardized system for the reporting of HRCT scans of asbestos-related disorders, similar to the ILO system and (b) studies on the specificity of lesions of the pleura visualized by HRCT as markers of asbestos exposure. This expert meeting concluded that "Computed tomography (CT) and high-resolution computed tomography (HRCT) can facilitate the detection of asbestosis and asbestos-related pleural abnormalities, as well as asbestos-related malignancies; they are not recommended as a screening tool but may be invaluable for individual clinical evaluation and research purposes."

C. Findings on Conventional CT and HRCT in Asbestosis

1. The following abnormal findings in asbestosis were seen on conventional CT in a large group of workers occupationally exposed to asbestos by Gamsu and colleagues (12):

subpleural lines: linear densities within 1 cm of the pleura and parallel to it,

parenchymal bands: linear densities 2 to 5 cm in length running through the lung, usually not in the direction of pulmonary blood vessels, and extending to the pleural surface,

pulmonary arcades: branching intralobular structures or interlobular septa that appear as arcades and are most prominent posteriorly in the dependent lung,

a subpleural dependent density: a 2- to 30-mm-thick band of poorly marginated, increased pulmonary density paralleling the dependent pleura and obscuring the underlying pulmonary morphology,

reticulation: a network of linear densities, often at the lung bases and often posterior,

a honeycomb pattern: multiple cystic spaces of 1 to 15 mm diameter, with thickened walls. Usually, this abnormality is subpleural in its distribution and predominates in the posterior lower lobes.

2. The following HRCT abnormalities were described by Aberle and colleagues (39) in persons with the radiographic and pulmonary function findings of clinical asbestosis:

interstitial thickening: abnormal interstitial thickening that is due to either septal lines or core structures. Thickening septal lines are evident as linear densities 1 to 2 cm in length, usually extending to the pleura. Thickened core structures are branching lines radiating from a central core, originating about 1 cm from pleura within the secondary pulmonary lobule,

a honeycomb pattern: multiple cystic spaces of 1 to 15 mm diameter, with thickened walls. Usually, this abnormality is subpleural in its distribution and predominates in the posterior lower lobes. Dilation of small bronchi and bronchioles may be evident with this abnormality,

parenchymal bands: linear densities 2 to 5 cm in length running through the lung, usually not in the direction of pulmonary blood vessels, and extending to the pleural surface,

subpleural lines: linear densities within 1 cm of the pleura and parallel to it,

a subpleural dependent density: a 2- to 30-mm-thick band of poorly marginated, increased pulmonary density paralleling the dependent pleura and obscuring the underlying pulmonary morphology.

ONGOING MEDICAL TESTING IN LIBBY, MONTANA

In response to the request for assistance from the U.S. Environmental Protection Agency (EPA) and Montana Senator Max Baucus, ATSDR recently proposed a plan for Medical Testing of Individuals Potentially Exposed to Asbestos Minerals Associated with the Vermiculite in Libby, Montana. This medical testing program was designed to (1) identify asbestos-related health effects among people exposed to asbestos from the Libby vermiculite mine and refer them for additional evaluation and treatment; (2) provide the EPA with information needed to identify and eliminate current exposures to asbestos in the community; (3) identify the types of illnesses experienced by these exposed people in order to better educate local physicians; and, (4) provide the local medical community with an estimate of the additional medical care the community will need over the next 10-20 years. This plan was developed by ATSDR in close collaboration with other federal, state, and local agencies.

This Medical Testing consists of (1) a face-to-face questionnaire designed to obtain exposure information, including detailed information about potential pathways of exposure and to collect information about respiratory symptoms, demographics, and smoking; (2) spirometry to provide an objective measurement of lung function including (a) the volume that can be expired forcefully after a maximum inhalation, called the forced vital capacity (FVC); (b) the volume of air expelled in the first second of such a forceful exhalation, called the forced expiratory volume in 1 second (FEV1); and (c) the calculated ratio (FEV1/FVC) of the preceding two measures; and (3) chest radiograph (CXR) to identify asbestos-related changes in the parenchyma and pleura of the lungs. CXR are to be evaluated and interpreted by the onsite clinical radiologist as well as a panel of three national experts in asbestos-related conditions.

