Room: AAPM ePoster Library
Purpose: To investigate the utility of Mobius3D for performing independent secondary dose calculations for HDR treatment plans.
Methods: For plan checks, Mobius3D (Varian) performs a point dose comparison for any point exported as part of the RTPLAN file (e.g., optimization or dose reporting points) using TG-43 formalism for a point source (i.e., without anisotropy corrections). To investigate the differences between the primary treatment planning system (Elekta Oncentra) and Mobius3D for a Flexitron Ir-192 afterloader, comparisons were performed for a single dwell position and a variety of clinical cases, including interstitial, superficial planar, GYN, and strut-based and balloon-based breast plans. For these comparisons, calculation points were created using a variety of methods (both manual and automatic) available in Oncentra.
Results: For a single dwell position, the calculated dose was compared at various radial distances (2-50mm) and polar angles. Due lack of anisotropy correction, the Mobius3D calculation disagreed with Oncentra by >10% for some calculation points, located close to the source or at angles of large anisotropy. For the clinical plans, of the 373 points calculated, the difference between Mobius and Oncentra was >3% for 20 points (5.4%) and >5% for one point (0.3%). The point with >5% discrepancy was located on the ring surface (6mm from the source channel) for a tandem and ring plan. For balloon and strut-based breast applicators, better agreement was found by placing points 1cm from the applicator surface to avoid placing calculation points too close to the source channels and the stem of the breast applicator.
Conclusion: Mobius3D secondary dose calculations for HDR brachytherapy are fast, appropriate for a variety of implant techniques and applicators, and result in agreement within 5% of a primary treatment planning system. Due to Mobius3D’s point source approximation, better agreement is achieved when comparing dose at locations less affected by anisotropy.