CT-guided versus MR-guided radiotherapy: Impact on gastrointestinal sparing in adrenal stereotactic body radiotherapy

Background and purpose: To quantify the indication for adaptive, gated breath-hold (BH) MR-guided radiotherapy (MRgRT(BH)) versus BH or free-breathing (FB) CT-based image-guided radiotherapy (CT-IGRT) for the ablative treatment of adrenal malignancies. Materials and methods: Twenty adrenal patients underwent adaptive IMRT MRgRT(BH) to a median dose of 50 Gy/5 fractions. Each patient was replanned for VMAT CT-IGRT(BH) and CT-IGRT(FB) on a c-arm linac. Only CT-IGRT(FB) used an ITV, summed from GTVs of all phases of the 4DCT respiratory evaluation. All used the same 5 mm GTV/ITV to PTV expansion. Metrics evaluated included: target volume and coverage, conformality, mean ipsilateral kidney and 0.5 cc gastrointestinal organ-at-risk (OAR) doses (D-0.5cc). Adaptive dose for MRgRT(BH) and predicted dose (i.e., initial plan re-calculated on anatomy of the day) was performed for CT-IGRT(BH) and MRgRT(BH) to assess frequency of OAR violations and coverage reductions for each fraction. Results: The more common VMAT CT-IGRT(FB), with its significantly larger target volumes, proved inferior to MRgRT(BH) in mean PTV and ITV/GTV coverage, as well as small bowel D-0.5cc. Conversely, VMAT CT-IGRT(BH) delivered a dosimetrically superior initial plan in terms of statistically significant (p <= 0.02) improvements in target coverage, conformality and D-0.5cc to the large bowel, duodenum and mean ipsilateral kidney compared to IMRT MRgRT(BH). However, non-adaptive CT-IGRT(BH) had a 71.8% frequency of predicted indications for adaptation and was 2.8 times more likely to experience a coverage reduction in PTV D95% than predicted for MRgRT(BH). Conclusion: Breath-hold VMAT radiotherapy provides superior target coverage and conformality over MRgRT(BH), but the ability of MRgRT(BH) to safely provide ablative doses to adrenal lesions near mobile luminal OAR through adaptation and direct, real-time motion tracking is unmatched. (C) 2021 The Authors. Published by Elsevier B.V.