Calibration of 90Sr + 90Y planar sources using thermoluminescent samples, a PMMA phantom and Monte Carlo simulation

Abstract

In certain regions of Brazil, 90Sr/90Y clinical applicators continue to be employed for dermatological and ophthalmic treatments, even though newer technologies, such as the 106Ru/106Rh Eye Applicator, are available worldwide. The use of these older applicators persists due to their lower cost and greater ease of use. However, it is crucial to calibrate and periodically recalibrate these applicators to ensure that the absorbed dose rates are accurate, and that the quality of clinical treatments is maintained. This study examines the thermoluminescent response of µLiF pellets to evaluate their reproducibility, linearity of response, and dose-response curves. Two radiation systems were used to characterize the dosimetric material. The first system is part of a Risö reader system (Risö TL/OSL-DA-200 model) with a 90Sr/90Y source, operating at a dose rate of 0.1 Gy/s (2010). The second system is a 90Sr/90Y source from the beta secondary standard system (BSS2) with a dose rate of 123.32 µGy/s (2005), from Amersham Buchler, calibrated in the German Primary Standard Laboratory, Physikalisch-Technische Bundesanstalt (PTB). The BSS2 system presents its calibration certificate in air and in ICRU 4-element tissue (k=2). The uncertainties in the thermoluminescent measurements for some measurement cycles of µLiF samples, irradiated with 1 Gy of beta radiation, Risö system, remained up to 5%. Linearity and dose-response curves were obtained over a dose range from 0.3 to 1 Gy using the BSS2 system. The R2 values for the µLiF samples ranged from 0.9974 to 0.9998. The dose rates for three clinical applicators were determined using µLiF pellets, a PMMA phantom, and the Monte Carlo method. This project followed the guidelines outlined in ISO 21439 (2009), which recommends the use of small detectors like µLiF for this type of calibration. Monte Carlo simulation was used to determine correction factors between absorbed dose in air, water, and PMMA, ensuring the use of PMMA in clinical applicator calibration. The PMMA has a cubic shape, a slot to keep the µLiF at a one-millimeter depth, and dimensions that ensure total absorption of beta radiation without interaction with air. The percentage differences between the µLiF calibration and the manufacturer calibration values were equal to or greater than 10%. The main reasons for these results are discussed in this work.