Nuclear Medicine Physics Course Laboratory Number 3: Dose Calibrator Quality Control

Transcription

1 Nuclear Medicine Physics Course Laboratory Number 3: Dose Calibrator Quality Control Purpose 1. To understand quality control tests for accuracy, linearity, and daily constancy that are necessary in the dose preparation area of a Nuclear Medicine Department. 2. To develop experience in verifying performance of the dose calibrator. Equipment and Materials Dose Calibrator Capintec CRC-25R Cs-137 standard sources Linearity Test Kit mci 99m-Tc (preferably 30 mci or more) Procedure Note: Not all facilities perform these tests the same way. This lab describes one way to get the job done. You may deviate from this type of test and follow the test procedures of the facility of your rotation. To receive full credit, you must answer the questions below and go through all the tests of the lab under the supervision of the person who normally does the tests. Have them sign and date your lab to verify your participation. A. Perform an accuracy and daily constancy test on the dose calibrator 1. Before commencing QC tests, put on laboratory gloves, wear a lab coat and personal radiation monitor, ensure that there are no sources in the calibrator and that all sources are properly stored in shielded containers. Why do you do this? 2. Press 'TEST' button and choose number 1, daily test. 3. The system first performs an auto zero test and instructs you to press any key to continue. After pressing a key the system instructs you to wait. 4. Once the system ascertains the zero adjust for the voltage applied to the dose calibrator you must press enter to accept the value. The value will fluctuate around values less than 1 mv, a typical reading is about mv. Press enter when a very low value like this is reached. 5. Next, the calibrator will auto-adjust for background. What does this mean? 6. Press 'Enter' when background level reading is very low (less than 1 microci). If not reached, look around for sources of radiation and move them away from the calibrator or place them behind a shield. Enter your results below. Lab #3 Dose Calibrator (Last Update May 2014) Page 1 of 7

2 Background activity = microci. Note: the system remembers this value and will automatically subtract it from all measurements so that the reading provided to you has the background already subtracted. 7. The system will now check the voltage that is applied to the dose calibrator. The voltage should be around 150 Volts. The system will indicate if the voltage is OK and prompt you the press enter when it is finished. When 'OK' appears, record value and press 'Enter'. Voltage must be above 150. What does the voltage do and why must it be at least 150V? 8. Data check will display. Press any key to continue. 9. After the system electronics check out OK, the daily system accuracy and constancy test will be performed. Measure 'Cs-137' will display. Using a tongs, place the Cs-137 source in the well and measure its activity. The display will indicate the serial number of the source to be used (S ). Some years ago, the radiation safety officer entered data about this source into this system. The data included the initial activity at calibration, the calibration date, and the serial number. Why is this information important? NOTE: to improve the number of significant figures in the readout, press the SPC button while the calibrator is making a measurement. 10. Wait until the reading has stabilized and record the activity. The system displays not only the activity but also the deviation in % from the expected activity. Record the activity and percentage deviation. = ; Deviation = % 11. Press Enter to accept the results. This completes the daily accuracy test. Note the results are normally entered into a log record to demonstrate to regulators that your equipment is tested as required for proper patient care. What purpose does this test serve? Does the calibrator pass this test? 12. Leave Cs-137 source in the well and sequentially push the other isotope buttons. Record values of activity readouts for these other isotopes when the Cs-137 source is in the well. Remember, most readings will take a little while for the readout to stabilize. This completes the constancy check. What is a constancy check? Why Lab #3 Dose Calibrator (Last Update May 2014) Page 2 of 7

