RADAR Patient Exposure Radiation Dose Calculator

This form gives dose calculations to physicians, nurses, family members or others from patients who have been given radiopharmaceuticals. Through relatively simple calculations, reasonable estimates of radiation dose can be calculated, to evaluate general safety issues, and in particular to evaluate the reasonableness of releasing patients to comply with a dose limit of 5 mSv (500 mrem).

Technical Basis

The technical basis for the calculations on this page, and worked examples (the same as appear at the bottom of this web page) are contained in this document. The document explains the basis for the calculations, gives guidance in choosing input parameters, and has other helpful information. A good discussion of this topic, with a handy little printable card that can be given to released patients can be found at this page. The page below contains several calculational tools and shows some sample calculations.

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Section I. Patient Release Calculations

As noted in our Health Physics Journal article ("Licensee Over-Reliance on Conservatisms in NRC Guidance Regarding the Release of Patients Treated with 131I", Health Phys. 93(6):667 677; 2007) and our web page on the subject, treatment of patients as unshielded point sources, with use of radioactive decay as the only means of removal of activity from the body, is an overly conservative method. This is intuitively obvious, and has been confirmed by measured data in a number of cases. In this section, we provide a more realistic approach to the problem, based on methods in the HPJ paper cited above. Here the terms are:
Occupancy factor = fraction of time exposed individual is assumed to be within the average distance chosen,
F1 = fraction of total administered activity associated with effective half-time Teff-1,
F2 = fraction of total administered activity associated with effective half-time Teff-2.

1. 131I-Sodium Iodide (NaI) - Hyperthyroid Patients

Occupancy Factor
F1 (extra-thyroidal)
F2 (thyroid)
Teff-1 (extra-thyroidal) days
Teff-2 (thyroid) days

Activity (mCi)
Average distance (m):

Total dose estimated for this exposure:

2. 131I-Sodium Iodide (NaI) - Thyroid Cancer Patients

Occupancy Factor
F1 (extra-thyroidal)
F2 (thyroid)
Teff-1 (extra-thyroidal) days
Teff-2 (thyroid) days

Activity (mCi)
Average distance (m):

Total dose estimated for this exposure:

3. Calculation of Restriction Time Needed - If we have a single exponential term, we can directly solve for the amount of time that a subject can spend at a given distance from a released patient to satisfy some dose criterion. With two exponentials, however, a simple web form like this cannot directly solve this. However, if you enter the usual input data and assumptions about contact with the patient, this form will calculate what dose was received after that time. For example, as in Siegel et al. (J Nucl Med 2002; 43:354-363), we might assume exposure at 1 m distance with an occupancy of 0.25 for 3 days, then normal contact at 0.1 m at 0.25 occupancy after three days. In the form below, to calculate the dose for this case, enter 1 m for distance 1, 0.25 for occupancy 1, and 0.1 m for distance 2 and 0.25 also for occupancy 2.

Teff-1 days
Teff-2 days

Activity (mCi)
Time period for restrictions (days)

Occupancy Factor 1 Average distance 1 (m):
Occupancy Factor 2 Average distance 2 (m):

Total dose estimated for this exposure:

4. Other Patient Release Calculations - if we have a patient with activity from any radionuclide study, we can obtain the dose by treating the subject as a 2 m line source of radiation. The best calculation will be made by using an effective half-time for estimating the cumulative exposure. A more conservative calculation will result from using just the radionuclide physical half-life. You can enter either below.

Activity (mCi)
Average distance (m)
Fraction 1     Half-time 1 (days):
Fraction 2     Half-time 2 (days):

Total dose estimated for this exposure:

Section II. Other Situations

Here we offer a tool that allows you to calculate doses for exposure to radioactive patients or other sources for fixed periods of time.
See the examples below for ways that this might be used. This section uses an unshielded, point source approximation. NOTE - the time entered here is in hours, not days.

Choose radionuclide and enter study data. Hit 'OK' when the data are ready:

Activity (mCi)
Average distance (m):
Time (hours):

Total dose estimated for this exposure:

  1. A patient has been given 5 mCi Ga-67 citrate for imaging infection. Three days later a surgeon, wishing to operate, hesitates to do so fearing radiation exposure. The surgical procedure would take about 3 hrs. Estimate the radiation dose to the surgeon, and compare it with common radiation exposure sources. The T1/2 of Ga-67 is 78 hrs. Very little is excreted over the 3-day period. Let's assume that 2.5 mCi is still in the patient. Using the Section III calculator we get:

    The assumption that the average distance between the patient and the surgeon is 2/3 meter is conservative; 1 meter might also be considered.

