Feasibility of ablative stereotactic body radiation therapy of pancreas cancer patients on a 1.5 Tesla magnetic resonance-linac system using abdominal compression

Abstract

Background and Purpose: Stereotactic body radiation therapy delivered using MR-guided radiotherapy (MRgRT) and automatic breathold gating has shown to improve overall survival for locally advanced pancreatic cancer (LAPC) patients. The goal of our study was to evaluate feasibility of treating LAPC patients using abdominal compression (AC) and impact of potential intrafraction motion on planned dose on a 1.5T MR-linac. Methods & Materials: Ten LAPC patients were treated with MRgRT to 50 Gy in 5 fractions with daily online plan adaptation and AC. Three orthogonal plane cine MRI were acquired to assess stability of AC pressure in minimizing tumor motion. Three sets of T2w MR scans, pre-treatment (MRI pre ), verification (MRI ver ) and posttreatment (MRI post ) MRI, were acquired for every fraction. A total of 150 MRIs and doses were evaluated. Impact of intrafraction organ motion was evaluated by propagating pre-treatment plan and structures to MRI ver and MRI post , editing contours and recalculating doses. Gross tumor volume (GTV) coverage and organs-at-risk (OARs) doses were evaluated on MRI ver and MRI post . Results: Median total treatment time was 75.5 (49-132) minutes. Median tumor motion in AC for all fractions was 1.7 (0.7-7), 2.1 (0.6-6.3) and 4.1 (1.4-10.0) mm in anterior-posterior, left-right and superior-inferior direction. Median GTV V50Gy was 78.7%. Median D5cm 3 stomach_duodenum was 24.2 (18.4-29.3) Gy on MRI ver and 24.2 (18.3-30.5) Gy on MRI post . Median D5cm 3 small bowel was 24.3 (18.2-32.8) Gy on MRI ver and 24.4 (16.0-33.6) Gy on MRI post . Conclusion: Dose-volume constraints for OARs were exceeded for some fractions on MRI ver and MRI post . Longer follow up is needed to see the dosimetric impact of intrafraction motion on gastrointestinal toxicity.

Clinical Implementational and Site-Specific Workflows for a 1.5T MR-Linac

“…However, targets and OARs in the abdomen may be highly mobile with daily variations in position, while targets located near the diaphragm (liver, pancreas, etc.) may also be influenced by respiratory motion [ 16 ].…”

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

“…Current contouring time is around 23 mins with the commercial method as the physician recontours the whole OAR within 2 cm from the PTV because of poor accuracies. In fact, contouring time represents the most time-consuming part of overall online adaptive planning process [19] .…”

“…The pneumatic pressure level was set in consultation with the patient to minimize gross tumor volume (GTV) and adjacent organ motion within 5 mm. The details of our treatment workflow were recently published [19] . During each treatment fraction, a set of three 3D T2w MRIs (TR/TE = 1300/87 ms, voxel size = 1 × 1 × 2 mm 3 , FOV = 400 × 450 × 250 mm 3 ) – pretreatment, verification and post-treatment were acquired.…”

“…This was followed by interfraction dose accumulation on the pretreatment MRI from the previous fraction to the current fraction. Accumulated dose-volume parameters such as D0.035 cm 3 and D5cm 3 were extracted for stomach_duodenum, small bowel, and large bowel and compared with our published departmental constraints for 50 Gy in five fractions [19] . To evaluate the dosimetric performance of DIRs, the agreements between the accumulated dose-volume parameters derived from deformed contours and the ground-truth manual contours between each fraction were compared using Bland-Altman plots [28] .…”

MRI-Guided Online Adaptive Stereotactic Body Radiation Therapy of Liver and Pancreas Tumors on an MR-Linac System

“…Liver and pancreas patients were treated in free-breathing using motionaveraged or mid-position 3D image data derived from retrospectively reconstructed 4D MRI data [7]. Pancreas patients with abdominal compression were treated using a single prescription of 50 Gy in five fractions and motion-averaged 3D image data [8].…”