Bone marrow uptake of (TC)-T-99m-MIBI in patients with multiple myeloma

In a previous study, we showed the ability of technetium-99m methoxyisobuty lisonitrile (Tc-99m-MIBI) scan to identify active disease in patients with multiple myeloma (Eur J Nucl Med 1998; 25: 714-720). In particular, a semiq uantitative score of the extension and intensity of bone marrow uptake was derived and correlated with both the clinical status of the disease and pla sma cell bone marrow infiltration. In order to estimate quantitatively (TC) -T-99m-MIBI bone marrow uptake and to verify the intracellular localization of the tracer, bone marrow samples obtained from 24 multiple myeloma patie nts, three patients with monoclonal gammopathy of undetermined significance (MGUS) and two healthy donors were studied for in vitro uptake. After cent rifugation over Fico11-Hypaque gradient, cell suspensions were incubated wi th Tc-99m-MIBI and the uptake was expressed as the percentage of radioactiv ity specifically retained within the cells. The cellular localization of th e tracer was assessed by micro-autoradiography. Twenty-two out of 27 patien ts underwent Tc-99m-MIBI scan within a week of bone marrow sampling. Whole- body images were obtained 10 min after intravenous injection of 555 MBq of the tracer; the extension and intensity of Tc-99m-MIBI uptake were graded u sing the semiquantitative score. A statistically significant correlation wa s found between in vitro uptake of Tc-99m-MIBI and both plasma cell infiltr ation (Pearson's coefficient of correlation r=0.69, P<0.0001) and in vivo s core (Spearman rank correlation coefficient r=0.60, P<0.01). No specific tr acer uptake was found in bone marrow samples obtained from the two healthy donors. Micro-autoradiography showed localization of Tc-99m-MIBI inside the plasma cells infiltrating the bone marrow. Therefore, our findings show th at the degree of tracer uptake both in vitro and in vivo is related to the percentage of infiltrating plasma cells which accumulate the tracer in thei r inner compartments.