ESTRO 2021 Abstract Book

S45

ESTRO 2021

Purpose or Objective Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i), in combination with endocrine therapy (ET), currently represent the standard of care for I-II line metastatic hormonal receptor (HR)-positive HER2-negative breast cancer, demonstrating a significant improvement in terms of both progression-free (PFS) and overall survival (OS) when compared to ET alone. In the metastatic setting, radiation therapy (RT) is often required with either palliative or ablative intent. The aim of our study was to evaluate the safety and efficacy of concomitant RT and CDK4/6i in metastatic HR+/HER2- breast cancer patients, comparing with patients who did not received RT. Materials and Methods We analysed data of 133 patients consecutively treated in two leading European institutions with I-II line CDK4/6i from September 2017 to February 2020. Both hematologic and non-hematologic acute toxicity have been evaluated and scored according to CTCAE v.5.0. Results We performed an analysis by simple cross-tables with chi-square test, and logistic analysis to confirm emerged associations between outcomes and several parameters. Median age of the series was 59 years (range 37-86). At the time of metastatic disease diagnosis, 111 patients (83.5%) were postmenopausal. One-hundred twelve patients (84.2%) were treated with palbociclib, 19 with ribociclib, 2 with abemaciclib. Fourty-six percent of patients received letrozole (I line) and 54% received fulvestrant (II line) as ET. At a median follow up of 13.2 months, PFS rate was 53.4%. Fifty-nine patients received concomitant RT with palliative (79.7%) or ablative intent (20.3%) during the course of CDK4/6i, while 74 patients did not. Bone was the most frequently treated RT site (81.4%), with 3-dimensional conformal radiation therapy (3D-CRT) as the most commonly technique used (79.7%). RT was not significantly associated with ≥G2 (p=0.45) and any grade toxicity (p=0.69). Furthermore, there was no association with RT and CDK4/6i dose reductions (p=1.0) and discontinuation (p=0.07) [Table 1]. Overall, the use of CDK4/6i plus ET in first or second line did not show an impact on ≥G2 toxicity development (p=0.71), as well as on dose reductions (p=0.39) and treatment discontinuation (p=0.66). Postmenopausal status was the only factor associated with a significantly increased risk of ≥G2 toxicity (p=0.005). Among patients who received RT, no significant associations were found regarding RT intent, technique, and bone or visceral RT site [Table 2] .

Conclusion Our study showed that concomitant administration of RT with either palliative or ablative intent during CDK4/6i is safe and effective, without increased toxicity and significant impact on systemic treatment conduction. Further studies on larger series are needed to confirm these findings. OC-0072 Hypofractionated radiotherapy for breast cancer: Findings from an international ESTRO-GIRO survey M. Mushonga 1,2 , J. Weiss 3 , A.Z. Liu 4,5 , O. Mohamad 6 , Y. Lievens 7 , D. Rodin 1,8 1 Princess Margaret Cancer Center, Radiation Oncology, Toronto, Canada; 2 Sally Mugabe Hospital, Department of Oncology, Harare, Zimbabwe; 3 Princess Margaret Cancer Centre, Department of Biostatistics, Toronto, Canada; 4 Princess Margaret Cancer Centre, Department of Biostatistics, Toronto, Canada; 5 Dalla Lana School of Public Health, University of Toronto, Biostatistics, Toronto, Canada; 6 University of California , , Department of Radiation Oncology, San Francisco, USA; 7 Ghent University Hospital and Ghent University,