  2. An ultrasound technologist balks at performing a gallbladder procedure in a patient who has just had a Tc-99m-disofenin ("HIDA") scan. The patient received 2 mCi Tc-99m 3 hrs. ago. What is the approximate radiation dose to the technologist, assuming that the ultrasound examination takes 30 minutes? Assume the technologist is ~ 2/3 meter from patient. Decay of Tc-99m:

    Using the Section III calculator we get the exposure to technologist:

    The form above does not give numerical estimates of dose below 1 mrem (0.01 mSv).

  3. A patient swallows 400 Ci 123NaI for a thyroid study and soon vomits a small amount on the floor. A calibrated ion chamber measures 0.2 mR/hr 50 cm above the vomitus. About how much activity did the patient vomit?

  4. A technologist inadvertently spills a mCi of Tc-99m on the floor while injecting a patient with 25 mCi of Tc-99m-MIBI. The technologist does not clean up the spill. What is the most radiation that could reasonably be absorbed by the most exposed person frequenting that area?. Assume the most exposed person spends 4 hrs. 1 meter from the contaminated spot (probably the technologist). Using the Section III calculator we get:

  5. A patient requires 50 mCi 131I for a Graves' disease therapy. Her thyroid uptake is 55%, her thyroid biological half-life is 5 days, and her renal function is normal. If you treated her as an outpatient, what is the highest expected radiation dose to someone with whom she shares a household? Assume she sleeps alone and does not share eating utensils. Assume that a person spends one fourth of all time at 1 meter from the subject (very conservative). Activity not taken up by the thyroid is assumed to have a 0.25 d biological half-life, which works out to be a 0.24 d effective half-time.

    Note: We used the estimate of 1.3 R-cm2/mCi-h for the non-thyroidal component, as described on page 4.

    Note: Since we integrated to infinity, the (1-e) terms become 1.0, and could have been left out.

    Note: The biological half-life of the non-thyroidal component in a person with normal renal function is about 8 hrs or 0.33 days, and the effective half-life is 0.32 days. In a hyperthyroid person with normal renal function it is about 6 hours or 0.25 days, and the effective half-life is 0.24 days. In a hypothyroid patient with normal renal function it is about 12 hours or 0.5 days, and the effective half-life is 0.47 days.

  6. A thyroid cancer patient requires 200 mCi 131I for therapy. Assume that uptake in the postoperative thyroid remnants is 1% and that the patient has normal renal function. If you treated him as an outpatient, what is the highest expected radiation dose to someone with whom he shares a household? The biological half-life of iodine in normal thyroid tissue is 80 days. If the patient is treated with Thyrogen (recombinant human TSH) assume a normal biological halflife of non-thyroidal iodine of 0.33 days. If the patient went naturally hypothyroid, assume a biological half-life of non-thyroidal iodine of 0.5 days. Here we assume rhTSH treatment.

    Note: we did not show the (1-e) terms this time, since they are equal to 1.0.

  7. A radiation inspector comes through your department and detects 10,000 dpm from a wastebasket in a public area. A patient injected for a 99mTc-MDP bone scan has been sitting near by drinking water as directed, and the activity appears to be coming from saliva on a foam cup he has discarded. Calculate the radiation dose to the most exposed person from this wastebasket, assuming the cup is left there. How would you respond to the radiation inspector? Assume that the person sits 1 m from the source for 4 hours (again very conservative). There is no regulation preventing Tc-99m patient-contaminated articles from being thrown in general trash.

  8. A patient receives 100 mCi Sm-153-EDTMP and is driven home from the nuclear medicine department in a car or taxi. The trip takes 2 hours, and there are no potty breaks. The patient sits in the back on the right. Calculate the dose to the driver. Using the Section III calculator we get:

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For any questions or comments about this form contact:

Michael Stabin, PhD, CHP
Vanderbilt University, Nashville, TN
(615) 343-4628