Abstract: Objective On rare occasions, yolk sac tumor may arise from extragonadal sites. Orbit is an unusual location for the primary development of this tumor. The presence of intracranial yolk sac tumor on biopsy always makes the diagnosis challenge for pathologists. Herein we report a case of intracranial primary yolk sac tumor in left orbital apex. The clinicopathology of this tumor and its differential diagnosis are discussed. Methods The clinical manifestation of a patient with primary yolk sac tumor occurring in left orbital apex was presented retrospectively. Resected orbital mass was routinely paraffin-embedded and stained with hematoxylin and eosin. Dako Envision immunohistochemical staining system was used to detect the tumor antigen expressions, including alpha-fetoprotein (AFP), cytokeratin, placental alkaline phosphatase (PLAP), CD30, CD34, CD45, CD99, CD117, synaptophysin (Syn) and chromogranin A (CgA). Results A 2-year-old boy presented with 3-month history of ptosis of eyelid and exophthalmos on left eye. Magnetic resonance imaging (MRI) scan revealed a lesion occupied the left orbital apex with infiltration of surrounding normal structure, including left posterior ethmoid sinus, the wall of left maxillary sinus and sphenoid. However, there was no evidence of tumor infiltrating in brain parenchyma. Craniotomy was performed and the tumor was removed by en bloc resection. Grossly, the biopsy specimen received in small pieces was 1.20 cm × 1.00 cm × 1.00 cm when aggregated. No fibrous capsule, necrosis, haemorrhage and gross calcification were found in the tissue fragments. Histological examination revealed that the tumor was arranged in a reticular pattern. The cells were relatively large with a clear cytoplasm and vesicular nuclei. Scattered mitotic figures were noted. Schiller-Duval bodies and periodic acid-Schiff (PAS) positive (eosinophilic bodies) were evident in the tumor tissue. Immunohistochemical staining showed that the tumor cells were diffusely positive for cytokeratin (AE1/AE3) and focal positive for AFP, CD99 and CD117, but negative for PLAP, CD30, S-100, CD45 and CD34. There was no evidence of mixture of other germ cell tumor component in this tumor by serial sections. Based on clinical presentation and histological findings, a final histological diagnosis of pure primary orbital yolk sac tumor, WHO grade Ⅳ, was made according to the criteria of WHO classification. The patient has not received chemotherapy and attended follow-up for 3 months, without any neurological deficit or signs of recurrence. Conclusion Despite the lower incidence, intracranial yolk sac tumors usually develop in the midline at the pineal or suprasellar regions occurring in children with distinctive histological features and immunohistochemical phenotypes. In general, intracranial yolk sac tumors are known to entail poor prognosis even after multidisciplinary treatment of operation, radiotherapy, and chemotherapy. It is noted that intracranial yolk sac tumor should be differentiated histologically from other types of germ cell tumors and mixed germ cell tumor.

Key words: Ovarian neoplasms, Endodermal sinus tumor, Orbital neoplasms, Immunohistochemistry

摘要： 目的 探讨临床少见原发于眶内卵黄囊瘤的病理学特征，并讨论颅内生殖细胞肿瘤的病理学鉴别要点。方法 分析1 例原发于左侧眶尖卵黄囊瘤患儿的临床表现，HE 和免疫组织化学染色观察颅内卵黄囊瘤的临床病理学特点及免疫表型。结果 患者男性，2 岁。表现为左眼上睑下垂并眼球突出。MRI 显示颅底肿瘤侵犯左侧后组筛窦、眼眶及翼窝并蝶骨翼板，左侧上颌窦后壁、筛骨及眼眶尖部骨质破坏，与硬膜分界尚清，未突破硬膜向脑实质浸润。术中可见肿瘤位于眶尖区之眶内外，累及左侧海绵窦区，位于硬膜外尚未侵入硬膜下区，无包膜，破坏眶尖区骨质，长入蝶窦和筛窦。肿瘤组织由许多相互交通的间隙形成疏松的网状结构，衬覆胞质透亮的圆形或多边形细胞，核深染、不规则且核仁突出，核分裂象多见；可见Schiller-Duval（S-D）小体结构，为单个圆形或长形乳头状结构，含有单个血管的纤维血管轴心，被覆柱状细胞，乳头占据的间隙衬覆立方、扁平或“鞋钉”样细胞；肿瘤细胞内或细胞间可见嗜伊红且PAS 阳性透明小体。肿瘤细胞弥漫表达细胞角蛋白，灶性表达甲胎蛋白、CD99 和CD117，但不表达CD30、CD45、胎盘碱性磷酸酶、突触素、嗜铬素A 和CD34。连续组织切片HE 染色未发现合并其他类型生殖细胞肿瘤，病理诊断左侧眶尖原发性卵黄囊瘤（WHOⅣ级）。结论 颅内卵黄囊瘤好发于青少年，多生长于中线结构，具有特殊的组织学构象和免疫组织化学表型，预后不良，诊断时应与其他生殖细胞肿瘤相鉴别，并注意是否同时合并其他生殖细胞肿瘤成分。

关键词: 卵巢肿瘤, 内胚层窦瘤, 眶肿瘤, 免疫组织化学