CHAPTER 3 - DOSE CALCULATION FOR EXTERNAL BEAMS

Abstract

This chapter provides an overview of dose calculation for external beams. The actual calculation of dose to a point in a patient is done by using the known, measured dose rate in rad/minute or rad/MU at a particular point in water or in air. Various techniques have been devised to determine the dose rate at other points. Included among these techniques are the source-surface distance technique (SSD) that involves the knowledge of how the dose varies with depth and field size for a fixed SSD, and the source-axis distance technique (SAD) that requires the knowledge of how the dose at a point, usually the axis of rotation of the therapy unit, varies as the amount of overlying tissue and field size changes. The absorbed dose delivered at a particular point in an irradiated medium is often referred to as the depth dose. The ratio of this depth dose to the dose at a reference depth, usually the depth of maximum dose, D max , along the central axis multiplied by 100% is referred to as the central axis percent depth dose. The depth dose at fixed depth in tissue can be considered to be made up of two components: (1) the primary dose component, and (2) a scatter dose component. The scatter dose, which is the dose provided to a point by all the scattered primary radiation, has a strong dependence on the field size and shape as well as on the depth in tissue. However, the primary dose does not change with the field size or shape; it changes only with depth.