The patient had suffered from shortness of breath two months earlier. The current CT was performed when he came to the ER with exacerbation of shortness of breath and fatigue, several weeks after he had undergone a pleural biopsy (initial CT unavailable). 

Histopathology:

Right pleura, CT-guided core needle biopsy:
Cores of spindle cell lesion. The spindle cells are moderately pleomorphic and arranged in a fascicular pattern. Occasional mitotic figures are present as well as areas of necrosis. Mostly round but also irregular large blood vessels are present. On immunostains, part of the spindle cells are positive for MNF116, AE1/AE3 and vimentin. The CD99 and TLE1 are weakly positive in most of the cells. The BAP1 shows preserved diffuse nuclear staining. The calretinin is equivocally positive (probably negative). The CD34 and CD31 highlight the blood vessels. The CD3 and CD5 show a small amount of mature small T lymphocytes. The CAM5.2, CK7, CK20, CK5/6, p63, p40, S-100, SOX10, TTF1, PAX8, D2-40, WT1, c-kit, DOG1, CD20, CD79a, TdT, CD1a, SALL4, STAT6, bcl-2, desmin, SMA and MUC4 are negative.
Fluorescence in situ hybridization (FISH) study shows deletion of 9p21 locus in 16% of the cells (in addition to 26% of monosomy of chromosome 9) and absence of SS18 gene rearrangements.

Discussion:
The presence of pleomorphism, mitotic activity and necrosis favor the possibility of a
malignant spindle cell lesion. The differential diagnosis includes: poorly differentiated carcinoma, sarcomatoid mesothelioma, thymoma type A, and sarcoma (including synovial sarcoma). The possibility of thymoma type A is less likely given mostly the pleomorphism of the cells. Given the weak positivity for TLE1 and the absence of SS18 gene rearrangements, the possibility of synovial sarcoma is excluded. However, the option of another kind of sarcoma with aberrant cytokeratin expression cannot be ruled out. The differentiation between undifferentiated carcinoma and sarcomatoid mesothelioma is very challenging in this case. As per radiological information, the tumor encroaches on the right lung which is clinically suspicious for mesothelioma. Moreover, all the additional mesothelial markers (calretinin, CK5/6, CK7, D2-40, WT1) are either negative or non-conclusive which can be seen in the sarcomatoid variant. However, the BAP1 immunostain is retained and the amount of cells with deletion of 9p21
locus is not enough to support this diagnosis.

Conclusion:
This is a malignant spindle cell neoplasm. The differential diagnosis is between sarcomatoid mesothelioma and undifferentiated carcinoma. The radiographic features favor the possibility of mesothelioma. However, the immunophenotypical and cytogenetic features are not conclusive in this regard.