A PROPOSED METHODOLOGY TO ASSESS HDR [superscript]192Ir RADIAL DOSE WITH FRICKE DOSIMETRY

Abstract

The radial absorbed dose to water, D(r,theta), a quantity not yet directly measured with primary standards, was assessed with the Fricke solution which has been considered as an absolute aqueous solution to perform dosimetry. To determine D(r=2 cm,theta=90°), a hollow quasi-water Fricke ring device filled with Fricke solution was designed to measure the change in the optical density in the Fricke solution at 303 nm wavelength caused by the exposure from a HDR 192Ir radioactive source positioned in its center inside a 15~15~15 cm[superscript]3 PMMA water phantom. A mini-irradiator was also designed and constructed to determine the product of the molar extinction coefficient and the chemical yield value of the Fricke solution at 303 nm against an A-12 ion chamber, in which its main dosimetric parameters to determine the absorbed dose to water were calculated using the PENELOPE Monte Carlo code. To enhance dosimetric accuracy, a large number of measurements were performed with both the quasi-water Fricke ring device and with the mini-irradiator calibration device. The radial absorbed dose to water at r = 2 cm and theta = 90° determined with the quasi-water Fricke ring device was compared against that determined with the HDR treatment planning system Ocentra. The results have shown that if the quantity D(r = 1 cm, theta = 90°) is replaced by D(r = 2 cm, theta= 90°), the methodology employed in this work may be used for clinical applications to determine the absolute radial absorbed dose to water with accuracy comparable to those assessed with Farmer and well-type ion chambers.