Increase of the surface dose in kilovoltage X-ray therapy beams due to a thin layer of plastic wrap

To absorb photoelectrons produced in a leaded glass cone by kilovoltage X-ray therapy beams, the authors have used a transparency film attached to the cone by a plastic wrap. They have shown that the transparency film attenuates adequately photoelectrons produced in the leaded glass cone. However, the plastic wrap generates a non-negligible amount of photoelectrons. Measurements indicate that surface dose can increase as much as 15% when plastic wrap is used. This increase of surface dose is due to the photoelectric interaction with the chlorine atoms present only in the plastic wrap. Calculations using a simplified model for the Bremsstrahlung X-ray spectrum explains the behavior of the measurements.