The photon attenuation and absorption properties of clear and white polystyrene

For many years clear polystyrene (C8H8)n has been used in electron dosimetry as a water or muscle substitute, and documents such as the SCRAD “Protocol for the Dosimetry of High Energy Electrons” (SCRAD, 1966) recommend the use of polystyrene calibration phantoms. Unfortunately, the “muscle equivalence” for low energy photons is poor, with the material having both mass attenuation and energy absorption coefficients (μ./ρ, μen/ρ) differing by more than 20% from the values for muscle at energies below 40 keV.