An innovative method for 192Ir HDR calibration by farmer chamber, V-film, and solid phantom

A simple, practical and economical technique was proposed to calibrate an Ir-192 HDR brachytherapy source in terms of air kerma strength. This technique makes use of the 0.6 cm(3) Farmer type ion chamber, radiographic film and polystyrene phantom. These tools are commonly used for dosimetry quality assurance of the clinical linear accelerator. In this study, the Exradin A19, PTW N30004 and TM30001 Farmer type ion chambers were used for the calibration of the Ir-192 HDR source. To perform the calibration, a 25.4 x 30.5 cm(2) radiographic film was taped on a piece of polystyrene plate, and a straight applicator probe of a HDR brachytherapy unit and the Farmer type ion chamber were affixed to the film envelope. The film was irradiated by the Ir-192 source, followed by an exposure in the simulator X-ray beam. The film set with the film removed was then placed on a 5 cm thick polystyrene phantom for calibration measurement. Based on the electrometer reading from the Farmer type ion chamber irradiated by Ir-192 and the measured source-to-chamber distance by means of the images on the developed film, we can calculate the air kerma strength of the Ir-192 using the new technique. Our calibration results were compared to the data provided by the manufacturer and that of five different well type ion chambers, namely, Sun Nuclear cooperation (SNC) 1008, Nucletron SOS 077.091, SOS 077.094, PTW TN33004 and Standard Imaging (SI) HDR-1000 Plus. The differences were all within 1.6%. Relative to the "7-distance measurement technique" by Stump et al., 2002, our method is more efficient if our empirical formula was used. In summary, our method is simpler and cost-effective to calibrate an Ir-192 NOR brachytherapy source for those hospitals without a calibration jig or a well type ion chamber. (C) 2011 Elsevier B.V. All rights reserved.