Study Rationale: There are limited studies evaluating real-time adaptive radiation therapy for locally advanced (Stage I-III) rectal adenocarcinoma with the goal of accomplishing organ preservation. We are testing higher doses of radiation therapy, using a novel method of real-time adaptive MRI-based radiation therapy treatment.~Hypothesis: We hypothesize that adaptive magnetic resonance (MR) -guided radiation therapy will result in acceptable toxicity and that a maximum-tolerated dose can be determined in a phase I setting.~Intervention Description: The intervention in this circumstance is higher doses of radiation therapy focused within the tumor given using adaptive MR guidance. Radiation Doses being applied in this study:~Cohort A- 64 Gy over 32 fractions, prophylactic nodes treated to 50 Gy over 25 fractions, boost to tumor and radiologically positive nodes to total dose of 64 Gy over 32 total fractions.~Cohort B- 68 Gy over approximately 34 fractions, prophylactic nodes treated to 50 Gy over 25 fractions, boost to tumor and radiologically positive nodes to 68 Gy over 34 total fractions.~Cohort C- 72 Gy over 36 total fractions, prophylactic nodes treated to 50 Gy over 25 fractions, boost to tumor and radiologically positive nodes to 72 Gy over 36 total fractions.~Dose-Limiting Toxicity from Radiation Therapy: A dose-limiting toxicity (DLT) as per NCI CTCAE version 5. Specifically, this encompasses the need for additional treatments including the following: transfusion, invasive intervention, or hospitalization indicated. The presence of such a DLT would result in the halting of radiation therapy and impact on the dose levels of radiation therapy applied in the trial.
This study is a prospective, open-label, phase I design.