Abstract: Objective To report the diagnosis and treatment of a case of extraskeletal myxoid chondrosarcoma (EMC) in the pineal region misdiagnosed as chordoid glioma of the third ventricle, and to summarize the clinicopathological features, diagnosis and differential diagnosis of this rare tumor. Methods and Results A 42-year-old male patient with head MRI showed a space-occupying lesion in the pineal region and moderate enhancement on enhanced scan. After the first surgery, the histological morphology showed the boundary between tumor and brain tissue was not clear, spindle or stellate tumor cells were bunched, braided or scattered, the nuclei were small and deeply stained, and the interstitium was rich in collagen fibers and a large number of basophilic myxoid matrix. Immunohistochemical staining showed vimentin (Vim) expression in the cytoplasm of tumor cells, some of which expressed glial fibrillary acid protein (GFAP), CD34, CD99, but not cytokeratin (CK), epithelial membrane antigen (EMA) and S-100 protein (S-100). The pathological diagnosis was "chordoid glioma of the third ventricle". The tumor recurred 7 months after the first surgery, and the histological morphology of the second surgery showed obvious epithelioid features of the tumor cells, which were arranged in a cord, bundle or braided shape, with enlarged nuclei and easy to see mitotic images (up to 8/10 high magnification field). Some tumor cells were rich in cytoplasm and nuclear deviation, which were rhabdoid. The immunophenotype was the same as before, maintaining the diagnosis of "notochord glioma". The patient died of tumor progression 5 months after the second surgery. Combined with the literature, supplemental immunohistochemical staining showed that the tumor cells did not express Brachyury, thyroid transcription factor 1 (TTF-1), and SMARCB1/INI-1 proteins. Fluorescence in situ hybridization (FISH) test was positive for NR4A3 (9q22.33) gene rupture, and finally confirmed the diagnosis of EMC. Conclusions EMC in the pineal region is extremely rare, and attention should be paid to distinguishing it from the frequent mucinous tumors in the pineal region, such as chordoma, chordoid meningioma and chordoid glioma. Definite diagnosis depends on histological morphology, immunohistochemical staining and molecular detection.

Key words: Chondrosarcoma, Pineal gland, Glioma, Immunohistochemistry

摘要： 目的 报道1例误诊为第三脑室脊索样胶质瘤的松果体区骨外黏液样软骨肉瘤患者诊断与治疗过程，并总结此类罕见肿瘤的临床病理学特点、诊断与鉴别诊断要点。方法与结果 男性患者，42岁，头部MRI提示松果体区占位性病变，增强扫描病灶呈中等强化。首次术后组织学形态，肿瘤与脑组织分界不清，梭形或星芒状肿瘤细胞呈束状、编织状或散在分布，胞核小而深染，间质富含胶原纤维和大量嗜碱性黏液样基质；免疫组化染色肿瘤细胞胞质表达波形蛋白，部分表达胶质纤维酸性蛋白、CD34、CD99，不表达细胞角蛋白、上皮膜抗原和S-100蛋白，病理诊断为“第三脑室脊索样胶质瘤”。首次术后7个月肿瘤复发，第2次术后组织学形态，肿瘤细胞上皮样特征明显，呈条索状、束状或编织状排列，胞核增大，核分裂象易见（8/10个高倍视野），部分肿瘤细胞胞质丰富红染、核偏位，呈横纹肌样；免疫表型同前，维持“脊索样胶质瘤”的诊断。第2次术后5个月患者死于肿瘤进展，结合文献补充免疫组化染色显示肿瘤细胞不表达Brachyury蛋白、甲状腺转录因子1、SMARCB1/INI-1蛋白；荧光原位杂交检测NR4A3（9q22.33）基因断裂信号阳性，最终明确诊断为骨外黏液样软骨肉瘤。结论 松果体区骨外黏液样软骨肉瘤极其罕见，应注意与松果体区好发的富含黏液肿瘤如脊索瘤、脊索样脑膜瘤和脊索样胶质瘤等相鉴别。明确诊断依靠组织学形态、免疫组化染色及分子检测。

关键词: 软骨肉瘤, 松果腺, 神经胶质瘤, 免疫组织化学