Confidence limit variation for a single IMRT system following the TG119 protocol

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Intensity-modulated radiation therapy (IMRT) is a complex radiotherapy technique.
• Robust quality assurance (QA) is crucial for ensuring IMRT treatment accuracy and patient safety.
• TG119 provides a framework for IMRT system QA, but its robustness needs evaluation.

Purpose of the Study:

• To assess the reliability and consistency of TG119-based QA metrics for an IMRT system.
• To identify potential sources of variation in IMRT QA results.

Main Methods:

• Four dosimetrists created IMRT plans for five TG119 test cases.
• Plans were delivered to a solid water phantom for composite and per-field measurements.
• Measurements utilized film, ionization chamber, and diode array, analyzed per TG119 specifications.

Main Results:

• Ion chamber measurements showed mean variance within 1.7% of planned dose.
• Confidence limits varied significantly between planners: composite film (8.06%-16.5%), per-field diode array (0.00%-2.89%).
• Ion chamber confidence limits ranged from 1.98% to 4.39%.

Conclusions:

• TG119-based QA metrics reveal significant planner-dependent variations in confidence limits for IMRT systems.
• Differences in treatment plan modulation likely contribute to observed variations.
• TG119 evaluations using single-planner plans may not adequately represent overall IMRT system accuracy.