PV QA 1 - Poster Viewing Q&A 1
Purpose/Objective(s): Erectile dysfunction is the most commonly affected quality of life (QOL) domain following radical therapy for primary prostate cancer. In contrast to surgery, data and technical ability supporting sparing of neurovascular elements (NEs) during radiotherapy have evolved slowly. MR/CT guided target delineation and SAbR, characterized by steep dose gradients and precise setup, offer the potential for such sparing. Combined with rectal hydrogel spacer to increase degrees of freedom in planning, an upcoming multi-center randomized phase II trial (POTEN-C) will evaluate whether NE-sparing SAbR will improve a patient-reported sexual QOL endpoint. In this dosimetric pilot study, we assess our ability to spare unilateral NEs with SAbR, while maintaining normal tissue constraints and target coverage.
Materials/Methods: The proposed study technique will allow low-intermediate risk patients without high-risk MR lesions (PIRADS≥3) within 5mm of one side’s NEs. Twelve patients meeting such criteria and previously enrolled on an institutional prospective trial of SAbR with hydrogel rectal spacer were evaluated. MRI/CT registered simulation images aided delineation of penile bulb, neurovascular bundles, and internal pudendal arteries delineated as NEs. Dose-painting with two PTVs was used: PTV30 (30Gy in 5 fx) = prostate (CTV)+3mm; PTV45 (45Gy in 5 fx) from PTV30 minus a 5mm avoidance on one side’s NEs (opposite to dominant disease). Constraints applied to the spared side were: NVB Dmax 30 Gy (V25Gy<3cc), IPA Dmax 20 Gy (V15Gy<3cc), and penile bulb Dmax 100%Rx (V30Gy<3cc). DVH data for sparing and non-sparing plans were compared by 2-tailed paired t-test.
Results: Unilaterally spared NEs demonstrated statistically significant lower max dose, mean dose and pre-specified volumetric constraints (see Table). PTV45 and PTV30 coverage were not compromised without increased heterogeneity. Rectum, bladder wall, and prostatic urethra dosimetry was not significantly changed and met the same constraints used for initial non-NE sparing.
|Structure||Dosimetric Parameter||Standard SAbR||Sparing SAbR||p-value|
|PTV45||Dmax||53.8 Gy||49.8 Gy||0.02|
|NVB (unil)||Dmax||51.3 Gy||25.5 Gy||2.0E-11|
|V20Gy||9.0 cc||1.5 cc||1.7E-06|
|IPA (unil)||Dmax||28.9 Gy||14.3 Gy||1.3E-05|
|V15Gy||3.6 cc||0.3 cc||4.9E-05|
|Penile Bulb||Dmax||23.2 Gy||10.3 Gy||0.01|
|Dmean||6.5 Gy||3.7 Gy||0.01|
Conclusion: Our data demonstrate NE-sparing SAbR with rectal spacer gel is feasible without compromising target coverage or normal organs at risk constraints, even at escalated doses and across an array of patients enrolled on a prior prospective trial. This new treatment technique will serve as the basis for a multi-institution phase II randomized trial (unilateral neurovascular sparing SAbR versus a non-sparing approach) with primary endpoint of EPIC health related QOL sexual domain score at 24 months.
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