Simulation of large x-ray fields using independently measured source and geometry details

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Accurate dose calculation is crucial for effective radiation therapy.
• Simulating radiation beams requires precise input parameters for treatment head components and incident radiation.
• Siemens Oncor linear accelerators are widely used in clinical settings.

Purpose of the Study:

• To achieve accurate dose simulations for 6 and 18 MV x-ray beams from a Siemens Oncor linear accelerator.
• To validate simulation accuracy by comparing it with direct measurements.
• To constrain simulations using independently determined parameters of the treatment head and incident beam, including energy and spot size.

Main Methods:

• Measurements were performed in three configurations: clinical, flattening filter removed, and target/flattening filter removed.
• Incident beam energy, spectral width, spot size, and flattening filter densities were measured directly.
• Simulations were conducted using EGSnrc/BEAMnrc code with an asymmetric setup accounting for offsets.

Main Results:

• Excellent agreement (1%/1 mm) between measurement and simulation was achieved at 6 MV, with or without the flattening filter.
• At 18 MV, agreement was 1.5%/1.5 mm with the filter and 1%/1 mm without, excluding the buildup region.
• Discrepancies in the buildup region were within 2%/2 mm at 18 MV and 1.5%/1.5 mm at 6 MV.

Conclusions:

• The study achieved good agreement between measured and simulated dose distributions, including the buildup region.
• Independent measurements of incident beam and treatment head parameters effectively constrained the simulations.
• The described methodology for parameter acquisition is practical for clinical linear accelerators.