ESTRO 2024 - Abstract Book

S376

Beachytherapy - Physics

ESTRO 2024

Feasibility testing of IAEA/WHO postal dosimetry audit methodology for high-dose rate brachytherapy

Alexis Dimitriadis 1 , Evelyn Granizo-Roman 1 , Roua Abdulrahim 1 , Geraldyne Ule-Duque 2 , Krzysztof Chelminski 1 , Pavel Kazantsev 1 , Godfrey Azangwe 1 , Mauro Carrara 1 , Jamema Swamidas 1 1 International Atomic Energy Agency, Dosimetry and Medical Radiation Physics Section, Vienna, Austria. 2 International, Dosimetry and Medical Radiation Physics Section, Vienna, Austria

Purpose/Objective:

To evaluate the sensitivity of a postal audit methodology for HDR brachytherapy in detecting deliberate dosimetric and geometric errors.

Material/Methods:

A compact 16 x 8 x 3 cm PMMA phantom was manufactured, accommodating two 5 or 6-French interstitial catheters that are 4 cm apart, with a cavity for a centrally placed radiophotoluminescent dosimeter (RPLD) (Model GD-302M, Asahi Technol Glass Corp, Tokyo, Japan) positioned in the same plane and equidistant from the two catheters. In close proximity to one of the two catheters an additional cavity for accommodating a strip of Gafchromic RTQA2 film (Ashland Advanced Materials, NJ, USA) can be placed and used for assessing the positional accuracy of the delivery (Figure 1). Based on a previously published “universal reference plan" 1 designed for reliable dose measurements, twenty treatment plan variations with deliberate errors were generated on SagiPlan v.2.0.2 (Eckert & Ziegler BEBIG Medical GmbH, Germany), ten of which had positional errors for all source dwell positions ranging from ± 1 to ± 5 mm and ten had dosimetric errors ranging from ± 1 to ± 10%. Three RPLD and film measurements were performed for each treatment plan that was delivered to phantom by a SagiNova 192 Ir HDR afterloader (Eckert & Ziegler BEBIG Medical GmbH, Germany). The planned TPS dose (D TPS ) in the centre of the RPLD (including the errors) was compared to the average measured dose (D MEAS ) from the three dosimeters used in each plan, after applying all necessary corrections for the dosimeter’s response and irradiation in non TG43 conditions. The planned dwell position shift (S TPS ) was compared to the average film-measured shift (S MEAS ) from the three films used in each plan. S MEAS was extracted using an in-house developed software (ImageJ macro).

Results:

The dose ratio for all delivered plans (D TPS / D MEAS ) ranged from 0.991 to 1.017 with an average of 1.002 (SD=0.008). The difference between S TPS and S MEAS , after accounting for the systematic source position error, ranged from -0.4 to 0.7 mm with an average distance of 0.0 mm (SD=0.3 mm). Table 1 shows the results for each plan. The magnitude and direction of both dosimetric and positional errors was successfully detected for all twenty treatment plans delivered. The average results and standard deviations are indicative of the methodology’s sensitivity in detecting dosimetric errors of ≥ 2% and positional errors of ≥ 1 mm.