Purpose: To assess the electron beam monitoring statistical process control (SPC) in linear accelerator
(linac) daily quality control. We present a long-term record of our measurements and evaluate which
SPC-led conditions are feasible for maintaining control.
Methods: We retrieved our linac beam calibration, symmetry, and flatness daily records for all electron
beam energies from January 2008 to December 2013, and retrospectively studied how SPC could have
been applied and which of its features could be used in the future. A set of adjustment interventions
designed to maintain these parameters under control was also simulated.
Results: All phase I data was under control. The dose plots were characterized by rising trends followed
by steep drops caused by our attempts to re-center the linac beam calibration. Where flatness and
symmetry trends were detected they were less-well defined. The process capability ratios ranged from
1.6 to 9.3 at a 2% specification level. Simulated interventions ranged from 2% to 34% of the total number of
measurement sessions.We also noted that if prospective SPC had been applied it would have met quality
control specifications.
Conclusions: SPC can be used to assess the inherent variability of our electron beam monitoring system.
It can also indicate whether a process is capable of maintaining electron parameters under control with
respect to established specifications by using a daily checking device, but this is not practical unless a
method to establish direct feedback from the device to the linac can be devised.