Histology: The biopsy reveals a lesion comprised of polygonal eosinophilic cells with round nuclei and prominent, cherry-red central nucleoli. Some cells contain distinct cytoplasmic inclusions reminiscent of intracellular hyaline bodies. In areas, the cells are arranged in “plates” of 3-4 cells thick. There are scattered mitotic figures. There is no appreciable cytoplasmic pigment suggestive of melanoma or bile. Immunostains show the lesional cells to be positive for HepAR1 and arginase, with canalicular pattern labeling for CD10; the cells are negative for PAX8, inhibin and S100.

Discussion:
The histology and immunophenotype of this lesion are characteristic of metastatic hepatocellular carcinoma (HCC). At the time of presentation with back pain and bony lesions, the patient did not have a known history of HCC. Although the diagnosis for this lesion may be suspected on the H&E alone, but immunostains are necessary to confirm the diagnosis in the absence of a known clinical history. Upon further clinical investigation, the patient was found to have large hepatic masses. The differential diagnosis of the bony lesions on the basis of the morphology alone is of a “pink cell tumor.” The differential diagnosis of malignant “pink cell tumors” must always include melanoma, melanoma, melanoma (which can have variable morphology and cytology), followed by adrenocortical carcinoma, hepatocellular carcinoma, and renal cell carcinoma, as well as rarer tumors such as alveolar soft parts sarcoma, clear cell sarcoma, oncocytic neoplasms of the thyroid (Hurthle cell) and parathyroid, and granular cell tumors. HCCs may also display a variety of architectural patterns such as trabecular/plate-like (as in this case) or pseudoglandular/acinar, as well a variety of cytologic patterns such as the presence of pleomorphic cells, clear cells, spindled cells, cytoplasmic bile pigment, cytoplasmic hyaline bodies, or ground glass inclusions. By immunohistochemistry, HCCs typically label positively for HepAR1 (~90%), arginase, glypican 3, and canalicular pattern polylonal CEA and CD10.