CHARACTERISTICS OF ORTHOVOLTAGE X-RAY THERAPY BEAMS AT EXTENDED SSD FOR APPLICATORS WITH END-PLATES

The effects of extended SSD (ESSD) on orthovoltage x-ray therapy dosim etry have been studied for a Therapax DXT300 treatment unit with sever al open- and close-ended cones. For the open-ended cones, the beam out put and percentage depth dose (PDD) at ESSD can be accurately estimate d from the measured data at the nominal SSD (NSSD) by an inverse squar e law (ISL) and a divergence factor for gaps (distance from cone end t o point of interest) up to 15 cm. In the case of close-ended applicato rs (with an end plate of 3.2 mm thick PMMA), extending SSD results in significant changes in output and in PDDs, but has little effect on be am penumbra and flatness for gaps up to 15 cm. The output at ESSD calc ulated by applying the ISL with the NSSD is found to be as much as 9% different from the measurement for the close-ended cone. The output at ESSD can, however, be calculated accurately by applying the ISL with an effective SSD or using a measured gap factor. The change in the PDD s with SSD can be calculated from a geometric divergence using a pseud o SSD which is approximately equal to 60% of the effective SSD determi ned in air. The calculated PDD curves at ESSD using this divergence fa ctor agree well with measurements (within +/-1% for beams of 100-300 k Vp with cone size ranging from 10x10 to 20x20 cm(2) and gap distance u p to 15 cm).