Modelling of Dosimetric Impact of Visceral Organ Variability During MRI-Guided Hypofractionated Stereotactic Ablative Radiotherapy (SABR) to the Pancreas

Abstract

Evaluation of visceral organ motion and dosimetric impact during pancreas SABR in non-patient subjects during the time frame of an MR-guided online adaptive radiotherapy (MRgART).Breath-hold MR images were acquired from six subjects on 0.35T MR-Linac at 30-minute intervals up to 90 minutes (intra-fraction images: F1a-d). A fifth image was acquired on a separate day (inter-fraction images: F2). SABR plans were generated to a prescription of 50 Gy in 5 fractions to mock pancreatic head tumors (median GTV 9 cc (range 8-10)) based on F1a images. Dose to visceral organs were recalculated on F1b-d and F2 images. For each visceral organ, the dose constraints used were V36 < 0.1 cc (volume of organ receiving 36 Gy < 0.1 cc), V33 < 0.5 cc and V25 < 10 cc. Volume of stomach, duodenum, small bowel and large bowel within 3 cm and 0.5 cm of PTV were recorded from each image and compared. Dosimetric impact of visceral organ motion was evaluated by comparing dose delivered to visceral organs at baseline (F1a) and the other time points (F1b-d, F2).For each subject, there was low degree of correlation between timepoint and volume of visceral organ within 3cm and .5cm of PTV (R = 0.01 and .23). Dose constraint violations were observed intra-fractionally. The highest number of dose constraint violations occurred for the duodenum, followed by stomach and large bowel (see table). No dose constraint violation was observed for the small bowel. The median magnitude of dose violations, defined as volume of organ receiving dose above constraints over mandatory volume limit, were statistically significantly lower in intra-fraction compared to inter-fraction images for the duodenum (median = .6 vs 1.9, P = 0.01).There is irregular visceral organ motion within the time frame of pancreas MRgART. This would not have been accounted for with respiratory gating. Although the magnitude of intra-fraction dose violation appears to be lower compared to inter-fraction, violations may have significant impact on toxicity. These violations are likely to be present in patients currently undergoing MRgART. However, given the reported low toxicity of dose-escalated MRgART, estimation of normal tissue toxicity based on current constraints may be limited. Further work is required to validate these findings in patients. Management of peristalsis may need to be considered. Table: Total number of dose constraint violations at each time point for 6 subjects. Each visceral organ has to meet 3 constraints V36, V33 and V25.