Impact of Breath-Hold MR-Guided Radiotherapy (MRgRTBH) vs. Free-Breathing CT Image-Guided Radiotherapy (CT-IGRTFB) on Gastrointestinal Sparing and Dose Conformality in Adrenal SBRT.

PURPOSE/OBJECTIVE(S) To quantitatively compare plan quality between mid-inspiration breath-hold (BH) MR-guided radiotherapy (MRgRTBH) and free-breathing (FB) CT-based intensity modulated radiotherapy (CT-IGRTFB) for the ablative treatment of adrenal malignancies. We hypothesized that MRgRTBH would provide improved target coverage and lower organ-at-risk (OAR) doses due to smaller target volumes compared to CT-IGRTFB. MATERIALS/METHODS Twenty adrenal metastasis patients were treated on a 0.35T MR-Linac with real-time sagittal tracking at four frames per second for gated BH delivery. All patients underwent IMRT planning to a median prescription dose of 50 Gy (range: 40-50 Gy) in 5 fractions. For MRgRTBH, the CTV was delineated on the primary 3D TRUFI scan with no internal target volume (ITV). Each patient was retrospectively replanned using a 3-arc, VMAT CT-IGRTFB treatment on a c-arm Linac (HD MLC). 4DCT respiratory evaluation was performed during MRgRTBH simulation. The average intensity projection CT was used to determine the ITV for the CT-IGRTFB plan. All plans used a 5 mm ITV to PTV margin. Target volume metrics used were: target coverage (TC) (PTV V100%/PTV vol), PTV D95%/Rx, PTV D90%/Rx, PTV D80%/Rx, homogeneity index (HI) (PTV D2%/ D98%), high dose conformity (PITV), low dose conformity (D2cm), and gradient (R50%). Additional dose metrics were evaluated for mean ipsilateral kidney and 0.5 cc gastrointestinal (GI) OARs. RESULTS The Wilcoxon signed-rank test showed a statistically significant increase (P 0.05). There were significant reductions (P 0.05) and significant GI OAR sparing (P < 0.05) resulted from the smaller target volumes and reduced OAR-to-PTV proximity. MRgRTBH was shown to be dosimetrically superior for OAR sparing to the small bowel, the large bowel and the duodenum (P < 0.05). Adaptive MRgRTBH was used for all patients, but only initial treatment doses were considered in quantifying the impact of gated BH versus ITV for this study.