ESTRO 2025 - Abstract Book

S639

Clinical - Breast

ESTRO 2025

Keywords: Breast cancer. 3DCRT. BCOi.

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Poster Discussion Dynamics of Circulating Tumor Cell Response with Adjuvant or Pre-Operative Radiotherapy in Locally Advanced Breast Cancer: A Prospective Cohort Study Chelain R Goodman 1 , Simona Shaitelman 1 , Karen Hoffman 1 , Melissa P. Mitchell 1 , Jonathan B Strauss 2 , Saleh Ramezani 3 , Anam Zia 1 , Nathan Comeaux 1 , Ritupreet Virk 1 , Ruitao Lin 3 , Kevin T Nead 1 , Michael C. Stauder 1 , Eric Strom 1 , George Perkins 1 , Elizabeth Bloom 1 , Valerie Reed 1 , Melissa Joyner 1 , Pamela Schlembach 1 , Bora Lim 4 , Carlos Barcenas 4 , Benjamin D. Smith 1 , Anthony Lucci 5 , Wendy A. Woodward 1 1 Department of Breast Radiation Oncology, UT MD Anderson Cancer Center, Houston, USA. 2 Department of Radiation Oncology, Northwestern University Feinberg School of Medicine, Chicago, USA. 3 Department of Radiation Oncology, UT MD Anderson Cancer Center, Houston, USA. 4 Department of Breast Medical Oncology, UT MD Anderson Cancer Center, Houston, USA. 5 Department of Breast Surgical Oncology, UT MD Anderson Cancer Center, Houston, USA Purpose/Objective: This is a per-protocol interim analysis of a prospective observational cohort study to evaluate the dynamics of circulating tumor cell (CTC) response with adjuvant or pre-operative breast radiotherapy (RT). Material/Methods: Eligible patients dispositioned to receive comprehensive regional nodal irradiation (RNI) were prospectively enrolled, including patients co-enrolled on a prospective trial evaluating pre-operative RT. CTCs were enumerated using the CellSearch® assay immediately prior to RT (Pre-RT), within 1 week of RT (End of RT), and at follow-up (Post-RT). Rates of CTC detection and clearance with RT were calculated. Associations between clinicopathologic variables and CTC status were determined using the Fisher exact, McNemar’s Chi-square, or Wilcoxon rank-sum tests. Results: At 12 months post study activation, 153 patients had a “Pre-RT” CTC assessment, of which 107 (70%) and 46 (30%) received adjuvant or pre-operative RT, respectively, of which 25 (23%) and 16 (35%) had ≥1 detectable CTC prior to RT. The percentage of patients with detectable CTCs significantly decreased with both adjuvant RT (23% versus 5%, p<0.001) and pre-operative RT (35% versus 9%, p<0.001). Of the CTC-positive patients who completed subsequent blood draws, 79% (n=32 of 41) and 100% (n=39 of 39) were CTC-negative at the “End of RT” and “Post-RT” timepoints, respectively. Among patients receiving adjuvant RT, “Pre-RT” CTC-positive status was significantly associated with advanced nodal disease (cN3, 32% vs 12% p=0.04) and a diagnosis of inflammatory breast cancer (20% vs 4%, p<0.05). Following pre-operative RT, 14 (88%) of CTC-positive patients had residual disease at surgery; CTC clearance was not associated with pathologic complete response (pCR, 17% vs 0%) or residual cancer burden (RCB) index (3.0 [1.4-3.4]