The dose tables used for the calculation of average glandular dose are based on a phantom that represents a patient with a compressed breast thickness of 4.2 cm and with breast tissue consisting of approximately 50% adipose (fat) tissue and 50% glandular tissue. While BR12 is manufactured to represent this tissue composition, it is commercially available in whole or half centimeter thicknesses. If a medical physicist is using this material to determine the dose, the physicist must use a thickness equal to or larger than 4.2 cm to assure that the x-ray system passes this test. 21 C.F.R. 900.12(e)(5)(vi).