Photon buildup in orthovoltage X-ray beams

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Orthovoltage x-ray beams present a depth dose buildup phenomenon not well-documented.
• This effect is primarily attributed to secondary (Compton) scattered photons initiating electron motion within the phantom.

Purpose of the Study:

• To elucidate the mechanisms behind depth dose buildup in orthovoltage x-ray beams.
• To investigate a secondary order primary dose buildup effect undetectable by conventional detectors.

Main Methods:

• Experimental validation and Monte Carlo simulations were employed.
• Monte Carlo techniques were used to describe a second-order primary dose buildup effect.

Main Results:

• The primary buildup is caused by secondary Compton scattered photons and longitudinal electronic disequilibrium.
• A second-order primary dose buildup effect, due to longitudinal electronic disequilibrium, was identified.
• The magnitude of dose buildup varies with beam parameters and measurement detectors.

Conclusions:

• Radiation oncology departments should evaluate depth dose buildup in their clinical data.
• Ensure accurate dose delivery by accounting for potential higher doses a few millimeters below the skin surface.
• Consider the impact of detector type and data collection methods on measured buildup.