Evaluation of three gamma detectors for intraoperative detection of tumorsusing In-111-labeled radiopharmaceuticals

Attempts to detect tumors with intraoperative scintillation using tumor-bin ding radiopharmaceuticals have intensified recently. in some cases previous ly unknown lesions were found, but in most cases no additional lesions were detected. in this study the physical characteristics of three detector sys tems and their ability to detect tumors through accumulation of an In-111-l abeled radiopharmaceutical were investigated. The first was a sodium iodide (Na[Ti]) detector; the second, a cesium iodide (CsI[TI]) detector; and the third, a cadmium telluride (CdTe) detector. Methods: A body phantom and tu mor phantoms (diameter 5-20 mm) made of water, agarose gel or epoxy with a density and attenuation coefficient similar to those of soft tissue were us ed to simulate a clinical situation. The activity concentration in the body phantom was based on reported Values of In-111-octreotide in normal tissue in humans. The In-111 activity concentration in the tumor phantoms varied from 3 to 80 times the In-111 activity concentration in the body phantom. D ata were processed to determine tumor detection levels. Results: The NaI(TI ) detector showed the lowest values for full width at half maximum because this detector had the best collimation, leading to a high ratio between cou nts from tumor and counts from background, i.e., small tumors could be dete cted. Because of high efficiency, the CsI(TI) detector sometimes required a somewhat shorter acquisition time to produce a statistically significant d ifference between tumor phantom and background. For deep-lying tumors the N aI(TI) detector was superior, whereas the CdTe detector was best suited for superficial tumors with a high activity concentration in the underlying ti ssue. Conclusion: At a maximum acquisition time of 30 s, almost all superfi cial tumors with a diameter of 10 mm or larger were detected if the ratio b etween the In-111 concentration in the tumor and the In-111 concentration i n the background exceeded 3. However, in clinical situations, biologic vari ations in the uptake of In-111-octreotide in tumors and in normal tissue ma kes difficult the determination of a distinct detection level. For such cli nical conditions, the NaI(TI) detector is the best choice because it has go od resolution despite a lower efficiency. Documentation of detector charact eristics is important so that clinicians can make an adequate device in rel ation to tumor location and receptor expression.