Tissue Equivalent Plastic
Tissue equivalent plastic (or TEP) is an A-150 plastic designed specifically to simulate human tissue. Its hydrogen and nitrogen percentages-by-composition are representative of that found in human skin and muscle.
In Nuclear Instruments and Methods in Physics Research, Al-Sulaiti notes that "For energies typically encountered in proton therapy, A-150 plastic appears to be a good representation of water (and thereby very similar to tissue.)"
The atomic-level effects that radiation has on the TEP are useful in deducing what similar effects may occur in humans.