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Thallium Tumor Imaging General Considerations
Thallium accumulates mainly within viable tumor tissue, and to a much lesser degree within connective tissue which contains inflammatory cells. Its accumulation is barely detectable in necrotic tissue. Localization of Thallium within tumors is likely multifactorial, involving blood flow, tumor viability, the sodium-potassium ATPase system, the non-energy dependent co-transport system, the calcium ion channel system, vascular immaturity with leakage, and increased cell membrane permeability [Nuclear Medicine Annual 94, p.183]. Cellular uptake of thallium is not affected by steroids, chemotherapy, or radiation therapy. By comparison, note that radiation therapy and chemotherapy do immediatelyinhibit gallium accumulation.
A baseline pretreatment determination of a tumor's thallium avidity is crucial to its efficacy in therapeutic response assessment. Early reports confirm an association between thallium uptake and histologic tumor response to treatment. The optimal time for thallium tumor imaging is 20 to 60 minutes post injection. Delayed images at 3 hours are recommended when imaging lymphoma because of an improved lesion to background ratio on the later images. The typical dose used for imaging is 3 to 4 mCi (or 30 to 40 uCi/kg). Spot views of the lesion should be 5 minute preset timed images using a high resolution collimator.
The normal distribution of thallium within the body is in the choroid plexus of the lateral ventricles, lacrimal glands, salivary glands, thyroid, myocardium, liver, spleen, splanchnic areas, kidneys, and testes. There is also uniform muscle uptake. Bone marrow activity should not be seen, and if noted indicates marrow hyperplasia. There is little uptake in healing surgical wounds. The elimination of thallium is slow, with a biologic half-life of 10 days. The kidneys are the critical organ. Unfortunately, thallium does not demonstrate a 100% specificity for tumors and false-positive uptake has been seen in histiocytosis X, benign bone tumors, stress fractures, and inflammation.
Technetium 99m Sestamibi Tumor Imaging - General Considerations:
There are many advantages to using Technetium rather than Thallium for scintigraphic imaging. Technetium's shorter physical half-life permits the use of a higher dose of the radiopharmaceutical. This translates to a higher count rate, which will shorten imaging times and provide sharper pictures. The gamma energy of Technetium (140 keV) is optimal for use with the detector crystal used in the gamma camera and will undergo less attenuation and scatter. Technetium is also readily available and produced daily from a Molybdenum generator in most Nuclear Medicine departments. Since its introduction, Tc-99m-Sestamibi has been shown to be of value in the evaluation of many tumors.
Normal brain uptake of the agent occurs from the CSF and is related to neuronal activity. There is normally little or no thallium uptake in the white matter. Thallium will not cross an intact BBB, yet disruption of the blood brain barrier is not the sole factor which affects thallium accumulation within a primary CNS lesion as little thallium accumulation is identified at sites of cerebral infarction. Uptake of thallium in CNS tumors is also likely dependent on the ATP-ase activity of the sodium-potassium pump and to active transmembrane transport via the K+/glucose co-transport system in viable tumor cells [JNM, Sept. 94, p.1414; Nuclear Medicine Annual 94, p.43]. Thallium accumulates in residual recurrent tumor in porportion to the malignant grade and total viable tumor bulk. Thallium can also accumulate in benign tumors such as meningiomas and pituitary adenomas [JNM, Dec95, p2202]. Thallium accumulation is minimal or negative at sites of radiation necrosis and resolving hematomas. Steroid administration does not affect thallium uptake.Thallium and Technetium 99m Imaging.doc