Histologically, pheochromocytoma is composed of large pleomorphic chromaffin cells. Electron microscopy reveals electron-dense neurosecretory granules. Approximately 10% of pheochromocytomas are considered to be malignant. No histologic features exist that distinguish benign from malignant tumors. Microscopic evidence for local invasion of tissue or blood vessels, however, suggests malignancy.[1] Criteria based on tumor size, mitotic index, and DNA ploidy have been reported to be helpful in some series, though they are not always reliable predictors of biologic behavior.[2-4] Because the distinction between benign and malignant tumors cannot be made with certainty, careful surveillance is needed for a prolonged period after the initial surgical resection.

References

1. Manger WM, Gifford RW: Pheochromocytoma. New York: Springer-Verlag, 1977. 
   
   
2. Amberson JB, Vaughan ED Jr, Gray GF, et al.: Flow cytometric determination of nuclear DNA content in benign adrenal pheochromocytomas. Urology 30 (2): 102-4, 1987.  [PUBMED Abstract]
   
   
3. Hosaka Y, Rainwater LM, Grant CS, et al.: Pheochromocytoma: nuclear deoxyribonucleic acid patterns studied by flow cytometry. Surgery 100 (6): 1003-10, 1986.  [PUBMED Abstract]
   
   
4. Capella C, Riva C, Cornaggia M, et al.: Histopathology, cytology and cytochemistry of pheochromocytomas and paragangliomas including chemodectomas. Pathol Res Pract 183 (2): 176-87, 1988.  [PUBMED Abstract]
   
   

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