Microscopically, it is a solid vascular-rich mass. About 20% of the tumor shows clear cell changes. Tumor invades into the pancreatic parenchyma, with perineural invasion.

This was a consultation case. If only one unstained(白片) available, which IHC stain do you choose to do?

IMPRESSION: Main features: A solid, cellular tumor with bland cytology. Accordingly, all well differentiated pancreatic epithelial tumors,   mainly including 1）高分化内分泌瘤 2）实体假乳头肿瘤 3）高分化腺泡细胞癌 4）胰腺母细胞瘤 5)高分化胰腺腺癌, are within the differential diagnosis. However, this case lacks primitive cells and squamoid corpuscle (required for pancreatoblastoma胰腺母细胞瘤), lacks single prominent nucleoli(frequently seen in well differentiated acinar cell carcinoma高分化腺泡细胞癌),  classic "salt and pepper" chromatin (seen in well differentiated pancreatic endocrine neoplasma高分化内分泌瘤), or glandular element (seen in acinar cell carcinoma, endocrine neoplasm or well differentiated adenocarcinoma高分化胰腺腺癌). In addition to lack of the above findings, this bland-looking tumor shows the solid-papillae:a thin vascular core convered by several layers of epithelial cells, and clear cell change/foamy cell changes are helpful features, suggestive of a solid-pseudopapillary tumor.  However, IHC is needed to confrim the impression. A beta-catenin stain is recommended, and shows diffuse positive in this case. For educational purposes, additional IHC stains were performed in this case, and showing tumor cells positive for CD10, negative for neuroendocrine markers and trypin.

ANSWER:  Solid-pseudopapillary Tumor, also known as Papillary and Solid Epithelial Neoplasma.

Key Points:

** Though usually(more than 90%) it occurs in young female (average 27 years old) with a huge mass (average 9.5cm in size), it can rarely occurs in male.

** Every tumor starts with one stem cell. When solid-pseudopapillary tumor is small like this case, it lacks pseudopapillary/cystic growth pattern, which is due to loss of cells after degenerative/necrosis when it grows larger.

** Beta-catenin mutation is the molecular alteration of this tumor, therefore, virtually it is positive universally. A reminder, only beta-catenin of nuclear staining is true positive.

** Prognosis of the disease is favorable, although it is invasive, and sometimes with perineural invasion (in this case), and rarely with metastasis.

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