Palliative Radiotherapy in Metastatic Breast Cancer Patients on CDK4/6 Inhibitors: Safety Analysis

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Purpose: CDK4/6 inhibitors require meticulous monitoring due to their potential to cause hematological toxicities and hepatotoxicity. This study evaluates the safety of combining CDK4/6 inhibitors with palliative radiotherapy in patients with metastatic hormone receptor-positive and HER2-negative breast cancer. Patients and Methods: This study included 188 patients treated with CDK4/6 inhibitors between January 2021 and June 2024. Data on patient demographics, tumor characteristics, and treatment interventions were extracted from medical records. The primary focus was on the incidence of grade ≥ 3 hematologic toxicities and hepatotoxicity, assessed according to CTCAE 5.0 criteria, in those receiving concurrent palliative radiotherapy. Results: With a median follow-up of 18.5 months, the 18-month PFS and OS rates were 67% and 85%, respectively. The median age was 57.5 years, and 79% of patients were post-menopausal. Bone and liver metastases were present in 66% and 23% of patients, respectively. Concurrent palliative radiotherapy was administered in 25% of the cohort. The incidence of grade ≥ 3 hematologic toxicity was comparable between those who received radiotherapy and those who did not. Ribociclib use was associated with lower rates of grade 3 hematologic toxicity (OR: 0.37), neutropenia (OR: 0.41), dose interruptions (OR: 0.30), and dose reductions (OR: 0.37). Pre-menopausal status was linked to fewer dose reductions (OR: 0.17). Rates of treatment interruption, dose reduction, and withdrawal were 55%, 24%, and 2%, respectively. Conclusions: The concurrent use of CDK4/6 inhibitors and palliative radiotherapy does not increase the incidence of hematological adverse events in patients with metastatic breast cancer.