Pleural disseminated cells mimicking malignant mesothelioma or adenocarcinoma metastasized from tongue squamous cell carcinoma: A case report and literature review

It is well known that malignant tumors often dedifferentiate along with invasion and metastasis. Marked dedifferentiation in the distal metastasis makes it difficult to determine whether the metastatic tumor is from known primary origin, from another unknown primary origin, or it is not metastasis but independent primary tumor. Here we present a case of tongue squamous cell carcinoma (SCC) accompanied by pulmonary lymphangitic carcinomatosis (PLC) and carcinomatous pleuritis (CP) showing marked dedifferentiation. A 65-year-old man visited our hospital with complaint of lingual pain. The ulcered induration was histologically diagnosed as well differentiated SCC. After the tumor resection and neck dissection, symptoms and imaging test suggested PLC and CP. Cell block specimens of the pleural effusion exhibited atypical round cells mimic malignant mesothelioma or signet ring cell carcinoma within numerous poorly differentiated cells and undifferentiated cells. Histological diagnosis of these atypical cells was challenging. Immunohistochemical examination according to World Health Organization (WHO) classification of tumors of the lung and pleura revealed that they were positive for AE1/AE3, CK5/6, D2–40 and negative for calretinin, WT1, MOC31, CEA, BerEP4. These findings indicated that pleural disseminated cells were derived from tongue SCC. The pleural disseminated cells in this case were difficult to determine the cell origin due to marked dedifferentiation. It would be impossible to make correct diagnosis without information of clinical course and immunohistochemical examination. We have to keep in mind that there is unexpected dedifferentiated change associating with tumor metastasis.