Reference kerma rate evaluation using Sievert integral for extended sources

Abstract

Computation of the dose distribution in brachytherapy requires a good knowledge of the geometric and physical characteristics of the sealed sources, as well as the composition of the medium in which it is calculated. Different dose calculation models have been used in practice. The chemical and physical forms of the radionuclide, as well as the shape and thickness of the capsule, influence the dosimetric properties of the source. The air Kerma rate in air is introduced in most treatment planning systems to define the characteristics of the irradiation. This value is measured by the manufacturer and communicated to the physicist via the calibration certificate. It is up to each physicist to control this value before its us e. In general, it will be measured using an ionization chamber for the source itself, in a plane perpendicular to the axis from the source. The present work aims to perform an analytical calculation of the air kerma rate in air using the Sievert integral for filtered extended sources to develop convenient method for quick verification of the dose or the treatment plan. The Sievert integral, is a special function commonly encountered in radiation transport calculations. It accounts for the photon attenuation in the source capsule of the brachytherapy line source. The computed air kerma rate in air will be used with the AAPM TG43 formalism as a convenient quality assurance method for the verification of dose distributions calculated by the treatment planning system for filtered extended sources. The choice of the Sievert integral is motivated by the fact that it takes into account the length of the source, whereas most of the conventional methods for calculating the air kerma rate in air are based on the hypothesis of the point source. The Sievert integral, involves the air kerma rate in air for point source, the active length of the source, the capsule thickness and an effective attenuation coefficient of the capsule. For the purpose of calculation, linear sources are assumed to consist of a number of small elementary point sources, each point source contributing to the total dose at the point of interest. The calculated air kerma rates in air for various extended filtered sources used in brachytherapy shows that the use of Sievert integral reduces the air kerma rate in air values, especially near the sources, compared to those obtained with the assumption of a point source, and to the corresponding quantity measured using a well-type chamber. The significant differences were observed for radial distances less than twice the active length of the sources and they decrease with the radial distance increase. The maximum differences were observed at the immediate neighborhood of the source, and increase with the source length increase.