Reducing low‐dose exposure in helical TomoTherapy for locally advanced left‐sided breast cancer with a deformable image registration–based dose‐mimicking workflow

BACKGROUND: Helical TomoTherapy provides highly conformal dose distributions for breast irradiation but is limited by extensive low-dose spillage ("low-dose bath"), contributing to increased integral dose and potential long-term toxicities. Complete blocks can suppress low-dose spread, but at the cost of prolonged treatment times on legacy TomoTherapy systems. PURPOSE: To develop and validate a deformable image registration (DIR)-based workflow that predicts patient-specific low-dose distributions and generates personalized complete blocks for TomoTherapy, aiming to reduce low-dose exposure and integral dose. A secondary objective was to determine whether Radixact, a modern helical platform, could mitigate treatment-time penalties while preserving dosimetric benefits. METHODS: Twenty-eight patients were retrospectively analyzed (18 tangential partial-arc volumetric modulated arc therapy [t-VMAT], 10 TomoTherapy). DIR-based dose prediction derived from t-VMAT atlases was used to construct complete blocks for replanning on Hi-Art (TOMO_RE) and Radixact (TOMO_FA). Dosimetric endpoints included target conformity, homogeneity, organ-at-risk (OAR) doses, and integral dose (ID). Statistical analyses used Mann-Whitney U test for independent cohorts and Friedman/Wilcoxon tests for paired TomoTherapy plans with Holm-Bonferroni correction. RESULTS: TOMO_FA significantly reduced low-dose exposure compared with TOMO_ORI, including lower contralateral lung mean dose (0.79 vs. 3.13 Gy, p < 0.01) and reduced Heart V5 (12.81% vs. 20.94%, p = 0.027). Body-PTV ID decreased meaningfully (103.14 vs. 114.52 Gy·L, p = 0.012). High-dose cardiac parameters (V25, V40) remained within clinically acceptable limits and comparable to t-VMAT. Treatment time improved substantially on Radixact (587.2 ± 44.3 s vs. 1118.0 ± 135.5 s). CONCLUSIONS: The proposed DIR-based complete block workflow effectively reduces low-dose exposure and integral dose in helical TomoTherapy without compromising delivery efficiency when implemented on Radixact. TOMO_FA represents a practical, personalized planning option, particularly for patients requiring stringent low-dose sparing.