Relationship Between Volume Of Bone Irradiated During Pelvic Radiation Therapy (Rt) For Rectal Cancer And Complete Blood Counts (Cbc)

551 Background: CBCs decrease during standard 3D-conformal pelvic RT for rectal cancer. We evaluated RT plans and CBCs to determine if this decrease is due to RT or only concurrent chemotherapy. Methods: Pelvic bones were contoured for 69 patients who had CT-based treatment plans and CBCs 3 months pre and during RT. CBCs included white blood cells (WBC), hemoglobin (Hgb) and platelets (PLT). The pelvic bones included the bilateral femoral heads and innominate bones, sacrum and L5. Dose volume histograms (DVH) for the pelvic bones were encoded as volumes (Vol) receiving 10 (V10) to 50 Gy (V50) in cc, and percent (Perc) (P10 to P50). The paired t-test was used to compare CBCs pre and during RT. CBCs were analyzed with a repeated measures general linear model for uneven time intervals (R 2.15.1). Results: Median age was 60 y, with 40 men (58%) and preoperative RT in 32 (46%). The median RT dose was 50.4 Gy (range 25.2-60.4 Gy), with all but 2 patients receiving ≥45 Gy. Among 677 CBCs, there were 675 WBC, 677 Hgb, and 672 PLT. The average WBC pre and during RT were 7.7 vs 5.4 bil/L (p<0.001), Hgb 12.9 vs 12.5 g/dL (p=0.005), and PLT 271 vs 227 bil/L (p<0.001). A statistical model with variables for time, the 3-week chemotherapy cycle, the pre-RT average CBC, and 1st and 2nd-order interactions of time with Vol and Perc was constructed. All Vol and Perc for all bones as one structure were significant predictors of the PLT timecourse during RT, with the first-order time-dose interaction encoding a decrease over time, and the second-order time-dose interaction a recovery towards the end of RT. For WBC, V40 and P40 were most predictive (p<0.001), with P40 more so. There were no time-dependent interactions for Hgb. All models had a significant effect of pre RT average CBC and the 3-week chemo cycle. Conclusions: Whereas chemotherapy acts systemically, RT affects a circumscribed area. As such, bone marrow outside the RT field can compensate for lost hematopoiesis over time. Pelvic bone RT dose correlates with PLT and WBC (circulation half-life 1-3 d) but not Hgb (half-life 120 d) supporting an effect of RT in decreased CBC during treatment. Research of approaches that minimize bone marrow dose during pelvic radiotherapy is warranted.