Introduction: Nowadays most of the clinics and hospitals use 131I in capsules, however, commercial radionuclide calibrators (or dose calibrators) commonly used in nuclear medicine centers don’t have calibrator factor to measure 131I activities in capsule geometry. In Brazil, the measurement of the activity that a patient will receive in a nuclear medicine procedure, according to the regulatory body, mustn’t overcome ±10% of accuracy. Radionuclide calibrators found in nuclear medicine centers as well as the LNMRI IG12 secondary standard ionization chamber (SSIC) don’t have calibration factor to measure 131I in capsule geometry. The SSIC has only calibration factor to measure 131I solution contained in glass ampoule.

Purpose: The main aim of this study is to calibrate the SSIC for 131I in capsule geometry. Thus secondary standards can be produced and be used to calibrate the radionuclide calibrators of nuclear medicine centers obtaining more accurate measurements of activity of 131I capsules, in order to optimize the radiation dose received by the patient achieving the intended medical objective.

Material and Methods: Two different procedures were carried out. The first one consisted in contaminates original inactive capsules. The other one consisted in dissolving contaminated capsules, identical to that provided to the nuclear medicine centers. In the first procedure mentioned a capsule containing inactive NaI salt is contaminated with a known activity of a solution of 131I and the ion current generated by this capsule in the IG12 ionization chamber is measured and related to added activity obtaining thus the intended calibration factor Fc in ampere/becquerel (A Bq-1). In the second procedure, the ion current of 131I capsule is measured in the ionization chamber and after quantitatively dissolved. Weighted aliquot of the dissolved solution is transferred to the standard glass ampoule and the original activity concentration (Bq g-1) of the capsule is determined in the SSIC using the calibration factor previously determined by the absolute 4pß-g coincidence counting method. Knowing the activity and the ion current generated in the ionization chamber the calibration factor Fc of 131I for geometry of capsule is then determined.

Results: The final Fc was taken as the weighted mean of all the results giving 6.46689 ± 0.03791 (0.59%) x 10-18 A Bq-1 which is only 0.24% higher than the glass ampoule standard geometry with 2 cm height of solution for SSIC.

Conclusions: The results of this work have demonstrated that the IG12 well-type ionization chamber is appropriated to measure the activity of 131I capsules without destroys then with accuracy suitable for nuclear medicine application. The small difference between calibration factors of capsule and solution geometries allows accurate activity measurements using commercial radionuclide calibrators.