METHODS

RATIONALE AND STUDY DESIGN

The utility of chest radiographs in screening for asbestos related disease has been well documented and is the standard for asbestos related screening programs for occupationally exposed cohorts. Irrespective of this history of use, many members of the community in Libby, Montana, who have undergone testing, have expressed interest in the use of CT scan testing.To address this concern, ATSDR is proposing this study. This study will evaluate the usefulness of HRCT in detecting chest lesions not found by chest radiograph in individuals exposed to asbestos minerals associated with vermiculite in Libby, Montana. At the time data collection for this study begins, findings from the above-mentioned Medical Testing will be available to researchers.

STUDY POPULATION

After completing the interview, chest radiograph, and spirometry during the above-mentioned Medical Testing, a group of males and females 18 years of age and older will be selected to have HRCT of the chest. For purposes of safety, pregnant women will not be included in the study.

For the medical testing program,four groups of people were presumed to be at risk of developing asbestos related changes (chest radiograph abnormalities) due to reported past exposures. The four groups of people presumed to be at some risk are 1) former vermiculite mine/mill workers, 2) household contacts of these former workers, 3) people with exposure to vermiculite due to past direct recreational behaviors, and 4) people who otherwise lived, worked or attended school in Libby, Montana. The eligibility criteria for medical testing program for all these categories is that they occurred for at least 6 months prior to December 31, 1990.

The magnitude of this risk is currently unknown, but can be roughly estimated for the purposes of planning this study. It is likely that the risk for these changes is not the same for the four groups. Based upon previous estimates of the potential for exposure, an assumption was made that exposure was greater for former workers, household contacts and people with reported direct recreational contact with vermiculite. Therefore, the power of detecting abnormalities among people with higher exposures will be greater. The power estimates for testing the hypotheses of this study would be dependent upon the prevalence of abnormalities in each of the subgroups. This is reviewed in the following section entitled "Sample Size."

Eligibility criteria for consideration for the HRCT study is that the person must have participated in the medical testing program and was found to have some indication of pleural disease by only 1 of the 3 physicians classifying the radiographs under the International Labor Organization (ILO) procedure. These 3 physicians have been certified in the ILO procedure as "B" readers by the National Institute for Occupational Safety and Health. Although all classification information will be provided to the participant's physician for clinical care considerations, the interpretation of no pleural changes by 2 of the 3 reviewers would be considered as an indeterminate chest radiographs (one of three physicians identified a possible abnormality) for the epidemiological analysis. By including people in this protocol to receive a HRCT who had one review report pleural changes, the participants would be those most likely to benefit from receiving a HRCT and will possibly bias toward finding plural changes on a HRCT scan.

Therefore, eligibility criterias for consideration for the HRCT study are as follows: (1) being either male or female 18 years of age or older; (2) being classified into the risk groups of former vermiculite mine/mill workers, household contacts of these former workers, or people with exposure to vermiculite due to past direct recreational behaviors; and (3) have an indeterminate chest radiographs in the recent Medical testing Program (only one of three physicians identified a possible pleural abnormality). Group 4, people who otherwise "lived, worked, or attended school in Libby, Montana" will not be included in this study based upon sample size considerations (see next section "Sample Size").

Sample Size

Our primary objective is to estimate the prevalence of 'test negative' cases reversed by HRCT. To do this we will select a sample of test negative cases (indeterminate chest radiographs), provide HRCT for these cases, and then count the number of test-negative results reversed by HRCT. In doing so, we will obtain a point estimate of the true proportion of reversals that would have been observed had we performed HRCT on the entire population of test negative cases instead of a sample. Next we will calculate a measure of our confidence in the point estimate called a confidence interval. We would like to be confident that our point estimate of the proportion of cases reversed reflects the true population proportion. In particular we would like the length of our confidence interval to be small, reflecting confidence in our point estimate. The length of the confidence interval is inversely proportional to sample size. Precision increases as sample size increases. For our calculations, we will express confidence interval length as a percent of expected prevalence of reversal.

Table 1. gives sample size requirements needed to obtain various levels of precision for various expected prevalence levels. For example, suppose the expected prevalence in a particular group is 10%. Then to obtain a confidence interval with length equal to 100% of the point estimate (100% of 10% is 10%), we would need 138 subjects. If the observed prevalence was in fact 10%, then we could conclude that the interval (5%, 15%) contains the true population prevalence 95% of the time. In contrast we would need 216 subjects to make the more precise conclusion that the interval (6%, 14%) contains the true population prevalence 95% of the time.

