Radiotherapy (RT) is a highly effective breast cancer treatment. However, RT can deliver radiation dose to the healthy tissues of the shoulder, increasing the potential for long-term shoulder morbidity. This study compared the dose delivered to key shoulder muscles between common RT techniques and patient positioning.

The treatment plans of 54 patients were analysed, including those treated in the prone and supine positions. Eight shoulder muscles were contoured on each patient’s computer tomography scan. The following breast treatment techniques were analysed: hybrid-intensity-modulated RT (hybrid IMRT), wedged field, two partial arcs volumetric-modulated arc therapy (VMAT), hybrid VMAT, and 3-field supraclavicular technique. Moreover, the effect of patient positioning was also evaluated (supine versus prone). Muscle radiation exposure was compared for the mean dose and the percent muscle volume exposed to V15 Gy and V30 Gy.

The mean dose and exposed volume for the pectoralis major and pectoralis minor were similar between hybrid IMRT, wedged field, VMAT, and hybrid VMAT. VMAT and hybrid VMAT delivered a greater mean dose to most posterior shoulder muscles compared to hybrid IMRT, though overall exposure remained generally low for these muscles. The 3-field supraclavicular technique increased radiation exposure to all shoulder muscles, particularly to the pectoralis major, the pectoralis minor and the supraspinatus. Prone positioning significantly reduced V15 Gy and V30 Gy exposure for both anterior and posterior shoulder muscles.

Shoulder muscle exposure was similar between hybrid IMRT, wedged field, VMAT and hybrid VMAT. The anterior shoulder muscles were consistently exposed to radiation with all RT techniques investigated in this study. In comparison, the 3-field supraclavicular technique substantially increased the radiation exposure to the posterior shoulder muscles. Lastly, delivering radiation in the prone position spared the anterior and posterior shoulder muscles. These findings inform treatment planning decisions aimed at mitigating the risk of long-term shoulder dysfunction.