Optimizing Dosimetry for Hypofractionated Breast Patients Treated Prone

Abstract

To analyze dosimetric factors allowing for optimal delivery of hypofractionated radiation in patients receiving whole breast irradiation. We propose the prone position will significantly reduce normal tissue exposure in these patients. We reviewed patients evaluated for whole breast irradiation and compared dosimetric parameters allowing for hypofractionation in clinically suitable patients. 138 consecutive patients underwent CT simulation and 3D conformal dosimetry planning in both the supine and prone position. Patients were treated with the plan that had optimal dosimetric characteristics as chosen by the physician (CQ). A total of 138 patients had prone and supine scans and had a target volume limited to just breast tissue. 102 patients were treated prone and 36 were treated supine. All 36 patients treated supine had better prone ipsilateral lung V20, and only 5 of the 36 had a heart V30 that was marginally worse in the prone position. 9 patients were treated supine due to patient comfort. As shown in the table below, V20 ipsilateral lung dose was significantly reduced in the prone position (p-value < 0.01) and the heart V30 in left-sided patients increased very slightly in the prone position. Average Dmax was the same for both positions. 39 patients with a Dmax less than 109% were treated using a hypofractionated dosing scheme. 34 of the 39 patients were treated in the prone position and 5 were treated supine. Of these 5 patients, 3 had optimal plans in the prone position but did not tolerate the position. For the 34 hypofractionated patients treated prone the mean ipsilateral lung V20 was 8.24% with the best supine plan, this was reduced to 1.37% when treated prone. 19 of these 34 patients were left-sided and their heart V30 was slightly higher in the prone position as compared to the supine position (0.36% v 0.16%). The table below compares the plans of all 39 hypofractionated patients in both the prone and supine positions. High energy 16 MV photons were more commonly used in the supine plans to increase the dose homogeneity (23 of 34 supine plans used 16 MV) while only 8 of 34 prone plans needed the higher energy. No difference in early toxicity was noted.Poster Viewing Abstracts 2105; Table 1All patients proneAll patients supineHypofractionated proneHypofractionated supineIpsilateral lung V201.99%8.78%0.82%8.19%Heart V30 (only Lt sided)0.38%0.23%0.31%0.17%Dmax109.62%109.65%108.00%108.10% Open table in a new tab The significant decrease in ipsilateral lung dose (p-value < 0.01) favors the use of prone positioning. As more women get treated with hypofractionated schemes, prone plans are simpler and normal tissue receives significantly less dose. More women were eligible for hypofractionation when treated in the prone position.