Answer to Question #2052 Submitted to "Ask the Experts"

Q

When taking care of a 70-kg patient who has recently had a dose of 69.6 mCi of quadramet, what are the risks to medical staff of radiation exposure? What are the precautions that should be taken? Could medical staff be exposed to radiation from sitting across a small hallway from the patient's room? Could medical staff be getting unsafe exposure to the radiation from close patient contact to body fluids such as sweat/urine/saliva without any kind of protection? What are safe limits from this type of exposure?

A

There are no risks to the medical staff. The beta particle emitted by samarium (¹⁵³Sm) is completely absorbed within the patient. The bremsstrahlung contribution to external dose is negligible. In 29% of the decay events, a 103 keV gamma ray is emitted. The specific gamma-ray constant for ¹⁵³Sm is 0.45 roentgens per mCi per hour at 1 cm, assuming that the source is an unshielded point source. The patient, however, is more like a shielded line source. One foot away from the patient, the line source nature of the patient decreases the constant by a factor of 0.43, and the patient's body absorbs roughly half the radiation, so the effective specific gamma-ray constant is (0.5)(0.43)(0.45)/(12)(2.54)(12)(2.54) = 0.0968/929 = 0.1 milliroentgens per mCi per hour. One meter from the patient, the patient "approaches" a point source, and the constant is decreased by only 0.823. Therefore at a meter, the effective specific gamma-ray constant is about 0.02 milliroentgens per mCi per hour. (These line source conversions come from Siegel JA, Marcus CS, Sparks RB: Calculating the absorbed dose to others from the radioactive patient: line source model versus point source model. In press, Journal of Nuclear Medicine). When such gamma-ray constants are plugged into the equation for dose received by a person near the patient, the actual doses are extremely small and need to be additionally decreased by self absorption by that person. No precautions need to be taken except that disposable plastic gloves should be worn when handling urine. Medical staff could be exposed to radiation from sitting across a small hallway from the patient's room, but to a miniscule extent. When in direct contact with body fluids, wearing disposable gloves is the standard of medical practice because of infection control, and this also provides contamination control. If inadvertent direct contact occurs, wash with soap and water and monitor to make sure contamination is gone. With regard to safe limits from this type of exposure to a member of the general public, 500 mrem per patient is the Nuclear Regulatory Commission (NRC) limit, although at least 10 times that is safe. Radiation workers are permitted up to 5,000 mrem per year from occupational radiation by the NRC, with no particular maximum from an individual patient. Carol S. Marcus, PhD, MD