Dosimetric comparison of the shoulder region between paired photon and proton plans in breast cancer patients.

[BACKGROUND] Adjuvant radiotherapy lowers regional recurrence in breast cancer but may cause arm symptoms. We compared photon (XRT) and proton therapy (PT) doses to eight shoulder structures, including seven muscles and the axillary-lateral thoracic junction (ALTJ).

[METHODS] We consecutively included all patients treated with PT in our centre for whom paired XRT-PT plans (2019-2024) were available. Eight shoulder structures were autodelineated with manual correction of the ALTJ region. For each parameter, medians (Q1-Q3) were calculated, and paired differences were defined as Δᵢ = XRTᵢ - PTᵢ and summarized by the median of Δᵢ for subject i; Statistical inference used the Wilcoxon signed-rank test with Hodges-Lehmann estimates (95% confidence intervals). Multiple comparisons were adjusted using the Benjamini-Hochberg procedure. Exploratory subgroup analyses were stratified by Regional nodal irradiation (RNI) levels. Analyses were performed in R (version 4.5.2).

[RESULTS] In 128 paired plans, XRT delivered significantly higher doses than PT to all shoulder muscles. For the five key muscles, XRT delivered 10.0-21.1 Gy higher D and 29.6-64.6% higher V, indicating a high advantage for PT. A significant moderate advantage was observed for the latissimus dorsi, with minimal yet significant for the trapezius and ALTJ. Subgroup analyses further showed that XRT delivered significantly higher doses than PT to the five key muscles in patients receiving RNI at levels III-IV or I-IV.

[CONCLUSION] PT offers substantial dosimetric advantages for five shoulder muscles compared with XRT. The clinical relevance of these dosimetric differences requires further study.