Table 1. Sample Sizes Based on Various Precision Levels
  Confidence Interval Length (expressed as a percent of the previous estimate
Prevalence Estimate 80% 100% 200%
0.1% 23986 15351 3838
0.5% 4778 3058 765
1% 2377 1521 380
5% 456 291 73
10% 216 138 35
20% 96 61 15
30% 56 36 9
40% 36 23 6
50% 24 15 4

The total number of eligible people is not known and sampling from the eligible pool is anticipated. This study is attempting to estimate the prevalence of asbestos related changes detected by HRCT among people with negative but suggestive changes on chest radiograph. This calls for a sampling procedure that will estimate the confidence interval around the point prevalence estimate. Table 1 can be used to choose sample sizes based on expected reversal rates and desired precision of estimates. Since we plan to examine reversal rates in 3 exposure group categories, total sample size will equal the sum of the exposure-specific sample sizes. Since the prevalence of identified changes is likely to be different among the four exposed risk groups, the sample size will vary in order to maintain similar confidence estimates. Using Table 1 and anticipated prevalence rates within each of the study groups, we can select the appropriate sample size to estimate prevalence with the specified precision (Table 2). Because we anticipate that the prevalence of asbestos related changes for general residents will be very small (.005), sample size requirements would be so high as to make sampling from this group unfeasible. For the purposes of selecting the sample size for this protocol, the following estimates are used:

Table 2
Group Estimated Group Size Estimated Prevalence Rate Calculated Sample Size
Former Workers 100 .25 50
Household Contacts 250 .10 138
Recreation Exposed 250 .10 138

STUDY INSTRUMENT

All eligible study participants will have a high-resolution CT (HRCT) of the chest. We will do HRCT scanning in both prone and supine positions at full inspiration without intravascular infusion of contrast material.

The following HRCT scanning protocol will be used:

Patient will start out in the prone position. We will have the patient take a deep breath and hold it. Then we will take our scout film. From there we will take a 1mm slice through each of the following areas:

  1. Top of aortic arch
  2. Carina
  3. 1/2 way between carina and the diaphragm
  4. 1/2 way between image 3 and 5
  5. Rt. dome of diaphragm
  6. 1/2 way between diaphragm dome and rt. angle of lung.

This will give us a total of 6 slices in the prone position. Then the patient will lay flat on their back and we take another scout film. Then we start at the apices of lungs and scan all the way through and out of the lung fields in 8mm x 8mm contiguous slices. Two window settings will be used, one for parenchymal visualization and another for mediastinal and pleural visualization. We will use a bone algorithm for all of the abovementioned scans. An exposure time of 1 second will be used with KV between 120 and 150, and mAs between 40 and 150, depending on the body habit.

Based on the literature review, radiation doses to the patient from HRCT of the chest vary substantially depending on exposure time, quantity and thickness of slices viewed, as well as kilovolt peak and milliamperage settings (see pp. 7-8). For this particular study, the radiation dose will be about 2-5 rad to the chest. This dose is approximately 15 times that of a posteroanterior chest radiograph at the skin surface of the breast and approximately three times the dose of a mammogram to the breast.

HRCT Technique and Limitations

The CT scan is an image produced by an X-ray source that rotates around the patient. The beams pass through the patient and are detected on the other side. This information is analyzed by computer to generate a cross-sectional view of the patient at that level. Its utility in the assessment of mediastinal disease has made CT an important tool in the staging of lung cancer, as an assessment of tumor involvement of mediastinal lymph nodes is critical to proper staging. With the additional use of intravenous contrast material, CT also makes it possible to distinguish vascular from nonvascular structures, particularly important in distinguishing lymph nodes and masses from vascular structures (14). Helical CT scanning allows the collection of continuous data over a larger volume of lung during a single breath-holding maneuver than is possible with conventional CT.

HRCT stands for "high resolution" CT; with HRCT, the cross-sectional images are about 1 to 2 mm instead of the 10 mm used in routine CT scans. The resolution (clarity) of an image depends on the radiation dose, the thickness of the slice viewed, the field of view, and the display capabilities. Cross-sectional images often make it possible to distinguish between densities that would be superimposed in a routine radiograph. HRCT is better at characterizing tissue density and determining the size of lesions; it is particularly valuable in visualizing disease adjacent to the chest wall or spine, hilar and mediastinal disease, and areas of fat density or calcification in pulmonary nodules. HRCT is sensitive for detection of early (asymptomatic) pulmonary or pleural lesions. The HRCT has greater spatial resolution than the CT. Also, HRCT is better at evaluating changes associated with low radiographic attenuation.

