Kinetics of Br-76-labeled anti-CEA antibodies in pigs; Aspects of dosimetry and PET imaging properties

A monoclonal antibody labeled with the positron-emitting radionuclide Br-76 (T(1/2)16.2h) has previously been shown useful for positron emission tomog raphy (PET) imaging of experimental tumors. Our aim in the present study wa s to investigate the effects of the complex decay scheme of this radionucli de on normal organ dosimetry and PET image quality. Three mini-pigs were in jected intravenously with 46-75 MBq of the Br-76-labeled anti-CEA antibody 38S1, and the whole-body kinetics followed by PET imaging for 19 h. From PE T data, absorbed doses in human organs were estimated using the MIRDOSE 3.0 software. The highest 76Br concentrations were found in lungs, after a cor rection for the air volume in this organ. The lungs received the highest ab sorbed dose (mGy/MBq, mean+/-maximum error), 0.84+/-0.16, followed by liver , 0.74+/- 0.28, and small intestine, 0.55+/- 0.05, while the effective dose equivalent was 0.41+/- 0.03 mSv/MBq. The PET imaging properties of Br-76 i n, a two-dimensional 2D PET camera, including central area resolution and s cattering effects, were investigated in phantoms and compared to those of F -18. I, a 0.97 g/cm(3) material, approximating soft tissue density, the FMH W ("full width at half-maximum") value of the point spread function was 7.7 +/- 0.2 mm for Br-76,and 6.0+/- 0.1 mm for F-18. In conclusion, radioimmuno PET using Br-76-labeled antibodies resulted in a fairly even distribution of the radiation dose, where the highest absorbed organ doses were only abo ut two to three times higher than the mean absorbed body dose. The high ene rgy beta(+) spectrum in the 76Br decay had only minor effects on the resolu tion, but may decrease the quantification accuracy, especially in organs wi th a lower density such as a lung. (C) 1999 American Association of Physici sts in Medicine. [S0094-2405(99)01202-X].