What are the precautions and considerations for using sodium 131 (radioactive isotope) in a diagnostic or therapeutic procedure?

Sodium Iodide I-131: Precautions and Considerations

Critical Contraindications

Sodium iodide I-131 is absolutely contraindicated in pregnancy because it crosses the placenta and causes severe, irreversible neonatal hypothyroidism. 1 Verify pregnancy status in all females of reproductive potential before administration. 1

• Multiple case reports document severe and irreversible hypothyroidism in neonates following in utero exposure to I-131. 1
• The fetal thyroid begins concentrating iodine after 10-12 weeks gestation, making exposure after this time particularly dangerous. 2
• Women should not breastfeed for four months after I-131 treatment. 2

Pre-Administration Patient Preparation

Patients must maintain a low-iodine diet starting two weeks before administration and continuing for several days during uptake/imaging. 1

Medication and Dietary Restrictions

Discontinue the following products before the procedure 1:

• Thionamide medications (propylthiouracil, methimazole, carbimazole): 3 days before
• Multivitamins containing iodide: 10 days before
• Triiodothyronine (T3): 2 weeks before
• Thyroxine (T4): 4 weeks before
• Iodine-containing foods (iodized salt, dairy, egg yolks, seafood, turkey, liver): 2 weeks before
• Kelp, agar, carrageenan, Lugol solution: 3 weeks before
• Saturated solution of potassium iodide: 3 weeks before
• Topical iodine preparations: variable duration based on formulation

Day-of-Procedure Instructions

• Fast at least 2 hours before and 2 hours after administration to ensure proper absorption. 1
• Hydrate adequately before and after administration and void frequently to ensure rapid excretion. 1

Radiation Safety for Healthcare Personnel

Handling and Administration Precautions

The most significant occupational exposure occurs during handling of radioactive materials, not during patient imaging. 2

Key safety measures 2:

• Wear gloves when handling radioactive materials; remove before leaving the area
• Use tongs to handle shielded vials of radioactive substances
• Keep vials and syringes in leaded storage holders at all times
• Transport shielded radioactive materials on a cart, never carry by hand
• Maintain minimum distance of 5 meters from injected patients when feasible
• Never pipette by mouth; use syringes or remote control devices
• Line all work surfaces with absorbent paper before using radioactive materials

Personnel Monitoring Requirements

• Wear film badge or thermoluminescent dosimeter (TLD) on chest at all times in the laboratory. 2
• Wear ring badge if directly handling radioisotopes. 2
• Conduct weekly wipe tests to assess contamination of laboratory surfaces. 2
• Mean annual occupational exposure in nuclear medicine is 100-140 mrem (1-1.4 mSv), well below maximum permissible doses. 2

Volatile Isotope Precautions

Sodium iodide used for thyroid scans is very volatile and poses airborne contamination risk. 2 Ensure proper ventilation and protective mechanisms are in place if the nuclear cardiology laboratory is located within a nuclear medicine department where I-131 is used. 2

Radioactive Waste Management

Storage and Disposal Requirements

Store radioactive waste for 10 half-lives (80 days for I-131), after which less than 0.1% of activity remains. 2

• The radioactive waste holding area must be appropriately shielded, secured, and surveyed weekly. 2
• Survey all waste material before disposal or shipping to verify decay to acceptable levels. 2
• Given I-131's 8-day half-life, use decay and delay in holding tanks as the standard method for abating radioactivity before discharge. 2

Environmental Considerations

Up to 90% of the administered radioactive dose can be excreted by treated patients, creating significant environmental contamination risk. 2

• Contamination from patient excreta can accumulate in sewer networks given the 8-day half-life. 2
• The International Commission on Radiological Protection states that storing patients' urine after therapy has minimal benefit, and radionuclides released into modern sewage systems result in doses to sewer workers and public well below dose limits. 2
• I-131 can be detected in the environment but has no measurable environmental impact when properly managed. 2

Patient Radiation Safety Post-Treatment

Caregiver Exposure Management

Radiation dose to caregivers of hospitalized patients receiving I-131 therapy ranges from 37 to 1,920 μSv depending on the level of support required, with maximum doses below the 5 mSv caregiver constraint. 3

Specific precautions for caregivers 4:

• Maintain appropriate distance from treated patients (specific hours/days depend on administered dose)
• Limit duration and frequency of close contact with patients
• Assess and examine patients before injection of radioactive materials to minimize post-injection contact time
• Thyroid uptake in caregivers is typically undetectable when proper precautions are followed. 3

Radiation Dosimetry and Cancer Risk

Sodium iodide I-131 contributes to long-term cumulative radiation exposure, which is associated with increased cancer risk. 1

Absorbed Dose Estimates

For diagnostic doses in adults (73.7 kg), absorbed doses vary significantly based on thyroid uptake 1:

• Thyroid (high uptake, 36% A0): 580 mGy/MBq
• Stomach wall: 0.66-0.87 mGy/MBq
• Kidneys: 0.27 mGy/MBq
• Urinary bladder wall: 0.34-0.54 mGy/MBq
• Effective dose: 0.28-29 mSv/MBq depending on thyroid uptake

The effective half-life of I-131 ranges from approximately 1.4 hours (blocked thyroid) to 80-90 hours (low to high uptake). 1

Hypersensitivity Risk Management

Hypersensitivity reactions including anaphylaxis may occur, particularly in patients with sulfite allergies. 1

• Although iodine itself is not an allergen, reactions may occur related to excipients or chemical components like sodium thiosulfate. 1
• Obtain and document complete allergy history, particularly sulfite allergies, before administration. 1
• Emergency resuscitation equipment and personnel must be immediately available during administration. 1

Common Adverse Reactions

Post-approval surveillance has identified the following adverse reactions 1:

• Gastrointestinal: vomiting, nausea, diarrhea
• Local effects: thyroid swelling
• Immune: hypersensitivity reactions
• Dermatologic: itching, rash, hives, erythema

Pregnant Healthcare Workers

The dose to the fetus from occupational exposure of a declared pregnant worker should not exceed 0.5 rem (5 mSv) over the entire pregnancy and 0.05 rem (0.5 mSv) during any single month. 2

• Women who adhere to recommended radiation safety precautions can safely work with I-131 during pregnancy with additional shielding and monitoring. 2
• Each woman has the prerogative to determine whether she will perform procedures during pregnancy. 2