Purpose: Daily fluctuation of Varian TrueBeam machine output was observed when the machine monitor chamber was deflated and reaching to the end of its life. This behavior is caused by monitor chamber responding to the atmospheric pressure fluctuation. In this work, we suggest monitoring the accumulated output drift since the installation of the monitor chamber. This drift can be used to predict the occurrence of output fluctuation before it is reaching to a level that impacts the treatment and can be detected by daily QA.
Methods: The accumulated output drift was monitored and recorded for two Varian Truebeam machines using monthly and annual QA output tuning data over 5 years. Both machines had output fluctuation observed and verified with deflated monitor chambers. By taking the atmospheric pressure into account, the initial fill pressure of monitor chamber is estimated, and an empirical formulation is suggested to predict the onset of daily output fluctuation using the accumulated dose drift.
Results: The results showed for 760 mmHg average atmospheric pressure, daily output fluctuation was detected when the average accumulated dose drift was around 11%. It is found that both sealed monitors were also deflated after 11% average accumulated output drift. The initial fill pressure of monitor chamber was then estimated to be 854 mmHg. Considering an external atmosphere pressure, it is suggested the occurrence of daily output fluctuation happens when the accumulated output drift percentage equals to D_(accumulated )=(854-P_atmospheric (mmHg))/854 ×100
Conclusion: Correlation of accumulated output drift with the onset of daily output fluctuation of two Varian machines showed the daily output fluctuation happens when the average accumulated output drift reaches to 11%. By considering the atmospheric pressure, an empirical equation is suggested to predict the onset of output daily fluctuation based on the accumulated output drift.
Not Applicable / None Entered.