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Cyclin-dependent kinase (CDK)4/6 inhibitors are utilized in locally advanced or as first-line therapy for metastatic hormone receptor positive breast cancer. However, there are limited data on safety of combined radiotherapy (RT) and CDK4/ 6 inhibition.

We conducted a multicentric retrospective study of women with breast cancer who received palliative RT or adiuvant RT within 15 days of CDK4/6 inhibitor use. The primary endpoint was toxicity per Common Terminology Criteria for Adverse Events v5.

Fourty-one patients received therapy with CDK inhibitor: 22 with ribocilcib (54%), 17 with palbociclib (41%) and 2 with abemaciclib (5%). These patients underwent 50 courses of radiotherapy. Among RT treatment (50) 11 patients (22%) received radiotherapy on breast/chestwall with an adjuvant intent with a median dose of 50 Gy in 25 fractions. The Median RT dose for the metastatic site was 30Gy. Treated sites included brain (n=10, 20%), spine (n=14, 28%), other bony sites (n=13, 26%) and others (n=2,  4%). Metastatic sites were treated with 3D technique in 21 cases and with SRS/SRT in the other 18 cases. RT was delivered concurrently or sequential CDK4/6 inhibitors in 28 (56%) and 22 (44%) cases respectively. One acute grade 3 hematologic toxicity occurred with interruption of CDKi before the RT course. No increased hematologic toxicity was attributable to RT with no grade 3 hematologic toxicities rates before, during, and 2 weeks after the end of RT

The use of RT within 2 weeks of CDK4/6 inhibitors had low acceptable toxicity and high efficacy, suggesting that it is safe for palliation of metastatic breast cancer.