Considerations for superficial photon dosimetry.

Dose measurements at superficial energies required special considerations. First, care must be taken in selecting appropriate phantom materials. Materials that are adequate tissue substitutes at megavoltage energies might not be adequate at superficial energies. The suitability of a material can be judged by comparing its mass attenuation and mass energy absorption coefficients at superficial energies to those of the tissue of interest. Second, very low energy x-ray and electron contaminants must be removed from the superficial beam before they reach the detector. For detectors with a very thin window, this can be achieved by placing thin film on top of the detector. Failure to properly eliminate contaminants can result in a large increase in dose measured directly at the surface.