Monte Carlo calculation of correction factors for determining the operational quantity [Formula: see text] in solid phantoms for ISO narrow series photon sources.

Abstract

Although previous studies already reported on backscatter and depth dose correction factors for a Polymethyl-methacrylate (PMMA) phantom to determine the operational quantity [Formula: see text], more comprehensive evaluations for a wider range of tissue-equivalent phantoms are limited. Besides addressing this gap, the present study also provides phantom scatter correction factors for various phantoms. Correction factors were calculated to determine the [Formula: see text] in solid phantoms (PMMA, Polystyrene, Solid Water, Plastic Water, Virtual Water, RW3, WE210, and A150) and the International Organisation for Standardisation (ISO)-recommended PMMA-walled water phantom involving detector materials such as air, LiF and Li₂B₄O₇ for ISO reference photon beams (N40, N80, N100, N150, N200, N250 x-rays and 662 keV gamma photon). The calculations were performed using the EGSnrc-based Monte Carlo code system. These correction factors include backscatter factor, depth dose factor and phantom scatter, for photon beams with normal incidence on the phantom. The calculated values of the backscatter and depth dose factors are in good agreement with published values for a PMMA phantom. The values of backscatter factor calculated in solid phantoms such as A150, Solid Water, Plastic Water, Virtual Water and WE210 were similar to those calculated in tissue phantom. The phantoms PMMA, Polystyrene and RW3 showed higher backscatter factor values in the energy range N40 - N100 as compared to the tissue phantom. The depth dose factors were comparable in all phantoms except in Polystyrene in which they were higher for N40 photons. The study shows that application of phantom scatter correction is important for phantoms such as PMMA (N40- N250), Polystyrene (N40- N150), RW3 (N40 & N80), Solid Water (N40 & N80), Virtual Water (N40 & N80) and WE210 (N40 & N80). A150, Plastic Water and PMMA-walled water phantoms behave like tissue-equivalent phantoms at all photon energies as the phantom scatter correction was in the range of 0.97-1.02, depending upon energy. This study demonstrates the importance of applying phantom scatter correction factors into the calculation of [Formula: see text], particularly for low-energy photon beams.