An investigation of entrance surface dose calculations for diagnostic radiology using Monte Carlo simulations and radiotherapy dosimetry formalisms

Abstract

Our aim in this work was to investigate the methodology used in the determination of the entrance surface dose (ESD) in diagnostic radiology. In kV x-rays for low-energy photons (tube potential up to 160 kV, HVL: 1-8 mm Al), the ESD is based on the use of the ratio of mass-energy absorption coefficients and backscatter factors. A full simulation of the photon and electron transport in a kilovoltage x-ray unit, using the Monte Carlo code BEAM/EGS4, was performed to obtain an accurate beam phase space for use in dose calculation. The modelled phase space was experimentally validated for the beam qualities (measured HVL: 3.3 mm Al-2.2 mm Cu) and showed good agreement between calculated and measured HVLs, air kerma and relative dose distributions. We have computed the conversion factors from air kerma to water or soft tissue absorbed dose at the surface of a phantom for beam qualities (HVL: 3.3-8.35 mm Al). The same model was also used to calculate the ESD in water and in soft tissue for the low-energy photon range considered. The results show that the numerical differences between the air kerma and the water kerma based backscatter factors are insignificant. The same conclusion was reached for the (mu(en)/rho) ratios, for soft tissue to air, evaluated using either the primary photon spectra or the spectra at the surface of a phantom. Furthermore, the good agreement obtained for the computation of the conversion factors with a full BEAM/EGS4 model confirms the previous studies which are based on different sources for the spectral distribution and different beam geometries (pencil beam or point source assumptions). On the other hand, the ESD in water or soft tissue is well described either with the B(air) or the B(w) formalism. Conversion factors from air kerma to ESD in these media are proposed in this work for several beam qualities in diagnostic radiology.

Backscatter factors and mass energy-absorption coefficient ratios for diagnostic radiology dosimetry

“…For matching kV, filtrations and HVL, the two datasets differ on the average by 0.6%, the maximum difference being 1.1%. Backscatter factors calculated in this work for 10 cm × 10 cm field sizes are also compared in figure 10 to the values published by Omrane et al (2003) and Chica et al (2008) to highlight the lack of published data covering a broad radiodiagnostic energy range. Additional comparisons have been made for 5 cm × 5 cm field sizes with Chica et al (2008) and Kim et al (2010), the latter calculated for a 6 cm diameter circular field, and for 20 cm × 20 cm field sizes with those of ICRU-74 and IAEA TRS-457.…”

“…For each kV and filtration, and the beam half-value layer (HVL) specified in mm of aluminium, backscatter factors were given for three field sizes, and 15 cm thick water, 'ICRU tissue' and PMMA phantoms; the data in these reports are taken from Petoussi-Henss et al (1998). More recent data on backscatter factors for similar diagnostic beam qualities, or empirical relations to derive them, have been published by Omrane et al (2003), Chica et al (2008) and Kim et al (2010). In a similar energy range, but for therapeutic beam qualities, the AAPM TG-61 dosimetry protocol (Ma et al 2001) provided backscatter factors as a function of HVL and field size for low-and medium-energy x-rays.…”

Advances in kilovoltage x-ray beam dosimetry

“…Alternatively, one can determine the primary x-ray spectra by using Monte Carlo calculations. However, this involves determining a complete model of the x-ray unit and requires a good knowledge of the geometry and materials in the x-ray tube (Ali and Rogers 2008a, 2008c, Mainegra-Hing and Kawrakow 2006, Mesbahi and Zakariaee 2013, Munck Af Rosenschold et al 2008, Omrane et al 2003.…”

Estimating the cancer risk and mortalities induced by routine digital radiography examinations on patient of different ages in Mazandaran province

“…Entrance surface dose (ESD) is the absorbed dose to the entrance skin of the patient at the central point of the irradiated area and it is used to determine the patient's effective dose (4,11) . The patient's effective dose also depends on the X-ray's penetrating power and the body region being examined (12) .…”