Cystic pancreatic neuroendocrine tumors: a clinicopathologic study.

Pancreatic neuroendocrine tumors (PanNETs) are typically solid neoplasms but in rare instances may present as cystic lesions. This unusual presentation can make clinical diagnosis challenging. In addition, the clinical and histopathologic characteristics of cystic PanNETs are poorly defined. We identified 53 cystic PanNETs in our single-institution experience of 491 surgically resected PanNETs. Similar to solid PanNETs, cystic PanNETs developed with an equal sex distribution and over a wide age range (23 to 91 y; mean, 52 y). The unusual cystic appearance made radiologic differentiation from other cystic pancreatic neoplasms difficult with a misdiagnosis in 23 of 53 (43%) cases. An association between cystic PanNETs and multiple endocrine neoplasia type 1 or multifocal disease [5 of 53 (9%) and 7 of 53 (13%), respectively] was not observed as compared with solid PanNETs (P=0.34 and P=0.31, respectively). Grossly, cystic PanNETs were predominantly located in the tail of the pancreas (n=28, 53%) and were similar in size (mean, 3.3 cm) to solid PanNETs (mean, 4.1 cm; P=0.12). All cysts were unilocular (n=53, 100%) and filled with clear to straw-colored fluid. Larger cysts were sometimes noted to be hemorrhagic. Histologically, the cysts were lined by a thin fibrous band that separated the cyst from the neoplastic cells. In comparison with their solid counterparts, cystic PanNETs were less likely to demonstrate tumor necrosis (6%; P=0.04), perineural invasion (8%; P<0.001), vascular invasion (4%; P<0.001), regional lymph node metastasis (13%; P<0.001), and synchronous distant metastasis (4%; P=0.015). The neoplastic cells of the cystic PanNETs were well differentiated (n=53, 100%) with a low mitotic rate and low Ki-67 proliferation index (range, 0.2% to 11%; mean, 1.8%). On the basis of both the American Joint Cancer Committee and European Neuroendocrine Tumor Society staging systems, the majority of cystic PanNETs presented at a lower pathologic stage as compared with solid PanNETs. In summary, cystic PanNETs are a distinctive subgroup of PanNETs with unique clinical, radiographic, and pathologic features.