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Evaluation of Linac Mechanical Systems Using Statistical Process Control

A Jones

A Jones1*, J Barnhardt2, J Treas3, (1) Geisinger Medical Center, Danville, PA, (2) Bloomsburg University, Bloomsburg, PA, (3) Geisinger Medical Center, Danville, PA

SU-E-T-137 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Measuring the accuracy of the mechanical readouts on a linear accelerator is a staple of monthly quality assurance in Radiation Oncology. This study uses statistical quality control to determine if the current specifications and the frequency of the checks are reasonable.

Statistical quality control looks at a set of data and determines the limits of the data. Changes can be made to the system with an eye toward improving the process. Previous measurements are used to determine the limits which are used to determine if the specifications are achievable or if they could be tightened.
The study analyzed monthly mechanical data measured by several physicists on 5 linacs over several years. We looked at the difference between digital readout and measured value in gantry, collimator, and table angles, table motion, and jaw readout. Cp, Cpk, range limits and confidence limits were calculated.

Cp values were greater than 1 indicating each system was capable of performing within specification. Cpk values were also greater than 1 showing that each distribution was within specification. Each system showed raw results well within specification. Upper range limits for each system were well under the specifications. Upper and lower confidence limits were also within the specifications. There were no readings outside of specifications, however there were differences exceeding the range limits and the confidence levels.

The general mechanical specifications set forth by manufacturers and national societies are easily achievable by modern linacs. The measurements and processes are able to be controlled. Range and control limits can be used to identify potential problems before they occur. Modern linacs can reliably and accurately display mechanical readouts well within specifications over long periods of time which may lead to a recommendation to decrease the frequency of mechanical measurements.

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