KRAS mutation-positive mucinous adenocarcinoma originating in the thymus

Thymic carcinoma is a rare, aggressive disease with a low 5-year survival rate. The most common histological neoplastic thymic tumor subtype is squamous cell. We describe an interesting case of a 39-year-old woman who presented with mucinous adenocarcinoma that originated in the thymus and was treated via radical resection and venoplasty of the superior vena cava (SVC). Macroscopically, the resected tumor contained a solid region and multiple cysts with abundant mucin. Microscopic examination showed a papillary growth pattern of goblet cells with round nuclei. Based on the histopathological and immunohistochemical findings and other inspections, the tumor was eventually diagnosed as a mucinous adenocarcinoma of the thymus. It was classified as Masaoka-Koga stage III owing to tumor invasion into the left brachiocephalic vein and pericardium. Polymerase chain reaction identified a Kirsten rat sarcoma viral oncogene homolog (KRAS) G12V mutation in the tumor. There were no mutations in the epidermal growth factor (EGFR) gene or a fusion gene of the echinoderm microtubule-associated protein-like 4 (EML4) and the anaplastic lymphoma kinase (ALK). A year later, multiple lung metastases were detected, and the patient underwent chemotherapy. She is alive 34 months after the initial surgery. This is the first report of a KRAS mutation-positive mucinous adenocarcinoma originating in the thymus. The treatment, diagnosis, and pathological findings of the patient are discussed.