History: A 44-year-old man presents for a chestradiograph following a positive purified protein derivative (PPD) skin test fortuberculosis. Patient has no respiratory or constitutional symptoms.
病史：44岁男性，因结核皮肤反应试验PPD阳性行胸部正侧位平片，患者无呼吸系统或全身症状。
Posteroanterior and lateral chest radiographs are shown below.
胸部后前位及侧位片如下所示。
Chest CT and MRI were attained for further characterization of the anterior mediastinal lesion seen on the radiographs.
平片发现前纵隔病变，进一步行胸部CT和MRI检查。
Findings 表现
Chest radiographs: Best seen on the lateral chest radiograph wall is a smoothly marginated anterior mediastinal opacity in the retrosternal clearspace, which measures up to approximately 4cm in length. The lungs are clear and without any evidence of tuberculosis. The right paratracheal stripe is within normal limits.
胸部平片：在侧位片上更易发现一边缘光滑的前纵隔阴影，位于胸骨后间隙内，病变长度约4cm，肺野清晰，未见肺结核相关表现。右侧气管旁线位置正常。
Chest CT: Contrast-enhanced axial and sagittal chest CT images demonstrate a well-circumscribed, oval, hypoattenuating anterior mediastinal lesion, corresponding to the abnormality seen on chest radiograph.The wall of the lesion is thin and noncalcified. The lesion is to the left ofmidline, anterior to the ascending aorta and main pulmonary artery. The adjacent fat planes between the lesion and the ascending aorta and main pulmonary artery are preserved. The lungs are clear.
胸部CT：轴位及矢状位胸部CT增强扫描可见一边界清楚，类圆形，低密度前纵隔病变，可对应胸片上发现的异常。病变壁薄、无钙化，位于中线左侧，位于升主动脉及肺动脉干前方，其间的脂肪间隙尚存在。肺野清晰。
Chest MRI: The well-circumscribed anterior mediastinal lesion seen on CT demonstrates homogeneous T2- and T1-hyperintense content, with a small amount of dependent T2-hypointense content, corresponding to layering debris. On the postgadolinium image, there is no significant mural enhancement and no enhancing mural nodularity or internal solid enhancing component.
胸部MRI：前纵隔病变边界清楚，呈均匀的T1及T2高信号，可见少量的T2低信号影沉积，与囊内碎屑层一致。增强扫描囊壁无明显强化，未见强化的壁结节或内部实性强化成分。
Differential diagnosis：
- Thymic cyst
- Lymphangioma
- Thymoma
- Teratoma
- Lymphoma
- Pericardial cyst
鉴别诊断：
- 胸腺囊肿
- 淋巴管囊肿
- 胸腺瘤
- 畸胎瘤
- 淋巴瘤
- 心包囊肿
Diagnosis: Thymic cyst
诊断：胸腺囊肿
Key points
Thymic cyst
The etiology of a thymic cyst can be congenital or acquired.
胸腺囊肿的病因：先天性或后天的。
Congenital: Cysts are more often unilocular and develop from thymopharyngeal duct components.
先天性：囊肿通常为单房，起源于胸腺咽管的组成部分。
Acquired: Cysts are more often multilocular and develop from infection/inflammation, resulting in cystic dilation of Hassall\’s corpuscles. Iatrogenic following thoracotomy or radiation therapy.
后天的：囊肿通常为多房，由于感染或炎症导致胸腺小体的囊样扩张；医源性原因有开胸术或放疗。
Natural history: 自然病程
Cysts can undergo spontaneous hemorrhage.
囊肿可自发性出血。
Cysts can become secondarily infected or arise as a result of inflammatory/infectious processes.
囊肿可继发感染，也可起源于炎性病变。
Very rare cases of malignant degeneration have been reported.
恶变罕有报道。
Follow-up cross-sectional imaging can be acquired in cases in which the distinction between thymic cyst versus thymoma is unclear.
部分病灶为了鉴别胸腺囊肿和胸腺瘤，需通过CT或MR进行随访。
Epidemiology 流行病学
Congenital cysts tend to occur in a younger population (10s to 20s).
先天性囊肿多见于年轻人（十几岁至二十几岁）。
Thymic cysts constitute approximately 3% of anterior mediastinal masses.
胸腺囊肿在前纵隔肿物中占大约3%。
Clinical presentation 临床表现
Thymic cysts are asymptomatic, incidental imaging findings.
胸腺囊肿多无症状，影像检查偶然发现。
Imaging features 影像表现
Chest radiograph: A thymic cyst appears as a well-circumscribed opacity in the anterior mediastinum.
胸片：胸腺囊肿表现为前纵隔阴影，边界清楚。
Chest CT: 胸部CT
Appears as an anterior mediastinal oval lesion.
表现为前纵隔类圆形肿物。
The cysts have thin walls (noting that acquired cases can have thicker walls from inflammation), without nodularity/solid component.
囊肿壁薄（值得注意的是后天的囊肿由于合并炎症，可囊壁较厚），无壁结节和实性成分。
Most cysts have fluid attenuation, but proteinaceous or hemorrhagic contents can have higher attenuation.
大部分囊肿为液性密度，当含蛋白或合并出血时可表现为较高密度。
There is no infiltration of adjacent mediastinal fat.
邻近纵隔脂肪无浸润。
Chest MRI: 胸部MRI
T2-weighted imaging: Homogeneously hyperintense contents.
T2WI：均匀高信号。
T1-weighted imaging: Variable signal depending on presence of proteinaceous or hemorrhagic content (which will be hyperintense).
T1WI：信号可变，可由于含蛋白或合并出血表现为高信号。
Postgadolinium: No enhancing mural or internal solid component.
增强扫描：无强化壁结节或内部实性成分。
对于本例而言，鉴别诊断还需要考虑到：支气管囊肿。大部分支气管囊肿位于中后纵隔，但少数也可位于前纵隔。