Charles M Able MS, Presenter: Supported, Varian Medical Systems, Inc

Alan Baydush PhD, Abstract Co-Author: Research Grant, Varian Medical Systems, Inc

Michael T. Munley PhD, Abstract Co-Author: Research Grant, Varian Medical Systems, Inc

PdM monitoring of beam uniformity using a new method for determining SPC control limits is more effective than using traditional control limits. SPC false alarms are reduced thereby eliminating unwarranted service intervention. We are working to develop code that will provide an SPC evaluation using the new formula.

The focus of this study is to determine the effectiveness of revisions to statistical process control (SPC) chart limits for linear accelerator steering coil current (beam uniformity) predictive maintenance (PdM) monitoring

We have re-evaluated calculation of the limits for steering coil current (SCC) SPC charts based on our observations that the changes in SCC required for 1% change in measured beam uniformity will generally exceed the traditional limits calculated using the standard deviation of the subgroup values alone. Using empirical data obtained from controlled experiments varying the SCC while measuring the beam uniformity, we have developed a formulism that incorporates a scaling factor (Cm) in the calculation of control limits.

SCC monitoring of an 18 MV photon beam using the new limits indicated a change in the transverse position coil operating current 2 weeks prior to multiple EXQ2 faults and unscheduled downtime. A change in beam uniformity of greater than 3% was found once the beam was scanned using a computerized water phantom and ion chamber. SPC charts of this steering coil using traditional limits would have resulted in 5 false positives prior to the first positive alarm indicated by the new limits

Able, C, Baydush, A, Munley, M, Analysis of Predictive Maintenance of Linear Accelerator Beam Uniformity Using Statistical Process Control.  Radiological Society of North America 2013 Scientific Assembly and Annual Meeting, December 1 - December 6, 2013 ,Chicago IL. http://archive.rsna.org/2013/13022135.html