Clinical practice patterns for advanced breast cancer patients on CDK4/6 inhibitors receiving palliative radiation therapy.

269 Background: Patients with hormone-receptor positive advanced breast cancer (ABC) often require palliative radiation therapy (RT) while receiving systemic treatment with CDK4/6 inhibitors (CDK4/6i). There are conflicting reports in the literature regarding whether concurrent administration of CDK4/6i and RT increases RT or hematologic toxicity and there are currently no formal guidelines for this realm. A Canadian national survey was conducted to evaluate local practice patterns of CDK4/6i management during palliative RT. Methods: An anonymized online survey was distributed to 162 Canadian breast cancer health care professionals between November 2020 and January 2021. The survey collected provider demographics and questions regarding practice, experiences and opinions on CDK4/6i management during palliative RT for ABC. Results: The survey was completed by 76 (47%) of the invited participants: 40% were medical oncologists, 26% radiation oncologists, 16% pharmacists and 18% nurses, physician assistants or radiation therapists. Nine provinces were represented. The respondents' clinical practice settings were predominantly at an academic/cancer centre (84%), while 16% of clinicians were based at a community setting. Interrupting the CDK4/6i during RT was recommended always by 21% of respondents, sometimes by 46% and never by 9%, while 24% had no opinion. The majority of opinions were based on personal experience (55%), colleagues’ practice (37%), medical literature (33%) and experience with chemotherapy agents (18%). Unexpected RT toxicity observed in patients on concomitant CDK4/6i was reported by 9% of respondents and prolonged cytopenias by 15%. Among responders who always or sometimes interrupt CDK4/6i during palliative RT, the timeframe to hold CDK4/6i prior to RT was 4-7 days 45%, 1-3 days 32%, 8-14 days 13% and 10% were unsure. Responses were similar for the timeframe used to resume the drug after RT. The majority (94%) thought that advising the patient on what to do with the CDK4/6i during RT was the role of the Medical Oncologist, while 48% also thought it was the role of the Radiation Oncologist. 23% of respondents though the patient should always be reassessed prior to restarting the CDK4/6i; 45% said sometimes, and 29% said not necessary. 82% of respondents indicated a standardized protocol or guideline would be valuable in this setting. Conclusions: Two thirds of Canadian breast cancer specialists sometimes or routinely interrupt CDK4/6i treatment during RT with 15% having observed increased toxicity with concurrent administration. Consensus guidelines for the management of CDK4/6i and RT are necessary to reduce treatment variability and improve the quality and safety of care for these patients.