IMRT QA: Selecting gamma criteria based on error detection sensitivity.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Gamma comparison is a standard method for evaluating agreement in intensity-modulated radiation therapy (IMRT) quality assurance (QA).
• Concerns exist regarding the sensitivity of commonly used gamma criteria in detecting errors.
• Understanding gamma criteria sensitivity is crucial for defining more effective IMRT QA protocols.

Purpose of the Study:

• To present a quantitative method for determining the sensitivity of various gamma criteria to induced errors in IMRT QA.
• To evaluate the sensitivity of different gamma criteria to specific error types, including MU, MLC, and penumbra width variations.
• To enable the definition of more meaningful gamma criteria and tolerance limits for IMRT QA.

Main Methods:

• 21 DMLC IMRT QA measurements using ArcCHECK® were compared to QA plans with induced errors (MU, MLC, penumbra width).
• Over 20,000 gamma comparisons were performed using a wide range of gamma criteria.
• Error curves were generated by graphing gamma passing rates against error magnitude to visualize missed errors.

Main Results:

• Error curve analysis effectively detected systematic and case-specific errors.
• Common criteria like 3%/3 mm (10% threshold) may miss substantial MU (15%) and MLC (±1 cm) errors.
• Increasing the dose threshold parameter significantly improved error sensitivity (up to twofold), while criteria like 2%/3 mm or 3%/2 mm with a 50% threshold showed appropriate sensitivity. Penumbra broadening proved difficult to detect.

Conclusions:

• The introduced error curve method quantitatively assesses gamma criteria sensitivity in IMRT QA.
• Commonly used gamma criteria can potentially miss significant errors, highlighting the need for improved methods.
• Increasing the dose threshold offers a straightforward way to enhance error detection sensitivity, leading to more robust IMRT QA.