Melanocytic differentiation in a solid pseudopapillary tumor of the pancreas.

Solid pseudopapillary tumor of the pancreas (SPT) is an uncommon neoplasm of low malignant potential, generally occurring in young women. The tumor is indolent, usually with long survival, even in the presence of extension into adjacent organs and metastases. Pathological features include solid, cellular, and cystic regions and degenerative pseudopapillae formation. Despite its distinctive morphology and cytological features, the cell lineage of this entity is unclear. Here we report a case of solid pseudopapillary tumor in a 48-year-old man with 10-year follow-up in which melanin pigment was found within the tumor cells. The tumor cells stained positive not only for melanocytic markers including S-100, HMB-45, and Fontana, but also other well-established markers for this kind of neoplasm such as alpha-antitrypsin (Alpha-AT), anti-alpha-chymotrypsin (AACT), NSE, CD10, cyclin D1, and beta-catenin. Electron microscopy confirmed the formation of premelanosomes and melanosome granules in the tumor cells. To our knowledge, this is the first report in which melanosomes were produced by SPT. Because melanocytes are derived from neurocrest, we hypothesize that the histogenesis of SPT is of neurocrest origin. This phenomenon may also be explained by ongoing research in which it has been shown that Wnt signaling/beta-catenin intranuclear localization promotes pigment cell formation by medial crest cells in embryos.