An analytical representation of equivalent square field size in relation to surface dose. Technical note

Analytical representation of the build up characteristics of megavoltage photon beams is achievable, producing accurate percentage dose results in the build up region for typical patient treatments. This approach requires conversion of an irregular shaped field to a equivalent square field for mathematical analysis. Surface dose for rectangular fields is expressed by an extension to Stirling's 4A P formula while irregular shaped fields follow an area integration technique. The equivalent square for a typical irregular treatment field is modelled within ± 0.5 cm2 for all measured fields. 12 refs., 1 tab., 1 fig