Limitations of HRCT:

  • HRCT provides many times (see pp. 7-8, 16) the radiation exposure of routine chest radiography.
  • There are no widely accepted, validated protocols for screening with HRCT while limiting the radiation exposure to the patient.
  • The Libby, Montana Asbestos Scientific Panel convened by EPA (Region VIII) and ATSDR on February 22-23, 2000 (40) "... agreed that HRCT scans are not suitable for screening because of the cost and radiation doses associated with the procedure."

HRCT is a noninvasive and reliable diagnostic procedure. The Expert Panel (above) noted that "... HRCT is more sensitive and specific in regards to pleural changes than the standard posterior-anterior chest radiograph..." Despite the fact that HRCT is expensive and necessitates additional radiation exposure, this technology mightbe useful for additional testing of some high risk individuals with normal chest radiographs who have: a) respiratory symptoms, such as cough, shortness of breath, or pleuritic chest pain; or, b) changes in their pulmonary function tests that are consistent with asbestosis.

INTERPRETATION OF HRCT

As described in the Literature Review section, there is no standardized system for the identification, categorizing, and classification of asbestos-related abnormalities by HRCT scans (similar to standardized International Labour Organization (ILO) system for classifying chest radiographs for pneumoconioses (29)).

Interstitial Disease

This study is to evaluate the prevalence of plural abnormalities and not interstitial disease. Therefore, no analytic evaluation will be conducted. However, interstitial disease might be identified by the reviewers and should be reported to the participant and their health care provider as a service to the participant. For the purposes of documentation and referral back to medical care providers, identified interstitial disease will be recorded in the following manner.The extent and severity of the asbestos-related abnormalities seen on HRCT could be assessed by either of the two of the following methods (30):

  • The first is a subjective semiquantitative method of determining the probability of interstitial fibrosis. It uses a 4-point scale incorporating extent and severity as follows:
    0 = normal, without interstitial lung disease;
    1 = a few sites of interstitial abnormality unlikely to represent diffuse interstitial fibrosis;
    2 = multifocal abnormalities consistent with asbestosis;
    3 = profuse bilateral interstitial abnormalities visible at multiple sites and on at least several CT scans.
  • The second method uses a cumulation of the different HRCT features described in the interstitial fibrosis. These had to be bilateral or on several scans in one hemithorax to be considered present and are as follows:
    a. thickening of the interlobular septa and centrilobular core structures, grouped together as interstitial lines;
    b. parenchymal bands;
    c. subpleural opacities;
    d. honeycombing;
    e. subpleural nodules;
    f. architectural distortion.

Pleural disease

Pleural disease should be scored using both a subjective evaluation with the following parameters and a quantitative system.

Subjective parameters:
(a) normal;
(b) minimal (questionable findings);

  • mild (definite findings of limited extent, scattered pleural plaques with involvement of only isolated areas of the posterior or anterior pleural spaces);
  • moderate (findings involving both posterior and anterior pleura with some areas of confluent pleural thickening, or mild chest wall findings with definite diaphragmatic involvement);
  • extensive (confluent sheet like areas of pleural thickening in more than one location at multiple levels, or moderate chest wall findings with definite extensive diaphragm involvement);
  • severe (extensive with massive involvement).

The quantitative system measures the percentage of the chest wall pleural surface that is involved with diffuse pleural thickening. Pleural plaques (small focal areas of pleural thickening with shelf like borders) are excluded. The percentage of the non mediastinal pleural surface involved with diffuse pleural thickening is estimated using a subjective analysis at the following five levels:

  • top of arch;
  • bottom of arch;
  • carina;
  • inferior pulmonary veins;
  • halfway between inferior pulmonary veins and last cut of left lung.

These five values are averaged to give the final score.

The onsite radiologist will evaluate HRCT scans for quality and provide a routine radiologic interpretation, including recording asbestos-related abnormalities. The purpose of this review is to provide an immediate referral as needed for medical care. Then, these HRCT scans will be independently evaluated by two national experts in CT assessment of asbestos-related conditions who have no clinical information. Documentation of the reviews will be recorded on a form modified from the ILO b-reader form. In addition to completing the standard B-reader form (Attachment C), panel members will summarize their overall interpretation of HRCT scans on a separate summary form (Attachment D), identifying any abnormalities that appear to be asbestos-related. The findings from the HRCT will be agreed upon by the two panel radiologists. If the two panel radiologists cannot reach an agreement and a tiebreaker review is needed, another radiologist will be used to achieve an agreement.

