D McIlrath, S Ahmad, E Schnell*, University of Oklahoma Health Sciences Center, Oklahoma City, OK

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  E Schnell

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PO-GePV-T-50 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To determine material selection for volumes in Elekta’s Oncentra brachytherapy planning software for non-listed materials for ACE TG-186 heterogeneity-corrected dose calculations using Gafchromic EBT3 film dosimetry.

Methods: A phantom was created to deliver dose to EBT3 Gafchromic film using a Nucletron MicroSelectron HDR afterloader with an Ir-192 source. The phantom was composed of bolus, five plastic catheters attached to a solid water support, a variable density layer (e.g. bolus, cork, 3D-printed polylactide (PLA)), and EBT3 film attached to a solid water base. The phantom was imaged using a GE Lightspeed CT simulator with a slice thickness of 2.5 mm. Solid water volumes were assigned as water program-defined material with uniform density. Materials of water and air were assigned to bolus/solid water and cork respectively. PLA materials were assigned varying materials in Oncentra with uniform density. Plans were created using Oncentra Brachy TPS to deliver a uniform dose of 600 cGy to points uniformly located across a portion of film. EBT3 film was calibrated for absolute dosimetry using a 6MV linear accelerator beam. Irradiated film doses were compared to ACE-calculated doses to determine agreement based on the material definition of PLA materials.

Results: Assigned materials to the variable density layer did not provide accurate doses when compared to film. PLA had no reasonable material assignment within Oncentra’s list.

Conclusion: Model-based dose calculation algorithms 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 provides insight on using Oncentra material definitions for non-included materials used in ACE-calculated plans.

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