Technical note: Determining equivalent squares of high-energetic photon fields.

Purpose: We developed a method based on a physical pencil beam model for accurate equivalent square calculations for rectangular and irregular fields, for different definitions of equivalent squares, for beams with and without flattening filter, different photon energies, and depths in water.

Methods: We considered two equivalent square definitions: equal dose at a point on the beam axis and equal depth dose, measured as tissue phantom ratio at 20 and 10 cm depth ( ). As dose engine, we used an analytical pencil beam model.