OBJECTIVE: To determine correction factors for the variation in volume of radiopharmaceuticals in containers of different geometries, comparing the influence of such factors on the determination of ^99mTc and ¹²³I activity with two types of calibrators – one with ionization chamber and another with Geiger-Müller (G-M) detector –; and to evaluate calibrators performance in the measurement of ^99mTc and ¹²³I activities. MATERIALS AND METHODS: Eight calibrators, 10R glass vials, 3 and 5 mL plastic syringes and ^99mTc and ¹²³I solutions were utilized. The correction factors were determined with basis on practical measurements of the variation in the calibrators' response according to the volume of radionuclide solution in the glass vials. The performance was evaluated according to the acceptance criterion of ±10% accuracy required by the Brazilian standard. RESULTS: The variation of the calibrators' response according to the variation in radionuclide volume was reasonably greater in the calibrator with G-M detector. It was also greater for ¹²³I than for ^99mTc. CONCLUSION: The results confirm that the calibrators' response depends on the radionuclide volume contained in the vials. Such dependence is more critical for the calibrators equipped with G-M detector and for ¹²³I as compared with ^99mTc.

Keywords: Radionuclides calibrator; Nuclear medicine; Activity; Tc-99m; I-123.