An improvement in IMRT QA results and beam matching in linacs using statistical process control

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Patient-specific quality assurance (QA) is crucial for intensity-modulated radiation therapy (IMRT).
• Multileaf collimator (MLC) calibration consistency across linear accelerators (linacs) is essential for accurate IMRT delivery.
• Statistical process control (SPC) principles can optimize QA processes.

Purpose of the Study:

• To apply SPC to patient-specific IMRT QA to establish clinic-specific action limits.
• To evaluate the impact of MLC calibration adjustments on IMRT QA outcomes.
• To improve the consistency of beam-matched linacs.

Main Methods:

• Retrospective analysis of 247 IMRT QA datasets from three linacs, focusing on gamma pass rate (GPR) and dose ratios.
• Implementation of SPC control charts and action limits.
• Monthly MLC sweep tests to assess leaf gap parameter and post-adjustment evaluation with 424 QA datasets.

Main Results:

• Initial average GPR was 98.6% (SPC limit 93.7%); average dose ratio was 1.003 (limits 0.989-1.017).
• MLC sweep tests revealed initial variations (-1.8% to +1.2%); post-adjustment, all were within 0.4%.
• Post-adjustment, average GPR improved to 99.4% (SPC limit 97.3%); average dose ratio became 0.997 (limits 0.981-1.011).

Conclusions:

• SPC effectively identified and reduced small variations between matched linacs in IMRT QA.
• Adjusting the MLC leaf gap parameter significantly improved QA consistency and accuracy.
• Ongoing SPC monitoring will refine clinical action limits for IMRT QA.