Retrospective study on performance of constancy check device in Linac beam monitoring using Statistical Process Control.

Abstract

To examine the application of Statistical Process Control (SPC) and Ishikawa diagrams for retrospective evaluation of machine Quality Assurance (QA) performance in radiotherapy. SPC is a popular method for supplementing the performance of QA techniques in healthcare. This work investigates the applicability of SPC techniques and Ishikawa charts in machine QA. SPC has been applied to recommend QA limits on the particular beam parameters using the QUICKCHECK webline QA portable constancy check device for 6MV and 10MV flattened photon beams from the Elekta Versa HD linear accelerator (Linac). Four machine QA parameters - beam flatness, beam symmetry along gun target direction and left-right direction, and beam quality factor (BQF) - were selected for retrospective analysis. Shewhart charts, Exponentially Weighted Moving Average (EWMA) charts and Cumulative Sum (CUSUM) charts were obtained for each parameter. The root causes for a failure in machine QA were broken down into an Ishikawa diagram enabling the user to identify the root cause of error and rectify the problem subsequently. Shewhart charts and EWMA charts applied could detect loss in control in one variable in the 6MV beams and in all four variables in 10MV beams. CUSUM charts detected offsets in the readings. The Ishikawa chart exhaustively included the possible errors that lead to loss of control. SPC is proven to be effective for detection of loss in control in machine QA. The Ishikawa chart provides the set of probable root causes of machine error useful while troubleshooting.