Neurovascular bundle-sparing radiotherapy for prostate cancer using MRI-CT registration: A dosimetric feasibility study.

128 Background: Sexual dysfunction after radiotherapy for prostate cancer remains an important late adverse toxicity. The neurovascular bundles (NVB) that lie postero-laterally to the prostate are typically spared during prostatectomy, but in traditional radiotherapy planning they are not contoured as an organ-at-risk with dose constraints. Our goal was to determine the feasibility in sparing the NVB while still delivering adequate dose to the prostate. Methods: Twelve consecutive patients with intermediate- or high-risk prostate cancer (with no gross disease within 2mm of either NVB) who were treated with external beam radiotherapy, with the same primary PTV margins, to a dose of 79.2 Gy were evaluated. Pelvic MRI and simulation CT scans were registered using dedicated software to allow for bilateral NVB target delineation on T2-weighted MRI. A volumetric modulated arc therapy (VMAT) plan was generated using the NVB bilaterally with 2mm margin as an organ to spare (with goal of reducing V70Gy by 30%) and compared to the patient’s delivered plan. Dose-volume histogram (DVH) endpoints for NVB, rectum, bladder, and PTV79.2 were compared between the two plans using a 2-tailed paired t-test. Results: The V70 for the NVB was significantly lower on the NVB sparing plan (p <0.01), while rectum and bladder endpoints were similar. Target V100% was lower (but still > 95%), and V105 was higher for the NVB Sparing plans (p <0.01). The table summarizes the DVH comparisons between the two plans. Conclusions: “NVB-sparing” radiotherapy is dosimetrically feasible using VMAT technology; target coverage is acceptable without increased dose to other normal structures, but with higher target dose inhomogeneity. The clinical impact of “NVH-sparing” radiotherapy is currently under study at our institution. [Table: see text]