A Monte Carlo study of accelerator head scatter

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiological Physics

Background:

• Off-focus x-ray production in medical linear accelerators (linacs) can impact radiation therapy dose distributions.
• Understanding these sources is crucial for accurate treatment planning and quality assurance.

Purpose of the Study:

• To investigate the origins and quantify the contributions of off-focus x-ray radiation within a 6 MV linac head.
• To identify specific linac components responsible for generating off-focus radiation.

Main Methods:

• Utilized the EGS4 Monte Carlo code to simulate radiation transport through a modeled 6 MV linac head.
• Tracked monoenergetic electrons and generated x-rays through components including the exit window, target, beam stopper, flattening filter, monitor chamber, and collimators.
• Analyzed the distribution and intensity of off-focus radiation reaching the isocenter.

Main Results:

• Off-focus radiation sources were found to be distributed throughout the linac head.
• The primary collimator, flattening filter, and beam stopper were identified as the most significant sources of intense off-focus radiation.
• Collimator effects are strongly influenced by the exposed volume of the head-scatter source.

Conclusions:

• The study provides a basis for developing advanced models to calculate collimator factors for various field sizes.
• Improved models can lead to better agreement between measured and calculated isodose distributions in radiation therapy.
• Findings are generally applicable to conventional linac head designs across different manufacturers and energies.