VARSKIN 4 calculates beta dose using the same method as in VARSKIN Mod 2 and VARSKIN 3. In general, VARSKIN 4 performs a five-dimensional integration of the source volume and the target area. With the exception of the slab model, the integration is simplified significantly because the dose is symmetric for a circular target area centered under the source.
The VARSKIN 4 backscatter correction model also accounts for material located between the source and the skin depth of interest. For sources with a thickness of less than 5 percent of the X99 distance, the BCF changes when the source is not in contact with the skin. The BCF does not change with depth for sources with a thickness greater than 5 percent of the X99 distance in the source material.
Cover materials and air gaps can be modeled using VARSKIN 4. The models use the concept of effective path length to determine the beta energy lost in either a cover material or air before it enters the skin. The path length is not the true path traversed by the beta particle; it is merely a mathematical convenience introduced to provide a measure of the energy lost in each layer. To prevent unintended applications of VARSKIN 4, the air gap is limited to a maximum of 5 cm.
The volume-averaged dose model allows the calculation of dose averaged over a given tissue volume. Any two planes of irradiated skin can be assigned to bound the skin volume. For sources in contact with the skin, the maximum penetration depth for beta particles is equal to the X99 distance. Doses averaged over the dose-averaging area are calculated at 10 skin depths between two limits set by the user, and a cubic spline (a third-order piecewise polynomial curve fit) is fit to this depth-dose distribution. When the user specifies the skin depths corresponding to the volume of interest, VARSKIN 4 integrates the depth dose function over the region of interest to obtain the volume-averaged dose.
The offset particle model allows calculation of skin dose averaged over areas that are not directly beneath the contaminant. This model was developed to determine dose to a single averaging area resulting from multiple hot particles. The offset particle model is available only for the point geometry. It requires only one input variable, the distance of the offset. For multiple particle irradiations, the dose from each particle must be calculated separately, with the user running VARSKIN 4 once for each particle. The offset particle model does not calculate the maximum dose to skin from several particles (Section 6.2 outlines the iterative process for determining the maximum dose to the dose-averaging area); rather, the user must manually add doses from each of the sources to a common dose-averaging disk at depth.
The photon dose model implemented by VARSKIN 4 is new and is an improvement to the basic photon model used in VARSKIN 3. The photon model uses a point kernel method that considers the buildup of CPE, transient CPE, photon attenuation, and off-axis scatter. The photon dose model has many of the basic assumptions carried in the beta dosimetry model, namely that the source can be a point, disk, cylinder, sphere, or slab and that dose is calculated to an averaging disk immediately beneath the surface of skin at a depth specified by the user. Photon dose is calculated for a specific skin averaging area, also specified by the user.