The geometry of the container (syringe or vial) that is used to measure a radioactive source can be a cause of assay errors in dose calibrators. Specifically, if there is a difference in the container used to obtain initial calibration settings and the container used to assay dosages in clinical practice. Ideally, the geometry of the standard source should be identical to the geometry of the source being measured. If the source geometry is not identical, the error in the measurement should be quantified and, if significant, either a new calibration setting determined or a correction factor applied.
When measuring patient doses in nuclear medicine, there is no standard syringe, type of syringe, volume of solution, or even needle length used. This may have a significant effect on the end measurements. Vial differences can also introduce assay errors. A measurement in a multi-dose vial that is different from the vial source used to calibrate the system may require a correction factor or new settings.
Position of the source in the chamber may introduce additional uncertainty if placed differently from the reference standard. Source holders in the dose calibrator generally ensure that the vertical position of the source is maintained. However, vertical and horizontal changes in source position within the holder (angle and height) may affect chamber response.
Geometry independence means that the indicated activity does not change with volume or configuration of the source. This test must be done according to protocols accepted by the appropriate state regulatory agencies or the Nuclear Regulatory Commission (NRC). Geometry independence testing is commonly performed at installation; after repair or moving the instrument.
Below is an example of how to perform a Geometry Test:
This test should be performed using a syringe that is normally used for injections. The following test assumes injections are done with 3 ml plastic syringes and that radiopharmaceutical kits are made in 30 ml glass vials. If you do not use these, change the procedure so that your syringes and vials are tested throughout the range of volumes commonly used. If a significant volume correction results from these procedures, the tests should be repeated to verify.
NOTE: There are two methods that can be used to calculate the results.
Syringe Test Example
1.In a small vial, mix 2.0 ml of a solution of Tc-99m with an activity concentration between 1 and 10 mCi/ml.
2.Set out a second small vial containing non-radioactive saline solution.
3. Draw 0.5 ml of the Tc-99m solution into the syringe and assay it.
4. Record the volume and activity of the first assayed sample (Figure 2).
5. Remove the syringe from the calibrator, draw an additional 0.5 ml of non-radioactive saline into the same syringe (total volume 1.0 ml), and assay again. Record the volume and measured activity on the form.
6. Repeat step 5 twice more until you have assayed 1.5 ml and 2.0 ml volumes and recorded them.
7. Assay the vial used to draw saline into the syringe. If the measured activity is greater than 1% of the 0.5 ml syringe assay, Tc-99m was lost during filling. Repeat the procedure.
8. Calculate results:
- Method one – select a normalized volume and decay correct the activity readings for the other volumes to the same time. Divide the calculated activity for each volume by the actual reading to get the correction factor for each volume.
- Method two – take the average of all the volume activity readings. Divide this average activity by the activity of each volume to get the correction factor for each volume. If the test takes longer than 10 minutes, decay correct the readings.
9. If any correction factors are greater than 1.05 or less than 0.95, it will be necessary to make a correction table that will allow you to convert from “indicated activity” to “true activity.” If this is necessary, be sure to label the table “syringe geometry dependence”, and note the date of the test as well as the model number and serial number of the dose calibrator.
Vial Test (10 ml) Example
1. To test the geometry dependence for a 10 ml glass vial, draw 1.0 ml of Tc-99m solution (between 1 and 10 mCi/ml) into a syringe and inject it into the vial. Assay the vial. Record the volume and activity indicated.
2. Remove the vial from the calibrator and, using a clean syringe, inject 2.0 or 3.0 ml of non-radioactive saline, and assay again. Record the volume and activity indicated on the form (Figure 2). Repeat the process until you have assayed a 8.0 ml volume. The entire process must be completed within 10 minutes, or, if not, decay-correct the activity.
3. Calculate results:
- Method one – select a normalized volume and decay correct the activity readings for the other volumes to the same time. Divide the calculated activity for each volume by the actual reading to get the correction factor for each volume.
- Method two – take the average of all the volume activity readings. Divide this average activity by the activity of each volume to get the correction factor for each volume. If the test takes longer than 10 minutes, decay correct the readings.
4. If any correction factors are greater than 1.05 or less than 0.95, it will be necessary to make a correction table that will allow you to convert from “indicated activity” to “true activity.” If this is necessary, be sure to label the table “vial geometry dependence”, and note the date of the test and the model number and serial number of the calibrator.
NOTE: Perform the vial test with the vial size you commonly use.
NOTE: Other isotopes can be used for performing geometry testing.
References:
AAPM REPORT NO. 181; The Selection, Use, Calibration, and Quality Assurance of Radionuclide Calibrators Used in Nuclear Medicine; Report of AAPM Task Group 181, June 2012.
APPENDIX J. Geometry Test; Atomlab Dose Calibrator Operation and Service Manual; page J-5 and J-6.
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