Purpose: To quantify dosimetric gains achieved with MR guided adaptive radiation therapy (MRgART) for pancreas cancer patients treated with stereotactic body radiotherapy (SBRT).
Methods: This IRB approved study evaluated SBRT plans of 15 pancreas cancer patients treated with MRgART on a 0.35T MR linac in our department between 2018 and 2020. Prescription dose was 40Gy to PTV and 50Gy to GTV in 5 fractions with 6MV FFF X-rays. For each fraction, a reference treatment plan was re-computed on the daily MR image for dose predictions. If OARs exceeded dose tolerance, the treatment plan was re-optimized and adapted. The decision to adapt was driven by an iso-toxicity approach: V36 ≤ 0.5cc for stomach, duodenum, small and large bowels (SB, LB). We looked at adapted plan specifications, reasons for adaptation, and dosimetric gains with adaptation for each OAR (Dmax and V36Gy) and PTV coverage (V40Gy) by comparing predicted vs. re-optimized plans.
Results: Based on a review of 72 treated fractions (42 adapted), duodenum was the most common reason for plan adaptation (38.1% fractions) owing to its close proximity to pancreas, followed by stomach (31%), SB(26.2%), and LB(9.5%). For 14.3% of fractions, PTV coverage was the decisive factor. For adapted fractions, average improvements in V36Gy (cc) for stomach, duodenum, SB, and LB were (mean ± s.d.) 1.7±1.2, 2.3±1.2, 1.1±0.7, and 4.0±3.2 cc respectively. The corresponding dose reductions in Dmax for Stomach, Duodenum, SB, LB were 18.1±7.0%, 16.7±10.3%, 15.0±10.8%, and 37.0±37.8% respectively. For target coverage driven MRgART, V40Gy for PTV improved on average by 5.0±4.7%.
Conclusion: Intra-abdominal OARs undergo significant motion due to soft-tissue deformation, patient weight loss, respiration, and peristalsis. MRgART can be effective in mitigating the dose impact of these anatomical variations.