Inter- and intrafraction motion assessment and accumulated dose quantification of upper gastrointestinal organs during magnetic resonance-guided ablative radiation therapy of pancreas patients

Abstract

Background and purpose: Stereotactic body radiation therapy (SBRT) of locally advanced pancreatic cancer (LAPC) is challenging due to significant motion of gastrointestinal (GI) organs. The goal of our study was to quantify inter and intrafraction deformations and dose accumulation of upper GI organs in LAPC patients.

Materials and methods: Five LAPC patients undergoing five-fraction magnetic resonance-guided radiation therapy (MRgRT) using abdominal compression and daily online plan adaptation to 50 Gy were analyzed. A pre-treatment, verification, and post-treatment MR imaging (MRI) for each of the five fractions (75 total) were used to calculate intra and interfraction motion. The MRIs were registered using Large Deformation Diffeomorphic Metric Mapping (LDDMM) deformable image registration (DIR) method and total dose delivered to stomach_duodenum, small bowel (SB) and large bowel (LB) were accumulated. Deformations were quantified using gradient magnitude and Jacobian integral of the Deformation Vector Fields (DVF). Registration DVFs were geometrically assessed using Dice and 95th percentile Hausdorff distance (HD95) between the deformed and physician's contours. Accumulated doses were then calculated from the DVFs.

Results: Median Dice and HD95 were: Stomach_duodenum (0.9, 1.0 mm), SB (0.9, 3.6 mm), and LB (0.9, 2.0 mm). Median (max) interfraction deformation for stomach_duodenum, SB and LB was 6.4 (25.8) mm, 7.9 (40.5) mm and 7.6 (35.9) mm. Median intrafraction deformation was 5.5 (22.6) mm, 8.2 (37.8) mm and 7.2 (26.5) mm. Accumulated doses for two patients exceeded institutional constraints for stomach_duodenum, one of whom experienced Grade1 acute and late abdominal toxicity.

Conclusion: LDDMM method indicates feasibility to measure large GI motion and accumulate dose. Further validation on larger cohort will allow quantitative dose accumulation to more reliably optimize online MRgRT.

Keywords: 1.5 T magnetic field; Deformable image registration (DIR); LDDMM; MR-guided radiation therapy; MR-linac.

Fig. 1

DIR propagated OAR segmentation produced using a) commercial DIR available in the Monaco treatment planning system for Elekta Unity MR-linac b) Physician ground truth segmentations and c) Large Deformation Diffeomorphic Metric Mapping (LDDMM) method. Segmentations for Liver (Yellow), Large Bowel (light brown), Small Bowel (green) and Stomach_duodenum (cyan) are shown. Poor or no segmentations resulting from commercial DIR method are shown in white arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

An example patient case with contours for stomach_duodenum (purple), small bowel (light green) and large bowel (dark green) for all five fractions indicating large inter-fraction motion of GI organs. Other structures include GTV (red) and kidneys (left orange; right blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Regions of largest displacement map of stomach_duodenum, small and large bowels between Fx1 and Fx2 for all five patients.

Fig. 4

Box plots showing accumulated dose metrics D0.035cm³ (left) and D5cm³ (right) for stomach_duodenum, small bowel and large bowel from interfraction (top row) and intrafraction (bottom row) deformation of all patients for all 5 fractions. Horizontal line represents the departmental constraint of 33 Gy (left) for D0.035cm³ and 25 Gy (right) for D5cm³.