Main Session
Sep 30
QP 46 - Imaging for Planning

1274 - Neurovascular Sparing in Prostate SBRT with Inhomogeneous, HDR-Like Dosimetry for Focal Integrated Boost to Dominant Intraprostatic Lesion: A Planning Feasibility Study

11:05am - 11:10am ET
Room 160

Presenter(s)

Elizaveta Lavrova, MD Headshot
Elizaveta Lavrova, MD - UNC Health, Chapel Hill, NC

E. Lavrova1, M. V. Lawrence2, E. S. Hollis3, A. Wijetunga4, S. Sud2, and M. C. Repka2; 1University of North Carolina, Department of Radiation Oncology, Chapel Hill, NC, 2Department of Radiation Oncology, University of North Carolina, Chapel Hill, NC, 3University of North Carolina Hospitals, Chapel Hill, NC, 4University of North Carolina, Chapel Hill, NC

Purpose/Objective(s): Surgical techniques have evolved to allow for neurovascular element (NE) sparing to preserve erectile function following radical treatment for prostate cancer, but minimal data is available for NE sparing techniques in radiotherapy. Stereotactic body radiotherapy (SBRT) with highly inhomogeneous, HDR-like dosimetry has been shown to produce lower post-treatment PSA nadirs than EBRT or SBRT with homogeneous planning. In this study, we aim to determine the feasibility of sparing unilateral neurovascular structures during SBRT planning with HDR-like dosimetry and focal integrated boost to the dominant intraprostatic lesion (DIL).

Materials/Methods: 13 patients previously treated for prostate cancer to 3625 cGy in 5 fractions with robotic SBRT using inhomogeneous, HDR-like dosimetry (planned hot spot in excess of 150% prescription dose) to 1 to 2 DILs (defined as PIRADS 4 or 5 lesions visualized on diagnostic prostate multiparametric MRI and subsequently confirmed as Gleason grade group =2 cancer via MRI-fusion biopsy or corresponding systematic core biopsy) were identified from an IRB-approved institutional database. NEs including neurovascular bundles (NVBs), internal pudendal arteries (IPAs), and penile bulb with crura of the corpus cavernosum were contoured using the Poten-C contouring atlas with assistance of MRI-CT fusion. New NE-sparing targets were defined: PTV30Gy = CTV + 2mm, or 5 mm on the side(s) with Gleason Grade 2+ disease; PTV36.25Gy = PTV30Gy cropped an additional 5 mm from the unilateral NEs. The NEs on the spared side were constrained as follows: NVB Dmax 30 Gy (V25Gy<3cc), IPA Dmax 20 Gy (V15Gy<3cc), and penile bulb Dmax 100%Rx (V30Gy<3cc). Planning metrics for the NEs, PTVs, DIL, and organs-at-risk were compared between plans using the Wilcoxon Signed-Rank test.

Results: 13 patients were identified and replanned. 12/13 (92.3%) of patients had rectal spacers. Dose to NEs on the spared side was significantly lower in the sparing plan than the original clinical treatment plan (NVB V25Gy 29.9% vs 64.7%, p < .01; IPA V15Gy 7.8% vs 22.8%, p < .01; Penile Bulb Dmean 614 cGy vs 735 cGy, p <.01, and Penile Bulb Dmax 1903 cGy vis 2089 cGy, p < .05). While coverage of the original PTV was lower in the sparing plan than the original plan (PTV V36.25Gy 85.0% vs 95.2%, p < .01), coverage of dose-painted PTV volumes differed by only a small, though significant, percentage (PTV36.25Gy V36.25Gy 95.2% vs 95.9%, p < .01, and PTV30Gy V30Gy 99.3% vs 100%, p < .01). Coverage of DIL was not compromised (DIL V45Gy 99.1% vs 99%, p = .22; Dmax 5595 vs 5575, p = .75). For 3/13 patients who had a second DIL (DIL2), coverage was also not compromised (DIL2 V45Gy 75.6% vs 76.8%, p = 1; Dmax 5161 vs 5127, p = .75).

Conclusion: Unilateral sparing of neurovascular elements is feasible in SBRT planning with highly inhomogeneous, HDR-like dosimetry. Focal integrated boost to DIL with a planned hot spot in excess of 150% prescription dose in the DIL is maintained when NEs are spared on one side.