Intrapulmonary Biphasic Mesothelioma Misdiagnosed as Adenocarcinoma: Case Report and a Potential Diagnostic Pitfall.

Background: Mesothelioma is an uncommon malignant tumor with variable clinical presentations, radiological features, and morphological patterns. Mesothelioma with predominantly intrapulmonary growth presents with an insidious onset, similar radiological and even morphological features to lung cancer, and poses a diagnostic pitfall.

Case presentation: Herein, we reported a 53-year-old female with biphasic mesothelioma misdiagnosed as poorly differentiated adenocarcinoma with focal sarcomatoid carcinoma. Computed tomography (CT) scan of the chest at the first visit revealed a solid lobulated nodule in the basal segment of the lower lobe of the right lung, which was suspicious of lung cancer. Microscopically, the tumor was composed of epithelioid and spindle cells, both of which were diffusely and strongly positive for CK7, and negative for TTF-1, Napsin A, P40, Melan A, S-100, SMA, and CD34. It was originally misdiagnosed as poorly differentiated adenocarcinoma with focal sarcomatoid carcinoma at initial presentation. Until her second admission with the discovery of a nodule in the right diaphragmatic angle, the peculiar location and biphasic component reminded us of biphasic mesothelioma. Immunohistochemically, tumor cells in both pulmonary and diaphragmatic nodules were positive for calretinin, D2-40, and WT-1, but negative for BerEP4 and MOC31. The patient was treated with a chemotherapy regimen of pemetrexed and carboplatin. After 11 months of follow-up, the patient recovers well without recurrence or metastasis.

Conclusion: Mesothelioma with predominantly intrapulmonary growth is extremely rare and poses a diagnostic pitfall. For this entity, subtle morphological features, selection of immunohistochemical markers, and electron microscopy are of great significance for definite diagnosis.