# The dosimetric and radiobiological effects of rotational errors in breast cancer radiotherapy

## TL;DR

This study shows that rotational errors during breast cancer radiotherapy can significantly affect treatment outcomes by reducing tumor control and increasing risks to healthy tissues.

## Contribution

The study introduces a detailed analysis of how rotational errors across multiple axes impact dosimetry and radiobiological outcomes in breast cancer radiotherapy.

## Key findings

- Rotational errors significantly altered dose distributions, affecting tumor control probability and normal tissue complication probability.
- Pitch and yaw rotations were most likely to cause dose deviations, with multi-axis rotations posing greater risks.
- Left- and right-sided breast cancers showed different patterns of dose deviation based on rotational direction.

## Abstract

This study aimed to assess the dosimetric and radiobiological consequences of rotational errors in breast cancer radiotherapy.

A retrospective analysis involving 80 breast cancer patients was performed. We simulated 124 rotational scenarios across three axes (yaw, pitch, and roll) to generate rotated dose distributions, then evaluated their dosimetric effects alongside tumor control probability (TCP) and normal tissue complication probability (NTCP) for the target and organs at risk (OARs).

Rotational errors caused significant dose deviations. The D95 for PTV, heart, ipsilateral lung, contralateral breast, and left anterior descending artery (LADV40) in rotated dose distributions showed statistical significance compared to the original dose (p < 0.05), except for LADV40 in the whole‐breast radiation therapy (WBRT) left4005 cohort (p = 0.058). LADV40 and PTVsc were primarily affected in left‐sided breast cancer, while PTVsc and PTVcw were predominantly affected in right‐sided breast cancer. TCP decreased from 0.91 to 0.81 in the WBRT4005 cohort, from 0.95 to 0.70 in the WBRT5000 cohort, and from 0.87 to 0.23 in postmastectomy radiation therapy (PMRT)5000 cohort. NTCP for the heart increased from 2.08 × 10−14 to 5.78 × 10−10, and NTCP for the lung increased from 1.82 × 10−3 to 1.37 × 10−2. When single‐axis rotations were involved, the pitch direction was more likely to cause dose deviations. For multi‐axis (≥2) rotations, opposite‐direction yaw and pitch rotations in left‐sided breast cancer and same‐direction yaw and pitch rotations in right‐sided breast were more prone to exceeding dose limits.

The dosimetric and radiobiological analysis of rotational errors demonstrated that rotational errors (particularly in pitch and yaw) induce significant dosimetric deviations, compromising tumor control probability and increasing normal tissue complication probability. It identifies rotational axes requiring correction during multi‐axis rotations and provides recommendations for prioritizing targets versus organs at risk during image‐guided registration. Implementing these corrections and prioritization strategies is essential for optimizing therapeutic efficacy in breast radiotherapy.