# Dose Constraints in Carbon-Ion Radiation Therapy to Minimize the Risk of Pectoral Myositis

## TL;DR

This study finds that high radiation doses in small muscle areas increase the risk of pectoral myositis in breast cancer patients treated with carbon-ion therapy.

## Contribution

The study is the first to evaluate pectoral myositis after carbon-ion radiation therapy for breast cancer.

## Key findings

- Higher RBE-weighted doses in small volumes of pectoralis major muscle were significantly associated with myositis.
- LET distributions showed no significant correlation with myositis development.
- D0.1cm3 to the pectoralis major muscle in patients without myositis was consistently below 33 Gy (RBE).

## Abstract

Carbon-ion radiation therapy (C-ion RT) is an emerging nonsurgical treatment for early stage breast cancer, offering biological advantages such as high linear energy transfer (LET) and precise dose distribution. However, the risk of radiation-induced toxicity, particularly pectoral myositis, remains unclear. This study evaluates the relationship between RBE-weighted dose, LET, and pectoral myositis in patients receiving C-ion RT.

Fourteen patients with cT0-1N0M0 breast cancer were treated with C-ion RT (46-50 Gy [RBE]) in the prone position. Magnetic resonance imaging was performed pretreatment and at 3-month intervals post treatment to assess pectoral myositis. RBE-weighted dose-volume histograms and LET distributions were analyzed. Statistical comparisons between patients with and without pectoral myositis were conducted using chi-square and t-tests.

Four of 14 patients (29%) developed pectoral myositis, all within 3 months of treatment. Higher RBE-weighted doses in small volumes of pectoralis major muscle were significantly associated with myositis (D2 cm3 >10 Gy [RBE], P = .014). The D0.1cm3 to the pectoralis major muscle in patients without pectoral myositis was consistently below 33 Gy (RBE). However, LET distributions showed no significant correlation with myositis development.

This study is the first to evaluate pectoral myositis after C-ion RT for breast cancer. The findings suggest that high RBE-weighted doses in small muscle volumes increase the risk of myositis, while LET is not a significant factor. Establishing dose constraints for the pectoralis major muscle is crucial to minimize radiation-induced toxicity. Further studies with larger cohorts are needed to validate these results.

•We analyzed if pectoral myositis after carbon-ion radiation therapy is dose-dependent.•Higher doses in small volumes of the muscle were associated with pectoral myositis.•A high rate of pectoral myositis was observed when D2 cm3 exceeded 10 Gy (RBE).•No correlation was observed between pectoral myositis and the linear energy transfer.

We analyzed if pectoral myositis after carbon-ion radiation therapy is dose-dependent.

Higher doses in small volumes of the muscle were associated with pectoral myositis.

A high rate of pectoral myositis was observed when D2 cm3 exceeded 10 Gy (RBE).

No correlation was observed between pectoral myositis and the linear energy transfer.