1 HRS SNM/VOICE CEU
 

 


DOSE CALIBRATOR OPERATION AND QUALITY CONTROL TESTING

One is required to evaluate calibrator performance on-site at specified intervals. The current NRC Regulations (January 2010) printed below were taken from the Code of Federal Regulations, Title 10, Part 35.60:

  • § 35.60 Possession, use, and calibration of instruments used to measure the activity of unsealed byproduct material. (a) For direct measurements performed in accordance with § 35.63, a licensee shall possess and use instrumentation to measure the activity of unsealed byproduct material before it is administered to each patient or human research subject. (b) A licensee shall calibrate the instrumentation required in paragraph (a) of this section in accordance with nationally recognized standards or the manufacturer’s instructions. (c) A licensee shall retain a record of each instrument calibration required by this section in accordance with § 35.2060.
  • The nationally recognized standards mentioned in (b) above are published by the NIST (National Institute for Standard and Technology) and are downloadable (for a fee) from their website. The condition of reliability exists when both accuracy and precision are maintained. To insure that the dose calibrator is reliable, there are certain mandatory quality control tests that must be performed on a routine basis.

REQUIRED DOSE CALIBRATOR TESTS AND FREQUENCY OF PERFORMANCE

The required dose calibrator tests and the frequency of their performance are listed in the following chart.

TEST REQUIRED FREQUENCY
Accuracy
at installation, then annually thereafter
Constancy
at installation, then daily thereafter
Linearity
at installation, then quarterly thereafter
Geometry
at Installation; after repair or moving instrument

It is essential to perform these test procedures correctly since patient safety is highly dependent upon the reliability of this instrument. All too often, procedural errors are made in performing QC testing of dose calibrators, even by experienced operators. It is critical to perform steps in the proper order to insure correct readings. The following paragraphs describe the recommended procedure for performance of mandatory dose calibrator quality control testing and are in compliance with guidelines set forth in the most recent NIST publication regulating dose calibrator quality control test procedures.

ACCURACY TEST
This test is designed to show that the calibrator is giving correct readings throughout the entire energy scale one is likely to encounter. Low, medium, and high energy standards (usually Co57, Ba133 or Cs137, and Co60, respectively), are measured in the dose calibrator using appropriate settings. The value on the label indicating the activity at a specific calibration time and date is mathematically decay-corrected to the testing date. The standard is then assayed in the dose calibrator and standard and measured values are compared. All values are recorded in the appropriate logbook. Measured values should be within ± 10% of the standard value. The following table displays the data collected during an annual Accuracy Test. The dose calibrator has passed the test.

Standard Energy (keV) expected value (mCi) measured value (mCi)
Co57
122
2.48
2.48
Cs137
662
3.38
3.29
Co60
1,332
1.55
1.52

CONSTANCY TEST
This test, performed at installation and daily, measures instrument precision and is designed to show that reproducible readings are obtained day after day on all the various isotope settings likely to be used. A long-lived source, usually 30 yr Cs137, is placed in the dose calibrator. Activity is then measured on the Cs137 setting (this actually represents a "mini" accuracy test) and on all other settings used on a daily basis. Values are recorded in the appropriate logbook and are compared with recent values to determine if the instrument is performing consistently on a day-to-day basis. Measured values should be within ± 10% of the standard value. It should be noted that, since we are reading the activity of a Cs137 source on settings for Tc99m, Tl201, I123, Xe133, and other isotopes, incorrect readings will be obtained. Our expectation is to obtain the same incorrect reading day after day. The following table displays the data collected during a daily constancy test over a one-week time period. The dose calibrator has passed the test.

ISOTOPE READING (mCi)
SETTING MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY
Cs137 123 124 122 126 124
Ga67 223 224 222 224 226
Tl201 163 164 162 166 164
Tc99m 243 242 244 246 244
I131 313 314 312 316 314
I123 193 192 194 196 194
In111 283 284 282 286 284
Xe133 433 434 432 436 434

LINEARITY TEST
This test is designed to prove that the dose calibrator readout is linear for sources varying from the μCi range through the mCi range. A high activity Tc99m source (50-300 mCi) is measured at T0 and at predetermined time intervals until activity reaches approximately 30 μCi. This can tak as long as 96 hours, depending upon initial activity. Using decay factors for Tc99m, one can take the activity at T0 and decay-correct it to predict what the activity should be at the predetermined times. Expected and actual measurements are recorded in the appropriate logbook (and may be analyzed graphically), and then are compared to determine if the instrument is linear throughout the usable activity range of the dose calibrator. The following table displays the data collected during a quarterly Linearity Test. The dose calibrator has passed the test.

Elapsed time
Expected reading
Measured reading
(hr)
(mCi)
(mCi)
0
300
300
1
267
272
2
238
241
3
212
209
6
150
148
12
75
72.4
24
18.75
19.1
48
1.17
1.19
72
0.073
0.074
78
0.036
0.037

 

GEOMETRY TEST
This test is designed to show that correct readings can be obtained regardless of the sample size or geometry. It therefore is necessary to perform this test on every different vial used (e.g., 10 ml, 30 ml)as well as every different syringe used (e.g., 1 ml, 3 ml, 5 ml, 10 ml). For example, to test a 10 ml syringe for linearity, one first places 1.0 ml of Tc99m in a 10 ml syringe (activity 25 mCi). The activity is then measured in the dose calibrator and the value obtained is recorded. The activity is then diluted with water to 2.0 ml, 3.0 ml, 4.0 ml, 5.0 m, etc, up to 10 ml. At each of these points a reading is taken and the value recorded. Data are then evaluated to determine the effect of sample geometry on the dose calibrator reading. If the instrument is geometry-dependent, ideally one should notify the manufacturer that the calibrator has failed acceptance testing and a new calibrator should be requested. If the decision is made to keep the instrument, it may be necessary to routinely correct readings obtained when using calibrator.

geometry

The following table displays the data collected during a Geometry Test. The dose calibrator has passed the test.

Sample Volume Activity (mCi)
0.5
25.5
1.0
25.3
2.0
25
3.0
24.8
4.0
24.7
5.0
24.5
6.0
24.4

 

SPECIFICATIONS FOR ALL DOSE CALIBRATOR QC TESTS

NRC and State Regulations require that, for all dose calibrator tests, deviation between standard and expected values must be within ±10%. If the deviation is greater than 10%, then one's obligation is to record the value, note that it is a test failure, repair or recalibrate the instrument, retest to insure that the equipment is now functioning properly, and record new values. In addition to the above steps, every dose must be mathematically corrected until the instrument is repaired. There is no longer a reporting requirement.

QC TEST FOR Mo99 BREAKTHROUGH USING A DOSE CALIBRATOR

Mo99 is assayed FIRST directly in the special lead pig supplied by the manufacturer of the dose calibrator using the Mo99 setting. Tc99m is THEN assayed directly in the plastic sleeve using the Tc99m setting. Activity (μCi) of Mo99 is divided by activity (mCi) of Tc-99m to obtain a ratio. If this ratio is <0.15 μCi Mo99 per mCi of Tc99m at time of administration, the generator eluate has passed the Mo99 Breakthrough Test.

 

moly

 

As a rule of thumb, if the ratio is <0.038 at time of elution, the material will be suitable for injection for at least 12 hours. If test is performed in reverse order, failure is extremely likely due to residual charge on ionization chamber that takes a few minutes to dissipate.

 

 


 

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