Sunday, June 19, 202211:30 AM - 12:30 PMPSOR1 Presentation Time: 11:30 AM: MRI-Guided Neurovascular Bundle Avoidance in HDR Prostate Brachytherapy

Abstract

<h3>Purpose</h3> Ultrasound or computed tomography (CT) are typically used for treatment planning in HDR prostate brachytherapy. However, the neurovascular bundles (NVB), which are hypothesized to have a role in erectile function, are not well visualized in these studies. Our group has shown that magnetic resonance imaging (MRI) can be used to segment the NVB as an avoidance structure for external beam radiation. In this study we hypothesize that treatment planning MRI will allow avoidance of the NVB during HDR prostate brachytherapy. <h3>Materials and Methods</h3> A retrospective chart review of all individuals with prostate cancer treated with radiation therapy at a single institution was used to identify patients who had HDR brachytherapy as monotherapy. All patients underwent TRUS-guided HDR catheter placement followed by CT and MRI for treatment planning. Patients were treated with two 13.5 Gy fractions utilizing standard clinical plans that optimized prostate coverage and minimized irradiation of the standard organs at risk (OARs) - bladder, rectum, and urethra. When applicable, MRI-defined GTVs were targeted for dose escalation to 150%. Experimental NVB-sparing plans were generated to maintain target coverage and OAR constraints but also to add avoidance of the NVBs. The left and right NVB were contoured on the treatment planning MRI and used as avoidance structures with 0 margin. Differences between target and OAR doses in the original and NVB-sparing plans were assessed pairwise using a two-tailed t-test. <h3>Results</h3> 15 prostate cancer patients at a single institution were treated with HDR monotherapy were included. Segmentation of the left and right NVB was feasible for all patients and 13 MRI-defined GTVs were targeted for dose escalation in 11 patients. Compared to their paired original plans, NVB-sparing plans reduced NVB D0.01cc (18.2 vs 14.6 Gy, p<0.01) and V100 (0.1 cc vs 0.0 cc, p<0.01). There was no difference in prostate D90 (108% vs 108%, p=0.19) and V100 (96.7 vs 95.9%, p=0.17) between plans; prostate V150% (36.5% vs. 34.3%, p<0.01) and V200% (14.4% vs 13.5%, p<0.01) slightly decreased in the NVB-sparing plans. Bladder V75 (0.3 cc vs 0.3 cc, p=1.0), rectum V75 (0.2 cc vs. 0.2 cc, p=0.75), and urethra V125 (0.03 cc vs. 0.03 cc, p=0.33) were similar between plans. In the subset of patients with GTVs, the mean GTV V150 decreased from 80.9% to 74.8% (p = 0.04) in the NVB-sparing plans; in one case NVB-sparing led to GTV min dose less than 100%. <h3>Conclusions</h3> NVB-sparing prostate HDR brachytherapy is feasible. Treatment planning MRI should be considered for NVB delineation. NVB-sparing should be approached with caution if GTVs are in close proximity. Evaluation of the clinical impact of MRI-delineated NVB irradiation is underway.