The quality of high-energy X-ray beams

Supplement 17 of the British Journal of Radiology is a survey of central-axis depth doses for radiotherapy machines, patterned largely on BJR Supplement 11 published some 11 years earlier. Inspection of the high-energy X-ray depth doses for a 10 x 10 cm field at an SSD of 100 cm disclosed large differences between the two sets of data, especially for qualities above 8 MV. For example, a depth dose of 80% at 10 cm is rated at about 19 MV according to BJR Supplement 11, and 23 MV according to BJR Supplement 17. It was found that the Supplement 17 depth-dose data above 8 MV were erratic, but that the Supplement 11 data could be represented by an analytical expression, providing a unique means of assigning MV quality. It was also found that the dose-weighted average energy of the filtered beam plotted smoothly against depth dose. For dosimetric purposes, it is suggested that this parameter be used as a true measure of beam quality, removing the discrepancies introduced by the use of nominal MV for this purpose.