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Statistical process control and verifying positional accuracy of a cobra motion couch using step-wedge quality
Diana Binny1,2, Craig M Lancaster1, Jamie V Trapp2
1Department of Radiation Oncology, Cancer Care Services, Royal Brisbane and Women's Hospital, Queensland, Australia.
Journal of Applied Clinical Medical Physics
|July 22, 2017
Summary
Statistical process control (SPC) enhances TomoTherapy couch positioning accuracy by identifying action limits for quality assurance tests. Implementing SPC and recalibration ensures positional accuracy, increasing confidence in daily treatment setups.
Area of Science:
- Medical Physics
- Radiation Oncology
- Quality Assurance
Background:
- Accurate patient positioning is critical in radiation therapy, particularly with advanced systems like TomoTherapy.
- Traditional quality assurance (QA) methods may not always detect subtle systematic or random errors in couch positioning.
- Statistical Process Control (SPC) offers a robust framework for monitoring and improving process stability and accuracy.
Purpose of the Study:
- To apply process control techniques to establish action limits for TomoTherapy couch positioning QA.
- To monitor and analyze couch positional accuracy using a 4-year dataset of IEC offset measurements.
- To evaluate the effectiveness of SPC in detecting and remediating couch positioning errors.
Main Methods:
- Utilized Statistical Process Control (SPC) techniques, including X-charts, process capability (cp), and acceptability (cpk) indices.
- Analyzed 4 years of couch IEC offset data and conducted a second study with physical couch offsets using the TQA 'Step-Wedge Helical' module and MVCT detector.
- Performed process capability tests on retrospective data to define tolerances based on user-specified levels.
Main Results:
- SPC identified systematic and random variations in couch positional accuracy, with mean variations up to ±0.5 mm over three months.
- Observed variations between applied and detected couch offsets, particularly in the vertical direction, requiring remediation.
- Couch encoding system recalibrations successfully reduced variations to user-specified levels.
Conclusions:
- SPC effectively detects systematic variations in couch positioning before machine tolerance levels are reached.
- Recommended a user-specified tolerance of ±2 mm with a 3-month test frequency for improved couch positional accuracy.
- SPC analysis enhances error identification, increasing confidence in daily treatment setups for TomoTherapy.
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