Thymoma Metastasis to the Semimembranosus Muscle

Liver Metastasis of Thymoma: Case Report and Review of the Literature

The 2021 "World Health Organization (WHO) Classification of Thoracic Tumours" classifies epithelial tumors of the thymus (thymomas) based on cytomorphology. Thymomas with benign cytomorphology are classified as type A, AB, B1, B2, and B3, while those with malignant cytomorphology are classified as thymic carcinoma. Although all thymomas have malignant potential, extra-thoracic metastasis of thymomas is exceedingly rare and the exact incidence is not known. Literature review demonstrated 39 cases of thymoma with extra-thoracic metastases reported since the publication of the 1999 WHO Classification of Thoracic Tumours. Nine of these cases presented with metastasis to the liver in the setting of concurrent metastasis to other organs, while only three cases metastasized solely to the liver. We herein report a well-documented case of type B1 thymoma with relatively limited stage (IIb) with an isolated, single liver metastasis occurring seven years after primary resection in a patient with concomitant myasthenia gravis. The following report includes a review of the literature, a discussion of thymoma classification and its relevance to prognosis, and an overview of other extra-thoracic metastatic thymoma cases.