Patient-Specific Lymphocyte Loss Kinetics as Biomarker of Spleen Dose in Patients Undergoing Radiation Therapy for Upper Abdominal Malignancies

Purpose: Radiation therapy (RT)einduced lymphopenia (RIL) is linked with inferior survival in esophageal and pancreatic cancers. Previous work has demonstrated a correlation between spleen dose and RIL risk. The present study correlates spleen dose-volume parameters with fractional lymphocyte loss rate (FLL) and total percent change in absolute lymphocyte count (%Delta ALC) and suggests spleen dose constraints to reduce RIL risk.Methods and Materials: This registry-based study included 140 patients who underwent RT for pancreatic (n = 67), gastroesophageal (n = 61), or biliary tract (n = 12) adenocarcinoma. Patient-specific parameters of lymphocyte loss kinetics, including FLL and %Delta ALC, were calculated based on serial ALCs obtained during RT. Spearman's rho was used to correlate spleen dose- volume parameters with %Delta ALC, end-treatment ALC, and FLL. Multivariable logistic regression was used to identify predictors of >= grade 3 and grade 4 RIL.Results: Spleen dose-volume parameters, including mean spleen dose (MSD), all correlated with %Delta ALC, end-treatment ALC, and FLL. Controlling for baseline ALC and planning target volume (PTV), an increase in any spleen dose-volume parameter increased the odds of developing >= grade 3 lymphopenia. Each 1-Gy increase in MSD increased the odds of >= grade 3 RIL by 18.6%, and each 100cm(3) increase in PTV increased the odds of >= grade 3 lymphopenia by 20%. Patients with baseline ALC < 1500 cells/mL had a high risk of >= grade 3 RIL regardless of MSD or PTV. FLL was an equally good predictor of >= grade 3 lymphopenia as any spleen dose-volume parameter.Conclusions: In patients undergoing RT for upper abdominal malignancies, higher spleen dose is associated with higher per-fraction lymphocyte loss rates, higher total %Delta ALC, and increased odds of severe lymphopenia. Spleen dose constraints should be individualized based on baseline ALC and PTV size to minimize RIL risk, although our findings require validation in larger, ideally prospective data sets. (C) 2020 The Authors. Published by Elsevier Inc. on behalf of American Society for Radiation Oncology.