Calibration of the LNMRI Secondary Standard Ionization Chamber for 131I capsules used in nuclear medicine

The radionuclide 131I has been increasingly used in nuclear medicine therapy procedures. Nowadays, the 131I source administered to the patient is manufactured in two different geometries: solution and capsules. The purpose of this study is the accurate measurement of the activity present in a 131I capsule without destroys it. The methodology to determine the capsules activity is to obtain the calibration factor of an IG12 secondary standard activity measurement system based on the IG12 well-type ionization chamber set up at Brazilian national metrology laboratory for ionizing radiation (LNMRI) of institute of radiation protection and dosimetry (IRD).The result obtained, 6.4670 ± 0.0381 × 10−18 A Bq−1, is quite similar to the calibration factor of the 131I solution contained in the standard ampoule geometry, 6.4515 ± 0.0368 × 10−18 A Bq−1. After obtaining the calibration factor it was used to measure 131I therapy capsules in order to check the performance of radionuclide calibrators of some Brazilian nuclear medicine centers.