ESTRO 2023 - Abstract Book

S414

Sunday 14 May 2023

ESTRO 2023

Conclusion We have presented a method to calibrate small point detectors, which is easily implementable and can be performed in less than 10 minutes. The method compensates for uncertainties in Δ Z, making it robust against positional uncertainties, which can lead to large calibration uncertainties, due to the steep BT dose gradients. The calibration factors vary both on the long and short term, based on both degradation of the material and coupling of the probe. This variation can be as large as a factor of three and confirms the need for regular recalibration. PD-0505 Monte Carlo simulated correction factors of a novel phantom for brachytherapy dosimetry audits K. Chelminski 1 , R. Abdulrahim 1 , A. Dimitriadis 1 , E. Granizo-Roman 1 , J. Kalinowski 2,3 , S.A. Enger 2,3 , G. Azangwe 1 , J. Swamidas 1 1 IAEA - International Atomic Energy Agency, Department of Nuclear Sciences and Applications, Division of Human Health, Vienna, Austria; 2 McGill University, Department of Oncology, Medical Physics Unit, Montreal, Canada; 3 Lady Davis Institute for Medical Research, Jewish General Hospital, Montreal, Canada Purpose or Objective Independent dosimetry audits have potential to prevent catastrophic errors and minimise systematic dose deviations in high dose rate HDR BT. The aim of this study was to obtain correction factors using MC method, to account for lack of scatter and non-water equivalence of phantom and dosimeter material as compared to the TG-43 conditions for a phantom geometry designed for IAEA/WHO postal dosimetry audits for HDR BT under the framework of a coordinated research project E24023. Materials and Methods A PMMA phantom of 16 cm × 8 cm × 3 cm containing two channels on either side of a RPLD located at the centre of a phantom was simulated using RapidBrachyMCTPS for HDR Ir-192, model Ir2.A85-2 source.

RPLDs are made of silver-activated phosphate glass (FD7) with a 1.5 mm diameter and 12 mm length located inside a capsule made of high-density polyethylene (HDPE) were used in the simulation. Absorbed dose was scored in 1.25x 1.25 x1.25 mm^3 voxels and 5 × 10^7 primary particles were simulated balancing computation time and uncertainty. Four cases were simulated (Table). Case 1 represents the TG-43 conditions of full scatter and water equivalent material, while case 2 mimic potential audit conditions, where the use of the PMMA, perturbation effects due to the presence of the RPLD, HDPE capsule, and the lack of scatter conditions of the phantom were simulated. Scenarios with different thicknesses of backscatter material (5 cm, 10 cm and 15 cm of G4_water) were also simulated in case 3 to mitigate the influence of a table material (case 4) on which the PMMA phantom may be positioned.

Simulation cases Surrounding medium Phantom Backscatter Table RPLD capsule RPLD glass 1 (TG-43) Water Water Water Water Water Water 2 (Audit) Air PMMA Air Air HDPE FD7 3 (Audit) Air PMMA Water Air HDPE FD7 4 (Audit) Air PMMA Water Steel HDPE FD7

Results The total estimated correction factor was 1.15 ± 0.02 for the audit case 2 in respect to case 1. The contributions of the individual correction factors were determined to be 1.06 ± 0.01, 1.05 ± 0.01 and 1.03 ± 0.01 for perturbation effects, the lack of scatter, and the use of PMMA, respectively. The varying thicknesses of the backscatter for the audit case 3 increased the dose calculated in the detector in respect to case 1 by 12% ± 1%, 11% ± 1% and 12% ± 1% for thicknesses of 5 cm, 10 and 15 cm, respectively. For the audit case 4 with a steel table the corresponding results were 12% ± 1%, 11% ± 1% and 12% ± 1%. Varying thickness of backscatter material resulted in a similar dose rise to the detector, suggesting that 5 cm backscatter is adequate. Moreover, 5 cm backscatter is sufficient to ensure that the audit result will not be influenced by the density of the underlying table.

Conclusion