Ultrahypofractionated Adjuvant Proton Therapy for Localized Stage Breast Cancer: Early Toxicity Report

Abstract

<h3>Purpose/Objective(s)</h3> To report early toxicity and dosimetry data in breast cancer patients treated with ultrahypofractionated proton therapy (Ultra-HF-PT). <h3>Materials/Methods</h3> Patients with localized breast cancer (Tis-T2, N0) who required postoperative whole-breast radiotherapy were eligible. The prescribed dose was 26 Gy in 5 daily fractions ± single fraction 5,2 Gy boost. Treatment planning was performed with 1 or 2 anterior fields (0°±40°) using multi-field optimization. Robust optimization was performed on the CTV structure with a 3- mm setup uncertainty and a 3.5% range uncertainty. For 69% of patients with left-sided cancer deep inspiration breath hold technique (DIBH) was used. The target coverage was defined as V_100%>98% (>98% of volume receiving 100% of the prescribed dose). Patients were seen weekly while on treatment, at 1 month after radiation therapy completion, and at 3-month intervals thereafter. Toxicity was scored using CTCAE ver. 5.0. <h3>Results</h3> From July 2020 to November 2021, 63 patients were treated with intensity-modulated proton therapy. Median follow-up was 9 months (2 to 18). During radiotherapy Grade 1 and 2 dermatitis occurred in 45 (71%) and 4 (6%) patients respectively. After the end of treatment Grade 1 or 2 dermatitis occurred in all patients, resolving in 4 weeks. Mean dose to the left anterior descending coronary artery (LAD) was 1,28 Gy (0,12-8,2) for left-sided breast cancer patients. Mean ipsilateral lung dose was 2,66 Gy (0,55-5,8). Compared to free breathing DIBH significantly reduced mean dose to the heart (0,4 to 0,1 Gy, p=0,03) and LAD (2,5 to 1,2 Gy, p=0,03) in patients with left-sided cancer. <h3>Conclusion</h3> Limited data exists demonstrating the clinical benefit of proton radiotherapy (PRT) in breast cancer. This study is the first report of clinical data for Ultra-HF-PT in patients with localized breast cancer. Treatment was well tolerated, with acceptable rates of skin toxicity, providing low dose to normal tissue without compromising target coverage. Dose reduction to organs at risk might translate into the reduction of late toxicities.