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

Abstract

<h3>Purpose/Objective(s)</h3> To quantitatively compare plan quality between mid-inspiration breath-hold (BH) MR-guided radiotherapy (MRgRT_BH) and free-breathing (FB) CT-based intensity modulated radiotherapy (CT-IGRT_FB) for the ablative treatment of adrenal malignancies. We hypothesized that MRgRT_BH would provide improved target coverage and lower organ-at-risk (OAR) doses due to smaller target volumes compared to CT-IGRT_FB. <h3>Materials/Methods</h3> 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 MRgRT_BH, 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-IGRT_FB treatment on a c-arm Linac (HD MLC). 4DCT respiratory evaluation was performed during MRgRT_BH simulation. The average intensity projection CT was used to determine the ITV for the CT-IGRT_FB 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 (D_2cm), and gradient (R50%). Additional dose metrics were evaluated for mean ipsilateral kidney and 0.5 cc gastrointestinal (GI) OARs. <h3>Results</h3> The Wilcoxon signed-rank test showed a statistically significant increase (<i>P</i> < 0.05) in PTV volumes for CT-IGRT_FB over MRgRT_BH, with median (range) at 91.9 cc (32.0 – 481.4 cc) versus 53.6 cc (20.6 – 377.4 cc); a 71% relative increase for CT attributable to the need for an ITV. Target coverage metrics were only marginally worse for CT-IGRT_FB (<i>P</i> > 0.05). There were significant reductions (<i>P</i> < 0.05) in conformality metrics for CT-IGRT_FB in PITV (10%) and R50% (24%) relative to MRgRT_BH, due to the significant increase in target volumes for CT-IGRT_FB. Low dose conformality was equivalent for D_2cm. MRgRT_BH exhibited significant reductions (<i>P</i> < 0.05) in the median dose to 0.5 cc of the large bowel (15%), small bowel (28%) and duodenum (45%) over CT-IGRT_FB. <h3>Conclusion</h3> Intrafraction BH motion management of MRgRT_BH enabled significant reductions of target volumes (<i>P</i> < 0.05) compared to the ITV-approach for CT-IGRT_FB. Marginally improved target coverage (<i>P</i> > 0.05) and significant GI OAR sparing (<i>P</i> < 0.05) resulted from the smaller target volumes and reduced OAR-to-PTV proximity. MRgRT_BH was shown to be dosimetrically superior for OAR sparing to the small bowel, the large bowel and the duodenum (<i>P</i> < 0.05). Adaptive MRgRT_BH was used for all patients, but only initial treatment doses were considered in quantifying the impact of gated BH versus ITV for this study.