Prone versus supine radiotherapy for right-sided breast cancer – a retrospective study of radiotherapy plans

Background In patients with lumpectomy for left-sided early breast cancer, whole breast irradiation (WBI) in prone position (PP) is an acceptable alternative to WBI in supine position (SP). We compared the less well explored right-sided PP WBI with SP WBI to identify the optimal organ at risk protection. Methods We retrospectively analyzed WBI-plans for 30 patients with right-sided early breast cancer in PP and SP. SP-plans were created with intensity-modulated radiotherapy (IMRT) and deep inspiratory breath hold (DIBH); PP-plans with 3D conformal radiotherapy and free breathing (FB). For better comparison of the different positioning, we additionally calculated IMRT-plans in PP and FB. We compared the extent of the clinical and planning target volumes (CTV and PTV) and the radiotherapy doses applied to CTV, PTV, right lung, heart, coronary arteries, left breast, and the right-sided axillary levels. Furthermore, we analyzed the possible impact of the breast size on the doses applied to the heart and right lung. Results Compared to SP, PP WBI was associated with lower doses applied to the right lung (Dmean right lung SP 5.6 Gy vs. 3.5 Gy PP, p < 0.001) but higher doses to the heart (Dmean heart SP 0.4 Gy vs. 0.7 Gy PP, p < 0.001) and coronary arteries, and a diminished dose coverage of the target volumes (V95 PTV PP 90.4% vs. 92.9% SP, p < 0.001). We found no statistically significant association between the CTV volume and the mean doses applied to the heart, right ventricle, right coronary artery, and right lung. Conclusions In patients scheduled for WBI in PP, the advantages for the right lung should be weighed against the cardiovascular disadvantages. However, the doses applied to both organs are not only related to the patient´s positioning but also to the used technique and could be reduced by employing a DIBH-technique in PP. A statistically significant relationship was not found between breast size and positioning suitability, but the study may be limited in power to detect small differences.