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Cyclin-dependent kinase (CDK) 4/6 inhibitors are utilized in the setting of advanced, hormone receptor (HR+) positive breast cancer. A potential synergy between radiation therapy (RT) and CDK4/6 inhibitors (CDKi) emerged from preclincial data. We assessed clinical outcomes of patients treated at our institution with the use of CDKi and stereotactic radiation (SRT) in the management of HR+ breast brain metastases

We conducted a retrospective analysis of patients who received stereotactic radiotherapy for HR+ brain metastases within 6 months of CDKi  administration. The primary endpoint was neurotoxicity during or after stereotactic radiation. Secondary endpoints were local control, brain control, distant control.

It was evaluated a  total of 24 lesions treated with stereotactic radiation in 10 patients. Four patients received  palbociclib (40%) and 6 patients ribociclib (60%).  RT was delivered concurrently in 7 lesions (30%) and sequential in17  lesions (70%). Median follow-up following stereotactic radiation was 11 months. One lesion (4%) developed radionecrosis without clincal symptoms. None patients developed acute severe toxicity. At 6 month 80% of patients presented brain control an 90% of patients distal control.  At 12 month 30% of patients presented brain and distal control.  None patients developed relapse in the treatment site.

Stereotactic radiation to breast brain metastases was well tolerated alongside CDK4/6 inhibitors. Compared to historical data, brain metastases control rates are similar.