TABLE 2

Determination of Activity Remaining in a Patient’s Body Based on Exposure Rate Measurements

Sample Calculation:

An ¹³¹I therapy patient is administered a therapy dose of 7400 MBq (200 mCi). An initial exposure rate reading is made a distance from the patient soon after dosing and is determined to be 50 μGy/h (5 mR/h). The dosing vial is assayed after the dose is administered and is found to contain 29.6 MBq (800 μCi) of residual activity. Approximately 24 h later, a second exposure rate reading of 22 μGy/h (2.2 mR/h) is obtained at the same distance from the patient as on the previous day. Based on the dose rate measurements provided, what activity remains in the patient’s body 24 h after dose administration?

Solution:

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Since exposure rate is directly proportional to the activity, the following proportion can be established:

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