CONSIDERATIONS FOR SUPERFICIAL PHOTON DOSIMETRY

Dose measurements at superficial energies required special considerati ons. First, care must be taken in selecting appropriate phantom materi als. Materials that are adequate tissue substitutes at megavoltage ene rgies might not be adequate at superficial energies. The suitability o f 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 contam inants 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 elim inate contaminants can result in a large increase in dose measured dir ectly at the surface.