Accelerated Partial Breast Irradiation Delivered with Helical Tomotherapy: Dosimetry and Volumetric Predictors of Ipsilateral Breast Dose

Background/Objectives: To evaluate the dosimetric performance and early clinical outcomes of five-fraction accelerated partial breast irradiation (APBI) delivered with helical tomotherapy (HT), and to identify volumetric parameters associated with ipsilateral breast dose exposure. Methods: We retrospectively reviewed early-stage breast cancer patients treated with APBI using HT (30 Gy in five fractions) between January 2024 and June 2025. Dosimetric parameters, acute toxicity, and early oncologic outcomes were analyzed. Correlation and receiver operating characteristic (ROC) analyses were performed to assess the association between volumetric parameters and ipsilateral breast V15Gy. Results: Sixteen patients were included with a median follow-up of 17.7 months. Mean PTV volume was 179.1 ± 53.9 mL, and mean PTV V95% was 98.0 ± 2.7%. Organ-at-risk sparing was favorable, with a mean heart dose of 0.82 ± 0.59 Gy and ipsilateral lung mean dose of 2.91 ± 0.45 Gy. The PTV-to-whole-breast volume ratio showed a significant correlation with ipsilateral breast V15Gy (β = 30.06, R2 = 0.34, p = 0.019) with a cutoff value of approximately 0.33 to predict ipsilateral breast V15Gy ≥ 50%. Acute toxicity was mild with no grade ≥ 2 dermatitis observed. Conclusions: Five-fraction APBI delivered with HT is dosimetrically feasible with favorable early toxicity profiles. The PTV-to-whole-breast volume ratio may serve as a simple and clinically applicable planning metric for anticipating ipsilateral breast dose.