Dosimetric comparison study of ultrahypofractionated photon versus proton treatment plans in post breast-conserving surgery breast cancer.

[BACKGROUND] There is an increasing utilization of ultrahypofractionation radiotherapy plans in whole breast radiotherapy. This study presents a comparative analysis of ultrahypofractionation using volumetric modulated arc therapy (VMAT) and intensity modulated proton therapy (IMPT) in treatment planning for patients undergoing breast conserving surgery (BCS).

[MATERIALS AND METHODS] CT datasets of twenty patients undergoing BCS, with ten on the left side and ten on the right side, were retrospectively replanned using both VMAT and IMPT techniques. The study included four scenarios: left/right breast only and left/right breast with regional nodes. A total dose of 26 Gy(RBE) was prescribed in 5 fractions. The CTV (clinical target volume) was optimized for IMPT, incorporating a 3 mm setup uncertainty and a 3.5% range uncertainty. The planning target volume (PTV) was used for VMAT optimization and evaluation for both techniques. Dose to the target volume and organs at risk (OARs) between IMPT and VMAT was analyzed.

[RESULTS] The PTV-D95, along with the V10 and V20 of the ipsilateral lungs, was comparable between VMAT and IMPT plans. The ipsilateral lung Dmean and V5 were significantly lower for the IMPT methods for breast-only radiotherapy and for the right breast with regional nodes. IMPT plans showed significantly reduced Dmean, V5, and V10 doses to the contralateral lung, as well as Dmean and V5 doses to the heart in all subgroups. The Dmean of the heart was less than 1 Gy for the left-sided and less than 0.5 Gy for the right-sided subgroup in IMPT plans. However, the IMPT plans showed a significantly higher Dmax for the skin surface in all subgroups, as well as for the esophagus in the left breast with regional nodes subgroup.

[CONCLUSION] IMPT plans significantly reduced radiation exposure to most surrounding OARs compared to VMAT. Clinical outcomes are needed to confirm the potential for reduced late toxicities. However, this benefit was accompanied by an increased skin surface dose and, in specific cases, a higher esophageal dose.