Aim: The aim of this study is to evaluate the constancy of the TB MPC over a 
long period of time, as well as to compare the performance of multiple TB MPC 
systems across different centers.
Methods: The TB MPC system was run daily at morning run-up on each 
Machine. The parameters tested by the MPC system and the corresponding 
tolerances are compared to the daily QA reports and the ionization chamber 
output. The results were statistically evaluated.
Results: the % difference in 6 MV output as measured by the TB MPC system 
compared with the corresponding monthly ionization chamber measurement on 
the 3 Linacs from January 2017 to November 2022. The maximum difference 
was 2%. The difference was also within 2% in consistency for the 3 TB Linacs. 
The rate of failure to detect any misalignment or output change for all other 
parameters was 0.1% or less.
Conclusion: our measurement indicates that MPC is appropriate as a daily 
output constancy check. There were a non-significant number of false negative 
results reported by MPC, and we would advocate the use of independent 
methods, such as the use of the Daily QA device, to quickly resolve these when 
they occur