Chronic expanding hematomas (CEH) are hematomas that continue to expand at least one month after onset. CEH often occur in the thoracic cavity or in the extremities secondary to trauma, surgery, anticoagulation therapy, or bleeding disorders. Few studies have reported CEH in retroperitoneal organs such as the adrenal glands and lesions without an apparent etiology. An 80-year-old woman was referred to our hospital for evaluation of a retroperitoneal tumor that was incidentally detected on ultrasonography. Computed tomography and magnetic resonance imaging showed a large left retroperitoneal mass (14 cm in diameter) with gradual peripheral enhancement and a suspected hemorrhagic component within the lesion. Parametric imaging using positron emission tomography (PET) showed mild accumulation of the tracer at the margins of the mass on the conventional standard uptake value images, relatively low accumulation on the slope images, and slightly higher accumulation on intercept images. Postoperative histopathological examination of the resected specimen revealed a mass that was encapsulated with fibrous tissue and primarily consisted of organized clots without evidence of tumor cells; the mass was diagnosed as an idiopathic CEH of the adrenal glands. The margins of the mass showed few inflammatory cell infiltrates consistent with the findings of parametric imaging, which appeared to accurately reflect the dynamics of fluorodeoxyglucose (FDG) in the body. PET parametric imaging can separately assess the metabolism and distribution of FDG in vivo and may be useful to determine the etiology and diagnosis of CEH.
