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An EPID based method for efficient and precise asymmetric jaw alignment quality assurance.

Luke Clews¹, Peter B Greer

¹Calvary Mater Newcastle Hospital, Newcastle, New South Wales, 2298, Australia.

Medical Physics

|January 26, 2010

Summary

This study demonstrates that electronic portal imaging devices (EPIDs) offer a precise and efficient method for quality assurance of linear accelerator asymmetric jaw junctioning, revealing high reproducibility in jaw positioning.

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Area of Science:

Medical Physics
Radiation Oncology
Quality Assurance

Background:

Accurate alignment of linear accelerator jaws is critical for precise radiation delivery.
Traditional methods for assessing jaw positioning may lack the required precision for modern radiotherapy techniques.
Electronic Portal Imaging Devices (EPIDs) offer a potential solution for enhanced quality assurance.

Purpose of the Study:

To investigate the efficacy of amorphous silicon EPIDs for routine quality assurance of linear accelerator asymmetric jaw junctioning.
To develop and validate a method for precise measurement of jaw positions using EPIDs.
To assess the long-term stability and reproducibility of jaw positioning.

Main Methods:

Utilized EPID imaging with opposing collimator angles to determine beam central axis position to subpixel accuracy.

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Employed penumbra interpolation on half-beam blocked images to precisely determine individual jaw positions.

Validated accuracy by measuring known translations of a simulated jaw using a translation stage.

Compared EPID junction dose measurements with radiographic film for utility assessment.

Main Results:

The EPID method achieved an accuracy of 0.14 mm (SD 0.037 mm) for jaw translation measurements.
EPID junction dose measurements differed from film due to scattering properties, necessitating a correction factor.
Long-term assessment showed high reproducibility of zero jaw positions (average SD 0.07 mm) over one year.
Consistent offsets in jaw positioning were observed at different gantry angles.

Conclusions:

EPID-based methods provide efficient and precise data for linear accelerator jaw positioning and reproducibility.
Results indicate zero jaw positions are reproducible to a precision finer than the displayed jaw resolution.
The findings highlight a need for improved calibration methods for linear accelerator jaw positioning.