Dose Calibrator Linearity Testing

Linearity testing assesses the ability of the dose calibrator to indicate the correct activity over the range of use of the calibrator.  This test is usually performed using a vial or syringe of Tc-99m whose activity is at least as large as the maximum activity normally measured in a prepared radiopharmaceutical kit, in a unit dosage syringe given to a patient, or in a radiopharmaceutical therapy, whichever is largest. For users of Mo-99/Tc-99m generators this activity may be the total eluate from a fresh generator. Other isotopes can be used to perform linearity such as F-18.

Linearity testing assesses the ability of the dose calibrator to indicate the correct activity over the range of use of the calibrator.  This test is usually performed using a vial or syringe of Tc-99m whose activity is at least as large as the maximum activity normally measured in a prepared radiopharmaceutical kit, in a unit dosage syringe given to a patient, or in a radiopharmaceutical therapy, whichever is largest. For users of Mo-99/Tc-99m generators this activity may be the total eluate from a fresh generator. Other isotopes can be used to perform linearity such as F-18.

The NRC requires that linearity be tested upon installation and at least quarterly thereafter as well as after repair. There are two methods for testing the linearity of a dose calibrator: the decay method and the attenuation tube or shielding method.

Decay method

Simply put, this method involves a single radioactive source which is periodically measured over several days

1. Assay the Tc-99m activity to be used in a syringe or vial in the dose calibrator, and subtract background to obtain the net activity in millicuries. Record the date, time to the nearest minute, and net activity on a Dose Calibrator Linearity Test Form. This first assay should be done in the morning at a regular time, for example, 8 a.m.
2. Repeat the assay at about noon, and again at about 4 p.m. Continue on subsequent days until the assayed activity is less than 30 microcuries. Intervals such as 6, 12, 24, 30, 48 hours can be chosen to provide a wide range of activities. Note: The exact timing of the decay intervals is not important, so long as the correct decay factor is used for that interval. This test may take a total of 3 to 4 days depending on the amount of the original activity.
3. Convert the time and date information you recorded to hours elapsed since the first assay.
4. The measured activities are then plotted versus time on a semi-logarithmic graph. Label the vertical axis in millicuries to represent the measured activity and label the horizontal axis in hours elapsed. At the top of the graph, note the date and the manufacturer, model number and serial number of the dose calibrator. Calculate the percent error for each activity level/time interval.
   Use the formula:
   
5. Draw a best-fit straight line through the data points. The best-fit straight line drawn through the data points is plotted either by eye or by using a least-squares curve fitting algorithm. For each data point, the difference between the measured activity and the activity on the best-fit straight line at that point should be less than +10%.
6. Place a sticker on the dose calibrator that alerts when the next linearity test is due.

Attenuation Tube (Shielding) Method

An alternative approach to testing linearity in a dose calibrator is the attenuation tube or shielding method which utilizes a set of lead “sleeves” of increasing thickness placed in the dose calibrator with a single Tc-99m source. Note that F-18 cannot be used with this method. The single radioactive source is measured multiple times in a short period of time using various leaded tubes interposing in increasing decay equivalent thicknesses simulating decay from mCi to uCi. The NRC and other licensing agencies may require a simple radioactive materials license amendment before use of the attenuation tube method.

Although this method is much faster than the decay method for checking linearity (taking minutes instead of days), an initial decay based calibration of the set of sleeves is recommended to accurately determine the actual decay equivalent activity (calibration) for each shield. On first use of the shields, the decay method must be performed at the same time to prove accurate correlation between the two methods. Thereafter, the shielding method alone is acceptable, unless damage to the shields is suspected.

The available attenuation tube systems are the Calicheck or Lineator System and it is important to follow the specific manufacturer’s directions when performing the test. The below steps are general instructions that may apply to linearity testing using the attenuation tube (shielding) method:

1. Begin the linearity test as described in the decay method above. After making the first measurement, the attenuation tube “sleeves” can be calibrated.
2. Remove all sources from the region of the calibrator to be tested.
3. Remove the dipper from the calibrator. Remove the chamber well liner, only if necessary, to allow insertion of the central or base tube.
4. Set the calibrator to measure Tc-99m, zero out the background reading or note the value for calculations. Note that background readings which vary widely may indicate a defective unit or a changing radiation environment which will affect the calibration.
5. Place the base sleeve in the calibrator chamber with the open end up. Use caution to avoid damaging the calibrator or the soft lead of the sleeve. The radioactive source and central tube will stay in place until the calibration procedure is complete.
6. Be prepared to work quickly by arranging the sleeves, data sheets and clock for ease of operation. A complete calibration requires less than five minutes. Completion in six minutes introduces only a one percent total error due to decay. If the linearity test duration exceeds six minutes, the procedure should be repeated.
7. If the dose calibrator has a range switch, set the range switch as necessary to read the activity to three significant figures.
8. Follow the manufacturer’s instructions as to the order in which the sleeves should be inserted. The calculations needed for this test may also vary according to manufacturer.
9. Ultimately once you have established initial calibration factors with each sleeve you may now perform the test again and this system may now be used to test dose calibrators for linearity.
10. Complete the decay method linearity test as described in steps 2 through 5 above under the decay method section.

Results and Corrective Action

Regulatory Guide 10.8 suggests a trigger level of +5 percent unless the licensee commits to this level in the license application, the requirement allows for a +10 percent variation.

1. If the largest percentage error is greater than 5 percent, the dose calibrator should be repaired or adjusted.
2. If the error is greater than 10 percent, the dose calibrator must be repaired or adjusted. Linearity testing shall be repeated following repair or adjustment, and if the percentage error is still greater than 10 percent, a correction table or graph shall be constructed that will indicate the correction factor to be applied to the measured activity to be converted to true activity.

Sources of error for the Linearity Test

Dose calibrators usually pass this test easily. Most failures are due to operator error rather than device failure.

1. Loss of bias battery voltage on the chamber.
2. Pressure leak from the chamber.
3. Temperature changes in the chamber due to environmental conditions (change in heat or air-conditioning systems).

If the voltage test indicates normal chamber voltage, and the daily constancy test passed, linearity failure is probably due to operator errors during the performance of the test.

Common operator errors during the performance of the linearity test using the decay method:

1. Recording time incorrectly- use the same timepiece for all points.
2. Failure to zero and background correct before each measurement.
3. Failure to wait for reading to stabilize before recording result.
4. Record reading on the lowest possible scale and slow response.
5. Failure to anticipate that the last measurement (30 uCi) should occur during normal work hours.
6. Failure to reproduce geometry during each measurement.

Common operator errors during the performance of the linearity test using the attenuation tube (shielding) method:

1. CaliCheck or Lineator devices must be calibrated before first use.
2. Do not recalibrate after initial calibration.
3. Failure to zero and background correct before measurement.
4. Failure to wait for the reading to stabilize before recording result.
5. Bent shielding tubes fail to seal properly around the source.
6. Too much time between first and last measurements (5-6 minute maximum depending on manufacturer recommendations).

References:

Regulatory Guide 10.8 Guide for the Preparation of Applications for Medical Use Programs; Revision 2 August 1987; APPENDIX C: Model Procedure for Calibrating Dose Calibrator.

US Nuclear Regulatory Commission Office of Nuclear Material Safety and Safeguards, Washington, D.C. 20555 Information Notice No. 93-10: DOSE CALIBRATOR QUALITY CONTROL; February 2, 1993. http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices Accessed 5/19/2014.

Wells, Patricia, C. Practical mathematics in nuclear medicine technology; Society of Nuclear Medicine 1999; pp.124-125.