VMC uses the Monte Carlo method to simulate mathematically the transport of photons through voxel phantoms representing the human body and geometrical representations of radiation detectors. A good description of the Monte Carlo method is given here and here.. Photons, in the form of gamma radiation or X-rays with energies above around 10 keV have enough energy to cause ionization, and therefore an ionizing radiation dose.
Lower doses give rise to stochastic effects. For high doses, such as those seen during radiation accidents, the radiation dose can cause short-term biological effects such as burns to the skin. These effects are called deterministic effects, and require specialized medical treatment. The effects of ionizing radiation on the human body are summarized here.
Photons interact with body tissues through photoelectric, Compton or pair production interactions. The mathematical simulation uses algorithms that reproduce as closely as possible the real interactions suffered by the photons. The range of photon energies considered in VMC is from 10 keV to 3 MeV. This is the energy range relevant to occupational, emergency and medical exposures, excluding radiotherapy with linear accelerators.
The small contribution to the dose from pair production interactions due to photons with energies above 1.02 MeV, less than 1%, is not considered in VMC dose calculation. A good discussion on photon interactions with matter can be found here.
As a result of the photoelectric and Compton interactions, charged particles in the form of electrons are produced. The maximum range in human tissue of electrons is small, around 4 mm for a 1 MeV electron. VMC dose calculation assumes electronic equilibrium for energy binning in organs and tissues.
The photons are transported through a "mathematical" human body called a "phantom" or "mathematical simulator". The human body is represented by a set of cubic or rectangular volumes called voxels. Each phantom has a different voxel size and each phantom is made up of voxels of the same size. Each voxel has its unique position inside the phantom and "voxel number". Each "voxel number" is associated with the tissue represented by the voxel. For example, in VMC in-vivo, the voxels with "voxel number" equal to 1 represent the skin.
For VMC dose calculation, the adult male and female ICRP reference voxel phantoms are provided. A complete description of the voxel phantoms is provided in publications ICRP 110 and ICRP 116. The VMC DC and VMC in-vivo voxel phantom files are encrypted, however the original phantoms can be obtained here.
For VMC in-vivo, the phantoms include encrypted files of the male and female ICRP reference phantoms and non-encrypted phantoms of the ANSI BOMAB, the Bfs head, the BPAM head, the LLNL lung phantom and the University of Cincinati knee phantom.
In the case of VMC in-vivo, the photons leaving the mathematical phantom in the direction of the detector are transported through the detector. The photons that deposit all their energy in the detector crystal are counted in the photopeak of the detector spectrum. A good description of photon interactions in solid state detectors is given here. The number of events in the photopeak divided by the total number of photons emitted in the respective organ, and taking the radionuclide's photon yield into account, will give an estimate of the calibration factor in net cps per Bq deposited in the tissue or organ of interest.
The two "help" files for VMC dose calculation and VMC in-vivo show the "workings" of the two Monte Carlo programs.
Download VMC dose calculation help (pdf).
Download VMC in-vivo help (pdf).