1. RSNA Meeting, Chicago, IL
Predictive Maintenance of Medical Accelerators: A Customized Dashboard Interface
CM Able, MS, AH Baydush, Ph.D., C Nguyen, BS, J Gersh, Ph.D., A Ndlovu, Ph.D., I Rebo, Ph.D., J Booth. Ph.D., M. Perez, Ph.D., B Sintay, Ph.D., and MT Munley, Ph.D.
December 3, 2014
RSNA Meeting, Chicago, IL

2. Disclosures:
This work was supported by a grant from Varian Medical Systems, Inc.

3. Accelerator Predictive Maintenance: Statistical Process Control (SPC)
Machine operational and performance data can be represented by a continuous probability density function – typically a Gaussian distribution.
Statistical Process Control (SPC) techniques detect non-random changes in a process or system
Process Control Charts are the primary tool of SPC
Individual – individual parameter versus time
Moving Range – difference between adjacent individual data points versus time
Standard SPC methods utilize the mean and standard deviation of the process or system being monitored to detect non-random changes, i.e. potential failures
The use of statistical process control techniques provides us the platform to determine when an accelerator subsystem’s operating characteristics have changed. The key characteristic that makes SPC effective is that given a homogeneous set of data, regardless of whether the data has a normal distribution, >98% of the data will fall within 3 standard deviations. When action limits are developed they can provide a guidepost to determine when service intervention may be required.

4. Automated and performed daily
VMAT QA Daily Delivery
VMAT delivery of a robust, technically challenging QA treatment (Snooker Cue*)
Maximizes gantry and MLC speed and speed changes, both with and against gravity
Automated and performed daily
5 sites consisting of 7 Truebeam linacs currently under investigation
Trajectory and text log files are decoded and analyzed
* VanEsch et al, Med Phys 38(9), (2011).

5. 525 total parameters analyzed 35 from text log file
490 from trajectory log file
482 of which are MLC/carriage related
40 are raw measured values
485 are software processed values
Speed - distance change over time
Cross correlation
With small, challenging subsection of baseline run
Analyze location and peak cross correlation value

6. PdM Software: Initial Launch

7. PdM Software: Machine Selection

8. PdM Software: 9 Major Sub-systems

9. PdM Software: RF Generation
5 Parameters

10. PdM Software: Gun Control
5 Parameters

11. PdM Software: Energy Control
4 Parameters

12. PdM Software: Beam Uniformity
11 Parameters

13. PdM Software: DC Voltage Generation
7 Parameters

14. PdM Software: System Regulation
3 Parameters

15. PdM Software: Jaws and Gantry
10 Parameters: 6 measured, 4 calculated

16. PdM Software: MLC Bank (A or B)
4 Parameters per leaf: all calculated

17. PdM Software: Analysis Pane

18. Individual and Moving Range Charts
Upper – Individual
Lower – MR
White line – Baseline Mean
Red Lines – Upper and lower control limits
Orange data – warning condition
Red data – Alarm condition

19. Horizontal Histogram and Parameter Summary Statistics
Upper – Horizontal frequency histogram
White line – Baseline Grand Mean
Red Lines – Upper and lower control limits
Middle – Parameter summary statistics
Lower – Saved user comments

20. Hybrid Upper and Lower Control Limits
Blue Line – Standard SPC limits (+/-3 SD) using measured data
Green Line – Limits using system specifications (MLC +/-2mm/s)
Red Line – Hybrid method combining standard SPC method and an empirical factor based on clinical requirements or global variation determined from a large sample of accelerators

21. PdM Software: Example Detection – Beam Steering
Radial Angle Steering Coil Current values deviated to an alarm state
Deviation was identified by morning flatness/symmetry checks
Change in symmetry was confirmed by scanning water phantom
Adjustments were made and values were back at baseline

22. PdM Software: MLC Motor Failure Prediction?
Cross correlation (CC) of baseline MLC position (location versus time during delivery) and its position during each daily delivery
This chart shows the CC maximum value is decreasing
Decreasing CC max value indicates the MLC is lagging compared to its baseline performance
MLC performance is evaluated by speed, CC maximum value and CC maximum value location

23. Predictive Maintenance: Next Steps
We have deployed an alpha version of the TrueBeam PdM software to our data collection partners.
We are now correlating our findings with field service reports in an effort to validate the utility and effectiveness of our technique for predictive ability.
Thank You!