Thymic mucoepidermoid carcinoma (MEC) is a rare tumor, and its characteristics remain to be clarified. Here we investigated 20 cases of thymic MEC to systematically characterize its clinical, histopathologic, and molecular features. The median age of the patients was 56 years (range, 19 to 80 y), there was a slight male predilection (3:2), and 44% of the patients were asymptomatic at diagnosis. The median tumor size was 6.8 cm in diameter, 55% were pT1 tumors, and 50% were TNM stage I tumors. When 4 tumor grading systems for salivary MEC (Armed Forces Institutes of Pathology, Brandwein, modified Healey, and the Memorial Sloan-Kettering) were employed, low-grade, intermediate-grade, and high-grade tumors accounted for 35% to 70%, 5% to 25%, and 25% to 50%, respectively. Many histologic variants were noted, and 70% of the cases were classified as nonclassic variants. MAML2 rearrangement was detected in 56% of cases, and the fusion partner was CRTC1 in all cases. CRTC1-MAML2 fusion was associated with lower pT classification and lower TNM stage. The overall survival rate of all patients was 69% and 43% at 5 and 10 years, respectively. Worse overall survival was associated with higher pT stage, higher TNM stage, residual tumors, greater tumor size, high-grade tumor histology (Armed Forces Institutes of Pathology and Memorial Sloan-Kettering, but not the other 2), and with the absence of CRTC1-MAML2 fusion. Of note, none of the patients with CRTC1-MAML2 fusion-positive tumors died during the follow-up. In conclusion, the clinicopathologic and molecular findings of thymic MEC presented here are expected to contribute to the management of this rare tumor.

胸腺粘液表皮样癌（MEC）是一种罕见的肿瘤，其特征仍有待阐明。在这里，我们研究了 20 例胸腺 MEC，系统地描述了其临床、组织病理学和分子特征。患者的中位年龄为 56 岁（范围为 19 至 80 岁），男性稍有好发性（3:2），44% 的患者在诊断时无症状。肿瘤大小中位直径为 6.8 cm，55% 为 pT1 肿瘤，50% 为 TNM I 期肿瘤。当采用4种唾液MEC肿瘤分级系统（武装部队病理学研究所、Brandwein、改良Healey和纪念斯隆-凯特林）时，低级别、中级别和高级别肿瘤占35%至70% 、5%至25%、25%至50%。许多组织学变异被注意到，70% 的病例被归类为非经典变异。56% 的病例中检测到MAML2重排，所有病例的融合伴侣均为CRTC1 。CRTC1-MAML2融合与较低的 pT 分类和较低的 TNM 分期相关。所有患者的 5 年和 10 年总生存率分别为 69% 和 43%。较差的总生存率与较高的 pT 分期、较高的 TNM 分期、残余肿瘤、较大的肿瘤大小、高级别肿瘤组织学相关（武装部队病理学研究所和纪念斯隆-凯特琳研究所，但与其他两个研究所无关），并且与缺乏CRTC1-MAML2融合。值得注意的是， CRTC1-MAML2融合阳性肿瘤患者在随访期间没有死亡。总之，本文提出的胸腺 MEC 的临床病理学和分子学发现有望有助于这种罕见肿瘤的治疗。