Abstract Abstract #5129 Introduction: Prone whole breast radiotherapy is feasible and can be delivered with a hypofractionated, concomitant boost IMRT regimen (JCO, vol. 25, 16, 2007). We hypothesized that in occasional patients a supine set-up could better spare heart and lung while optimally treating the target breast. We designed NYU 05-181 protocol to test this hypothesis. Herein we performed a dosimetric analysis by comparing treatment plans in both the prone and supine position for patients with left breast cancer.
 Methods: Early stage breast cancer patients after segmental mastectomy are eligible for this IRB-approved prospective study. A supine CT simulation with images acquired every 2.5 mm is performed. The patient is then repositioned prone using a dedicated breast mattress and a second CT is obtained. The treatment volume (TV) is the whole index breast, determined according to conventional breast simulation with the patient in the supine position. Each patient is planned in both positions and is treated based on the optimal plan defined as the one that best satisfies each of the following criteria: 1) inclusion of the TV; 2) maximum heart sparing; 3) maximum lung sparing. Dosimetric analysis of the prone and supine treatment plans was performed to compare coverage of the whole breast, PTV coverage of the tumor bed, and the dose to lung and heart.
 Results: Twenty-five patients with a left breast cancer were analyzed. Five patients were treated supine, and 20 patients were treated prone because this position enabled optimal breast coverage and heart and lung sparing. However, all patients were simulated and planned in both positions. As shown in Table 1, whole breast coverage is better for the prone position although not statistically significant (p=0.38), PTV coverage was superior in the prone position (p<0.001).
 
 Importantly, heart dose was not increased in the prone position (p<0.001). Additionally, lung dose was markedly reduced in the prone position (p<0.001).
 Conclusions: Contrary to many concerns that the prone position leads to anterior displacement of the heart within the treatment field this was only observed in 5 of the 25 patients. This dosimetric analysis demonstrates that in fact prone breast irradiation when compared to supine results in reduced exposure to both heart and lung while enabling complete coverage of the whole breast and tumor bed for the vast majority of patients. There are few left breast patients who are better positioned supine and the NYU protocol 05-181 is currently accruing to fully characterize this infrequent subset. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5129.