Answer: Solid pseudopapillary tumor

Histology: The tumor in this case has a predominantly solid appearance with a prominent microvasculature. The tumor cells are polygonal with uniform nuclei, and generally have eosinophilic cytoplasm, though vacuoles are present in some cells. Focal areas show degeneration yielding a pseudopapillary pattern. The tumor cells were strongly immunoreactive for CD10, and showed nuclear labeling for ß-catenin, supporting the diagnosis of solid pseudopapillary tumor. While synaptophysin was positive, chromogranin was negative, which is entirely consistent with the diagnosis.

Discussion: An islet cell tumor of the pancreas is the main differential diagnostic consideration in this case, since it too would have a solid, nested architecture and be composed of small cells with even, fine chromatin. The synaptophysin positivity in the current case was particularly strong, and raised this differential diagnostic consideration. Importantly, however, the current lesion was negative for chromogranin, which is almost always true of solid pseudopapillary tumors. The strong labeling for CD10 and nuclear ß-catenin also helped exclude islet cell tumor. Acinar cell carcinoma only infrequently demonstrates nuclear ß-catenin labeling (approximately 25%), and cytologically shows more prominent nucleoli with glandular or acinar patterns. Labeling for trypsin or lipase would be positive in acinar cell carcinomas. Pancreatoblastoma should feature more undifferentiated small blue cells with focal squamoid islands.

Solid pseudopapillary tumor typically affects females at a median age of 27 years; hence, the current patient is outside of the usual age and gender distribution. These tumors are essentially low-grade malignancies. Most act in a benign fashion but some will metastasize to lymph nodes or liver. Even malignant tumors, when completely resected, usually have a good outcome.

Reference(s):
– American Journal of Pathology 2002; 1(60): 1361-1369;
– American Journal of Pathology 2000; 24:1361-1371.