Figure 6. I-123 MIBG whole body scan 24 hours after injection of 185 MBq in a 78-year-old patient known with MTC, hyperparathyroidism and suspected of pheochromocytoma ( MEN-2a). It shows intense uptake in the neck with bilateral paratracheal expansion (arrow); it also shows uptake in the left adrenal region (arrow).
may have some impact on tumoral uptake (e.g. alpha receptor blocking agents). Whole body and spot imaging may be supplemented by SPECT imaging.
There is no place for MIBG scintigraphy in patients with differentiated thyroid cancer, but in medullary cancer the method may be helpful.
In medullary thyroid cancer, MIBG scans are positive in only a limited number of patients with medullary thyroid carcinoma with mentioned sensitivities of 12-30% (114,115).
A pitfall in imaging arises when liberated free iodine localizes in thyroid remnants (in the rare patients that were not ablated after thyroidectomy), but this can usually be differentiated from uptake in medullary thyroid cancer metastases. In cases of doubt Tc-pertechnetate imaging can be helpful.
A number of patients with medullary thyroid cancer have been treated with 131I-MIBG and a palliative response has been reported in 50% of the patients (116). Pen-tavalent 99m Technetium Dimercaptosuccinic Acid (99mTc (V)DMSA).
Pentevalent Tc-DMSA (called DMSA-V) is derived from DMSA, but includes the Tc99m label in a 5+ molecular charge, instead of 7+, as is the common chemical form of Tc99m. 99mTc(V) DMSA is not taken up by the normal thyroid gland, but can be applied in the diagnosis of medullary thyroid cancer due to an increased turnover of calcium and phosphate ions.The compound localizes in a number of tumours. The precise mechanism is not well known, uptake may be related to the intracellular phosphate concentration (117). 99mTc(V) DMSA exists in three isomeric forms and the biodistribution of the individual isomers differs from the whole radiopharmaceutical.
Figure 7. Tc-99m(V)-DMSA scan 2 hours after injection of 290 MBq in a patient known with MTC (MEN-2a) with persistent elevated serum calcitonin. The scan shows uptake in the neck, chest and in the right proximal femur (arrow). This patient also has severe scoliosis of the thoracal spine.
Images are acquired 2-3 hours after injection. SPECT imaging of suspected areas may be helpful to improve the sensitivity of tumour detection.
The normal biodistribution is seen after 2 hours in the nasal mucosa and faintly in the skeleton, with breast uptake in women. Excretion is through the kidneys, liver uptake is not prominent. Some blood pool activity may also be present.
99mTc(v) DMSA is not commercially available in the United States but is well used in other countries. Tumor lesion sensitivity is reported to be between 50 and 95% (118,119).
RADIOLABELED ANTI CARCINOEMBRYONIC ANTIGEN ANTIBODY General mechanism
Serum calcitonin and carcinoembryonic antigen (CEA) are used as tumormarkers in medullary thyroid carcinoma (MTC).
The specific positive immuno-histochemical property of a positive staining for calcitonin and carcinoembryonic antigen (CEA) and the expression of CEA levels at the surface ofthe cells were the basis for the development of specific anti-CEA monoclonal antibodies, the so-called radioimmunoscintigraphy, to image patients with MTC. Various anti-CEA antigen antibodies can be labeled with 99m-Tc, lll-In,123I or 1311. The disadvantages of using monoclonal antibodies include the low tumor-background ratio and the forming of human anti-mouse antibodies (HAMA) making repeated studies difficult.
The injected dose depends on the radionuclide the antibodies are labeled with, just like the timing of scanning and the choice of collimator (120,121,122). Injected activity are 555-1110 MBq (15-30 mCi) for 99m-Tc labeled antibodies, 74-370 MBq (210 mCi) for 131I labeled antibodies and 111-185 MBq (3-5 mCi) for 111-In labeled antibodies. With the 99m-Tc labeled anti-CEA antibodies planar and SPECT imaging of the neck/chest, abdomen and pelvis can be acquired 4 and 24 hours postinjection with an LEHR-collimator. With the 111-In labeled anti-CEA antibodies imaging can be performed up to 72 hours postinjection with a medium energy collimator. Imaging of labeled anti-CEA antibodies can be performed 4 hours and up to 7 days postinjection. Blood pool activity may be prominent, kidney and bone marrow uptake can be seen.
Reported lesion based sensitivity of various anti-CEA antibodies in medullary thyroid carcinoma is around 70-100%, for both known and occult disease (121,122,123). However, only a limited number of groups have published about these results. Apart from the limited application in medullary thyroid cancer, the anti-CEA antibodies scan has been extensively used in the detection of metastatic colon cancer, where it appears to have a sensitivity of 50-70% and provides clinically relevant information, especially in combination with CT scanning.
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