Impact of excluding internal mammary node coverage on musculoskeletal dosimetry in breast radiotherapy.

Purpose: Breast cancer radiotherapy (RT) can lead to shoulder complications including weakness, restricted motion, and discomfort, affecting up to 40% of patients. The necessity to include the internal mammary nodes (IMNs) during breast and nodal irradiation for every patient remains under discussion, particularly for early-stage breast cancers. The dosimetric effect on nearby musculoskeletal (MSK) structures when targeting the IMNs remains to be completely understood; hence the focus of this current study. Methods: This retrospective study included breast cancer patients who underwent lumpectomy and nodal sampling followed by adjuvant hypofractionated whole breast and regional nodal RT (4005 cGy in 15 fractions) who were treated between January 1, 2022, and November 30, 2023 at a single institution. MSK structures such as the bones (ribs, scapula), muscles (pectoralis, rhomboids), and joints (glenohumeral, acromioclavicular) were retrospectively contoured on the CT simulation images. Two RT plans (one with and one without IMN coverage) were created, and dosimetric parameters including mean (Dmean), near maximum (D2), near minimum (D98) and volumes received 15, 20 and 40 Gy (V15, V20, V40) were compared. Standardized mean difference between the plans was calculated for each dosimetric parameter, and Wilxocon's signed-rank test was used for comparison. Univariable linear regression analysis was used to identify patient and tumor factors that were associated with more significant dosimetric differences. Results: A total of 30 breast cancer patients with a median age of 63 (range 30-82 years) were selected for analysis. The location of tumours included 15 (50 %) in the right breast, and 15 (50 %) in the left breast; with 10 (33 %) centrally, 10 (33 %) medially, and 10 (33 %) were laterally-located within the breast. The pathologic T stage included 6 (20 %) T0/Tis, 11 (37 %) T1, and 13 (43 %) T2. Seven patients (23 %) were N0, 19 (63 %) were N1, and 4 (13 %) were N2a. Nine patients received adjuvant chemotherapy, 11 neoadjuvant chemotherapy, and 10 patients received no chemotherapy.The exclusion of IMN coverage led to significantly decreased Dmean for muscle groups in the posterior, posterolateral, lateral, anterior and antero-lateral-posterior regions. Specifically, the largest absolute reductions included teres major (Dmean 340 cGy), subscapularis (320 cGy), serratus anterior (241 cGy), latissimus dorsi (232 cGy), chestwall (209 cGy), and the pectoralis minor (37 cGy) muscles. Significant differences were also observed for V40 and V15 of pectoralis minor (V40 of 12 %), pectoralis major (V40 of 10 %), and for both subscapularis and teres major (V15 of 8 %) muscles.Patient factors that were associated with greater dosimetric differences included younger age, larger breast size, larger tumor cavity, and non-central tumor locations. Conclusion: This retrospective in silico dosmetric analysis clearly demonstrated that excluding IMN coverage for breast RT significantly reduced the dose received by the MSK structures of the neck, chest wall, and shoulder, particularly in the posterior muscle groups. Future research with larger cohorts and prospective designs would be required to validate these dosimetric benefits and their relationship to MSK-related side effects post-radiotherapy.