Biological consequences of MLC calibration errors in IMRT delivery and QA

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Intensity modulated radiotherapy (IMRT) utilizes multileaf collimators (MLCs) for precise dose delivery.
• MLC calibration errors can lead to unintended dose variations in planning target volumes (PTVs) and organs at risk.
• Accurate quality assurance (QA) is crucial for ensuring treatment efficacy and patient safety in IMRT.

Purpose of the Study:

• To investigate the biological impact of MLC calibration errors in prostate and head and neck IMRT.
• To assess PTV and normal tissue under- or overdose levels using clinical QA action limits.
• To inform the development of biologically-based MLC and IMRT QA action limits.

Main Methods:

• Ten prostate and ten head and neck IMRT cases were analyzed.
• Systematic MLC offsets (-2 to +2 mm) were introduced to simulate calibration errors.
• Generalized equivalent uniform dose (gEUD) was used to evaluate biological impact on targets and organs at risk.
• Plans were recalculated and compared using ion chamber dosimetry and 3D gamma analysis against QA criteria.

Main Results:

• Prostate IMRT: >2% differences in PTV and rectum gEUD were observed, with some passing ion chamber QA but failing 3D gamma QA.
• Head and neck IMRT: QA flagged >4% PTV gEUD and >5% brainstem maximum dose differences.
• Relaxing QA criteria allowed plans with larger gEUD deviations to pass, highlighting sensitivity differences between QA methods.

Conclusions:

• 3D gamma analysis is more sensitive to MLC errors in low modulation IMRT plans, while ion chamber dosimetry is better for high modulation plans.
• Both QA methods can detect >2% gEUD changes in PTVs and critical structures for low modulation plans.
• For high modulation plans, >2% PTV gEUD and >5% critical structure gEUD changes are detectable; smaller changes have similar detection rates for both QA methods.