Pectoralis muscle dosimetry and post-treatment rehabilitation utilization for early-stage breast cancer patients

Up to 50% of women treated for localized breast cancer will experience some degree of arm or shoulder morbidity. While radiation is thought to contribute to this morbidity, the mechanism remains unclear. Prior studies have shown biologic and radiographic changes in the pectoralis muscles after radiation. This study thus aims to investigate the relationship between radiation to the pectoralis muscles and referrals for rehabilitation services post-treatment for arm and shoulder morbidity. A retrospective 1:1 matched case-control study was conducted for breast cancer patients who were and were not referred for breast or shoulder rehabilitation services between 2014-2019 at a single academic institution. Patients were included if they had a lumpectomy and adjuvant radiation. Patients who underwent an axillary lymph node dissection (ALND) were excluded. Cohorts were matched based on age, axillary surgery, and use of radiation boost. Muscle doses were converted to EQD2 assuming an α/β ratio of 2.5 and were compared between the two groups. In our cohort of 50 patients of a median age 60 years (interquartile range (IQR) 53-68 years), 36 patients (72%) underwent a sentinel lymph node biopsy (SLNB) in addition to a lumpectomy. While pectoralis muscle doses were generally higher in those receiving rehabilitation services, this was not statistically significant. Pectoralis major V20-40Gy reached borderline significance, as did pectoralis major mean dose (17.69Gy vs. 20.89Gy, p=0.06). In this limited cohort of patients, we could not definitively conclude a relationship between pectoralis muscle doses and use of rehabilitation services. Given the borderline significant findings, this should be further investigated in a larger cohort.