ESTRO 2025 - Abstract Book

S1048

Clinical – Head & neck

ESTRO 2025

Purpose/Objective: The aim of this study is to evaluate the comparative prognostic value of pre-treatment (pre-ALC) and mid-treatment (mid-ALC) absolute lymphocyte counts (ALC), and their change (∆-ALC = mid-ALC – pre-ALC) throughout treatment. Material/Methods: All patients with HPV-associated oropharyngeal carcinoma (OPC) treated with definitive chemoradiotherapy (CRT) between 2005 and 2018 at a tertiary cancer centre were considered in this retrospective analysis. Eligible patients had a pre-ALC (reading within 6 weeks of treatment initiation) and mid-ALC (reading between weeks 3-5 of radiotherapy) result available, permitting calculation of ∆-ALC. Per patient overall survival, disease-free survival, locoregional control and distant control was identified from a prospectively maintained anthology of outcomes. The competing risks and Kaplan-Meier method were applied to outcomes and the Fine-Gray competing risk regression and Cox proportional hazards regression were applied for the univariate and multivariate analyses. The C-index was calculated to assess the relative performance of the metrics. Results: A total of 494 patients were included (Table 1). Median follow-up was 5.2 (0.4-11.3) years. 68 patients had died during follow up and 78 disease recurrences had occurred. On univariate and multivariate analysis, all three metrics were found to be significantly associated with multiple disease outcomes (Table 2). We found that patients with higher pre-ALC and mid-ALC counts had more favourable outcomes. Likewise, larger falls in lymphocytes during treatment had higher association with death or disease progression. The C-index for overall survival, disease-free survival and distant control was highest for pre-ALC, while for locoregional control, ∆-ALC was found to be most prognostic.