3 is it valid if we use a Cs-137 source with the buttons for other radionuclides? Deviations greater than 5% but less than 10% from the prior days values must be investigated by the RSO or Nuclear Medicine Manager who will decide on further actions, if necessary. Deviations of 10% or greater indicate a serious fault in the system and this must be repaired before the calibrator can be further used. Why is this necessary? 13. Remove Cs-137 source from the well and return it to the shielded container. Press HOME button on the dose calibrator to end the test. B. Perform a quarterly linearity test on the dose calibrator Note: This test is performed quarterly and assures that the dose calibrator has a "linear response. What does linear mean? The purpose of this test is to verify that across the entire spectrum activities used at your facility, the dose calibrator provides an accurate result on activity, regardless of magnitude. This linearity in response to activity can be measured by ordering a 100 mci source of Tc-99m. When it arrives a first reading is measured. Then, for every few hours, another reading is acquired. Knowing how Tc-99m decays, the measured activity can be compared to the expected decayed activity until the source reaches levels below which the calibrator is not used, typically on the order of a few tenths of a microcurie. This method takes a long time and it is inconvenient to make measurements during hours when the facility is closed. An alternative method is to use a kit composed of lead-lined cylinders to simulate different levels of activity from a single high source. The kit might have cylinders of different colors representing different thicknesses of lead-lining. Or the kit might be comprised of numbered and lettered cylinders of different diameters and different linings that are used in combination, each combination having different amounts of shielding to simulate various stages of 99m-Tc decay (e.g., 0, 6, 12, 24, 30, 40 and 50 hours). These kits are calibrated when first acquired by ordering a 100 mci Tc-99m source and measuring the linearity by the decay method as well as by the cylinder method. At the start of the decay calibration, measurements are also made with the tubes. The source is placed inside the first tube that has no lead in the walls. Then another measurement is made with the thinnest lined tube. Measurements are made with the activity in each tube or tube combination until all tubes have been measured. Meanwhile the decay of the source is measured periodically over the period of about 3 days. With the data all in, the decayed measurements are used to verify that the calibrator had a linear response at the time of the cylinder calibrations. If the calibrator passes that test, then the tube attenuation measurements can be used as a calibration for all future linearity tests. A 100 mci source is acquired and measured inside each of the tubes. The measured activity of each tube relative to no tube should be the same as Lab #3 Dose Calibrator (Last Update May 2014) Page 3 of 7

4 the calibrated ratios. This then confirms maintenance of a linear response and is completed in just a few minutes. IMPORTANT: the background activity must be carefully assessed and set for the calibrator before beginning this test! 1. Place the 99m-Tc source in the central unlined tube and insert the tube in the dose calibrator. Record the activity on the data sheet provided with this laboratory exercise. Now place the six other tubes over the central tube according to the combination specified in the datasheet and record the readout for each combination. Multiple the initial unattenuated activity by each tube(s) attenuation factor and compare to activity measured for each combination of tubes. Record results on the data sheet as you acquire them. 2. Verify that all seven individual readings are within the +/- 5% limits. (Deviations of less than 5% should be achievable with modern units and this represents acceptable performance. Deviations greater than 10% indicate a fault in the system and use of the calibrator must be terminated until repaired.) Does the calibrator pass the linearity test? Explain your answer. Lab #3 Dose Calibrator (Last Update May 2014) Page 4 of 7

5 LBJ Dose Calibrator Data Worksheet Accuracy and Daily Constancy Test: System voltage Accuracy/Constancy source: Type of vial: Cs-137; Cal. ; Cal. Date Placed in Well Button Selected Constancy Isotope Calc Times Isotope Cs-137 Cs Constancy Test for Other Selected or Isotope Settings: Constancy Calc Button Isotope Times Placed in Well Selected Isotope Cs-137 Ga Cs-137 Tc-99m 1.84 Cs-137 I (liquid) Cs-137 I-123 (tablet) 0.66 Cs-137 I Cs-137 Xe Cs-137 Tl What is the constancy isotope factor or coefficient? Are the results acceptable? Does the type of vial of the calibration source matter for these experiments: Why? Quarterly Linearity Test: Linearity source: ; Cal. ; Cal. Date/Time Data Order Combination of Tubes Measured Vial Inside #1 Tube Attenuation Calculated Measured Attenuated E F E x F G (G-(E x F))/G x a a a Lab #3 Dose Calibrator (Last Update May 2014) Page 5 of 7

6 Memorial Hermann Dose Calibrator Data Worksheet Accuracy and Daily Constancy Test: System voltage Accuracy/Constancy source: Type of vial: Cs-137; Cal. ; Cal. Date Placed in Well Button Selected Constancy Isotope Calc Times Isotope Cs-137 Cs Constancy Test for Other Selected or Isotope Settings: Constancy Calc Placed in Button Isotope Times Well Selected Isotope Cs-137 Ga Cs-137 Tc-99m 1.84 Cs-137 I Cs-137 I Cs-137 Xe Cs-137 Tl What is the constancy isotope factor or coefficient? Are the results acceptable? Does the type of vial of the calibration source matter for these experiments: Why? Quarterly Linearity Test: Linearity source: ; Cal. ; Cal. Date/Time Data Order Combination of Tubes Measured Vial Inside Black Tube Attenuation Calculated Measured Attenuated E F E x F G (G-(E x F))/G x Black Blk + Red Blk + Orange Blk + Yellow Blk + Green Blk + Blue Blk + Violet Lab #3 Dose Calibrator (Last Update May 2014) Page 6 of 7

7 Are the results acceptable? Resident s name (printed): Laboratory personnel to verify resident performance of lab: Print Name Signature Date of completion: After you complete the lab and answer all questions TURN IN a copy of THIS ENTIRE DOCUMENT for credit to the Physics and Education Offices for credit. Lab #3 Dose Calibrator (Last Update May 2014) Page 7 of 7