DATA MANAGEMENT

Data Collection

Data collection will begin with the recruitment effort. Community members eligible for the study will be contacted by mail and asked to come to a local health facility for a HRCT scanning of the chest. Key findings from HRCTs will be collected.

Data Entry and Management

Unique identifiers assigned to all participants in the Libby Community Environmental Health Project medical testing will be used in this study. This will allow investigators to compare individual HRCT results with findings from chest radiographs. Data from HRCT tests will be abstracted from standardized forms after visually inspecting for completeness and consistency. Following data entry, internal consistency computer programs, as well as validity and range checks, will be used to identify possible coding and data entry errors.

Data from this study will be kept as a separate computer file. Data will be submitted in electronic form in ASCII and SAS 6.12 formats. ATSDR will receive the original data set and merge it with the chest radiograph data collected from the above mentioned medical testing by unique identifiers to produce reports. The merged data will be analyzed using SAS (Release 6.12, SAS Institute, Cary, NC) and S-Plus software (Release 4.5, Mathsoft, Seattle, WA).

Data Protection and Privacy

As a federal public health agency, ATSDR is bound by federal law to protect the confidentiality of study participants. Information collected on participants will be kept in accordance with the Federal Privacy Act of 1974. Reports written about this testing program will provide only group information and will not identify specific individuals. Confidential records will be kept out of sight of unauthorized persons, stored in locked cabinets or locked rooms when not being used, copied only when absolute necessary, and stored in sealed containers when transferred to archives.

Informed Consent

Informed consent will be obtained from each study participant when they report for their appointment, and prior to HRCT scanning. The consent form (Attachment A) include a description of the study purpose, study procedures, risks, benefits, financial burden, use of data and confidentiality, and rights and questions.

Individual Notification

The individual HRCT results will be released to each participant and to a designated physician if requested. A person with an HRCT abnormality identified in this study without a personal physician will be referred to a local physician through a referral system coordinated by the Lincoln County Health Officer. HRCT results indicating immediate clinical significance will be given high priority and reported to a participant as soon as possible. In order to determine whether medical referrals are completed and ascertain the participant's final diagnosis and disposition, they will be asked to give written permission for ATSDR to contact the physician they identified (see Attachment E). This contact will determine whether the referral appointment was made and request the participant's final diagnosis.

Statistical Analysis

Evaluating Screening Tests Background

The adequacy of screening tests (as opposed to screening programs) is generally assessed by evaluating the validity, reliability and predictive value of the test (41). The validity of a screening test is measured by its ability to correctly categorize persons who have preclinical disease as test-positive and those without preclinical disease as test-negative. Validity is measured by calculating various statistical measures including: the sensitivity of the test, the specificity of the test, and the probability of false positive and negative results. Together, these measures paint a picture of the screening test's ability to properly identify preclinical disease.

The reliability (reproducibility) of a screening test is measured by its ability to produce consistent results when repeated examinations are performed on the same person under the same conditions. Reliability of a screening test is assessed by measuring various sources of variability including: biological variation (an individual's vital capacity may vary with time and circumstances), measurement variability (variability of medical instruments), intraobserver variability (an individual B-reader's reliability), and interobserver variability (variation between B-readers). Together, these measures paint a picture of the screening test's ability to give reproducible results.

The predictive value of a screening test is a measure of its potential yield and is closely related to the underlying prevalence of preclinical disease in the screening population. Predictive value provides a measure the likelihood that a screened individual has disease, given the results of the screening test. Predictive value is measured by calculating probabilities similar to those used to assess validity. Together, these measures paint a picture of the number of cases that will be detected by a screening program and are useful for planning follow-up to medical services for those who test positive in a screening program.

Data Requirements and Statistical Measures

For this analysis, we wish to evaluate the effectiveness of chest radiograph compared to HRCT scan. Specifically, we will evaluate the adequacy of chest radiograph as a screening tool by calculating the percent of test-negative cases that are reversed because the 'gold standard' results in a test-positive classification. This is equivalent to calculating the complement of the negative predictive value, P(D+|T-). (It should be noted that this is different than calculating the percent of positive diagnoses missed. Moreover this approach will not allow for the assessment of the validity or reliability of chest radiograph as a screening tool.)

