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  Solid pseudopapillary neoplasm is a solitary tumor. In adults, it is slightly more frequently located in the tail, whereas in children it is located in the pancreatic head. Tumors are round, well demarcated, and generally large, with an average size of 8 to 10 cm (range, 0.5–25.0 cm). The cut surface shows a variable appearance from case to case: some cases are completely cystic, whereas others show a variable combination of solid, cystic, hemorrhagic, and necrotic areas. More rarely, SPNs have a predominantly solid appearance, especially when small. Although generally well circumscribed, rare tumors extending into the duodenal wall or other adjacent structures have been reported.
  At low magnification, SPNs generally show a heterogeneous appearance, including various proportions of solid and pseudopapillary structures (Figure 4, A and B).
  Neoplastic cells are rather monomorphic, with eosinophilic or vacuolated cytoplasm often containing small diastaseresistant, periodic acid-Schiff–positive hyaline globules (Figure 4, C).
  The solid component is composed of uniform cells admixed with numerous delicate capillary-sized blood vessels.
  Nuclei are round to oval, often grooved or indented, with finely dispersed chromatin without a prominent nucleolus. Bizarre nuclei may occasionally be observed. Mitoses are uncommon. Vascular and perineural invasion is rarely found.
  Additional features that can be observed include hemorrhage areas, pseudocystic changes, the presence of foamy macrophages (Figure 4, D), and depositsof cholesterol crystals.
  Figure 4. Solid pseudopapillary neoplasm shows various proportions of solid and pseudopapillary structures. A, The solid component is composed of uniform cells admixed with numerous delicate, capillary-sized blood vessels. B, Detaching of neoplastic cells from the capillary-sized blood vessels results in the pseudopapillary appearance. C, Some neoplastic cells may contain small hyaline globules. D, Frequently, foamy macrophages can be observed (hematoxylin-eosin).
  图4 实性假乳头状肿瘤显示不同比例的实性和假乳头状结构:A、实性成分是由一致细胞和许多纤细的毛细血管大小血管混合而成。B、从毛细血管大小的血管中分离肿瘤细胞导致假乳头状外观。C、一些肿瘤细胞可能含有小的透明球。D、通常可以观察到泡沫样巨噬细胞(苏木精-伊红染色)。
  In addition to these typical features, some variants have been described, including oncocytic, pigmented, and clear cells subtypes. These variants can give diagnostic difficulties, especially on cytologic preparations or small biopsies.
  In the oncocytic variant the cytoplasm of cells is abundantly eosinophilic and filled with mitochondria, and may simulate oncocytoma or chromophobe renal cell carcinoma.
  In pigmented SPNs the brown pigments can be due either to lipofuscin or melanin.
  In clear cell SPNs, cells present clear cytoplasm resulting from the accumulation of multiple cytoplasmic vacuoles, which seem to be the result of distended mitochondria or endoplasmic reticulum.
  In cases reported by Albores-Saavedra et al, clear cells represented more than 90% of neoplastic cells. However, in some cases the clear cell component may be less predominant and confined only to a few areas of the tumor.
  Clear cell SPNs or SPNs with clear cell areas can be a challenging task for pathologists because other pancreatic tumor types can show a more or less abundant clear cell component. Although ductal adenocarcinomas with clear cell features do not generally represent a difficult differential diagnosis, acinar cell carcinoma or pancreatic neuroendocrine neoplasms with clear cells can cause difficulties.
  Immunohistochemistry including acinar cell markers (trypsin, chymotrypsin, BCL10) or neuroendocrine markers (chromogranin and pancreatic hormones) is mandatory for the differential diagnosis.
  During the last years, several attempts have been made to clarify the immunophenotype of SPNs (Table 2) with the aim of finding useful diagnostic and prognostic markers. Considerable data are now available, but the diagnostic and prognostic utility of all investigated markers needs to be critically evaluated.
  It is well known that tumor cells characteristically show nuclear/cytoplasmic immunoreactivity for β-catenin. In addition, they are also positive for CD10, progesterone receptor, cyclin D1, and vimentin. A very characteristic feature is represented by the immunoreactivity of CD99 that shows a peculiar dotlike paranuclear expression. Aberrant expression of E-cadherin is a typical feature of SPN, and 2 distinct patterns of immunoreactivity have been well documented and depend on the antibody used. With the antibody directed against the cytoplasmic domain tumor, cells show nuclear E-cadherin positivity, whereas when using the antibody for extracellular fragments they are E-cadherin negative. More recently, several other markers have been investigated. Solid pseudopapillary neoplasms have been found to be positive for glutamine synthase, a-methylacyl-CoA racemase (P504s), transcription factor E3 (TFE3), SOX11, lymphoid enhancer-binding factor 1 (LEF1), androgen receptor (AR), fused in sarcoma (FUS), WNT inhibitor factor-1 (WIF-1), CD138, and CD200. Immunoreactivity for cytokeratins, synaptophysin, and CD56 can be observed in 30% to 70% of cases, whereas chromogranin A is negative. Up to 50% of SPNs can be positive for CD117, but KIT mutation has not been demonstrated. Solid pseudopapillary neoplasms are negative for PDX1 and acinar cell markers, including trypsin and BCL10, whereas the diastase-resistant periodic acid Schiff–positive hyaline globules, which ultrastructurally correspond to zymogen-like α1-antitrypsin granules, are positive for both α1-antitrypsin and a1-antichymotrypsin.
  众所周知,肿瘤细胞β-catenin为细胞核/细胞质阳性。此外还表达CD10、PR、cyclin D1和vimentin。典型特征是CD99为核旁逗点状阳性。E-cadherin的异常表达是SPN的一个典型特征,已有2种不同的免疫反应模式被证实,并依赖于所使用的抗体。针对细胞质的抗体,显示出细胞核E-cadherin阳性,而当使用细胞外片段的抗体时,E-cadherin则为阴性。最近,一些其他的标记也被研究过。已发现实性假乳头状肿瘤对谷氨酰胺合酶、a-甲基酰基辅酶a消旋酶(P504s)、转录因子E3(TFE3)、SOX11、淋巴增强因子结合因子1(LEF1)、雄激素受体(AR)、融合肉瘤(FUS)、WNT抑制因子1(WIF-1)、CD138和CD200呈阳性反应。30-70%的病例表达细胞角蛋白、突触素和CD56,但CgA为阴性。高达50%的SPNs表达CD117,但KIT突变尚未被证实。实性假乳头状肿瘤PDX1和腺泡细胞标记物(包括胰蛋白酶和BCL10)呈阴性,而抗淀粉酶的PAS阳性透明质球(其超微结构与酶原样α1抗胰蛋白酶颗粒相对应)表达α1抗胰蛋白酶和α1抗胰蛋白酶。
  Because the specificity and sensitivity of each positive or negative marker used alone is variable, the use of panels increases the diagnostic power of immunohistochemistry. In Table 3 an immunohistochemical panel for the routine pathology workup is proposed, presuming that the listed antibodies are available in most pathology laboratories, including the smaller ones.

  [1] Pancreatic Solid Pseudopapillary Neoplasm: Key Pathologic and Genetic Features[J].Arch Pathol Lab Med,2020.
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