Littoral cell angioma. A novel splenic vascular lesion demonstrating histiocytic differentiation.

Seventeen cases of a novel type of vascular tumor of the spleen are described. The lesions, whose size ranges from minute foci to large nodules almost completely replacing the splenic tissue, are composed of anastomosing vascular channels resembling splenic sinus and have irregular lumina, often featuring papillary projections and cyst-like spaces; they are lined by tall endothelial cells that slough off into the vascular lumina and show hemophagocytosis. Atypical cells are absent and mitotic activity very low. In contrast to normal sinus endothelia, which express only FVIIIag, neoplastic cells express both endothelial (FVIII-AG, BMA 120) and histiocytic (KP1, lysozyme) antigens; occasionally S-100 protein is also present. The morphologic and immunohistochemical findings in this tumor reflect the dual differentiation potential of the reticuloendothelial cells lining the splenic sinus, justifying the term littoral cell angioma, and recognize a distinct entity that is different from other vascular lesions of the spleen, notably angiosarcoma. This distinction is all the more important because the clinical behaviour of this lesion is apparently benign.