I-131 Therapy Patient Discharged to a Small Apartment
A 58-year-old woman with Graves' disease receives 1.1 GBq (30 mCi) of I-131 as an outpatient. Her measured dose rate at 1 m is 22 µSv/h immediately after administration. She lives in a 55 m² apartment with her husband and a 7-year-old grandson who visits every weekend. She works as a cashier and wants to return to work in 3 days. The clinic uses a 1 mSv public dose constraint and a 0.5 mSv constraint for the child. The licensee's written directive and release criteria are based on NRC guidance.
1. Using the dose-rate measurement, estimate whether she can be released and what instructions are needed for the child.
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The release decision is based on the measured dose rate and the licensee's procedure. A common criterion is that the dose rate at 1 m be low enough that a member of the public would not exceed the applicable constraint with reasonable behavior. At 22 µSv/h at 1 m, the time to reach 0.5 mSv at 1 m is 0.5 mSv ÷ 22 µSv/h ≈ 23 hours of continuous occupancy at 1 m - but the child is not at 1 m continuously, and dose falls with distance and decay (8-day half-life, effective half-life shorter). The RSO should compute a conservative household estimate: if the child spent 8 h/weekend at 1 m, weekly dose ≈ 22 × 8 = 176 µSv initially, decreasing with decay; over several weeks this could exceed 0.5 mSv. The practical answer is that release is permitted if instructions are given: maintain distance (sleep in a separate bed for several days), limit close contact with the child (no lap sitting, keep >1 m when possible), use separate towels/utensils, flush twice, and delay return to work if it involves close public contact. The RSO should document the calculation and the instructions.
2. What should the RSO advise about returning to work as a cashier in 3 days?
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A cashier has prolonged close public contact at <1 m. The RSO should estimate the dose to a nearby coworker or customer: at 22 µSv/h at 1 m and decay over 3 days (3 × 8 h = 24 h, about 3 half-lives of 8 days is only 0.375 half-life, so dose rate falls to about 22 × 2^(-72/192) ≈ 17 µSv/h), close contact at 0.5 m would roughly quadruple the rate to ~68 µSv/h. A coworker spending 8 h at 0.5 m would receive ~0.5 mSv in a day, which is at the constraint. The RSO should advise delaying return to work or restricting close contact, and should provide a letter and instructions. The decision should be documented and the patient given written precautions consistent with the licensee's procedure.