Influence of phantom thickness and material on the backscatter factors for diagnostic x-ray beam dosimetry

Abstract

Most of the existing backscatter factors for the dosimetry of clinical diagnostic x-ray beams have been calculated for 15 cm thick phantoms; these data are used for skin dose determinations which in general ignore the influence of phantom material and thickness. The former should strictly be required whenever dosimetry measurements are made on phantom materials different from those used for the backscatter factor calculations. The phantom or patient thickness is of special importance when skin dose determinations are made for infants or paediatric patients. In this work, the recently published formalism for reference dosimetry and comprehensive database of backscatter factors for clinical beams and water phantoms have been extended using two correction factors which account for phantom material and thickness. These were determined with simulations using the PENELOPE Monte Carlo system, for PMMA to analyse the influence of the phantom material relative to water, and for a broad range of thicknesses of water and PMMA to investigate the role of this parameter in patient dose estimates. The material correction factor was found to be in the range 3-10%, depending on the field size and the HVL. The thickness correction factor was in the range 2-12% for a 5 cm thick phantom and square field sizes between 5 and 35 cm, reaching a plateau of about ±1% for thicknesses beyond 13 cm. Expressions in the form of surface fits over the calculated data are provided which streamline the determination of backscatter factors for arbitrary thicknesses and phantom materials, as well as field sizes. Results demonstrate the inadequacy of using conventional backscatter factors (calculated for 15 cm thick phantoms) without correction factors that take into account the phantom material and its thickness.

Review of backscatter measurement in kilovoltage radiotherapy using novel detectors and reduction from lack of underlying scattering material

“…Recently, influence of backscatter material and thickness has been modeled for diagnostic beams also, showing reductions of up to 12% for the highest energy, largest field size, and 5 cm thick phantoms compared to 15 cm full scatter conditions (25) . Correction factors for the use of PMMA instead of water were 3%‐10%.…”

Proof of principle of helium‐beam radiography using silicon pixel detectors for energy deposition measurement, identification, and tracking of single ions

“…From this DAP, the entrance dose D E was calculated to be 1440 μGy using the following equation [51, page 379 ff. ]where A is the irradiated area of the phantom, B is the backscatter factor of 1.38 according to the study of Benmakhlouf et al., ( μ en / ρ ) PMMA and ( μ en / ρ ) Air are the mass energy‐absorption coefficients for PMMA and air for photons with an energy of 30 keV, respectively , ¶. The dose profile of the x‐ray field along the depth of the PMMA phantom was obtained by performing a MC simulation with Fluka .…”

“…where A is the irradiated area of the phantom, B is the backscatter factor of 1.38 according to the study of Benmakhlouf et al, 55 (l en /q) PMMA and (l en /q) Air are the mass energy-absorption coefficients for PMMA and air for photons with an energy of 30 keV, respectively. 56,-The dose profile of the x-ray field along the depth of the PMMA phantom was obtained by performing a MC simulation with FLUKA.…”

A framework for organ dose estimation in x-ray angiography and interventional radiology based on dose-related data in DICOM structured reports

“…Vano et al, 2001;Theodorakou and Horrocks, 2003). Alternative methods have been employed based on the use of precalculated Monte Carlo (MC) factors and coefficients (McCollough and Schueler, 2000;Schlattl et al, 2007;Benmakhlouf et al, 2011Benmakhlouf et al, , 2013 for the determination of absorbed dose to internal organs and to the skin. These methods rely on dose-related data that describe the physical context of the individual exposures of the performed XA procedure to generate an accurate dose estimate.…”