Nuclear medicine imaging in medullary thyroid carcinoma (MTC)

The diagnosis and follow up of patients with MTC depend mainly on the determination of Calcitonin (Ct, basal and/or after pentagastrin stimulation) and CEA levels and on the use of various imaging modalities. Imaging techniques with a high spatial resolution can often detect macroscopic disease; however their low specificity may cause problems in image interpretation. A number of radiopharmaceuticals which selectively concentrate in MTC can allow the scintigraphic visualization of primary and metastatic lesions, thus offering advantages in diagnosis and prognosis. Radioiodinated MIBG (labeled with 123I or 131I) which is specific for tumors originating from the neural crest, shows in MTC a high specificity but a low sensitivity (30% approximately); this seems to be higher for both residual and local recurrent tumor than for metastatic lesions. MIBG scan allows the simultaneous imaging of pheochromocytoma in MEN syndromes. Furthermore,131 I-MIBG at high specific activity (\gr 1.1 GBq/mg) may be used for therapy in patients which show uptake of the tracer in tumoral lesions. 99mTc(V)DMSA has a higher sensitivity (70% approximately) even in metastatic disease. Moreover it has the advantage of low cost, easy availability and high quality images; the use of SPECT may further improve the diagnostic sensitivity. However a number of false positive results can occur. 201Tl uptake has been reported both in primary and recurrent tumor of MTC, with sensitivity of more than 80%; 201Tl is not specific but, in selected patients, can be useful. Imaging agents using anti-CEA or anti-calcitonin monoclonal antibodies have been used in the diagnostic assessment of MTC. The results reported have been variable, ranging from a very low sensitivity with anti-calcitonin antibodies to a higher sensitivity (78%) with radiolabeled anti-CEA antibodies. A novel immunotargeted method which employs a bispecific antibody and 131I as a radiolabel, seems to be very promising. 111In-octreotide is widely employed in the imaging of neuroendocrine tumors, including MTC. Sensitivity of approximately 70% has been reported in detecting MTC lesions, except liver metastases in which the sensitivity is lower due to nonspecific uptake in the liver.