Teletherapy beam characteristics: the first second.

The beam characteristics of radiotherapy treatment units are measured and recorded under circumstances in which transient effects are eliminated. Output measurements are done by averaging several readings. Isodose curves are obtained during a period of irradiation which is large compared with that used in patient treatment. When treatments are given, the therapy unit is assumed to deliver radiation beams that match the data acquired during calibration. We have looked at the energy, field uniformity, and output constancy typical of a number of these machines in their initial stages of operation. Although most can provide their "reference" beam characteristics in a short time, there are exceptions. The implications for patient treatment, port filming, and film dosimetry are discussed.