Purpose: To quantify inter-fraction deformations and dose accumulation of upper GI organs using T2w MRI in locally advanced pancreatic cancer (LAPC) patients undergoing MR-guided ablative SBRT on Elekta Unity MR-linac.
Methods: Five LAPC patients underwent 5 fraction SBRT treatment using daily online plan adaptation to 50Gy. A pneumatic compression belt was used for all treatments to minimize GTV motion within 5mm. Between each consecutive fraction, pre-treatment MRI was deformably registered to next fraction using in-house Large Deformation Diffeomorphic Metric Mapping (LDDMM) deformable image registration (DIR) method and the total dose delivered to the duodenum-stomach, small and large bowels were accumulated. Inter-fraction deformations were quantified for all three directions using gradient magnitude of the Deformation Vector Fields (DVF). The DVFs were fit to a B-spline to provide elasticity to the converging/diverging vectors resulting from large organ deformations. Registrations were assessed using Dice and Hausdorff distance (HD) calculated between the deformed and physician’s contours. DIR accumulated dose metrics (Dmax, D0.035cc, D2cc and D5cc) were compared against rigidly accumulated doses.
Results: A total of 25 MRIs were analyzed. Average Dice and 95th percentile HD were: Stomach (0.93, 1.2mm), Small bowel (0.84, 4.4mm), and Large bowel (0.88, 3.2mm). Small bowels showed the largest motion with maximum (mean±SD) inter-fraction deformation of 21.9mm (9.1±2.5mm) compared to stomach: 15.8mm (6.8±2.8mm) and large bowels: 20.9mm (8.8±3.6mm). Mean difference between rigid and DIR accumulated dose for D0.035cc and D5cc was 1.52±0.59, 1.29±0.96 and 1.56±0.9 Gy and 0.36±0.26, 0.53±0.4 and 1.43±0.44 Gy for stomach, small bowel and large bowel respectively for all the fractions.
Conclusion: LDDMM registration was able to account for large deformations of upper GI organs greater than 2cm. Rigidly accumulated doses to all organs at risk were significantly larger compared to DIR, suggesting use of deformable dose accumulation to spare major mobile structures for LAPC patients.