Kinetics and dosimetric predictors of acute radiation-induced lymphopenia in pancreatic cancer.

300Background: Radiation (RT) induced lymphopenia (RIL) is an adverse prognostic factor in pancreatic cancer (PC) and is likely due to the irradiation of lymphocytes in the RT field. The goal of this study was to identify dosimetric predictors for high rates of absolute lymphocyte count (ALC) loss during RT for PC. Methods: This was a retrospective study of 34 PC patients in an institutional database who had received concurrent 5-FU or gemcitabine-based chemoradiation (50-54 Gy) and had ≥ 3 ALCs measured during RT. Baseline ALC was normal (>1000 cells/uL) in 28/34 (82%) and grade 3-4 RIL occurred in 24/34 (71%). ALC was plotted against fraction # and a best-fit line for each patient was created to determine per-fraction loss in ALC (PFLAC). Linear regression was used to correlate PFLAC with dosimetric parameters including mean dose to gut, liver, kidney, spleen, and cisterna chyli, as well as estimated dose to immune cells (EDIC), which calculates dose to immune cells according to the % of body lymphocyte...