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 dosimetry have been studied for a Therapax DXT300 treatment unit with several open- and close-ended cones. For the open-ended cones, the beam output and percentage depth dose (PDD) at ESSD can be accurately estimated from the measured data at the nominal SSD (NSSD) by an inverse square law (ISL) and a divergence factor for gaps (distance from cone end to point of interest) up to 15 cm. In the case of close-ended applicators (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 beam penumbra and flatness for gaps up to 15 cm. The output at ESSD calculated 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 PDDs with SSD can be calculated from a geometric divergence using a pseudo SSD which is approximately equal to 60% of the effective SSD determined in air. The calculated PDD curves at ESSD using this divergence factor agree well with measurements (within +/- 1% for beams of 100-300 kVp with cone size ranging from 10 x 10 to 20 x 20 cm2 and gap distance up to 15 cm).