For this analysis, a test-negative screening result will be defined as indeterminate chest radiograph. A test-positive screening result will be defined as a positive chest radiograph.

In addition to test-negative count data, we will also collect demographic data, and data that will allow us to categorize potential risk to exposure (for example, former vermiculite workers may be compared to household contacts).

A limitation of our approach (collecting data on test-negative results only) is that we will not be able to calculate the following statistical measures for X-ray screening:

Sensitivity P(T+|D+)

Probability of False Negative P(T-|D+)

Specificity P(T-|D-)

Probability of False Positive P(T+|D-)

Positive Predictive Value P(D+|T+)

Complement of Positive PV P(D-|T+),

where P(T-|D-), for example, is the probability (P) of observing a negative test (T-) for a person without disease (D-).

Based on the results of the analysis we will be able to determine the number of test-negative results reversed by the 'gold standard'. The results will apply to those participants who meet the eligibility requirements for the study. Logistic regression will be used to compare the odds of reversal among the three exposure groups.

REFERENCES

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13. Gamsu G. Computed Tomography and High-Resolution Computed Tomography of Pneumoconioses. Journal of Occupational Medicine. 1991;33(7/July):794-796.

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15. Neri S, Antonelli A, Boraschi P, Falaschi F, Rizzini D, Baschieri L. Asbestos-Related Lesions Detected by High-Resolution CT Scanning in Asymptomatic Workers. Specificity, Relation to the Duration of Exposure and Cigarette Smoking. Clin Ter 1994 Aug;145(8):97-106.

16. Zerhouni EA, Naidich DP, Stitik FP, et al. Computed Tomography of the Pulmonary Parenchyma. Part 2: Intestinal Disease. J Thorac Imag 1985;1(1):54-64.

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20. Henschke CI, McCauley DI, Yankelevitz DF, Naidich DP, McGuiness G, Miertinen OS, Libby DM, Pasmantier MW, Koizumi J, Altorki NK, Smith JP. Early Lung Cancer Action Project Overall Design and Findings from baseline Screening. Lancet 1999;354:99-105.

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25. Aberle DR, Gamsu G, Ray CS. High-Resolution CT of Benign Asbestos-Related Diseases: Clinical and Radiographic Correlation. AJR 1988;151:883-891.

26. Staples CA, Gamsu G, Ray CS, Webb WR. High Resolution Computed Tomography and Lung Function in Asbestos-Exposed Workers with Normal Chest Radiographs. Am Rev Respir Dis 1989;139:1502-1508.

27. Lerman Y, Scidman H, Gelb S, Miller A, Selikoff IJ. Spirometric Abnormalities Among Asbestos Insulation Workers. J Occup Med 1988;30:228-233.

28. Levin SM, Kann PE, Lax MB. Medical Examination for Asbestos-Related Disease. Industrial Medicine 2000;37:6-22.

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30. Gamsu G, Salmon CJ, Warnock ML, Blanc PD. CT Quantification of Intestinal Fibrosis in Patients with Asbestosis: A Comparison of Two Methods. AJR 1995;164:63-68.

31. Müller NL. Clinical Value of High-Resolution CT in Chronic Diffuse Lung Disease. ARJ 1991;157(December):1163-1170.

32. Evans SH, Davis R, Cookie J, Anderson W. A Comparison of Radiation Doses to the Breast in Computed Tomographic Chest Examinations for Two Scanning Protocols. Clin Radiol 1989;40:45-46.

33. Murphy F, Heaton B. Patient Doses Received During Whole Body Scanning Using an Elscint 905 CT Scanner. Br J Radiol 1985;58:1197-1201.

34. Rueter FG, Conway BJ, McCrohan ML, Suleiman OH. Average Radiation Exposure Values for Three Diagnostic Radiographic Examinations. Radiology 1990;177:341-345.

35. Mayo JR, Webb WR, Gould R et al. High-Resolution CT of the Lungs: An Optimal Approach. Radiology 1987;163:507-510.

36. Lucaya J, Piqueras J, Garcia-Pena P, et al: Low-Dose High-Resolution CT of the Chest in Children and Young Adults: Dose, Cooperation, Artifact Incidence, and Image Quality. AJR 2000; 175 (October): 985-992.

