The __Monte Carlo method__ as applied to Visual Monte Carlo is the simulation through mathematics of real elementary particles called __photons__. These photons, in the form of __gamma rays __ or __X-rays__, have enough energy to cause ionization in the human body, and can therefore cause a __radiation dose__. For high doses, such as those seen during accidents, the radiation can cause short term biological effects such as burns to the skin. These effects are called __deterministic effects__, and will require specialized medical treatment.

The photons interact with the body tissues through the __Photoelectric__, __Compton__ or __Pair production__ interactions. The mathematical simulation, made through the use of mathematical equations on a computer, reproduces 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. The small contribution to the dose from pair production interactions due to photons with energy above 1.02 MeV, around 1% or less, is not considered in the simulation.

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.

The mathematical photons are transported through a “mathematical” human body called a “phantom” or “mathematical simulator”. In VMC, the human body is represented by a set of cubic or rectangular volumes called __voxels__. Each phantom has a different voxel size, but each phantom is made up of voxels of the same size. For example, the voxel used in the __Yale phantom__ has a dimension of 0.36 cm x 0.36 cm x 0.36 cm. Each voxel has its unique position inside the phantom, and its “voxel number”. The “voxel number” is related to the tissue which is represented by the voxel. For example, in VMC in-vivo, the voxels with “voxel number” equal to 1 represent the skin.

In the case of VMC in-vivo, the photons leaving the mathematical phantom in the direction of the detector are then transported through the detector. Those photons that deposit all their energy in the detector crystal are then counted in the __photopeak__ of the detector spectrum. The number of events in the photopeak over a given time, multiplied by the calibration factor and taking the yield into consideration, will give an estimate of the activity deposited in the tissue or organ of interest.

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