In the medical field of radiation oncology, buildup refers to blocks of clear plastic which are used by physicists.
Buildup is heavy, similar to plexiglass, but it is specially formulated to have a density comparable to that of ordinary water. When a radiation beam passes through water or buildup, it is attenuated and scattered. This same effect occurs when a radiation beam enters the human body during radiation therapy.
Because radiation is harmful to living tissue, radiation oncologists must be able to simulate and quantify the scatter effects in order to ensure the safest possible conditions for the patient while they are irradiated. This process of verification is known as radiation dosimetry, and it is vital for patient safety.

See: radiation oncology, radiation dosimetry