The immune system may influence prognosis, and lymphopenia is a frequent side effect of concurrent chemo-radiotherapy (CCRT). Radical irradiation for locally advanced esophageal cancer (LA-EC) exposes significant vascular and heart volumes, and we hypothesized that lymphopenia is linked to cardiac and pericardial doses and affects patient prognosis. We identified 190 LA-EC patients who received radical CCRT between 2011 and 2019. Cardiac, pericardial, and lung dosimetric parameters were obtained and multivariate analysis (MVA) was performed to correlate clinical factors and dosimetric parameters with overall survival (OS). Absolute lymphocyte count (ALC), absolute platelet count (PLT), absolute white blood cell count (WBC), absolute neutrophil count (ANC), neutrophil-lymphocyte ratio (NLR = ANC/ALC), and platelet-lymphocyte ratio (PLR = PLT/ALC) were collected before and during CRT. Grade 4 (G4) lymphopenia was defined as Lymphocyte count nadir <0.2 103/mL during CRT and it was used to dichotomize the lymphocyte count nadir. MVA was performed to correlate hematologic toxicity with OS. Logistic stepwise regression was performed to determine the relationship between dosimetric parameters and G4 lymphopenia. Finally, a nomogram of G4 lymphopenia was developed and validated externally. Median follow-up time for all patients was 27.5 months (range 12-118 months). On MVA for OS (n = 190), higher pericardial V30 (PV30) was linked to worse survival (HR = 1.013, 95% CI 1.001-1.026, p = 0.039). The median OS stratified by PV30>55.3% and PV30≤55.3% was 24 months and 54 months, respectively (p = 0.004). G4 lymphopenia was shown to be linked with worse OS in the MVA of hematological toxicity (n = 161) with OS (HR = 2.042, 95% CI 1.335-3.126, p = 0.001). 24 (24%) of the 100 patients in the training set had G4 lymphopenia. Our final model comprised Stage-IVA (p = 0.017), PLR during CRT (p = 0.008), Heart V50 (p = 0.046), and PV30 (p = 0.048). External validation 26 of 90 patients (29%) had grade 4 lymphocytopenia. The ROC curve displays an AUC for internal validation of 0.775 and external validation of 0.843. Higher doses of pericardial radiation might affect LA-EC patients' prognosis by inducing G4 lymphopenia in CCRT process. Further prospective studies are warranted to confirm these findings, especially in the era of immune-checkpoint inhibitor treatment.
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