MRI-guided neurovascular bundle avoidance in high dose rate prostate brachytherapy.

[PURPOSE] The neurovascular bundles (NVBs) play a role in erectile function and are not well visualized on ultrasound or computed tomography (CT). In this study we hypothesize that treatment planning MRI will allow avoidance of the NVB during HDR prostate brachytherapy.

[MATERIALS AND METHODS] This was a retrospective review of patients who had HDR prostate brachytherapy as monotherapy (two implants 1-2 weeks apart each delivering 13.5 Gy). All patients underwent TRUS-guided HDR catheter placement followed by CT and MRI simulation. The left and right NVB were contoured on the treatment planning MRI. Standard clinical plans were optimized for prostate coverage and avoidance of organs at risk (OARs). Experimental NVB-sparing plans were generated to add avoidance of the NVBs. Differences between target and OAR doses in the original and NVB-sparing plans were assessed pairwise using a 2-tailed t-test.

[RESULTS] 34 patients were included and segmentation of the left and right NVB was feasible for all cases. Compared to their paired original plans, NVB-sparing plans reduced left and right NVB D0.01cc (130% vs. 102% and 130% vs. 108%, p < 0.01, respectively). There was no difference in prostate D90 (108% vs 108%, p = 0.19) and V100 (96.1 vs 95.9%, p = 0.17) between plans; prostate V150% (35.8% vs. 34.7%, p < 0.01) and V200% (14.0% vs 13.4%, p < 0.01) slightly decreased in the NVB-sparing plans. Mean bladder V75 (0.3 cc), rectum V75 (0.2 cc), and urethra V125 (0.03 cc) were similar between plans.

[CONCLUSIONS] Using MRI-based treatment planning, NVB-sparing prostate HDR brachytherapy is feasible and merits further study.