The application of the approximation method of equivalent square for magavoltage photon beams

In treatment planning of radiation fields, it is essential to know the dose received at various tissue levels in the irradiated volume. For a certain radiation quality and source to skin distance, radiation dose at maximum build-up or at a depth in tissue is a function of the size and shape of the radiation field. It has been established that the quilibrium dose as the depth dose along the central axis of the beam for a rectangular field can be determined by applying the equivalent square technique. The approximation method of area/perimeter of different portal sizes used in practice for finding the equivalent squares of a rectangular field has been tested on a 35 MV Clinac Linear Accelerator for the purpose of treatment planning. Data were taken for 8 and 25 MV x-rays at depths of maximum build-up regions and at 10 cm depth below the surface of an adequate water phantom. Maximum variation of dose calculation from the above method is less than 1.6% for routine clinical usage, while less than 3.2% for extreme rectangular field sizes. Dose variation is improved as the depth in phantom is increased.