PCXMC is a Monte Carlo program for calculating patients' organ doses and effective doses in medical x-ray examinations. The organs and tissues considered in the program are: active bone marrow, adrenals, brain, breasts, colon (upper and lower large intestine), extrathoracic airways, gall bladder, heart, kidneys, liver, lungs, lymph nodes, muscle, oesophagus, oral mucosa, ovaries, pancreas, prostate, salivary glands, skeleton, skin, small intestine, spleen, stomach, testicles, thymus, thyroid, urinary bladder and uterus.
The program calculates the effective dose with both the present tissue weighting factors of ICRP Publication 103 (2007) and the old tissue weighting factors of ICRP Publication 60 (1991). The anatomical data are based on the mathematical hermaphrodite phantom models of Cristy and Eckerman (1987), which describe patients of six different ages: new-born, 1, 5, 10, 15-year-old and adult patients. Some changes are made to these phantoms in order to make them more realistic for external irradiation conditions and to enable the calculation of the effective dose according to the new ICRP Publication 103 tissue weighting factors. The phantom sizes are adjustable to mimic patients of an arbitrary weight and height.
PCXMC allows a free adjustment of the x-ray beam projection and other examination conditions of projection radiography and fluoroscopy. All organ doses calculated by PCXMC are relative to the incident air kerma, Ka,i. This quantity represents the air kerma at the point where the central axis of the x-ray beam enters the patient. It is given in units of milligray (mGy), free-in-air, without backscatter; see ICRU 74 (2005). The user must supply this datum to the program. It can also be input as the entrance exposure (mR, free-in-air, without backscatter), air kerma-area product or dose-area product (mGycm2), or exposure-area product (Rcm2). If radiation measurements are not available, the program is able to estimate the incident air kerma also from an input of the x-ray tube current-time product (mAs).
The dose calculation method in PCXMC is the Monte Carlo method. The Monte Carlo calculation of photon transport is based on stochastic mathematical simulation of interactions between photons and matter. Photons are emitted (in a fictitious mathematical sense) from a point source into the solid angle specified by the focal distance and the x-ray field dimensions, and followed while they interact with the phantom according to the probability distributions of the physical processes that they may undergo: photo-electric absorption, coherent (Rayleigh) scattering or incoherent (Compton) scattering. At each interaction point the energy deposition to the organ at that position is calculated and stored for dose calculation. Other interactions are not considered in PCXMC because the maximum photon energy is limited to 150 keV. This chain of interactions forms a so-called history of an individual photon. A large number of independent photon histories is generated, and estimates of the mean values of energy depositions in the various organs of the phantom are used for calculating the doses in these organs.
Calculated organ doses can be used for the assessment of the risk of exposure-induced cancer. The risk estimates are based on the combined absolute and relative risk models of BEIR VII committee (BEIR 2006). PCXMC calculates the risk of exposure-induced death for leukaemia, cancers in colon, stomach, lung, urinary bladder, prostate, uterus, ovaries, breast, liver, thyroid and for all other solid cancers combined. The user may use the risk calculation module for estimating the cancer risk resulting from a single exposure or multiple exposures simulated in PCXMC. The user may also edit the organ doses without calculating doses with PCXMC: radiation risk assessment can be made for arbitrary irradiation cases.
The present version (2.0) of the program runs in a PC under Windows 95/98/NT/2000/XP/Vista. The Monte Carlo simulation time depends on the desired accuracy and on the speed of the PC, but is typically less than a minute in a PC with a 1.8 GHz processor. The same Monte Carlo data can be used for calculating doses for any x-ray spectrum of interest when the other conditions of the examination remain unchanged; in this case the calculation time is very short because no further Monte Carlo simulations are needed.
The data calculated by PCXMC have been earlier compared to the organ dose conversion factors calculated in NRPB by Jones and Wall (1985) and Hart et al. (1994, 1996) and were found to agree well (Tapiovaara et al 1997). The excellent agreement with the NRPB data still exists for most irradiation conditions. Small differences are noted in some irradiation conditions, because the composition and density of the phantom tissues have been changed and the phantoms have been modified from the earlier versions of the program. Reasonable agreement of PCXMC results has also been found in many comparisons with dose measurements and calculations with other phantom models, e.g., Schmidt et al (2000), Schultz et al (2003), Helmrot et al (2007). The usability of the phantom size modification feature of PCXMC has been demonstrated in an extreme case by Smans et al (2008) who calculated doses in two premature babies with weights of 590 g and 1910 g. The differences between the doses calculated with their voxel phantoms and PCXMC were explained by the differences in the phantom models and the difficulty to place an x-ray field in them in an equivalent fashion. Similar differences between computational and voxel phantoms have also been seen in the papers of Staton et al (2003), Lee et al (2006) and Pazik et al (2007); dose differences of the same order are obtained also in dose calculations with different voxel phantoms (Zankl et al. 2002, Schlattl et al. 2007).
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