Perturbation of electron beam doses as a function of SSD due to the use of shielding blocks on the Clinac-18a.

Many radiotherapy linear accelerators use electron beam applicators which extended close to the patient's surface when treating at the regular distance. The relatively large size of these applicators often necessitates the use of a larger SSD than that designed by the manufacturer. In such cases, the addition of shielding blocks to the applicator can significantly alter the factors which should be used to calculate the dose rate at the new SSD as compared with the unshielded beam condition. In some typical clinical situations, errors of greater than 60% may result from failure to account for this perturbation. This paper presents the proper correction of the dose vs SSD function for the presence of such shielding blocks for the Clinac-18 linear accelerator.