Purpose: Hypofractionated (60Gy in 20fx) prostate radiotherapy (HPR) achieves clinical outcomes comparable to standard fractionation and shorter treatment times that are highly desirable for patients. One limiting factor in HPR is rectal toxicity due to high dose per fraction. SpaceOAR is a synthetic hydrogel placed at the prostate-rectum interface to increase spacing and reduce rectal dose. Despite these benefits, SpaceOAR utilization is not widespread among patients receiving radiation therapy, with uncertainty about which patients derive the greatest benefit. This study investigates the patient-specific benefits of SpaceOAR in HPR.
Methods: Five patients with diagnostic CT scans prior to SpaceOAR were studied to determine dosimetric differences between plans with (clinical CT scan) and without (diagnostic CT scan) SpaceOAR. Average time between scans was 61.8 days (range: 23-89). SpaceOAR was placed 7-8 days before simulation. Contouring was performed by a single radiation oncologist. Planning was done with RapidArc (Eclipse V11.0, PRO). The clinical plans were mimicked on diagnostic scans by using the same fields and converting relative volumetric optimization objectives into absolute volume constraints to account for anatomical changes. All plans were normalized to 95% PTV coverage. Dosimetric analysis included: Rectum â€“ Dmax, V100%, V95%, V90%; Body â€“ Dmax; SpaceOAR â€“ Dmax.
Results: In both cases, the contoured PTV volumes were comparable, average difference 7.7% (range: 1.4-13.6%). With SpaceOAR, the rectum V100%, V95%, and V90% decreased by 3.85cc, 4.91cc and 5.24cc, respectively. Global max dose differences between diagnostic and clinical plans were within 3.7%. Max SpaceOAR dose was greater than the max rectum dose in all clinical plans. One patient had a minimal decrease in dose to the rectum, indicating they may not be a good candidate for SpaceOAR.
Conclusion: SpaceOAR successfully reduced the high dose to the rectum in patients receiving HPR. Further work is warranted to determine recommendations for patient selection.