WE-E-BRB-07: Direct 3D Fluence Calculation from Machine Beam Parameters for VMAT Delivery Verification.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Machine log files are used for quality assurance in intensity-modulated radiation therapy (IMRT).
• Current methods derive 2D fluence maps, limiting application to fixed-gantry IMRT.
• Extending quality assurance to rotational-gantry VMAT requires advanced computational methods.

Purpose of the Study:

• To introduce Direct 3D Fluence Calculation (D3DFC), a computationally efficient method for 3D fluence volume computation.
• To extend dynamic log file-based quality assurance from fixed-gantry IMRT to rotational-gantry VMAT.
• To improve the speed and accuracy of fluence map derivation for VMAT.

Main Methods:

• D3DFC directly calculates 3D photon fluence distribution from machine parameters in log files or DICOM plans.
• Avoids intermediate conversion to 2D fluence maps per gantry angle, enhancing computation speed and accuracy.
• Verified with film-in-air measurements and compared VMAT delivery records against treatment plans to identify errors.

Main Results:

• D3DFC is implemented in MATLAB, supporting DICOM, Varian MLC dynamic logs, and Varian Truebeam logs.
• Computation time is 10–20 seconds for a single-arc VMAT plan or delivery record.
• Delivery errors, such as 1 mm MLC errors, are clearly detectable through delivery-to-plan fluence volume comparison.

Conclusions:

• Direct computation from machine parameters offers superior speed and accuracy for beam delivery verification.
• D3DFC is advantageous for quality assurance where rapid verification is needed, reducing reliance on slower dose computations.
• The calculated 3D photon fluence volume effectively detects and visualizes VMAT delivery discrepancies.