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Most adrenocortical carcinomas accumulate and retain FDG and thus can be visualized by PET. However, false-negative findings are possible, especially with very small lesions.

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The purpose of this report was to determine the pattern of thallium (T1-201) uptake in pigmented villonodular synovitis (PVNS) and giant cell tumor of the tendon sheath (GCTTS) and to evaluate for features that might help to distinguish these lesions from malignant disease. Scintigraphic images from patients subsequently found to have pigmented villonodular synovitis confirmed by histopathologic analysis were assessed. The patients had been evaluated with T1-201 scintigraphy for a suspicious musculoskeletal lesion. All 6 patients with PVNS had significant T1-201 uptake on the early images with retention on the delayed images. The T1-201 activity was in a juxta-articular location and generally conformed to the synovial cavity with scattered irregular nodular components. Of 4 cases of giant cell tumor of the tendon sheath, all had T1-201 uptake on the early images, and 3 cases (75%) also had uptake on the delayed images. T1-201 uptake is observed almost invariably in pigmented villonodular synovitis on both early and delayed images and can simulate the findings of malignant disease. Features that should raise suspicion of PVNS include close proximity to a joint, especially with distribution corresponding to that of the synovial cavity. When T1-201 activity is detected in a lesion in the hand or foot, giant cell tumor of the tendon sheath should be considered. Magnetic resonance imaging will often aid in the differentiation by demonstrating features of hemosiderin degradation products.