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 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.