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Online adaptive MR-guided SBRT versus CT-based planning in pancreatic cancer: A single-center dosimetric comparative
Randa Kamel1, Thierry Gevaert2, Dirk Van den Berge2
1Faculteit Geneeskunde en Farmacie, Vrije Universiteit Brussel, Jette, Brussels, Belgium.
Background:
Stereotactic body radiotherapy (SBRT) for pancreatic cancer is limited by the proximity of tumors to gastrointestinal (GI) organs, increasing the risk of toxicity. Magnetic resonance-guided radiotherapy (MRgRT) offers potential advantages through superior soft tissue visualization, real-time tumor tracking, and online adaptive planning. This study aimed to quantitatively compare dosimetric outcomes of pancreatic cancer SBRT using online adaptive MRgRT versus conventional computed tomography-based image-guided radiotherapy (CT-IGRT).
Methods:
A retrospective dosimetric analysis was conducted on 100 plans from 10 patients with primary (n = 3) or recurrent (n = 7) pancreatic adenocarcinoma treated between July 2021 and December 2022 at UZ Brussel. Treatment included 80 adaptive MRgRT fractions, 10 initial MR plans, and 10 CT-based volumetric-modulated arc therapy (VMAT) plans using an internal target volume (ITV) approach. All patients were treated on the MRIdian system with daily online adaptation and real-time beam gating. Dosimetric endpoints included target coverage, plan quality metrics, and organ-at-risk (OAR) doses. Statistical comparisons were performed using Wilcoxon signed-rank tests.
Results:
MRgRT significantly reduced target volumes compared to CT-IGRT (median gross tumor volume (GTV): 40.65 cc vs. 62.56 cc; p = 0.005; median planning target volume (PTV): 64.4 cc vs. 94.4 cc; p = 0.005) and improved dose coverage of both GTV and PTV. Intermediate dose spillage (R50%) was also lower with MRgRT (5.18 vs. 6.56, p = 0.04). MRgRT plans provided superior sparing of critical GI OARs, with median D5cc relative dose reductions of 42% to the small bowel (p = 0.02), 23% to the duodenum (p = 0.02), and 13% to the stomach (p = 0.01). No significant differences were observed for the liver, kidneys, or large bowel. Treatment was well tolerated, with only grade I-II toxicities reported.
Conclusions:
MR-guided adaptive SBRT demonstrated dosimetric superiority over CT-IGRT in pancreatic cancer, with improved target coverage and enhanced GI OAR sparing. These findings support the use of MRgRT to expand the therapeutic window for safe dose escalation. Prospective studies are warranted to confirm clinical benefits.
