SU-F-T-110: LKB Modeling of Acute Hematologic Toxicity in Rectal Cancer.

PURPOSE: To investigate dosimetric parameters correlated with acute hematologic toxicity (HT) in patients with rectal cancer treated with IMRT and concurrent chemotherapy.

METHODS: We analyzed 127 rectal cancer patients receiving IMRT and concurrent chemotherapy. Whole pelvic bone marrow (PBM) was contoured for each patient including lumbosacral spine (LSSP), ilium and lower pelvic (LP). The equivalent uniform dose (EUD) of each region and the PBM were calculated. Endpoints for HT is grade ≥3 (HT3+) leukopenia, neutropenia or thrombocytopenia. Normal tissue complication probability (NTCP) was evaluated with the Lyman-Kutcher-Burman (LKB) model. On the basis of our NTCP modeling, we get the value of TD5 , TD10 , TD50 and other predictors of hematologic toxicity.

RESULTS: Twenty six patients experienced HT3+. Constrained optimization of the LKB model for HT3+ yielded the parameters m=0.583, n=1, and TD50 =59.6Gy. With this model, mean PBM doses of 1.5Gy, 15Gy, 30.5Gy, 42Gy and 51Gy result in a 5%, 10%, 20%, 30% and 40% risk of HT3+, respectively. Our model suggested chemotherapy alone induced a HT3+ complication rate of about 4.6%.

CONCLUSION: LKB modeling indicates that PBM acts like a parallel organ, implying that the mean dose to the organ is a useful predictor for toxicity. Compared with a Stanford study about anal cancer patients, the patients of anal cancer received chemotherapy with MMC and 5-FU which can cause much more severe HT than 5-FU or FOLFOX in our rectal cancer study. Therefore, the incidence of HT3+ is much higher in anal cancer and resulted in a much lower TD50 than our study.