Performance evaluation of the reference system for calibration of ipen activimeters

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Elaine Wirney Martins and Maria da Penha A. Potiens
Instituto de Pesquisas Energéticas e Nucleares (IPEN / CNEN - SP),
The formation of good quality image in nuclear medicine services depends on several factors, including the radiopharmaceutical activity,which must be well determined by a specific apparatus, in perfect operating condition, called activimeter. Therefore, the establishment of a quality control program for measuring the radiopharmaceuticals radioactivity before being administered to the patient is crucial to the safe and effective use of the radiopharmaceuticals used in diagnostic and therapeutical procedures. Due to the largenumber of procedures performed with the use of ionizing radiation in medicine, it is necessary to use of a substantial number of activity equipment meters in order to cover all NMS, and concerns about the development of quality control programs as well as radiological safety are inevitable. Several factors influence the accuracy of the dose given by activimeters. Before becoming available for sale, theactivimeter, after its manufacture, is submitted to calibration using a set of certified sources. The manufacturer limits its initial accuracy to a range between +/- 1% and +/- 5%. Over time this accuracy can vary due to pressure changes in the gas of its chamber and electronic attraction. To ensure the proper operation of the instrument, due to these variations, regular tests were established tobe applied at specific intervals. There are three types of tests which are part of a quality control program: (I) acceptance tests, (II) reference tests, (III) operational tests.

Precision: the maximum percentage deviation between the individual activity measure (Ai) with respect to the measured mean activities (Ā) was less than 0,32% for all the tests performed, not exceeding therecommended limit of ± 5%, thus showing satisfactory results with reliability of 95%. Accuracy: the acceptance limit is ± 10% with a reliability of 90% recommended by CNEN standard. All tests performed were satisfactory, not exceeding the recommended limit. In Table 2, it is possible to observe the results of accuracy test, that is the percentage deviation between mean of activity measured (10measures) and the standard source activity with the application of the decay correction factor. TABLE 2: Results obtained for the accuracy test with reference sources in both activimeters. NPL-CRC® Ā (MBq) A (MBq) Accuracy (%) Bário -133 8.03 8.01 0.27 Césio -137 6.93 6.92 0.14 Cobalto – 57 169.3 169.2 0.06 CRC®-15BT Ā (MBq) A (MBq) Accuracy (%) 7.93 7.90 0.29 7.40 7.38 0.26 170.0 169.9 0.06MATERIALS AND METHODS
In this study the following activimeters were tested: the reference system, the secondary standard, NPL-CRC® radionuclide calibrator, and the tertiary system, Capintec basic CRC®-15BT. They consist of two parts: the console in which the electrometer is situated, and the ionization chamber, both by Capintec, as presented in Figure 1. The reference system is a secondary standard,model NPL-CRC® radionuclide calibrator, serial number 111113, manufactured by Southern Scientific plc, with traceability to National Physical Laboratory (NPL). The system to be used as work standard is a model CRC®15BT, serial number 180020, calibrated by Accredited Dosimetry Calibration Laboratory of Medical Radiation Research Center – University of Wisconsin, with traceability to National Instituteof Standard and Technology (NIST).

For the stability test a sequence of measurements was performed. 10 first measurements were employed to determine the value to be used as reference, the maximum coefficient of variation obtained was 1% (acceptable variation is 5%). Figure 3 shows the reference and work system stability over time (standard deviation of the average), respectively. For the...
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