Methodologies for Critical Body Organ Space Radiation Risk Assessments

One of the risks associated with long-term space flights is cancer incidence resulting from chronic exposure to space radiation. Assessment of incurred risk from radiation exposure requires quantifying the dose throughout the body. The space radiation exposure received by Space Shuttle astronauts is measured by thermoluminescent dosimeters (TLDs) worn during every mission. These dosimeters measure the absorbed dose to the skin, but the dose to internal organs is required for estimating the cancer risk induced by space radiation. A method to extrapolate these skin dose measurements to realistic organ specific dose estimates, using the Computerized Anatomical Man (CAM) and Computerized Anatomical Female (CAF) models, is discussed in detail. A transport code, which propagates high energy nucleon and charged particles, is combined with the CAM/CAF-generated shielding areal distributions to evaluate the absorbed dose at selected organ sites. The most radiosensitive organs and their respective cancer incidence factors have been tabulated by the National Council on Radiation Protection and Measurements (NCRP). These critical organs include the lung, female breast, thyroid, esophagus, stomach, colon, liver, pancreas, kidney/bladder, and the blood-forming organs (BFO). The NCRP cancer incidence and mortality risk coefficients vary with gender and age at first exposure and are used to compute the cancer risk for the identified organs. The total cancer mortality risk is evaluated for estimated exposures for several mission profiles.