Pitfalls in dose calculation using a commercial treatment planning computer for Clinac-4 4MV x-ray beam

Dose calculations on a commercial treatment planning computer based on the storage of profile data along a principal axis for Clinac-4 4MV x-ray can lead to significant error in calculated dose in the corners of a large field. These errors are due to the unique nature of the lead flattening filter on Cl-4. A method is suggested to remedy this problem by storing profile data along a diagonal of a large field.