Thymic carcinoma

Thymic carcinoma is a part of the malignant spectrum of thymic epithelial tumors, along with malignant thymomas and neuroendocrine carcinomas.

Epidemiology

Patients are typically 50 to 70 years of age at presentation ⁹.

Pathology

The incidence of paraneoplastic syndromes is thought to be low. At least 10 different histologic variants have been described ⁴. The most common subtypes are squamous cell carcinoma and lymphoepithelial-like carcinoma ¹.

Classification

The WHO classification of thymic tumors (5th ed.) divides thymic carcinomas into the following morphological entities ¹⁰:

• squamous cell carcinomas

  • squamous cell carcinoma, not otherwise specified

  • basaloid carcinoma

  • lymphoepithelial carcinoma

• NUT carcinoma of the thorax

• salivary gland-like carcinomas

  • mucoepidermoid carcinoma

  • clear cell carcinoma

  • sarcomatoid carcinoma

• adenocarcinomas

  • adenocarcinoma, not otherwise specified

  • low-grade papillary adenocarcinoma

  • thymic carcinoma with adenoid cystic carcinoma-like features

  • enteric-type adenocarcinoma

• adenosquamous carcinoma

• undifferentiated carcinoma

• thymic carcinoma, not otherwise specified

​The category of thymic carcinoma NOS includes entities also known as hepatoid carcinoma, rhabdoid carcinoma, undifferentiated large cell carcinoma associated with Castleman disease-like reaction, and sebaceous carcinoma, although these fall under the same morphology code in this classification. Carcinosarcoma is a subtype of sarcomatoid carcinoma.

Radiographic features

CT

Useful features for differential from more benign thymic epithelial tumors include ¹:

• larger ⁵ and highly aggressive anterior mediastinal mass

• areas of necrosis, hemorrhage, calcification, or cyst formation

• gross invasion of contiguous mediastinal structures and wide spread to involve distant intrathoracic sites

• high incidence of extrathoracic metastases

Nuclear medicine

FDG PET-CT may be useful in differentiating thymic carcinoma from other thymic neoplasms, thymic hyperplasia, and normal physiologic uptake. The standardized uptake value (SUV) for thymic carcinoma is considered to be significantly greater than that for invasive or noninvasive thymoma, often with an SUV cutoff point of 5.0, thymic carcinoma can be differentiated from thymoma with reasonably high sensitivity (84.6%), specificity (92.3%), and accuracy (88.5%) ⁶. 

Treatment and prognosis

They are often associated with a poor prognosis.

Differential diagnosis

For an invasive anterior mediastinal mass lesion consider:

• aggressive mediastinal germ cell tumor

• primary mediastinal lymphoma with invasive spread: lack infiltration

• lung cancer with mediastinal invasion