Mechanism of gallium-67 accumulation in tumors.

Neoplasms are characterized by increased perfusion, increased permeability of their capillary beds to macromolecules, and a delay in new lymphatic vessel growth. These lead to the increased entry and residency time of macromolecules in the interstitial space of tumors. Multiple factors contribute to the localization of 67Ga in tumors. Adequate blood supply is essential; at areas with no blood supply such as the necrotic center of a large tumor, there is no 67Ga accumulation. Gallium-67, mainly in the form of transferrin-67Ga complex, is delivered to the tumor through capillaries with increased permeability. In tumors, some 67Ga is taken up by tumor cells; some may also be taken up by inflammatory cells when they are present. Gallium-67 binding proteins, such as lactoferrin or ferritin, may also contribute to the accumulation and retention of 67Ga in tumors; however, their roles are less clear. The intensity of these various factors determine their relative contribution and the degree of 67Ga accumulation in tumors.