Usefulness of gallium imaging in the evaluation of lung cancer.

The current enthusiasm for gallium (Ga) citrate as a tumor imaging agent reflects the need of clinical medicine for a good tumor imaging agent. To date, Ga-67 is probably the best tumor imaging agent available for clinical use. Initially, Ga-67 was investigated for its bone scanning potential. In this process, in 1969, Edwards and Hayes accidentally found that Ga-67 concentrated in soft tissue tumors, mainly lymphomas, in patients. Later studies reported the clinical experience with Ga-67 concentration in many different tumor types. Great variation was noted in the ability of different tumors to concentrate Ga-67. However, Ga-67 was most consistently and reliably taken up in lung tumors, with sensitivities of Ga imaging positivity in lung cancer ranging from 85 to 95%. Within the lung cancer group, squamous cell carcinoma consistently has been much more reliably positive than adenocarcinoma or alveolar cell carcinoma. Subsequent studies on Ga-67 led to the recognition of its preferential concentration in inflammatory lesions and abscess. These reports resulted in the clinical application of Ga-67 imaging as a diagnostic tool in the evaluation of patients with suspected abscesses. Mechanisms of Ga localization in tumor and inflammatory lesions are not currently well understood. Electron microscopic studies have revealed some information regarding the intracellular localization of Ga, but the mechanism by which it is taken up by the cell remains unproven, although several explanations have been suggested. The biodistribution of Ga-67 is responsible for the great difficulty experienced in interpreting Ga images of the abdomen, mainly because of the normal of the normal excretion in the bowel. Clinical studies have shown that the Ga scan can be used in the workup of patients with lung cancer as a sensitive tool in excluding the presence of mediastinal metastases. In some institutions, a negative Ga mediastinal scan in the presence of positive Ga uptake in the presumed primary tumor in patients with lung cancers has been used in lieu of a staging mediastinoscopy. Data regarding the thresholds of various factors which determine visibility of a lung tumor by Ga-67 imaging have been described in some detail. The factors include lesion size, depth in tissue, gallium concentration in tumor relative to background, type of film and instrumentation used, and count rates obtained. The data suggest the need for very high radiopharmaceutical concentrations in small tumors relative to background activity for identification of the tumor on an image.