Abstract A47: Circulating tumor cell-defined minimal residual disease in locally advanced rectal cancer treated with multimodality therapy

Abstract Background: Current treatment for locally advanced rectal cancer (LARC) includes multimodality therapy in the neoadjuvant and adjuvant settings. Response to neoadjuvant therapy (NT) is prognostic of long-term outcomes. While long-course chemoradiation (CRT) had been the traditional NT regimen, novel regimens adding systemic therapy and/or eliminating radiation have been introduced. We examined if circulating tumor cells provide prognostic information independent of the specific treatment regimen utilized. Specifically, we aimed to define the significance of CTC-defined minimal residual disease (MRD) in LARC. Methods: A prospective longitudinal protocol enrolled LARC patients (N=100; clinical stage II=4; stage III=96) undergoing NT. Peripheral blood was collected at baseline (t1, treatment-naive), after NT (t2, intraoperatively before tumor manipulation), after resection (t3, 2-8 weeks postoperatively), and after completion of adjuvant therapy (t4, 2-12 weeks after completion). CTC was enumerated by the CellSearch® platform within 72 hours of collection. Patients were followed for disease-free survival (DFS). Results: At t1, CTCs were detected in 28/90 (31.1%) patients with available samples with a median CTC count of 1.5 [IQR: 1.0, 2.75]. NT consisted of long-course pelvic CRT (65%), extended NT with systemic chemotherapy and long- or short-course pelvic CRT (22%), and a radiation-sparing regimen with systemic chemotherapy only (13%). At t2, CTCs were detected in 34/75 (45.3%) patients with a median CTC count of 1 [IQR: 1, 1]. Type of NAT did not correlate with CTC positivity (p=0.637). Ten (9.4%) patients declined surgery, while cPR occurred in 12/93 (12.9%) surgical patients. Postoperatively (t3), CTC was detected in 12/47 (25.5%) patients, with a median count of 1.5 (IQR: 1.0-3.0). After completing adjuvant chemotherapy (t4), only 3/29 (10.3%) patients had detectable CTC. With a median follow-up of 47.50 months from diagnosis for the entire cohort, CTC positivity at t4 significantly stratified DFS (p=0.035). Conclusions: One third of locally advanced RC patients harbored detectable CTCs at baseline. CTC detection after completion of curative-intent multimodality therapy (i.e., MRD) correlated with long-term DFS. Citation Format: Lucas Lee, Carol Hall, Antony Lucci, Brian Bednarski, Miguel Rodriguez-Bigas, George Chang, Y. Nancy You. Circulating tumor cell-defined minimal residual disease in locally advanced rectal cancer treated with multimodality therapy [abstract]. In: Proceedings of the AACR Special Conference on Advances in Liquid Biopsies; Jan 13-16, 2020; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(11_Suppl):Abstract nr A47.