3212 - Radiation Induced Lymphopenia Depends on Lymph Node Irradiation in Prostate Cancer Radiotherapy

L. Kuncman¹, K. Stawiski², M. Bilski³, J. Nowak-Potemska⁴, M. Bilewicz¹, and J. Fijuth¹; ¹Medical University of Lodz, Lodz, Poland, ²Medical University of Lodz, Lodz, NS, Poland, ³Medical University of Lublin, Lublin, Poland, ⁴Copernicus Memorial Hospital in Lodz Comprehensive Cancer Center and Traumatology, Lodz, Poland

Purpose/Objective(s): Radiation-induced lymphopenia is gaining prominence due to implications in cancer survivorship. No consensus is established regarding lymph node irradiation (LNI) and the optimal treatment strategy in prostate cancer (definitive radiotherapy (dRT) vs radical prostatectomy with possible following post-operative radiotherapy (poRT)). Herein we assess the impact of LNI and radiotherapy (RT) strategies on lymphopenia, trying to fill a gap in existing research.

Materials/

Methods: In this retrospective cohort study, we analyzed data from patients who received definitive radiotherapy (dRT) or post-operative radiotherapy (poRT) with or without LNI for localized prostate adenocarcinoma (stages T1-T4N0-1M0) at our tertiary cancer center in 2021.We retrieved complete blood count (CBC) of all eligible patients from the initiation of RT to January 2024. The primary endpoint was to compare depth of lymphocyte nadir between different treatment groups: 1) RT with LNI vs RT without LNI and 2) dRT vs poRT. Secondarily, we assessed the time of nadir occurrence and the time taken for recovery from nadir. We developed a generalized additive model (GAM) using LOESS for absolute lymphocyte count (ALC). U-Mann-Whitney test was used for intergroup comparison. Analyses were performed in R (version 4.2.3).
Results: 1901 CBC of 263 patients have been analyzed. Lymphopenia CTCAE =G2 occurred in 124 (47.1%) and =G3 in 50 patients (19%) and was not correlated with age or RT duration. The ALC nadirs differs (p < 0.001) between the RT with LNI group (median=0.51x10³ cells/µL) and the RT without LNI group (median= 1.01x10³ cells/µL). No similar differences were seen between different treatment strategies groups (dRT vs poRT).

Our GAM (R2=0.200) indicated that ALC nadir should be expected on the 75th, 75th, and 64th day from initiation of RT for the overall cohort, those with RT with LNI, and those with RT without LNI, respectively. Recovery from nadir is anticipated on the 142nd, 142^nd and 113th day from initiation of RT, for the overall cohort, those with RT with LNI, and those with RT without LNI, respectively.
Conclusion: Lymphopenia is induced by lymph node irradiation and occurs typically in third month after start of radiotherapy. Our results shed new light on the discussion regarding lymph node irradiation strategies in prostate cancer.