ESTRO 2025 - Abstract Book

S562

Clinical - Breast

ESTRO 2025

3150

Digital Poster A dosimetric and radiobiological evaluation of simultaneous integrated boost and FFF beam during locoregional radiotherapy for left breast cancer Hela Ayouni 1,2 , Bilel Daoud 3 , Sarra Saidi 2 , Amal Riahi 2 , Lotfi Ben salem 4 , Semia Zarraa 2 , Chiraz Nasr 2 1 Medical physics, Higher Institute of Medical Technology of Tunis, Tunis, Tunisia. 2 Radiotherapy oncology, Institute Salah Azaiez, Tunis, Tunisia. 3 Imaging physics research, MD anderson cancer center, Texas, USA. 4 Medical physics, Institute Salah Azaiez, Tunis, Tunisia Purpose/Objective: In the context of whole breast irradiation, there is currently no consensus regarding the most appropriate technique for delivering a simultaneous integrated boost (SIB) to the tumor bed. Considering the benefits of the FFF beam, particularly in terms of accelerating treatment while reducing peripheral dose, a comprehensive evaluation of a large irradiation field is warranted. This study aimed to compare the dosimetric and radiobiological benefits of SIB versus sequential boost (SEQ), while evaluating the impact of two irradiation modes with FFF and flattening filter (FF) using IMRT and VMAT. Material/Methods: A total of 30 patients with left breast cancer who had undergone breast-conserving surgery and received regional nodal irradiation were included in this retrospective study. The VMAT-SEQ and VMAT-SIB plans were generated and subsequently replanned in IMRT-SEQ and IMRT_SIB, respectively. Each of these plans was created using FFF and FF beams. Conventional fractionation was used, and an assessment of NTCP and several dosimetric parameters with respect to the PTVs and four organs at risk (OARs) was performed. Results: The PTVs coverage (breast, boost, supra/infraclavicular and internal mammary nodes), conformity and homogeneity were comparable between the different plans. Irrespective of the irradiation mode, the VMAT-SIB technique provided significant preservation of the heart (Dmean and V30 Gy) and left lung (Dmean, V5 and V20 Gy) in comparison to SEQ. VMAT-SIB-FFF was identified as the optimal option, demonstrating the greatest heart and left lung sparing in terms of Dmean, V30 and V5 Gy (p<0.001). Nevertheless, the reduction in V20 Gy of the left lung was observed to favor VMAT-SIB-FF (p < 0.001). For FF planning, the Dmean of the right breast was the lowest with VMAT-SIB (2.12 Gy; p<0.001), while the V5 Gy tended to be more reduced with IMRT-SIB, but with no significant difference (p=0.167). For FFF planning, IMRT-SIB was the most effective in terms of right breast (p < 0.001). For the right lung, a significant preservation of Dmean was observed with IMRT-SIB utilizing both FFF (0.74 Gy) and FF (0.86 Gy) beams. The superiority of IMRT-SIB-FFF was highlighted by the sparing of contralateral OARs (p<0.001). The value of NTCP for pneumonitis, grade ≥1 was reduced with VMAT-SIB-FFF and VMAT-SIB-FF modes, with an advantage for the FF mode (p<0.001).

Figure1: Linear regression between Dmean, V20 Gy and NTCP for left lung