Determination of a beam quality conversion factor from 60Co to 192Ir

Abstract

Radiation dosimetry is an integral part of the radiation therapy process. The ultimate goal of radiation dosimetry is to determine the dose delivered to the tumor and to the normal tissues in a patient undergoing radiotherapy. The importance of HDR 192Ir brachytherapy as the main mode of treatment calls for an accurate dosimetry standard. A dosimetry standard for the direct measurement of absolute dose to water in 192Ir sources is not available.The AAPM TG-43, along with its update, constitutes the accepted protocol for dose to water determination based on an air kerma strength Sk,air measurement. The air kerma strength of the radioactive source is converted to dose to water via dose rate constant Λ (a calculated absolute quantify) and several relative correction factors accounting for scatter attenuation and anisotropy of the dose distribution among other effects.In this work, we proposed an absorbed dose conversion by determination of a beam quality factor (kQ) with an ionization chamber. Ideally, the kQ should be measured directly for each chamber at the same quality as the user beam. However, this is not achievable in most standards laboratories. When no experimental data are available, or it is difficult to measure kQ directly to realistic beams, in many cases the correction factors can be calculated theoretically with Monte Carlo method. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry. To validate the source geometry, a few dosimetry parameters had to be calculated according to the AAPM TG-43U1 formalism. The dosimetry studies included the calculation of the air kerma strength Sk, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The XCOM photon cross-section library was used in subsequent simulations. Consequently, the photoelectric effect, pair production, Rayleigh scattering and bound Compton scattering were included in the simulation. Variance reduction techniques were also used in the simulations to speed up the calculation. The obtained radial dose function and dose rate constant of the Gammamed Plus 192Ir source in this work were compared with those obtained by Ballester et al. and Taylor &Rogers et al.A value of 1.109 ± 0.003 for the dose to the chamber to dose to water ratio was determined for PTW 30013 ionization chamber. The agreement between the EGSnrc calculated and TRS-398 reported result is encouragingly similar. The beam quality correction factor, kQ, was obtained at 2 cm. This ratio corresponds to the beam quality conversion factor from 60Co to 192Ir. With this kQ it is possible measure the absorbed dose to water at any point for HDR 192Ir source.