AB047. The challenges in the diagnosis of sarcomatous mesothelioma—a case report

Background: To report the case of a male patient, 65, who presented with a mass of the left lower lobe of the lung, incidentially found during a CT scan of the thorax.

Methods: At first, we received bioptic material, following CT-guided biopsy, and, secondly, we received a surgical specimen of left lower lobectomy of the lung.

Results: The bioptic material consisted of 2 tissue cylinders, 1.6 cm in total length. The microscopic examination revealed scattered cells with severe atypia, within desmoplastic stroma. Necrotic areas and an increased number of mitoses were observed. Immunohistochemistry showed a positive expression for CK8/18, CK7 and Vimentin, while Calretinin and WT-1 were only lightly and focally expressed. Negative staining was observed for CK5/6, CD56, TTF-1, BerEP4 and Napsin. The surgical specimen consisted of a lung lobe, 10×8×8 cm, with a peripheral mass, 3.5 cm in greatest diameter. The microscopic study confirmed the presence of a malignant neoplasm, consisting mainly of spindle cells, with pleomorphy and nuclear atypia, in compact growth pattern. Necrosis was present. The neoplasm infiltrated the pulmonary parenchyma, as well as the skeletal muscles and fat tissue of the parietal pleura. An inflammatory stromal reaction involved mainly lymphocytes and plasmocytes. The immunohistochemical findings were similar to the previous ones, with the exception of Calretinin and WT-1, which were better expressed this time. The proliferation marker Ki67 was positive in 30% of the neoplastic cells. According to these findings, the diagnosis was sarcomatous mesothelioma.

Conclusions: The diagnosis of sarcomatous mesothelioma, especially in bioptic tissue material, can be problematic. Immunohistochemical stains that are frequently used for epithelial mesothelioma, may not have the same results for the sarcomatous subtype. The differential diagnosis includes a malignant mesenchymal neoplasm (sarcoma) and sarcomatoid carcinoma. Positive cytokeratin staining is usually considered necessary for the diagnosis of mesothelioma. The mesothelial markers Calretinin and WT-1 are not so intensely expressed in cases of sarcomatous subtype, compared to the epithelial subtype, and may as well be positive in sarcoma or sarcomatoid carcinoma. The combination of immunohistochemistry, morphology and clinical-imaging findings should be estimated for the final diagnosis.

Keywords: Immunohistochemical findings; clinical findings; imaging findings