Abstract

Esil Kara¹, Ayse Hicsonmez¹

¹Onko Ankara Oncology Center, Radiation Oncology, Ankara, Turkey

Optically stimulated luminescence dosimeters (OSLDs) have properties that make them so important to radiation dosimetry for in vivo dosimetry. The aim of the study is to figure out the physical and the dosimetric characteristics of the Beryllium Oxide (BeO) OSL dosimetric system before using it routinely in clinical practice in radiotherapy.

In this study, OSLs were irradiated by using 6 MV, 10 MV, and 15 MV photon beams with the Elekta Versa HD linear accelerator. The output dose of the linear accelerator was calibrated by using the absorbed dose calibration protocol of the IAEA TRS-398 in the water at the depth of d_max. Monitor units per cGy for a 10x10cm² field size at a source-to-surface distance (SSD) of 100cm was calibrated 1cGy for 1MU. Absolute dose measurements were made with a 0.6cc cylindrical ion chamber, PTW 30013 Farmer. PTW 30x30cm2 solid water slab phantoms and PTW Octavius 4D phantom was used at the measurements. The build-up thicknesses varied depending on the irradiated photon energy, back scatter thickness was 10 cm for all measurements. BeO OSLs were first annealed at 700 ^oC for 3 hours. They were placed in a plastic mold and given an ID number. In the optical eraser section, the eraser process was performed for 30 minutes and the base level was determined in the reader. The OSL decay curve was obtained by irradiating at a known dose with the irradiation system with Cs-137. Calibration curves were defined by matching each curve with the ID numbers of the OSLs. Bolus was used to filling the cavity around the OSL dosimeters. The measurements were taken by placing BeO OSLs at the maximum dose depth and ion chamber at the reference depth for each energy at the source surface distance (SSD) at 100 cm. For the angular/directional dependence test, the setup for the cylindrical phantom was the same as the slab phantom, but a SAD of 100 cm was set to the isocenter of the phantom and center of the OSL dosimeter. The tests performed were reproducibility/sensitivity, dose-response linearity, energy dependence, angular dependence and field size dependence. As a result of the tests, correction factors and functions were obtained, and the OSL measurement accuracy was increased by using these factors and functions.

Reproducibility/sensitivity and angular dependence are smaller than %2, dose-response linearity and energy dependence are smaller than %1.  After making SSD corrections recommends in IAEA Human Health Reports No:8, the dependency of SSD is %3 for SSD 80 and smaller than %2 for other SSDs. The field size correction factor should be used if a field size other than 10x10 cm² during irradiation. 

By obtaining factors as mentioned in the AAPM TG 191 the accuracy of OSLD system could be increase. The use of BeO OSL in radiotherapy applications is promising with advanced measurements.