Errors in off-axis treatment planning for a 4-MeV machine.
Computer treatment planning systems allow dose computation in planes parallel to the central one (off-axis plans). The beam data may consist of, e.g., percentage depth doses along the central axis plus off-axis ratios (OAR) at several depths. In some systems, the calculation of an off-axis plan is based on the assumption that the OAR can be represented by a separable function: OAR(x,y) = f(x).g(y), where x and y are the symmetry axes perpendicular to the beam axis and the functions f and g are equal for a square open field. The errors of this assumption for a 4-MeV machine were measured for open fields and wedged fields at five different depths. The measured dose was compared with that predicted by the above equation for 50%, 75%, and 88% of the half field width from the beam axis. Maximum deviation of more than 10% was observed with the probable error of the measurement being 1%.