Implementation of a spreadsheet program for calculation of equivalent square field size.

Equivalent square field size is a parameter widely used in calculations for external beam treatment planning. The algorithm and features of a spreadsheet program used to calculate the equivalent square, independently of the treatment planning system, are described. The program uses an approximation method based on the application of Lamé curves. Comparison of the output, for a variety of field shapes and sizes, with tabulated data and other calculation methods yielded differences typically