Conversion Factors between Energy Imparted to the Patient and Air Collision Kerma Integrated over Beam Area in Pediatric Radiology

Abstract

Conversion factors between the energy imparted to the patient in pédiatrie radiography and air collision kerma integrated over beam area are presented. The values have been derived from Monte Carlo calculations in soft tissue phantoms and extend results published earlier to cover children from early infancy to the age of 15 years. Variations related to phantom size as well as to focus-phantom distance, radiation field size, orientation of view (a.p., lateral), tube potential, and beam filtration are given. We show that the conver sion factor increases with increasing half-value layer of the Xray beam and the anterioposterior width of the simulated child. Increasing the focus-phantom distance increases the conversion fac tor, while increasing the field size decreases the factors due to more scattered radiation escaping laterally from the phantom.In optimizing pédiatrie radiography, the image quality and the risk to the child from the irradiation are of interest (10). The degree of correlation of the concept "energy im parted to the patient" (22) to risk may in many cases be just as good as that of the more complicated estimate of effective dose (1,27). Tissue weighting factors (w T ) for calcu lation of effective dose (E) (23) for different organs have been published for a population comprising equal numbers of both sexes. So far, however, tissue weighting factors for children have not been published; nor have values been adopted by the International Commission of Radiation Pro tection (ICRP) (9). ICRP states that data applicable for adults are not generally applicable to children. Furthermore, as compared to E, the energy imparted to the patient (ε) and the mean absorbed dose (ε/m; m = patient mass) will not change with time as knowledge of the radiosensitivities of organs increases and are also less dependent on exact knowledge of field position and field size. In practice, a transparent ion chamber is placed in the irradiation field close to the beam diaphragm to measure the air collision kerma, K cair) integrated over beam area. These measure ments together with conversion factors can give the energy imparted to the patient, which in turn is related to the risk.Conversion factors based on human shaped phantoms for use in calculating absorbed dose in specified organs for pédiatrie radiology have been published (7,25,30). ROSENSTEIN et al. (25) used 3 sizes of phantoms, representing the ages: newborn, 1 year, and 5 years; CRISTY et al. (7) newborn, 1,5,10, 15 year and adult; and VEIT et al. (30) phantoms based on CT images from 2 children, 2 months, and 7 years old. Our previously published conversion factors between the energy imparted and air collision kerma integ rated over beam area using brick-shaped phantoms repre sentative for an adult (2) have now been extended to include children. The conversion factors are derived using Monte Carlo calculations in soft tissue phantoms corresponding in size to children from the newborn to 15 years of age.

Radiation risk assessment in neonatal radiographic examinations of the chest and abdomen: a clinical and Monte Carlo dosimetry study

“…A number of surveys regarding E and the risk to neonates from simple x-ray examinations have been carried out (Schneider et al 1992, Wraith et al 1995, Almen and Mattsson 1995, McParland et al 1996, Lowe et al 1999, Jones et al 2001, Armpilia et al 2002, Gogos et al 2003, Brindhaban and Al-Khalifah 2004. In these studies E and energy imparted (ε), which can be reliably used in correlation with E for risk estimation, were calculated from entrance surface dose (ESD) or kerma area product (KAP) using appropriate conversion coefficients provided by other investigators (Persliden and Sandborg 1993, Chapple et al 1994, Hart et al 1996. These coefficients are derived using Monte Carlo (MC) simulation which is a well-established method used in paediatric and non-paediatric radiology for the calculation of H, E and ε (Shrimpton et al 1986, Zankl et al 1989, Drexler et al 1990, Hart et al 1994, Huda and Gkanatsios 1997, Wise et al 1999, Alonso et al 1999, Schimdt et al 2000.…”

Ionization chambers for measuring air kerma integrated over beam area. Deviations in calibration values using simplified calibration methods

“…It is easy to handle and is in position during the whole x-ray examination without interfering with the light beam for alignment. Its reading is proportional to the air collision kerma integrated over beam area ( A K c,air dA) and can be converted to effective dose or energy imparted to the patient through appropriate conversion factors , 1996, LeHeron 1992, Persliden and Sandborg 1993, Alm Carlsson et al 1984.…”

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

“…It is well known that the amount of material beyond the surface (i.e. phantom material thickness in Monte Carlo simulations or patient thickness) affects the absorbed dose to the skin (cf Persliden and Sandborg 1993, Stamm and Saure 1998, Mooney and Thomas 1998, mostly due to the amount of backscattered photons. Because of the wide differences in paediatric and adult population, especially with regard to their thickness, it is not feasible to assign a unique general paediatric patient thickness for backscatter factor calculations.…”