Timer error and beam quality variation during "ramp-up" of a superficial X-ray therapy unit

Superficial and orthovoltage therapy machines generally exhibit a "ramp-up region" during which the dose rate increases from zero to the steady state output. This can produce a significant dose deficit, which can be made up by adding a correction to the treatment time. This correction, which can be determined from the intercept of the dose versus time plot, takes no account of either the true duration of the ramp-up region or the beam quality variation while it takes place. A new method for investigating the ramp-up region is proposed, and the results are presented for a 100 kVp beam. Measurements of beam quality in terms of half value layer (HVL) have been made as a function of time. A change in HVL from an average of 2.9 mm aluminium in the ramp-up region to 3.1 mm for the steady state region has been observed. A significant random variation in the time between initiation of the radiation and the beginning of the ramp-up region was also observed which was traced to a time varying signal in the control system.