Online Adaptive MRI-Guided Stereotactic Body Radiotherapy for Pancreatic and Other Intra-Abdominal Cancers

MRI can provide better visualization of tumors and nearby organs at risk (OAR) than CT for fast and accurate contouring during online adaptive MRI-guided stereotactic body radiation treatment (MRI-guided SBRT) for pancreatic and other intra-abdominal cancers. Pre-set MRI sequences provided in a 1.5T MRI scanner hybrid with a linear accelerator can be used during MRI-guided SBRT, but they often limit tumor and OAR visualization and require a long image acquisition time. This study retrospectively analyzed 26 patients with pancreatic and intra-abdominal cancers that underwent CT and MR simulations and 3-5 fractionated MRI-guided SBRT. The visualization of tumors and OAR was improved with T1W imaging, which is essential for online adaptive planning and resulted in fast and accurate contouring in a shorter imaging time. A 1.5T MRI combined with a linear accelerator (Unity (R), Elekta; Stockholm, Sweden) is a device that shows promise in MRI-guided stereotactic body radiation treatment (SBRT). Previous studies utilized the manufacturer's pre-set MRI sequences (i.e., T2 Weighted (T2W)), which limited the visualization of pancreatic and intra-abdominal tumors and organs at risk (OAR). Here, a T1 Weighted (T1W) sequence was utilized to improve the visualization of tumors and OAR for online adapted-to-position (ATP) and adapted-to-shape (ATS) during MRI-guided SBRT. Twenty-six patients, 19 with pancreatic and 7 with intra-abdominal cancers, underwent CT and MRI simulations for SBRT planning before being treated with multi-fractionated MRI-guided SBRT. The boundary of tumors and OAR was more clearly seen on T1W image sets, resulting in fast and accurate contouring during online ATP/ATS planning. Plan quality in 26 patients was dependent on OAR proximity to the target tumor and achieved 96 +/- 5% and 92 +/- 9% in gross tumor volume D90% and planning target volume D90%. We utilized T1W imaging (about 120 s) to shorten imaging time by 67% compared to T2W imaging (about 360 s) and improve tumor visualization, minimizing target/OAR delineation uncertainty and the treatment margin for sparing OAR. The average time-consumption of MRI-guided SBRT for the first 21 patients was 55 +/- 15 min for ATP and 79 +/- 20 min for ATS.