A clinically observed discrepancy between image-based and log-based MLC positions.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Accurate radiation delivery relies on precise multileaf collimator (MLC) positioning.
• Log-file data is typically used to verify MLC positions during treatment.
• Potential discrepancies between logged and actual MLC positions may compromise treatment accuracy.

Purpose of the Study:

• To report a clinical case where real-time intratreatment imaging detected a significant MLC leaf positional error.
• To emphasize the limitations of relying solely on log-file data for MLC quality assurance.

Main Methods:

• Utilized an Electronic Portal Imaging Device (EPID)-based exit-fluence dosimetry system for cine imaging during treatment.
• Visually identified a suspected MLC leaf displacement not detected by standard systems.
• Quantified the MLC leaf position discrepancy using EPID images and standard test patterns, comparing with log-file data.

Main Results:

• Log-file data indicated no difference between planned and recorded MLC positions.
• Image-based measurements revealed a consistent medial displacement of 1.3 ± 0.1 mm for the affected MLC leaf.
• The observed offset was validated through independent test pattern irradiations.

Conclusions:

• MLC leaf positions derived solely from log-file data can deviate significantly (>1 mm) from actual positions.
• Independent verification of MLC positions is crucial for reliable patient and machine quality assurance in radiotherapy.
• Intratreatment EPID imaging offers a valuable method for detecting deviations in MLC planned positions during treatment delivery.