The sensitivity of patient specific IMRT QC to systematic MLC leaf bank offset errors

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Patient-specific quality control (QC) is crucial in intensity-modulated radiation therapy (IMRT) to mitigate errors during planning and delivery.
• Systematic errors in multileaf collimator (MLC) leaf positioning can compromise treatment accuracy.
• Evaluating the efficacy of patient-specific checks in detecting MLC errors is vital for radiation oncology practices.

Purpose of the Study:

• To assess the feasibility of identifying systematic MLC leaf bank position errors using patient-specific quality control (QC) methods.
• To determine if current QC protocols are sensitive enough to detect clinically significant MLC inaccuracies.

Main Methods:

• Delivered 9 head and neck (H&N) and 14 prostate IMRT beams with simulated systematic MLC leaf offsets (0.5-1 mm).
• Measured dose distributions using MAPCHECK and electronic portal imaging devices (EPID).
• Analyzed data using absolute dose difference, relative dose difference, distance to agreement (DTA), and gamma index criteria.

Main Results:

• The gamma index with 2% absolute dose (AD) and 2 mm distance to agreement (DTA) for EPID, and 3% AD, 3 mm DTA for MAPCHECK were most sensitive to MLC offsets.
• Relative dose difference criteria showed the least sensitivity to systematic MLC errors.
• Highly modulated fields (H&N) exhibited more significant changes in passing points due to MLC inaccuracy compared to prostate fields.

Conclusions:

• Current patient-specific QC methods and criteria are not sufficiently sensitive to detect systematic MLC offsets at clinically significant levels for individual IMRT fields.
• Patient-specific QC cannot replace routine quality control (QC) procedures for the MLC itself.
• Ensuring the accuracy of MLC performance requires ongoing, dedicated quality control measures beyond patient-specific checks.