The histological features of the current case has the typical microscopic features of so-called "sclerosing hemangioma" of the lung. Four different histological patterns are described including solid, papillary, sclerotic and hemorrhagic regions:

1Papillary pattern: complex papillae lined by cuboidal surface cells. The stalk of the papillary projections contains the round cells. It can be sclerotic or occasionally myxoid.

2Sclerotic pattern:  dense foci of hyaline collagen at the periphery of the hemorrhagic areas, within papillary stalks, or within the solid areas.

3Solid pattern: sheets of round cells, with scattered cuboidal surface cells forming small tubules. The predominant cell type consists of sheets of round cells with relatively uniform nuclei and eosinophilic cytoplasm. Superimposed clear cell change may also be found in these regions. The nuclei display occasional folds and grooves, and mitotic activity is not seen. In other hand, the cuboidal surface cells lining the surface of the papillae contain abundant vacuolated, foamy cytoplasm and round nuclei that may have intranuclear inclusions

4Hemorrhagic pattern: large blood-filled spaces lined by epithelial cells or foci of hemorrhage and debris containing hemosiderin deposits, foamy macrophages, and cholesterol clefts rarely surrounded by granulomatous and chronic inflammation.

Round cells express thyroid transcription factor-1 (TTF-1) and epithelial membrane antigen (EMA), but are pancytokeratin negative. Cuboidal surface cells express TTF-1, EMA, surfactant apoprotein A and pancytokeratin. Antibodies against TTF-1 have been used to demonstrate that both cell types are of pulmonary epithelial origin, while the stains for thyroglobulin are uniformly negative. EMA is also expressed by both cell types, although somewhat less intensely by the round cells. On the basis of the female predominance, studies have been done to find estrogen and progesterone receptors. Although progesterone receptor expression has been detected, estrogen receptor expression has been generally only focally and faintly stained by immunohistochemical examinations.

Differential Diagnosis:

 The differential diagnosis includes clear cell tumors involving the lung (metastatic renal cell carcinoma, clear cell sugar tumors, and clear cell carcinomas of the lung), carcinoid tumors, papillary pulmonary epithelial neoplasms and inflammatory pseudotumors. Benign clear cell tumors differ from this tumor in that the tumor cells have clear, glycogen-rich cytoplasm and the delicate vascular spaces are thin-walled and sinusoidal. Sclerosing hemangiomas differ from papillary carcinomas by the presence of the distinct round cells within the papillary stalks as well as the characteristic solid, hemorrhagic or sclerotic patterns. Low-grade mucoepidermoid carcinomas differ from sclerosing hemangiomas in that an intimate admixture of mucinous and squamous epithelium. Intravascular sclerosing bronchoalveolar tumor (IVBAT) differs from this tumor by the relative circumscription of the lesion, the absence of alveolar filling by the cells at the periphery of the lesion, and the character of the cells themselves. Metastatic renal cell carcinoma may have tumor cells with clear cytoplasm which can resemble sclerosing hemangiomas; however renal cell carcinoma often shows malignant cytological features, and generally lacks the distinct various patterns seen in sclerosing hemangiomas. Sclerosing hemangiomas differ from carcinoid tumors in that they lack the organoid, trabecular, rosette, or spindle cell histological patterns and they do not have neuroendocrine features by immunohistochemtstry. Sclerosing hemangiomas differ from inflammatory pseudotumor which lacks the distinct epithelioid cells, papillary growth, and vascular spaces of sclerosing hemangiomas but consists primarily of a mass of inflammatory and fibrous tissue.

Pulmonary nodules are frequently first diagnosed by frozen section, immediately followed by lobectomy or other procedures. The frozen section diagnosis of pulmonary nodules is sometimes very difficult, especially in case of this tumor. It should be noted that the mesenchymal element of this tumor is key to the diagnosis.