Gallium67 Scintigraphy

Gallium-67 citrate has been used since the 1970s for imaging of lymphoma and other tumors. Many malignant cells, including some lymphomas, take up large amounts of iron for use as a cofactor for ribonu-cleotide reductase, a key enzyme in DNA synthesis. Many malignant cells overexpress the transferrin receptor,1-3 facilitating uptake of iron bound to trans-ferrin. Gallium-bound transferrin binds the transfer-rin receptor and is therefore taken up into cells in a manner similar to iron. Ga-67 is administered intravenously and, following a distribution period of 1-2 days, can be imaged, with the best sensitivity and accuracy demonstrated by single photon emission computed tomography (SPECT).45

Using Ga-67 SPECT and higher doses of tracer, Ga-67 scintigraphy has been studied in staging and in follow-up of patients with lymphoma. It was found to be beneficial following treatment for restaging and detection of recurrence.6 7 Different lymphoma grades and histologies have variable rates of detection, however. For example, while Ga-67 was sensitive and accurate in restaging of aggressive lymphomas, it was inadequate for detection of indolent lymphomas, and did not predict Richter's transformation in chronic lymphocytic leukemia (CLL).8 Baseline scanning is therefore critical to establish the utility of this modality in individual tumors.

In recent years, the emergence of FDG-PET has led investigators to compare the utility of these two imaging modalities in the evaluation of lymphoma. In both Hodgkin's lymphoma (HL) and non-Hodgkin's lymphoma (NHL), FDG-PET shows higher sensitivity for staging and follow-up after treat-ment.9-11 In addition, FDG-PET detects indolent lymphomas with better accuracy than does Ga-67 scintigraphy. These findings, combined with a shorter half-life of FDG and more convenient imaging characteristics, have led to FDG-PET largely replacing Ga-67 scintigraphy where FDG-PET is available.

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