Calculation of dose in off-central axis planes using off-axis ratios.

Accuracy of dose calculation for regular fields in off-central axis planes was investigated on a RAD-8 treatment planning computer for 4- and 10-MV x-ray beams produced by Varian Clinac -4 and Clinac -18 linear accelerators. These calculations, which are based on central axis depth dose and off-axis ratios in the principal planes, can be in error by as much as 25%-30% at locations well within the irradiated volume for the 4-MV x-ray beam. These large errors for the Clinac -4 beam result from the falloff in dose beyond the peak dose along a diagonal of a large field at distances greater than 14 cm from the central axis due to the lead flattening filter. The profile data stored in the computer along the principal planes cannot be used to calculate the dose accurately in such a situation. Computed doses for the 10-MV x-ray beam agreed with the measured doses within 4%-6% at all locations.