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Evaluation of Three Gamma Detectors for Intraoperative Detection of Tumors Using ¹¹¹In-Labeled Radiopharmaceuticals

Departments of Radiation Physics and Surgery, Sahlgrenska University Hospital, Göteborg University, Göteborg, Sweden
Medical Physics and Bioengineering Department, Southampton General Hospital, Southampton, United Kingdom

Correspondence: For correspondence of reprints contact: Sven Anders Benjegård, MSc, Department of Radiation Physics, Göteborg University, Sahlgrenska University Hospital, S-413 45 Göteborg, Sweden.

ABSTRACT

Attempts to detect tumors with intraoperative scintillation using tumor-binding radiopharmaceuticals have intensified recently. In some cases previously unknown lesions were found, but in most cases no additional lesions were detected. In this study the physical characteristics of three detector systems and their ability to detect tumors through accumulation of an ¹¹¹In-labeled radio-pharmaceutical were investigated. The first was a sodium iodide (Nal[TI]) detector; the second, a cesium iodide (Csl[TI]) detector; and the third, a cadmium telluride (CdTe) detector. Methods: A body phantom and tumor 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 used to simulate a clinical situation. The activity concentration in the body phantom was based on reported values of ¹¹¹In-octreotide in normal tissue in humans. The ¹¹¹In activity concentration in the tumor phantoms varied from 3 to 80 times the ¹¹¹In activity concentration in the body phantom. Data were processed to determine tumor detection levels. Results: The Nal(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 counts from tumor and counts from background, i.e., small tumors could be detected. Because of high efficiency, the Csl(TI) detector sometimes required a somewhat shorter acquisition time to produce a statistically significant difference between tumor phantom and background. For deep-lying tumors the Nal(TI) detector was superior, whereas the CdTe detector was best suited for superficial tumors with a high activity concentration in the underlying tissue. Conclusion: At a maximum acquisition time of 30 s, almost all superficial tumors with a diameter of 10 mm or larger were detected if the ratio between the ¹¹¹In concentration in the tumor and the ¹¹¹In concentration in the background exceeded 3. However, in clinical situations, biologic variations in the uptake of ¹¹¹In-octreotide in tumors and in normal tissue makes difficult the determination of a distinct detection level. For such clinical conditions, the Nal(TI) detector is the best choice because it has good resolution despite a lower efficiency. Documentation of detector characteristics is important so that clinicians can make an adequate device in relation to tumor location and receptor expression.

Key Words: intraoperative detection • gamma detectors • ¹¹¹In-labeled radiopharmaceuticals