Within each GTV stratum, the dose escalation will be determined as follows: Accrue 3 patients in the first dose level based on the determined stratum. A Dose Limiting Toxicity is defined as the development of > Grade 3 acute GI toxicity, per the RTOG acute toxicity scale. If no DLT is observed at the first dose level, 3 patients will be enrolled at the next dose level. If one patient experiences DLT at a given dose level, 3 additional patients will be enrolled at that dose level. If 0 of these 3 additional patients experience DLT, dosing of the next dose level is begun. If 1 or more of these 3 additional patients experience DLT at the highest dose level below the maximally administered dose, this dose becomes the recommended dose. At least 6 patients must be entered at this recommended dose. The Maximum Tolerated Dose (MTD) is defined as the dose preceding that at which 2 of 6 patients experience dose limiting toxicity.