On the sensitivity of patient-specific IMRT QA to MLC positioning errors

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Accurate multileaf collimator (MLC) leaf positioning is critical for effective intensity modulated radiation therapy (IMRT).
• Patient-specific IMRT quality assurance (QA) procedures are essential for verifying treatment accuracy.
• Understanding the sensitivity of current QA methods to minor MLC positioning errors is crucial.

Purpose of the Study:

• To evaluate the sensitivity of patient-specific IMRT QA procedures to minor MLC leaf positioning errors.
• To compare the effectiveness of radiochromic film and diode matrix measurements in detecting these errors.
• To assess the impact of different analysis criteria (DTA, gamma index) on error detection.

Main Methods:

• Introduced random errors (up to 2 mm) and systematic errors (+/-1 mm, +/-2 mm) into 8 clinical IMRT patient plans.
• Compared calculated planar dose distributions with measurements from radiochromic films and a diode matrix.
• Analyzed agreement using absolute distance-to-agreement (DTA) and gamma index (2%/2 mm, 3%/3 mm criteria).

Main Results:

• Both film and diode matrix detected systematic errors of 2 mm or greater.
• Diode arrays showed higher sensitivity than film due to superior detector response characteristics.
• Gamma index with 2%/2 mm criteria using a diode array demonstrated the highest sensitivity to MLC positioning errors.

Conclusions:

• Patient-specific IMRT QA can detect systematic MLC positioning errors of 2 mm or more.
• Diode arrays with tighter gamma criteria (2%/2 mm) offer enhanced sensitivity for MLC QA.
• Combining patient-specific IMRT QA with periodic MLC QA is recommended to ensure accurate radiation delivery.