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39. Aberle DR, Gamsu G, Ray CS, et al: Asbestos-Related Pleural and Parenchymal Fibrosis: Detection with High-Resolution CT. Radiology 1988;166:729-734.

40. Libby, Montana Asbestos Scientific Panel, EPA (Region VIII) and ATSDR, Cincinnati, OH, February 22-23, 2000.

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ATTACHMENTS

ATTACHMENT A

Consent Form

Computed Tomography Scanning in Libby, Montana

CONSENT FORM

(Participants 18 years of age or more)

Program Sponsors

The Lincoln County Health Department, the Agency for Toxic Substances and Disease Registry (ATSDR), Department of Health and Human Services (DHHS) Region VIII Office, and the Montana Department of Health and Human Services (MDHHS) are conducting a research study involving high-resolution Computed Tomography (HRCT) scanning of selected individuals from Libby and nearby towns. About 330 people will be asked to take part in the study.

Purpose

The purpose of this study is to evaluate the usefulness of chest radiograph (chest X-rays) by using high resolution computed tomography (CT scans).

Procedures

As a participant in this study you will be asked to lay on a table in a CT scanner. The scanner will rotate around you. X-ray beams will pass through you and a compter will analyze the X-rays to produce a view of you on X-ray film. At times, you will be asked to hold your breath for several seconds. The procedure is not painful. The actual time in the CT scanner is less than 30 minutes, but the entire time in the department will take about 45 minutes.

The testing program does not take the place of your regular medical care. If we find a problem and refer you to a doctor for follow up, we would like to contact him/her in 3 - 6 months to find out your diagnosis.

Exclusion

If you are pregnant or think you could be pregnant, you should not take part in this study. We will give you a pregnancy test if you want one.

Benefits

You could benefit directly if you get early diagnosis of some asbestos-related disease. Findings from this scanning will be used to give advice about health care for each person who is tested. But, if a problem is found, we will not know for sure what caused it.

Risks

This test is a standard hospital procedure and involves minimal risk. But, the amount of radiation from this test is about 15 times more than a chest X-ray and about 3 times more than a mammogram. You may want to contact your doctor to find out whether there is a better test for you.

Financial Burden

The HRCT scanning will be provided free to people who participate in this study.You will be asked to provide your own transportation to the program clinic at a scheduled time. The Libby Community Environmental Health Program (LCEHP) and ATSDR cannot pay for any further evaluation or medical treatment of any abnormalities detected by the CT scan.

Use of Data and Confidentiality

You will receive a copy of your medical test results. Reports written about this study will give only group information and will not identify specific individuals. We will keep your information private to the extent permitted by law.

Rights and Questions

Taking part in this HRCT scanning is completely voluntary. If you decide not to take part, it will not affect either your legal rights or your access to medical care. You may stop at any time, even after signing any forms. If you stop, there is no penalty and you will not lose any benefits to which you are now entitled. St. John's Lutheran Hospital will not keep a copy or report of your CT scan unless you give specific permission to Dr. Brad Black later in this Consent Form.

If you have any questions about the testing program or feel that you were injured by being in the testing program, you may call or email Dr. Oleg Muravov (ATSDR), 1-888-42ATSDR or

1-404-639-5131 or e-mail oim0@cdc.gov

If you have any questions about your rights while taking part in this research and testing, you can call Dr. Robin Wagner, 1-888-42ATSDR or riw8@cdc.gov

Consent to Participate

I have read the information above, and agree to take part in the HRCT scanning. This scanning is being conducted by the Libby Community Environmental Health Program (LCEHP). I have been told the purpose and intended use of the data that I will provide. I have been told who will have access to these data. I am aware that this study may use some data collected by the Medical Testing Program. I have been told that taking part is voluntary, and that I can stop at any time, even after signing this consent form. I have been told what I have to do to take part in this testing program and the risks of taking part have been explained to me. I have been told that I will be able to ask any questions at any stage of the process. I know that the tests will not cost me anything. I have been told that the tests are limited to an initial evaluation. No treatment or ongoing care can be provided by this program. I have been told that I will not loose and benefits or medical care I now get if I decide not to be part of this program. I agree to take part.

Please print your name and mailing address:

Name: ___________________________________________________
Address: ________________________________________________
_________________________________________________________
Your Signature: ___________________________________________ 
Date: _______________

Release of information to your private doctor

If you would like us to send your results to your doctor, please fill in the name and address of your doctor and sign below.

