Ssion of CD99 (SP, 400?. c) Diffusely infiltrative tumor cells were present in the pancreas parenchyma (HE, 200?.characteristics and was difficult to distinguish from the surrounding pancreas parenchyma. Laparotomy showed a 3.0 cm ?4.0 cm solid and cystic mass on the surface of the uncinate process of the pancreas. The boundary between the mass and pancreas parenchyma was obscure and the mass appeared unencapsulated. An en bloc resection of the mass was accomplished through the standard Whipple procedure [5]. Diffuse infiltrative tumor cells were present in the surrounding tissue (Figure 2c). Immunohistochemical staining of the tumor cells confirmed strong membranous expression of CD99 and focal expressions of vimentin and synaptophysin. The immunophenotyping results and patient history of pulmonary PNET indicated a postoperative pathologic diagnosis of metastatic pancreatic PNET. Again, the patient refused adjunct radiation and chemotherapy. To date, no evidence of tumor recurrence or metastasis has been found. The patient has survived seven years after the mass was initially detected, and four years after the first radical resection.Discussion pPNETs belong to the family of "small round cell tumors" that show varying degrees of neuroectodermal differentiation and are derived from cells originating from the neural crest [6] and are characterized by a specific chromosomal translocation, t(11;22)(q24;q12), in most cases. Among the reported cases of PNET, tumors involving the thoracopulmonary region were first reported as "malignant small cell tumors of the thoracopulmonary region in childhood" by Askin et al. in 1979, which led to these tumors being referred to as Askin's tumors [7]. Conventional light microscopy analysis of PNETs shows undifferentiated small, round cells with uniform, unconspicuous nuclei and scanty cytoplasm arranged in lobules with rosettes and pseudorosettes formation; in addition, there is little or Gemcitabine (hydrochloride) no stroma. Immunohistochemically, PNETs are positive for CD99 antigen, but CD99 immunostaining isnot specific and the results must be interpreted in combination with other findings. T lymphoblastic lymphoma, poorly differentiated synovial sarcomas, stromal tumors, and rare rhabdomyosarcoma may show CD99 positivity. Vimentin stains most tumor cells and neural markers, such as NSE, and is frequently expressed by tumor cells [6]. Cytokeratin-positive staining has also been reported in some cases of primitive neuroectodermal tumors [6]. To diagnose a tumor as PNET, it should display small round cells forming rosette and pseudorosettes, and should be positive for at least two of PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/16989806 the neural markers. Ultrastructural analysis usually shows PNET cells to have complex cytoplasmic processes, microtubules, and few neurosecretory granules. The following chromosomal translocations have been associated with PNET specimens: t(21;22)(q22;q12), t(11;22)(q24;q12), t(7;22)(p22;q12), and t(7;22)(q22;q12) [8]. Thus, the diagnosis of PNET necessitates histopathological, immunohistochemical, ultrastructural, and, if possible, genetic analyses. The differential diagnosis of PNETs includes neuroblastoma, lymphoma, small-cell carcinoma, rhabdomyosarcoma, monophasic synovial sarcoma, and desmoplastic small round cell tumor, all of which are indistinguishable by conventional light microscopy [6]. However, due to the different prognostic characteristics and therapeutic schedules for each of these tumor types, differential diagnoses are essential. Imm.