SU-F-J-145: MRI-Guided Interventional Boost Radiotherapy for Rectal Cancer: Investigating the Feasibility of Adapting the Anatomy.

PURPOSE: The recent development of an MRI-linac allows adaptation of treatments to the anatomy of the moment. This anatomy, in turn, could be altered into a more favorable situation for radiotherapy purposes. The purpose of this study is to investigate the potential dosimetric benefits of manipulating rectal anatomy in MRI-guided interventional external-beam radiotherapy for rectal cancer.

METHODS: For this retrospective analysis, four patients (1M/3F) diagnosed with rectal cancer were included. These underwent MR-imaging using sonography transmission gel as endorectal contrast at time of diagnosis and standard, non-contrast, MR-imaging prior to radiotherapy planning. In the contrast scan, the rectum is inflated by the inserted contrast gel, thereby potentially increasing the distance between tumor and the organs-at-risk (OAR). Both anatomies were delineated and 7- beam IMRT-plans were calculated for both situations (RT-standard and RT-inflated), using in-house developed treatment planning software. Each plan was aimed to deliver 15Gy to the planning target volume (PTV; tumor+3mm margin) with a D99>95% and Dmax<120% of the planned dose. The D2cc dose to the OAR were then compared for both situations.

RESULTS: At equal (or better) target coverage, we found a mean reduction in D2cc of 4.1Gy/237% [range 2.6Gy-6.3Gy/70%-621%] for the bladder and of 2.0Gy/145% [range -0.7Gy-7.9Gy/-73%-442%] for the small-bowel, for the RT-inflated compared to the RT-standard plans. For the three female patients, a reduction in D2cc of 5.2Gy/191% [range 3.2Gy-9.2Gy/44%-475%] for the gynecological organs was found. We found all D2cc doses to be better for the RT-inflated plans, except for one patient for whom the bladder D2cc dose was slightly increased.

CONCLUSION: Reduction of OAR dose by manipulation of anatomy is feasible. Inflation of the rectum results in more distance between OAR and PTV. This leads to a substantial reduction in dose to OAR at equal or better target coverage.