ESTRO 2025 - Abstract Book

Brachytherapy - Gastro-intestinal, paediatric brachytherapy, miscellaneous

S185

ESTRO 2025

Purpose/Objective: This study investigates the impact of neoadjuvant radiotherapy (nRT) on absolute lymphocyte count (ALC) and the association between ALC and pathological complete response (pCR) in rectal cancer patients. External beam radiation therapy (EBRT) in nRT has proven to be an effective downstaging treatment when given prior to rectal tumor surgical resection, increasing survival among rectal cancer patients 1,2 . However, EBRT is also associated with lymphopenia, likely due to the inclusion of primary and secondary lymphoid organs in the radiation field, which may reduce the immune response against malignancy 3 . High-dose rate endorectal brachytherapy (HDRBT), a more localized form of nRT, could potentially reduce lymphocyte depletion by sparing the lymphoid immune structures from radiation exposure 4 . We hypothesize that HDRBT will yield a higher post-treatment ALC, and that elevated ALC correlates with higher pCR rates, enhancing long-term survival outcomes. Material/Methods: A retrospective chart review identified 197 patients with rectal adenocarcinoma treated with nRT (HDRBT or EBRT) followed by surgical resection between 2008 and 2024 at the Jewish General Hospital in Montreal, Canada. The ALC values were recorded before and after nRT, and patients were categorized into ALC LOW and ALC HIGH based on the study population’s ALC median post-nRT. The pCR, defined as no viable tumor post nRT, and survival outcomes were assessed. Statistical analyses, including Mann-Whitney U test, Kaplan-Meier survival curves, and Cox regression, evaluated the associations between ALC, pCR, and survival. Results: Patients treated with HDRBT had significantly higher median post-nRT ALC (1.25 x10^9 cells/L) compared to those receiving EBRT (0.7 x10^9 cells/L, p < 0.0001) (Figure 1). Patients identified as ALC HIGH had a pCR rate of 26% compared to 13% of patients in the ALC LOW group (p = 0.034). Kaplan-Meier analysis demonstrated improved 5-year overall survival of 83.5% compared to 70.4% respectively in ALC HIGH patients compared to ALC LOW patients with (p=0.017) (Figure 2C). Similarly, disease free survival was improved in ALC HIGH (71.9%) compared to ALC LOW patients (48.5%) respectively (p=0.016) (Figure 2D).