Evaluation of Scatter Contribution and Distance Error by Iterative Methods for Strength Determination of HDR ¹⁹²Ir Brachytherapy Source

Abstract 

High-dose rate (HDR) 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 because 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, commonly called a distance error (c). The contribution of scatter radiation (M_s) 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 (pC/s) of a cylindrical ionization chamber. Monte Carlo simulation was also used to cross verify the results of the 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. 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 M_s required to determine the strength of HDR ¹⁹²Ir brachytherapy sources.

Key words: Brachytherapy, Source strength, Iterative method, Scatter factor, Monte Carlo