Source standardization benchmark - strength determination of HDR 192Ir brachytherapy source

High dose rate 192Ir brachytherapy sources are commonly used for management of malignancies by brachytherapy applications. Measurement of source strength at the hospital is an important dosimetry requirement. The use of 0.6 cm3 cylindrical ionization chamber is one of the methods of measuring the source strength at the hospitals as this chamber is readily available for beam calibration and dosimetry. While using the cylindrical chamber for this purpose, it is also required to determine the positioning error of the ionization chamber with respect to the source which is commonly called as distance error (c). The contribution of scatter radiation (Ms) from floor, walls, ceiling and other materials available in the treatment room also need to be determined accurately so that appropriate correction can be applied while calculating the source strength from the meter reading. Iterative methods of Newton-Raphson and Least-Squares were used in this work to determine scatter contribution in the experimentally observed meter reading of a cylindrical ionization chamber. Monte Carlo simulation was also used to cross verify the results of Least-Squares method. The experimentally observed, Least-Squares calculated and Monte Carlo estimated values of meter readings from HDR 192Ir brachytherapy source were in good agreement to one another. Considering procedural simplicity, the method of Least-Squares is recommended for use at the hospitals to estimate values of f (constant of proportionality), c, and Ms required to determine the strength of HDR 192Ir brachytherapy sources. (author)