P175] Simultaneous boost in breast radiotherapy requires increased margins or revised match strategy

Abstract

Purpose In radiotherapy simultaneous boost compared to sequential boost allows shortening of the overall treatment time. Challenges remain in obtaining good image match concurrently within the boost volume and total target volume. The present study evaluated the difference between match based on boost volume and total target volume, with the overall aim to suggest treatment margins if treating both boost and total target volume using the same registration. Methods A total of 358 cone-beam CT (CBCT) s from 55 patients were analysed retrospectively. Of these 250 CBCTs were performed before each fraction on 16 patients receiving radiotherapy after breast conserving surgery and with lymph node involvement, and 39 patients receiving radiotherapy after breast conserving surgery without lymph node involvement were scheduled for CBCT once a week, and reached a total of 108 CBCTs. Two matches were performed with each CBCT: A chest wall-match where CBCTs were auto-registered to the planning CT, lymph nodes included if relevant (a match on the total volume). Further a boost specific match using manual registration of surgical clips. Results Based on the systematic (Σ) and random errors (σ) between the chest wall-match and the boost match the CTV-to-PTV margins of the boost volume were calculated. Margins needed for the boost volume to receive the prescribed dose using a match based on the total target volume are (lateral, longitudinal, vertical) = (7.3, 8.4, 8.5) mm and (5.1, 7.5, 4.3) mm, for the patients with lymph node involvement and without lymph node involvement data, respectively. Conclusions CTV-to-PTV margin for the boost area are normally set to 5 mm when using a sequential boost strategy. Our study suggests that with simultaneous boost radiotherapy larger margins are required. Further, patients with lymph node involvement compared to patients with only the breast involved requires a larger CTV-to-PTV margin. Another solution can be to use an adaptive protocol for daily IGRT which can reduce the required CTV to PTV margin for the boost region by re-scanning and re-matching the boost region for patients where the two regions differ by more than a pre-set threshold.