Cyclin-Dependent Kinase 4/6 Inhibitors Combined With Radiotherapy for Patients With Metastatic Breast Cancer

Background: The cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) represent the standard treatment for hormone receptor–positive, human epidermal growth factor receptor 2–negative metastatic breast cancer. Data about the balance between efficacy and toxicity of combined palliative radiotherapy (RT) and CDK4/6 inhibition are lacking. Patients and Methods: We undertook a review of 46 patients with metastatic breast cancer on systemic treatment with CDK4/6i who underwent 62 metastases-directed RT. Clinical, laboratory, and RT treatment planning data were collected. Statistical analyses included Student t test, paired sample t test, and logistic regression modeling. Results: Thirty patients (65.2%) received palbociclib, 15 (32.6%) received ribociclib, and one patient received abemaciclib (2.2%). Median total prescribed RT dose was 20 Gy (range, 8-63 Gy). Sites of RT were bone (n = 50; 80.7%), visceral (n = 7; 11.3%), or brain metastases (n = 3; 4.8%), as well as primary tumor of the breast (n = 2; 3.2%). Overall, the rates of grade 3 or higher adverse events (AEs) were 6.5%, 4.3%, 15.2%, and 23.9% before the start of RT, during RT, 2 and 6 weeks after RT completion, respectively. We found no correlation between dose distribution to organs at risk and the development of AEs. The local control rates for the entire cohort were 98% at 6 months and 90% at 12 months. Overall, pain relief (complete or partial) was experienced by 80% (24/30) of patients who initially reported pain at the treated metastatic site. Conclusion: We observed a modest increase in the rates of grade 3 or higher AEs after combined RT and CDK4/6i, with maintained efficacy of concomitant RT.