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Retrospective analysis of linear accelerator output constancy checks using process control techniques
Taweap Sanghangthum1, Sivalee Suriyapee, Somyot Srisatit
1Department of Nuclear Technology, Chulalongkorn University, Bangkok, Thailand.
Journal of Applied Clinical Medical Physics
|January 16, 2013
Summary
This study demonstrates that comprehensive process control using Shewhart and EWMA charts, along with capability indices, effectively monitors linear accelerator output constancy for quality assurance. Optimal parameters were identified for these tools to ensure reliable linac performance.
Area of Science:
- Medical Physics
- Radiation Oncology
- Quality Assurance
Background:
- Linear accelerators (linacs) require routine output verification for patient safety.
- Shewhart control charts have been proposed for linac output constancy checks.
- A comprehensive process control approach for linac output QA has not been fully investigated.
Purpose of the Study:
- To investigate a comprehensive process control approach for linac output constancy quality assurance (QA).
- To evaluate the effectiveness of Shewhart control charts, EWMA charts, and capability indices for linac output verification.
Main Methods:
- Utilized RBA-3 dose constancy check data from a Varian Clinac 21EX linac (2009-2010).
- Applied Shewhart-type individuals charts (X-charts), EWMA charts with varying parameters (λ, L), and capability indices (C(p), C(pk)).
- Investigated the impact of the number of data points on control limits and process capability calculations.
Main Results:
- X-charts effectively detected systematic errors, with optimal data points for control limits ranging from 8-12.
- EWMA charts demonstrated longer in-control runs (9-33 measurements) with recommended parameters (λ=0.10, L=2.703) and 4 data points for initial calculations.
- Capability indices (C(p), C(pk)) generally exceeded 1.0, indicating acceptable process capability, with 25-30 in-control points recommended for calculation.
Conclusions:
- A comprehensive process control strategy using Shewhart and EWMA charts enhances linac output constancy QA.
- Specific parameters and data point requirements were identified for optimal implementation of these statistical tools.
- This approach contributes to improved reliability and safety in radiation therapy through robust quality assurance.
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