PurposeThis study aimed at investigating whether theirradiated volume of pelvic bone marrow (PBM) andspecific subsites may predict the occurrence of acutehematologic toxicity (HT) in anal cancer patients under-going concurrent chemo-radiation.Methods50 patients, submitted to IMRT and concurrentchemotherapy, were analyzed. Several bony structureswere defined on planning-CT: PBM and lumbar-sacral(LSBM), lower pelvis (LPBM) and iliac (IBM) bonemarrow. On dose-volume histograms, dosimetric parame-ters were taken. Endpoints included white blood-cell-count(WBC), absolute-neutrophil-count (ANC), hemoglobin(Hb) and platelet nadirs and acute hematologic toxicity(HT) according to RTOG scoring scale. Generalized linearmodeling was used to find correlations between dosimetricvariables and blood cell nadirs, while logistic regressionanalysis was used to test correlation withCG3 HT.Receiver Operating Characteristic (ROC) curve analysiswas used to evaluate the optimal cut-off points for pre-dictive dosimetric variables with the Youden method.ResultsMaximum detected acute HT comprised 38 %ofCG3 leukopenia and 32 % ofCG3 neutropenia. Grade 2anemia was observed in 4 % of patients andCG3 throm-bocytopenia in 10 %. On multivariate analysis a higherPBM-V20was associated with lower WBC nadir. IncreasedLSBM-V40was correlated with a higher likelihood todevelopCG3 HT. A cut-off point at 41 % for LSBM-V40was found. Patients with LSBM-V40C41 % were morelikely to developCG3 HT (55.3 vs. 32.4 %;p\0.01).ConclusionsIncreased low-dose to pelvic bony structuressignificantly predicted for WBC decrease. Medium–highdose to specific osseous subsites was associated with ahigher probability of HT. LSBM-V40was a strong predictorofCG3 HT. A threshold at 41 % for LSBM-V40could beused to limit HT