Answer: Solitary fibrous tumor

Histology: This is a highly collagenous lesion which replaces the existing hepatocytes and leaves portal tracts with prominent bile duct proliferation behind. These bile ducts are mainly present at the periphery of the neoplasm, but extend some ways towards the center of it. The spindled lesional cells are bland, and are associated with dense collagen deposition. Some of the blood vessels within the lesion demonstrate a staghorn (hemangiopericytomous) appearance. The lesion is diffusely immunoreactive for CD34, supporting the diagnosis of solitary fibrous tumor.

Discussion: Differential Diagnosis: Gastrointestinal stromal tumor may become fibrotic, particularly after therapy. These lesions would label for CD117 in addition to CD34, and typically do not show hemangiopericytomatous vessels. A nonspecific fibroma would not demonstrate the more cellular areas in the current lesion, and the diffuse CD34 immunoreactivity. A leiomyoma would feature spindle cells with prominent eosinophilic cytoplasm, as opposed to the eosinophilic extracellular collagen seen in the current case.