ESTRO 2023 - Abstract Book

S1951

Digital Posters

ESTRO 2023

1 International Atomic Energy Agency, Nuclear Sciences and Applications, Division of Human Health, Vienna, Austria

Purpose or Objective To determine a reference plan geometry in high dose rate (HDR) brachytherapy that delivers a homogenous dose region to a Radiophotoluminescent dosimeter (RPLD) suitable for postal dosimetry audits with an objective to audit Reference Air Kerma Rate (RAKR). The plan geometry must be achievable by all commercially available brachytherapy TPSs to ensure universal compatibility with the methodology. Materials and Methods The IAEA/WHO network have coordinated dosimetry audits in radiotherapy since 1969. Since 2021, the photon beam audit service has been expanded to include electron beams and through the organisation of a coordinated research project, efforts have been initiated to develop a new service for HDR brachytherapy dosimetry audits. To enable a seamless integration with the existing services, RPLD rods type FD-7, will be employed and irradiated in a standardised reference plan that is achievable by all auditees. SagiPlan v.2.0.2 with both 192Ir and 60Co source models was employed to investigate various irradiation geometries, to determine an optimum geometry for the audit methodology. The parameters investigated included the number of applicators, the number of dosimeters, the distance from the applicator(s) to the dosimeter(s), the number of dwell positions in each applicator, step size, dose homogeneity in the measurement region, sensitivity to source/RPLD position errors, RAKR errors and compatibility with commercially available brachytherapy TPSs. Results The resulting reference plan geometry consists of two catheters positioned 4 cm apart, in the same plane and parallel to each other (Fig 1a&b). An RPLD is positioned in the same plane and in the middle of the two catheters. Each catheter has 13 active dwell positions with 5 mm spacing, with uniform dwell time, totalling a 6 cm active length in each catheter (Fig 1c). The central dwell position of each catheter is aligned with a control point placed at the RPLD’s centre and a prescription dose of 2 Gy is normalised to this point. The resulting isodose distribution creates a homogenous dose region over the RPLD’s active volume. TPS-based simulated source position errors of +/-5 mm along the catheters corresponded to a <1% dose difference in the RPLD for both 60Co and 192Ir source models. A +/-1 mm lateral shift of the RPLD also corresponded to a <1% dose difference for both source models. The process of generating the same plan geometry was achievable in BrachyVision and Oncentra.

Conclusion The resulting plan geometry can enable reliable dose measurement to RPLDs, once all relevant correction factors are considered. This dose measurement can be used as a means of verifying the RAKR. The low sensitivity to positional errors is a desirable trait in designing a postal audit methodology for brachytherapy. A two-catheter geometry allows for a compact phantom design that is suitable for postal audits and is achievable by all TPSs. Further work will focus on producing a phantom prototype and determining the correction factors required.