K. BIOASSAY PROGRAM
EXAMPLE PROCEDURE AND CALCULATION
The following bioassay schedules apply to volatile forms of licensed material or forms that have a potential for ingestion, inhalation or skin absorption and are based upon investigation levels (A) for each radionuclide, the probability of an incorporation of an investigation level, the ease of detection and the type of procedures in which these radionuclides are used. Additional bioassays will be performed when indicated by findings of surface or airborne contamination. Bioassays shall be performed if an individual is likely to receive, in excess of, 10% of the applicable ALI.
|Nuclide Use Level||Frequency||Method||ALI*|
H-3 10 mCi < A ≤ 100 mCi
100 mCi < A
Within 1 week
I-125 1 mCi ≤ A
I-131 1 mCi ≤ A
* Annual Limits on Intake from 10 CFR Part 20. Appendix B
** Non-stochastic dose limit to thyroid by ingestion
*** Non-stochastic dose limit to thyroid by inhalation
Routine pre-operational and post-operational bioassays will be performed on individuals using material from the above listed use levels. Additional diagnostic bioassays will be performed when action levels are exceeded.
If bioassay results indicate that an ALI is exceeded, work shall immediately be suspended until the cause of the exposures has been identified and corrected. As is appropriate for some radioactive material, therapeutic procedures shall be followed to block specific organs or accelerate excretion of the material.
EXAMPLE PROCEDURE AND CALCULATION
The following is an example procedure and calculation for performing a direct thyroid count bioassay.
A Ludlum model 2200 scaler with adjustable threshold and window settings and a sodium iodide thin-window probe may be used to determine thyroid burden. This system has an approximate I-125 efficiency of 1.5% in a standard thyroid phantom geometry. Calibration of the system used will be performed annually.
- Set Window
The window is set to detect emissions for the radionuclide of interest. For I-125 the window will include emitted photons between 27 kev to 35 keV. For I-131 the window will include the major 364 keV emitted photon.
- Establish Background
Hold probe on thigh (ensure thigh and/or lab coat are not contaminated) and count for at least 1 minute. The count results are recorded.
- Count Standard
A standard of known activity will be used to determine instrument efficiency. When assaying for I-131, a corrected for decay I-131 liquid or capsule standard (or a standard source of known activity that emits photons of approximately the same energy as I-131, e.g., Ba-133) will be used. For I-125 a liquid or a mock I-125 source (i.e. I-129) may be used.
The standard is placed in a thyroid phantom. The probe is held against the phantom in an established geometry, similar to the geometry to be used when performing a bioassay on an individual, for a specified length of time such as 1 minute. The count results are recorded.
- Determine System Efficiency
Standard CPM - Background CPM = Net Standard CPM
Net Standard CPM x 100 = % Efficiency
Stardard Activity(µCi) 2.2 x 106 DPM/µCi
- Investigation Limits
- The RSO shall be notified whenever the thyroid burden at the time of measurement exceeds 0.03 µCi of I-125 or 0.04 µCi of I-131. The RSO shall perform an investigation into the cause of the exposure and the potential for further exposure, and develop corrective actions to prevent recurrence.
- The RSO shall be notified immediately whenever the thyroid burden at the time of measurement exceeds 0.09 µCi of I-125 or 0.14 µCi of I-131. The RSO must perform an investigation, as described above, and must perform weekly bioassays on the individual until the individual's thyroid burden is less than 0.04 µCi of I-131.
- Measure Thyroid Gland
- Perform measurements in a low-background area.
- Hold probe on thigh (ensure thigh and/or lab coat are not contaminated) for a count. Record results.
- Hold probe in the center of neck near Adam's apple for required amount of time. Record results.
- Subtract background from thyroid count to obtain net counts. Record results.
- Calculate and record the amount of radioactivity in thyroid by using the equation:
Net counts (CPM) x 100 = _______ µCi% Efficiency x 2.2 x 106 DPM/µCi
- If results are less than the investigation limits established in E.I. above, record results.
- If results are more than the investigation limits established in E. above, notify the RSO immediately. The RSO may restrict the employee's further handling of I-131 or I-125 until the thyroid burden is measured to be below the reporting limits established in E above.