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 D 95% 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.

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