Irradiation of pelvic bone marrow (PBM) has been correlated with hematologic toxicity (HT) in patients undergoing chemoradiation for anal cancer. Multiple studies have suggested that irradiation of the lumbosacral spine (LSBM) may be the primary cause of HT. We hypothesized that irradiation of the LSBM may be a superior predictor of absolute neutrophil count (ANC) nadir than the PBM or other anatomically-defined subsites. The cohort included 45 patients with non-metastatic anal cancer who were treated with definitive chemoradiation with mitomycin-C and 5-fluorouracil. PBM was defined as the external contour of the pelvic bones from the top of the L5 vertebral body to the bottom of the ischial tuberosity. PBM was divided into three subsites: LSBM included the entire sacrum and L5 vertebral body, iliac subsite (IBM) extended from the iliac crests to the superior border of the femoral head, and lower pelvis (LPBM) included the pubic bones, ischia, acetabula, and proximal femora. The primary endpoint was the absolute neutrophil count (ANC) nadir during or within 2 weeks of completion of treatment. Generalized linear modeling was used to analyze the correlation between the equivalent uniform doses (EUD) with “a” value of 0.5 to the PBM, LSBM, IBM, and LPBM structures and the ANC. Age, BMI, sex, baseline ANC count, immunosuppression were analyzed as potential covariates. Mean ANC nadir was 0.77 x 109/L (standard deviation: 0.66 x 109/L). Grade 3 and 4 ANC toxicity occurred in 26.7% and 44.4% of patients, respectively. Baseline ANC was the only significant covariate and was included in all models. EUD of PBM, LSBM and IBM were significantly associated with ANC nadir (Table). Model performance (R2) was not superior for the LSBM compared to the IBM and PBM models. Grade 4 ANC toxicity was significantly associated with the EUD of PBM (odds ratio per Gy (OR)): 1.04; P = 0.038), LSBM (OR: 1.02; P = 0.029), and IBM (OR: 1.04; P = 0.035). The LSBM model was not superior to the PBM model, likely because of the functional importance of the IBM region. Our results support further investigation of techniques designed to reduce irradiation of both the LSBM and IBM subsites. Whether the LPBM should be considered functionally important for avoidance of HT requires further study.Abstract 2487; Table 1.Modelß95% Confidence IntervalP-valueAdjusted R2PBM-0.065-0.109 to -0.0200.0070.226LSBM-0.031-0.059 to -0.0130.0130.206IBM-0.066-0.107 to -0.0240.0040.249LPBM-0.029-0.078 to 0.0200.2520.106 Open table in a new tab