Validation of ACE: Advanced Collapsed Cone Engine Heterogeneity-Corrected Brachytherapy Dose Calculation with Gafchromic EBT3 Film
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PO-GePV-T-29 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: To validate Elekta’s Oncentra ACE TG-186 heterogeneity-corrected calculation of dose with heterogeneous materials using Gafchromic EBT3 film dosimetry.
Methods: A phantom was created to deliver a uniform dose to a 5 cm by 10 cm area of EBT3 Gafchromic film using a Nucletron MicroSelectron HDR afterloader with an Ir-192 source. The phantom composed of bolus, five plastic catheters attached to a solid water support, a variable density layer (e.g. bolus, cork, bone), and EBT3 film attached to a solid water base. The phantom was imaged using a GE Lightspeed CT simulator with a slice thickness of 1.25 mm. Layers were contoured within Varian’s Eclipse and assigned program-defined material with uniform density values. Materials of water and air were assigned to bolus/solid water and cork respectively. Plans were created using Oncentra Brachy TPS. Doses of 200 cGy were delivered to a normalization point located at the center of film. EBT3 film was calibrated for absolute dosimetry using a 6MV linear accelerator beam.
Results: Film irradiated through a layer of cork produced an average dose of 194 cGy at the normalization point. Doses for each trial were less than the ACE-calculated 200 cGy in Oncentra.
Conclusion: Model-based dose calculation algorithms (MBDCAs) are of great importance in cases where heterogeneities are present, and its accuracy is paramount for research. There is a need for experimental validation of these engines to assure meaningful results, as well as guidance for best practice when assigning material definitions. Gafchromic EBT3 film is a great practical option since it offers minimal additional heterogeneities into the dose validation calculation. This work indicates the possibility of using film as a means for dosimetric validation of ACE calculated plans.
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