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SU‐E‐T‐54: Initial Investigation Applying Statistical Process Control to Accelerator Beam Quality
Author(s) -
Able C,
Hampton C,
Baydush A
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3612005
Subject(s) - control limits , interlock , standard deviation , statistical process control , beam (structure) , control chart , chart , calibration , transverse plane , alarm , mathematics , optics , physics , statistics , computer science , engineering , process (computing) , structural engineering , electrical engineering , operating system
Purpose: This study retrospectively applies statistical process control (SPC) methods to determine its utility in detecting changes in linear accelerator beam steering parameters prior to equipment failure (interlocks actuated) thereby increasing patient safety. Methods: Steering coil currents (SCC) for the transverse and radial planes are set such that a reproducibly useful photon or electron beam is available. SCC are sampled and stored in the control console computer each day during the morning warm‐up. The transverse and radial ‐ positioning and angle SCC for photon beam energies were evaluated using average and range (Xbar‐R) process control charts (PCC). The weekly average and range values (subgroup n=5) for each steering coil were used to develop the PCC. SCC from September‐2009 (annual calibration) until two weeks following a beam steering failure on June 28th were evaluated. PCC limits were calculated using the first twenty subgroups. Appropriate action limits were developed using conventional SPC guidelines. Results: PCC high alarm action limit was set at six standard deviations from the mean. Low alarm indicators were: (1) nine points in a row on either side of the mean, and (2) two out of three points in a row greater than two standard deviations from the mean. Alarms received following establishment of limits (week twenty) are indicative of a special cause for deviation (Xbar chart) and/or an uncontrolled process (R chart). Transverse angle SCC for 15X and 6X indicated a high alarm on March 11th and March 29th respectively. A downward trend in this parameter continued, with high alarm, until failure. Transverse position and radial angle SCC for both energies indicated a high or low alarm starting in February or March. Conclusions: Patient safety may be improved by instituting SPC methods to monitor the beam steering process and detect abnormal changes prior to equipment failure.