ESTRO 2023 - Abstract Book

S1514

Digital Posters

ESTRO 2023

Figure 2. R2 according to dose for recipes 2 and 3.

Conclusion The next step of the project is to manufacture phantoms of an optimized recipe. The dynamic range, the response to dose and the sensibility of the gel will be characterize, as well as a method for correcting the non-uniform sensitivity response in three dimensions.

PO-1791 3D printed inhomogeneity phantom for radiotherapy commissioning and QA measurements

D. Banjade 1 , S. How 1

1 Western NSW Local Health District, District Radiation Oncology Service, Orange, Australia

Purpose or Objective An inhomogeneity phantom with accurate geometrical and heterogeneity simulation can be fabricated with a 3D printer for the commissioning and quality assurance (QA) of radiotherapy treatment. Clinical departments can fabricate phantoms specific to patient treatment with accurately simulated tissue, lung, and bone at a lower cost. A relatively simple and novel method to simulate bone is presented here by combining an epoxy resin potting mix with a clinically available radio opacifier, barium sulfate (BaSO4). Materials and Methods A CT dataset of an SBRT patient’s thorax was used to design and 3D print an anthropomorphic thorax slab phantom with substitutes for tissue, lung and bone (Figure 1). Tissue and lung equivalency was fabricated using polylactic acid (PLA) with an in-fill density of 80% and 26% respectively. Bone equivalency was produced by mixing a tissue equivalent epoxy resin potting mix with the radio-opacifier barium sulfate (BaSO4), which was added 6% w/w to produce similar CT density to bone. Validation of the 3D printed substitutes were performed with slabs of 3D printed tissue, lung and bone placed on top of solid water phantom. EBT3 film was used to compare PDDs of TPS vs Film. The 3D printed inhomogeneity phantom had two inserts which allowed the placement of Gafchromic™ EBT3 film strips through the phantom. SBRT radiotherapy plans were then delivered targeting the lung and bone respectively and then analysed with SNC patient software.

Results The average HU number on the 3D printed phantom for tissue was 0 HU ( ρ _e/ ρ _(e,w)= 0.99), lung was -650 HU ( ρ _e/ ρ _(e,w)= 0.35) and bone was +700 HU ( ρ _e/ ρ _(e,w)= 1.40), and the results were found to be consistent with the HU values quoted for commercial inserts with the Gammex 467 RMI phantom. The TPS versus Film PDDs shows approximate agreement after the build-up region. In the build-up region where there is a lack of charged particle equilibrium (CPE), the TPS will underestimate the dose compared to the radiochromic film measurement. The SBRT plans measured with radiochromic film were analysed with a low dose threshold of 10% and a gamma

criterion of 3%/2mm. The resulting gamma passing rate for lung and bone were ≥ 95% and 85% respectively.

Conclusion 3D printers have allowed customizable anthropomorphic phantoms to be fabricated at clinical radiotherapy departments at a low cost. The total cost of the phantom minus the printer can be approximately 5-10 times lower than purchasing commercially available phantoms. Mixing a radio-opacifier such as barium sulfate (BaSO4) with an epoxy resin potting mix allows one to achieve bone equivalency with reasonable accuracy. A phantom similar to the one in this study provides commissioning and quality assurance towards small field stereotactic type radiotherapy for lesions located within heterogeneity. More research and procedural adaptation is recommended to establish QA of these customised 3D printed phantom to avoid any unnoticed errors.