I would like my test results sent to the doctor whose name and address is shown below:

Doctor's Name: ___________________________________________________
Mailing Address: _________________________________________________
__________________________________________________________________
Doctor's Phone: ( ) _______________________
Your Signature: ___________________________________________ 
Date: _______________

Release of information to EPA

To assist only in finding out how I might have been exposed and to remove current exposures, it is ok to give a copy of my test results and address with my name to EPA.

[control: checkbox] Yes [control: checkbox] No
Your Signature: ___________________________________________ 
Date: _______________

Release of information to Dr. Brad Black, Medical Director

Clinic for Asbestos-Related Disease, St. John's Lutheran Hospital

To assist local health care providers with possible treatment I might need, it is ok to give a copy of my test results including a computer copy of my CT scan with my name to the clinic.

[control: checkbox] Yes [control: checkbox]No
Your Signature: ___________________________________________ 
Date: _______________

ATTACHMENT B

Summary Form for Onsite Screening Radiologist

In addition to completing the usual clinical CT interpretation, please summarize each set of CT scans using this form.

1. Which best describes this CT? abnormal (continue) normal (skip to END)

2. Is there evidence of interstitial abnormalities?........................... [control: checkbox] yes [control: checkbox] no

If yes, choose the best category for the interstitial abnormalities from the following:

[control: checkbox] probably asbestos-related.

[control: checkbox] probably not asbestos-related.

[control: checkbox] cannot categorize regarding asbestos.

3. Is there evidence of pleural abnormalities?................................. [control: checkbox] yes [control: checkbox] no

If yes, choose the best category for these pleural abnormalities from the following:

[control: checkbox] probably asbestos-related.

[control: checkbox] probably not asbestos-related.

[control: checkbox] cannot categorize regarding asbestos.

4. Is there a need for clinical referral? ............................................ [control: checkbox] yes [control: checkbox] no

(refer any asbestos-related abnormality, as well as any other abnormalities that need clinical evaluation)

If yes, what is the basis:_______________________________________

5. Is the need for clinical referral urgent ?.................................... [control: checkbox] yes [control: checkbox]no

END

ATTACHMENT C

Standard B-Reader Form For Panel Radiologists

[image=ct-ir0image0.gif]

ATTACHMENT D

Summary Form For Panel Radiologists

Summary Form for Panel Members

In addition to completing the standard form for B-readers, please summarize each set of CT scans using this form.

1. Which best describes this CT? abnormal (continue) normal (skip to END)

2. Is there evidence of interstitial abnormalities?....................... [control: checkbox] yes [control: checkbox] no

If yes, choose the best category for the interstitial abnormalities from the following:

[control: checkbox] probably asbestos-related.

[control: checkbox] probably not asbestos-related.

[control: checkbox] cannot categorize regarding asbestos.

3. Is there evidence of pleural abnormalities?.............................. [control: checkbox] yes [control: checkbox] no

If yes, choose the best category for these pleural abnormalities from the following:

[control: checkbox] probably asbestos-related.

[control: checkbox] probably not asbestos-related.

[control: checkbox] cannot categorize regarding asbestos.

END

ATTACHMENT E

Medical Records Release Form

(This medical release form is entirely revised and replaces the original submission.)

MEDICAL RECORDS RELEASE FORM

Dr. [blank]

I request that you give researchers at the Agency for Toxic Substances and Disease Registry (ATSDR) access to my CT scan films. I have been told that the CT scan films will be taken by ATSDR researchers from your office and copied. I understand that my original CT scan films will be returned to your office as soon as the copy is made. I understand that my personal identifying information will be removed from the copy and the CT scan films will then be sent to independent radiologists for review.

I have been told that providing this information will not cost me anything and I will not receive any treatment or money as a part of this study. I understand that releasing this information is voluntary. I have been told that I will not lose any benefits or medical care I now get if I do not sign this form. I know that I can ask any questions I would like to ask and I can change my mind, even after I sign this form.

I know that this information will be used for public health purposes. I know that ATSDR will take every reasonable step to make sure no information is released that could identify me. This request expires one year from the date I signed it. A copy of this document with my signature is as good as the original.

I agree to have my CT scan films released for review.

PARTICIPANT NAME (PLEASE PRINT):

SIGNATURE (INK):

DATE:

This page last updated on October 29, 2008