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Determination of absorbed dose in water at the reference point D ( r 0 , θ 0 ) for an Ir 192 HDR brachytherapy source using a Fricke system
Author(s) -
Austerlitz C.,
Mota H. C.,
Sempau J.,
Benhabib S. M.,
Campos D.,
Allison R.,
deAlmeida C. E.,
Zhu D.,
Sibata C. H.
Publication year - 2008
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2996178
Subject(s) - brachytherapy , materials science , ionization chamber , dosimetry , optics , dose profile , nuclear medicine , ionization , physics , imaging phantom , radiation therapy , medicine , ion , quantum mechanics
A ring‐shaped Fricke device was developed to measure the absolute dose on the transverse bisector of a Ir192 high dose rate (HDR) source at 1 cm from its center in water, D ( r 0 , θ 0 ) . It consists of a polymethylmethacrylate (PMMA) rod (axial axis) with a cylindrical cavity at its center to insert the Ir192 radioactive source. A ring cavity around the source with 1.5 mm thickness and 5 mm height is centered at 1 cm from the central axis of the source. This ring cavity is etched in a disk shaped base with 2.65 cm diameter and 0.90 cm thickness. The cavity has a wall around it 0.25 cm thick. This ring is filled with Fricke solution, sealed, and the whole assembly is immersed in water during irradiations. The device takes advantage of the cylindrical geometry to measure D ( r 0 , θ 0 ) . Irradiations were performed with a Nucletron microselectron HDR unit loaded with an Ir192 Alpha Omega radioactive source. A Spectronic ® 1001 spectrophotometer was used to measure the optical absorbance using a 1 mL quartz cuvette with 1.00 cm light pathlength. The PENELOPE Monte Carlo code (MC) was utilized to simulate the Fricke device and the Ir192 Alpha Omega source in detail to calculate the perturbation introduced by the PMMA material. A NIST traceable calibrated well type ionization chamber was used to determine the air‐kerma strength, and a published dose‐rate constant was used to determine the dose rate at the reference point. The time to deliver 30.00 Gy to the reference point was calculated. This absorbed dose was then compared to the absorbed dose measured by the Fricke solution. Based on MC simulation, the PMMA of the Fricke device increases the D ( r 0 , θ 0 )by 2.0%. Applying the corresponding correction factor, the D ( r 0 , θ 0 )value assessed with the Fricke device agrees within 2.0% with the expected value with a total combined uncertainty of 3.43% ( k = 1 ) . The Fricke device provides a promising method towards calibration of brachytherapy radiation sources in terms of D ( r 0 , θ 0 )and audit HDR source calibrations.