Confidence limits for patient-specific IMRT dose QA: a multi-institutional study in Korea

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Patient-specific dose quality assurance (DQA) is crucial for intensity-modulated radiation therapy (IMRT).
• Establishing appropriate tolerance levels for IMRT DQA is essential for patient safety and treatment efficacy.
• Confidence limits (CL) have been proposed as a metric for defining these tolerance levels.

Purpose of the Study:

• To investigate the suitability of confidence limits (CL) for determining patient-specific IMRT dose QA (DQA) tolerance levels.
• To evaluate the impact of data distribution and statistical methods on CL calculations.
• To assess the performance of CL across different treatment sites and DQA metrics.

Main Methods:

• A multi-institutional study involving 11 Korean institutions and 155 IMRT DQA measurements.
• Analysis of point-dose differences and gamma passing rates for various cancer types (brain, H&N, abdomen, prostate).
• Application of Shapiro-Wilk, Student's t-distribution, and Spearman's tests to assess data normality and CL sensitivity.

Main Results:

• Differences in CLs between normal and t-distributions were negligible for 3%/3 mm criteria but significant for 2%/2 mm criteria (1.6% composite, 2.2% per-field).
• The increased sensitivity of 2%/2 mm criteria, coupled with large standard deviations, led to notable CL discrepancies.
• No significant correlation was observed among different DQA methods.

Conclusions:

• Confidence limits may not be a suitable metric for IMRT DQA tolerance levels when sample group statistics lack normality and exhibit large standard deviations.
• The choice of DQA criteria (e.g., 2%/2 mm vs. 3%/3 mm) significantly impacts the calculated CLs.
• Further research is needed to establish robust tolerance levels for patient-specific IMRT DQA.