Malignant Transformation of Metaplastic Thymoma into High-Grade Sarcomatoid Carcinoma: A Case Report

Abstract

The correlation of histogenesis between metaplastic thymoma and thymic sarcomatoid carcinoma is unclear. We report a case of metaplastic thymoma transformed into high-grade sarcomatoid carcinoma. A 64 × 54 × 32 mm anterior mediastinal mass in a 61-year-old woman microscopically consisted mainly of classic metaplastic thymoma, with the center dominated by high-grade sarcomatoid carcinoma. In some areas, both epithelial and spindle cell components of the metaplastic thymoma showed increased cellular atypia, mitotic activity, and focal necrosis and gradually transformed into the polygonal/pleomorphic and spindle cell components of sarcomatoid carcinoma. Immunohistochemically, the characteristics of the polygonal/pleomorphic sarcomatoid cells were similar to those of the epithelial component of metaplastic thymoma, while the spindle sarcomatoid cells were more similar to the spindle cells component of metaplastic thymoma. The Ki-67 index was less than 5% in the metaplastic thymoma areas but up to 70% in the sarcomatoid carcinoma area. Radical operation and postoperative radiotherapy were performed. Multifocal relapses at the pleura occurred 13 months after surgery.

A Rare Case of Metaplastic Thymoma Presenting With Myasthenia Gravis

“…Most of the patients had an excellent outcome, except for those tumors demonstrating sarcomatous component or transformation. 7,9,15 Previously described as thymoma with pseudosarcomatous stroma, low-grade metaplastic carcinoma, biphasic thymoma, mixed polygonal and spindle cell type, the WHO classification of thoracic tumors recognized metaplastic thymoma as its own entity in 2004. 4 The diagnosis is based on histologic findings of a biphasic tumor with islands of polygonal thymic epithelial cells separated by abundant spindle cells with no cytologic atypia.…”

“…Unless there is sarcomatoid transformation, as it has been described in 3 cases previously mentioned, there is no cytologic atypia, increased mitotic activity, or areas of necrosis. 7,9,15 The origin of the spindle cells is unclear, and whether they are simply stromal cells or the result of a mesenchymal transformation of the epithelioid tumor cells is debated. In 2012, Biao Liu et al considered the spindle cell component to be a mesenchymal metaplasia of tumor cells through a metaplastic phenomenon known as epithelial-mesenchymal transition (EMT).…”

“…Most of the patients had an excellent outcome, except for those tumors demonstrating sarcomatous component or transformation. 7,9,15…”

Loss of YAP1 C‐terminus expression as an ancillary marker for metaplastic thymoma: a potential pitfall in detecting YAP1::MAML2 gene rearrangement

“…[6][7][8] Most reported metaplastic thymomas follow a benign to indolent clinical course, with only rare reports of recurrence 2 or malignant transformation to sarcomatoid carcinoma. [9][10][11] Histologically, the most important differentiation from metaplastic thymoma is type A thymoma and sarcomatoid carcinoma. 5,12 Type A thymoma may exhibit variable epithelial cell growth patterns that mimic a biphasic growth pattern, whereas sarcomatoid carcinoma can show variable spindle and epithelial cell components with significant nuclear atypia.…”

“…Patients rarely present with myasthenia gravis 6–8 . Most reported metaplastic thymomas follow a benign to indolent clinical course, with only rare reports of recurrence 2 or malignant transformation to sarcomatoid carcinoma 9–11 . Histologically, the most important differentiation from metaplastic thymoma is type A thymoma and sarcomatoid carcinoma 5,12 .…”