• Difference in lung NTDmean (biologically weighted [normalized] mean of total dose to lung normalized to 2 Gy fractions) in supine vs prone positions [ Designated as safety issue: No ]
  

• Closeness of agreement between MRI-based vs CT scan/clip- based delineation of tumor bed [ Designated as safety issue: No ]
  
• Closeness of agreement between clinical target volumes (CTVs) defined using uniform margin vs those defined using knowledge of all excision margins [ Designated as safety issue: No ]
  
• Difference in NTDmean to ipsilateral non-planning target volume (PTV) breast tissue, chest wall muscle, and heart [ Designated as safety issue: No ]
  
• Difference in mean volume of PTV in supine versus prone positions [ Designated as safety issue: No ]
  
• Difference in dose homogeneity within PTV in each position [ Designated as safety issue: No ]
  
• Difference in complexity of beams needed to achieve above dose homogeneity within PTV [ Designated as safety issue: No ]
  
• Patient comfort with each positioning technique (assessed by questionnaire) [ Designated as safety issue: No ]
  

OBJECTIVES:

• To improve precision of tumor bed localization and definition of clinical target volume, and to reduce normal tissue irradiation in women undergoing partial breast or breast boost radiotherapy.
• To test whether post-operative MRI improves the precision of tumor bed delineation after wide-local excision in comparison with the current standard CT scan/clip method.
• To determine the impact of tumor position within the excision specimen upon the localization of clinical target volume in relation to the tumor bed.
• To compare theoretical non-target tissue exposure from partial breast irradiation planned in the supine and prone (face-down) positions.

OUTLINE: Patients undergo planned breast-conservation surgery and placement of titanium clips to the four radial, the deep, and superficial margins of the excision cavity (for localization of tumor bed).

Within 2 weeks after surgery, patients undergo supine radiotherapy-planning CT scan as standard analysis. Patients then undergo a radiotherapy-planning CT scan in the prone position. Patients complete a linear analogue questionnaire after both scans designed to assess patient comfort and anxiety in each position. Patients then undergo multifunctional MRI (including dynamic contrast-enhancement MRI and diffusion-weighted MRI) of the ipsilateral breast in the prone position (≥ 3 weeks after surgery). If suspicious lesions ≥ 5 mm are found on MRI, patients are referred for a second-look ultrasound with biopsy (if lesion visible on ultrasound); where suspicious lesions are seen only on MRI, patients undergo MRI-guided biopsy. Lesions < 5 mm are included in the whole-breast radiotherapy treatment.