A Novel Approach to Vessel-Sparing Prostate Radiotherapy Using MR-Only Simulation and Non-Contrast MR Angiography: Vessel Delineation Feasibility and SBRT Dosimetric Analysis

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<h3>Purpose/Objective(s)</h3> Erectile dysfunction is a common side effect of prostate cancer treatment, including prostate radiotherapy (RT). Vessel-sparing RT with avoidance of the internal pudendal artery (IPA) has shown promising rates of erectile function in non-randomized data. Implementation of vessel-sparing RT can be complicated by factors such as difficulty of MRI registration to planning CT and clinical desires to reduce routine use of IV contrast. We optimized a non-contrast MR angiography (ncMRA) sequence and compared IPA delineation on ncMRA with contrast MRA in a cohort of patients undergoing prostate stereotactic body RT (SBRT). We further performed dosimetric analysis on the IPAs. <h3>Materials/Methods</h3> Prostate cancer patients were prospectively enrolled on an IRB-approved MR imaging protocol. Treatment was with SBRT to a dose of 37.5 – 40 Gy in 5 fractions every other day. MR-only simulation was performed including MRA (2D T1w fast field echo (FFE) time-of-flight) and ncMRA (3D T1 FFE, 12 dynamics, temporal resolution = 14 seconds) sequences. Bilateral IPAs were delineated by a single blinded observer on MRA and ncMRA sequences. Analytical comparison between IPA contours on each patient was performed using mean distance to agreement (MDA), Dice similarity coefficient (DSC), and Jaccard index (JI). DSC and JI were also performed on IPA contour expansions of 2mm and 5mm. Dosimetry evaluated D50 and D05 on raw contours and 5mm expansion, and t-test compared dose on MRA and ncMRA. <h3>Results</h3> 10 patients were enrolled; 8 received 40 Gy, and 2 received 37.5 Gy. The median (range) analytical difference between IPA contours on MRA and ncMRA for MDA, DSC, and JI were 2.21 mm (0.99 – 4.16 mm), 0.54 (0.51 – 0.65), 0.37 (0.34 – 0.48), respectively. Using 2 mm contour expansion, DSC and JI were 0.69 (0.61 – 0.77) and 0.53 (0.44 – 0.63), while 5 mm expansion produced DSC and JI of 0.77 (0.69 – 0.85) and 0.62 (0.52 – 0.73). Dosimetrically, there was no difference in left and right IPA D50 or D05 between contours on MRA and ncMRA (Table). On 5mm expansion, the median (range) of D50 and D05 to IPA contours on ncMRA were 16.2 Gy (11.2 – 22.0 Gy) and 26.7 Gy (17.5 – 31.7 Gy). <h3>Conclusion</h3> This study found that delineation of IPAs using ncMRA was feasible and produced contours similar to those delineated on contrast MRA with no dosimetric differences. We feel that this agreement between imaging modalities demonstrates that ncMRA can be used alone for IPA delineation in vessel-sparing RT. Furthermore, the dosimetric findings of this study can serve as a baseline for expected IPA dose in prostate SBRT.