Changes in Pectoral Muscle Volume During Subacute Period after Radiation Therapy for Breast Cancer: A Retrospective up to 4-year Follow-up Study.

Radiotherapy (RT) is an effective treatment for managing breast cancer patients with breast conserving surgery, but patients may experience radiation-induced shoulder problems. Even though the course of shoulder morbidity is unknown, pectoral muscle changes after radiotherapy can be a major cause of shoulder problems. Twenty-two patients treated with RT for unilateral breast cancer were included in the study. All patients underwent serial computed tomography (CT) imaging before and immediately after RT, as well as 2 months, 6 months, 2 years, and 3-4 years after RT. These CT scans were used to compare muscle volume changes. The pectoral muscle volume and muscle volume surrounding the scapular measurement was performed using 3D modelling after segmentation of the CT scans. In all patients, the pectoral muscle volume increased during the 2 months after RT, and there was continuous volume reduction from 2-48 months after RT. Changes in muscle volume ratio over time were analysed by repeated measure ANOVA and it was found that there was a significant change in the pectoral muscle volume (p < 0.001) from Just before RT and Immediately after RT at 2 month after RT. On the other hand, the changes in the muscle volume of the surrounding scapular were not significant.