In Vivo Counting Program
An in vivo counting program is an important component of an occupational radiation protection program for quantifying the amount of radioactive material in the body. Anthropomorphic phantoms containing known amounts of radioactive material are used to establish calibration factors for these in vivo measurement instrument systems. The design, fabrication, and use of anthropomorphic calibration phantoms must include tissue substitute materials with radiation interaction properties similar to the human tissue of interest. This is most important for radionuclides with weakly penetrating radiation and/or low activities. This helps ensure the best practicable assessment of the radioactive materials in human subjects. Variability in past phantom designs still used to calibrate some in vivo counting systems can result in large differences in activity estimates due to the sensitivity of emitted radiations relative to differences in phantom material composition.
In vivo counting programs use radiation detectors to measure radioactive emissions from radioactive material contained inside an individual. These detectors are variously configured to measure lungs, thyroid, or the wholebody. Lung counting systems place detectors on the chest, or less typically at the posterior upper torso, in order to monitor inhaled radionuclides. Lung counting systems are important for inhalation intakes of moderately soluble to insoluble radioactive materials that remain fixed in the lungs for a period of time. Thyroid counting systems are important for monitoring radioactive iodine intakes. The location of the thyroid near the skin surface and its ability to concentrate iodines provide a practical mechanism for obtaining enhanced radioiodine information. Finally, for radionuclides that are readily soluble in body fluids or distributed throughout the body (e.g., within bones), whole-body counting systems can be used.
Counting programs are established for routine and non-routine monitoring of radioactive material workers. Routine monitoring occurs on a set schedule for workers directly involved in handling unsealed radioactive materials or for workers who frequent locations with unsealed materials. Routine monitoring also occurs at some facilities for pre-employment screening to establish or confirm background levels of radioactive materials in an individual. Other facilities also monitor at the end of a workers employment to assure that an individual did not incur a previously unsuspected radioactive material intake. Non-routine monitoring occurs when a radioactive material intake is known or suspected.
