Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

labeled with 1-131. 1-131 decays with an 8-day half-life and emits high-energy betaparticles and gamma rays. These decay properties, together with the facts that 1-131can be released as a gas under certain conditions, can be absorbed through the skin,and concentrates and is retained in the thyroid, necessitate a number of radiationprotection precautions with the administration of therapeutic quantities ofI-131.Following administration, 1-131 is secretedl excreted in all body fluids includingurine, saliva, and perspiration. Before 1-131 is administered, surfaces of the patient'sroom likely to become contaminated, such as the floor, bed controls, mattress, lightswitches, toilet, and telephone, are covered with plastic or absorbent plastic-backedpaper to prevent contamination. In addition, the patient's meals are served on disposabletrays. Waste containers are placed in the patient's room to dispose of usedmeal trays and to hold contaminated linens for decay. Radiation safety staff will measureexposure rates at the bedside, at the doorway, and in neighboring rooms. In somecases, immediately adjacent rooms must be posted off limits to control radiation exposureto other patients and nursing staff These measurements are posted, together withinstructions to the nurses and visitors including maximal permissible visiting times.Nursing staff members are required to wear dosimeters and are trained in the radiationsafety precautions necessary to care for these patients safely and efficiently. Visitorsare required to wear disposable shoe covers, and staff members wear both shoecovers and disposable gloves to prevent contamination. Visiting times are limited andpatients are instructed to stay in their beds and avoid direct physical contact to keepradiation exposure and contamination to a minimum. Visitors and staff are usuallyrestricted to nonpregnant adults. After the patient is discharged, the health physics ornuclear medicine staff decontaminates the room and verifies through wipe tests andGM surveys that the room is completely decontaminated. Federal or state regulationsmay require thyroid bioassays of staff technologists and physicians directly involvedwith dose preparation or administration oflarge activities ofI-131.The NRC regulations (Title 10 Part 35.75) require that patients receiving therapeuticradionuclides be hospitalized until or unless it can be demonstrated that the totaleffective dose equivalent to any other individual from exposure to the released patientis not likely to exceed 5 mSv (0.5 rem). Guidance on making this determination canbe found in two documents from the NRC: NUREG-1556, Vol. 9, entitled "A ConsolidatedGuidance about Materials Licenses: Program-Specific Guidance about MedicalLicenses" and Regulatory Guide 8.39, entitled "Release of Patients AdministeredRadioactive Materials." These documents describe methods for calculating doses toother individuals and contain tables of activities not likely to cause doses exceeding 5mSv (0.5 rem). For example, patients may be released from the hospital following 1-131 therapy when the activity in the patient is at or below 1.2 GBq (33 mCi) or whenthe dose rate at 1 m from the patient is at or below 0.07 mSv/hr (7mrem/hr).To monitor the activity ofI-131 in a patient, an initial exposure measurementis obtained at 1 m from the patient. This exposure rate is proportional to the administeredactivity. Exposure rate measurements are repeated daily until the exposurerate associated with 1.2 GBq (33 mCi) is obtained. The exposure rate from 33 mCiin a patient will vary with the mass of the patient. The activity remaining in thepatient can be estimated by comparing initial and subsequent exposure rate measurementsmade at a fixed, reproducible geometry. The exposure rate equivalent to1.2 GBq (33 mCi) remaining the patient (Xu) is:Xu = (1.2 GBg)EoAo