Analysis of daily beam output data for teletherapy machines

Quality assurance protocol of radiotherapy machines recommends a daily constancy check of beam output. In our institution we use a Keithley TRACKER^TM system to monitor daily variation of beam output, flatness, and symmetry. The monitoring device does not correct the readings of ionization chambers for temperature and pressure. We have analyzed the daily beam output data of a Co-60 teletherapy unit and a Varian Clinac 2100CD unit. The data were collected for a five-year period from December 1994 to December 1999. We have found that the TRACKER™ readings of the Co-60 unit show a sinusoidal oscillation with one-year period. Decay corrected data were analyzed by using a time-series model. The results indicate the amplitude of 2.0% for the seasonal oscillation and a random fluctuation of 1.0% about the sinusoidal trend curve. Temperature and pressure measurements for monthly quality assurance dose not indicate such a periodic oscillation in the temperature and pressure correction factor. The Co-60 data were used to remove the seasonal variation from the TRACKER™ readings of the 2100CD photon-beam output. The resulting curve shows a linear trend with occasional discontinuities due to machine output calibration. We also have demonstrated that the daily output data, which are not corrected for temperature and pressure, can be used to estimate the output deviation from a reference value and predict a future trend of the beam output.