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Detection of systematic errors in external radiotherapy before treatment delivery
R Calandrino1, G M Cattaneo, C Fiorino
1Servizio di Fisica Sanitaria, H.S. Raffaele, Milan, Italy.
Summary
Independent quality control of radiation therapy plans significantly reduces systematic errors. A 5-year analysis of over 6000 checks revealed operator-dependent error rates and a downward trend, though some errors still required in vivo dosimetry detection.
Area of Science:
- Medical Physics
- Radiation Oncology
- Quality Assurance
Background:
- Systematic errors in radiation therapy can lead to significant deviations in prescribed daily dose.
- Independent checks of monitor units (MU), dose distribution, and treatment charts are crucial for error reduction.
- Operator dependency and temporal trends in error rates require careful analysis.
Purpose of the Study:
- To evaluate the effectiveness of an independent quality control process in reducing systematic errors in radiation therapy.
- To analyze the error rates, identify contributing factors (e.g., operator dependency), and assess trends over time.
- To determine the residual error rate not caught by pre-treatment checks.
Main Methods:
- Analysis of 6272 independent quality control checks performed over 5 years for approximately 5000 patients.
- Categorization and quantification of serious (deviation >5%) and minor errors detected before treatment delivery.
- Time-trend analysis of error rates and comparison of operator-specific error frequencies.
- Identification of errors missed by the independent check and detected by in vivo dosimetry.
Main Results:
- A total of 70 serious and 147 minor errors were detected and corrected pre-treatment.
- Serious error rates varied significantly between operators, ranging from 0.3% to 2.5%.
- A significant reduction in serious errors was observed over time, from 1.5% (1991-1994) to 0.9% (1994-1996).
- Three serious systematic errors were missed by the independent check and detected by in vivo dosimetry in 650 patients.
Conclusions:
- Independent quality control is highly effective in reducing systematic errors in radiation therapy planning and delivery.
- Operator performance significantly influences the error rate, highlighting the need for consistent training and protocols.
- While effective, the independent check is not infallible, and in vivo dosimetry remains essential for detecting residual errors.