Room: Exhibit Hall | Forum 1
Purpose: In this study, we investigate VMAT planning strategies for bilateral hip prostheses prostate patients with respect to plan quality and deliverability, while limiting entrance dose to the prostheses.
Methods: Three VMAT plans were retrospectively created for nine patients: 1) partial arcs (PA), 2) two full arcs optimized with 5Gy max prostheses dose (MD), and 3) two full arcs optimized with max DVH constraint of 5Gy to 10% prostheses volume (MDVH). PA techniques contained six partial arcs with beam angles that avoid entering each prosthesis. For each patient, other than prostheses constraints, the same Pinnacle VMAT optimization objectives were used. Plans were normalized with PTV D95% = 79.2Gy prescription dose. OAR DVH metrics, MUs, conformality, gradient, and homogeneity indices were evaluated for each plan. Mean entrance prosthesis dose was determined in Pinnacle by converting each arc into static beams and turning on dose for control points traversing each prosthesis. Plan deliverability was evaluated with ArcCheck measurements (gamma criteria 3%/2mm) on an Elekta machine.
Results: MD and MDVH had similar dosimetric quality, both improved DVH metrics for rectum and bladder compared to PA. Plan complexity amongst all plans was similar (average MUs: 469-515). Conformality, homogeneity and gradient indices were significantly improved in MD and MDVH versus PA (p<0.001). Gamma pass rates for MD (98.4Â±1.5%) and MDVH (98.7Â±1.7%) were comparable. A significant difference over PA was observed (96.2Â±1.5%, p<0.02). Field-by-field analysis demonstrated 8/9 PA plans resulted in fields with pass rates <95% vs. 1/9 plans for MD and none for MDVH. Cumulative mean entrance doses to each prosthesis was 60.6Â±20.6cGy for MD plans and 83.9Â±36.1cGy for MDVH plans.
Conclusion: MD and MDVH plans had improved dosimetric quality and deliverability over PA plans with minimal entrance doses (~1% of prescription) to each prosthesis and are an improved alternative for bilateral prostheses prostate patients.