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

## TL;DR

This study introduces a new workflow using image registration to reduce low-dose radiation in breast cancer treatment with TomoTherapy, improving safety and efficiency.

## Contribution

A deformable image registration-based workflow is proposed to personalize low-dose suppression in TomoTherapy.

## Key findings

- TOMO_FA reduced contralateral lung dose and heart V5 compared to original TomoTherapy plans.
- Radixact significantly decreased treatment time while maintaining dosimetric benefits.
- Integral dose to the body-PTV was meaningfully reduced with the new workflow.

## Abstract

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.

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.

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.

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).

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.