Answer: Acinar cell carcinoma

Histology: The biopsy specimen shows a solid sheet-like proliferation of tumor cells. Focally, the tumor cells for dilated acinar structures. The tumor cells have abundant pink granular cytoplasm, and vesicular nuclei with prominent nucleoli. A special stain for mucin is negative. Immunohistochemical studies show that the tumor cells are immunoreactive for lipase (diffuse), trypsin (diffuse), and chromogranin A (patchy).

Discussion: Acinar cell carcinoma is an uncommon tumor of the exocrine pancreas that, be definition, demonstrates acinar cell differentiation. In most tumors an acinar growth pattern is well developed. The diagnosis becomes more challenging when the architectural pattern is dominated by solid growth. In these instances where a solid growth pattern may cause confusion with a low-grade endocrine tumor, immunohistochemical documentation of acinar cell differentiation is critical. All acinar cell carcinomas stain strongly for a least one of the following pancreatic enzymes: trypsin, lipase, chymotrypsin and phospholipase A2. Notably, about one-fourth of acinar cell carcinomas show some focal evidence of endocrine differentiation in the form of synaptophysin or chromogranin staining. Accordingly, the presence of staining for endocrine markers does not by itself exclude the diagnosis of acinar cell carcinoma. Although the presence of dilated acini may give the appearance of glandular differentiation, mucin stains are consistently negative.

Given the aggressive nature of acinar cell carcinoma, it is important not to confuse it with the indolent islet cell tumor. More than one-half of patients present with metastatic spread at the time of presentation, and few patients survive beyond 5 years.