Source:http://linkedlifedata.com/resource/umls/id/C1517468
NCI: Gel dosimetry involves filling a model of a body part, such as the head, neck or breast, with a gel mixture. The gel-filled model (phantom) then undergoes MRI, CT, ultrasound, or radiation therapy, to gauge how effective the same procedure might be for the patient. Radiation alters the chemical structure of the gel, and the changes appear when scientists view a cross-section of the phantom on the computer screen. (from Medscape)