Angular dependence of the output of a kilovoltage X-ray therapy unit

During the recommissioning of a Philips RT-250 kilovoltage X-ray unit, unexpected output variations with tube head rotation (cross-plane) and tube head tilt (in-plane) were observed. The measured output showed an increase of up to 7.3% relative to the neutral position (0? in-plane and 0? cross-plane) over the possible range of angles of in-plane rotation for 75 kVp (half-value layer, HVL = 1.84 mm Al). A less pronounced but noticeable output change (with respect to the neutral position) was observed for cross-plane rotation reaching 2% for the 225 kVp beam (HVL = 0.90 mm Cu). This output variation was observed while manually adjusting the current to maintain constancy according to the current meter gauge. In order to address the observed output dependence with head orientation, the dose rate monitor chamber of the kilovoltage unit was calibrated to monitor the beam output in real time. The dose rate was manually adjusted to maintain a constant dose rate (in r/min) as displayed on the r/min gauge. This approach resulted in maintaining beam output for the 75 kVp and the 225 kVp beams within +/- 2% for the in-plane angle variation and +/- 0.5% for the cross-plane angle variation. A daily output check that includes ion chamber-based measurements at the neutral position and an in-plane angle of 45? has been implemented using the constant dose rate approach to monitor the stability of the X-ray beams. As a result of the output variations with in/cross-plane rotation, the quality control (QC) procedures that are typically used for clinical setup have been modified to test the stability of the beams under the non-neutral positioning of the X-ray tube.