Characterization of fluorine-18-fluorodeoxyglucose accumulation in Blastomyces dermatitidis-associated inflammation

Whole-body positron emission tomography (PET) using the radiopharmaceutical fluorine-I8-fluorodeoxyglucose (FDG) is widely used for tumor imaging in people. Lesions caused by fungal infections can have high accumulation of FDG and result in misdiagnosis for malignancy. Little information was available in the literature to draw firm conclusions regarding the uptake of FDG by fungal lesions. The purpose of this research was to determine the typical intensity of uptake of FDG by fungal lesions, to describe the distribution of FDG in fungal lesions, and to determine if fungal-associated inflammatory lesions could be detected using whole-body FDG-PET. A technique for creating lesions caused by the fungal agent Blastomyces dermatitidis in rats was developed. Accumulation of FDG in these experimentally created fungal lesions was compared with uptake by experimentally induced non-fungal inflammation (turpentine abscesses) and malignant tumors (lymphomas) in rats using the differential uptake ratio. Results showed that experimental Blastomyces lesions typically exhibit high intensity uptake of FDG. The intensity of uptake of FDG by the Blastomyces granulomas was higher than for the turpentine abscesses and as high as for malignant lymphomas. The uptake of FDG in the experimentally created Blastomyces lesions was associated with the region occupied by the granulomatous inflammatory reaction and the yeast organisms. Uptake of FDG was then evaluated in dogs with naturally occurring blastomycosis and compared with uptake in dogs with naturally occurring lymphoma using the standardized uptake value (SUV). The mean (+/- sd) SUV for naturally occurring lesions of blastomycosis in dogs was 7.7 +/- 2.0, and was higher than the mean SUV for naturally occurring lymphomas in dogs of 4.8 +/- 1.8. A cutoff SUV of 7.0 gave 100% specificity for blastomycosis. FDG-PET reliably detected lesions of Blastomyces-associated inflammation and may have a role in determining extent of disease and monitoring therapy in patients with blastomycosis. Because of the similarities of blastomycosis and lymphoma in dogs and people, similar results would be expected in human patients. Fungal granulomas should be considered a differential diagnosis for lesions with high intensity accumulation of FDG in areas where blastomycosis is endemic.