PSH is an uncommon lung tumor initially thought to be of vascular origin
. However, immunohistochemical and molecular findings strongly suggest that PSH originates from primitive respiratory epithelium
[4, 5, 8]. As an epithelial tumor, PSH is histologically distinctive. It is characterized by two cell types, namely cuboidal cells and polygonal cells, which significantly differ in histological patterns, cellular morphologies, and immunophenotypes. However, in our previous study
, we performed clonality analysis using the X-linked androgen receptor and phosphoglycerate kinase gene polymorphisms, and the results revealed that the cuboidal cells and polygonal cells had the same cellular origin. The two cell types form a distinct constellation of four architectural patterns including papillary, sclerotic, solid, and hemorrhagic pattern. The correct diagnosis is easily made if one is familiar with the two cell populations and four architectural patterns. Herein, we present a peculiar case of PSH which was predominantly composed of spindle cells. The spindle cells were arranged into solid sheets. Focally, the cells displayed bundles or whirling pattern. The presence of diffuse spindle cells in this case may cause a great diagnostic confusion.
It was well documented that both the cuboidal cells and polygonal cells showed reactivity for TTF-1 and EMA in nearly all cases of PSH. In contrast to the strong expression of CK, CK7, SPA and SPB in cuboidal cells, the polygonal cells usually lack strong expression of the above markers, but show diffuse and strong positivity for Vimentin, and rarely for Actin(SM)
. In our case, the cuboidal cells and polygonal cells show consistent staining results with the reports in the literatures
. Similar to polygonal cells, the spindle cells were diffusely positive for TTF-1, EMA, and Vimentin, focally positive for Actin(SM); this indicates that the spindle cells may be simply the variants of polygonal cells. However, it is unclear why and when the polygonal cells display the spindle cells change. Moreover, the significance of spindle cells change is also unclear to us.
Although many scholars believe that the tumor is benign, there are reports of lymph node metastasis
[10, 11], pleural dissemination
, and metastatic spread to the stomach
. So, some authors advocate it may belong to be a tumor with low malignant potential
. Dacic et al.
 proposed that PSH has a similar molecular pathogenesis as bronchioloalveolar carcinoma, so might have a biological behavior similar to BAC. In our case, the size of the tumor increased slowly, no lymph node metastasis was found. But, it is essential for long-term follow-up to further elucidate the clinical behavior of this tumor and significance of spindle cells change.
Because of the extensive presence of spindle cells, a series of mesenchymal tumors including inflammatory myofibroblastic tumor, synovial sarcoma, solitary fibrous tumor, leiomyoma and mesothelioma should be excluded in this case. In addition, the differential diagnosis should also include some uncommon tumors, such as pulmonary epithelioid hemangioendothelioma
. Inflammatory myofibroblastic tumor is characterized by the admixture of spindle-shaped and ovoid cells with a prominent inflammatory infiltrate
. It can also show expression of Actin(SM), but the majority also exhibits the reactivity for ALK. Synovial sarcoma is usually composed of two morphologically different types of cells, epithelial cells and spindle cells
. Imunohistochemically, the presence of CK and CD99 expression favor the diagnosis of synovial sarsarcoma. Solitary fibrous tumor usually involves the pleura, and is characterized by the random arrangement of the tumor cells and striking hyalinization
. The lack of CD34 and CD99 expression in our case can also help to exclude this tumor. Mesothelioma is chracterized by a mixture of epithelial and sarcomatous components with marked hyperchromasia and nuclear pleomorphism
. Mesothelioma is usually positive for CK5/6 and Caretinin. Leiomyoma can also be ruled out for lacking Desmin and strong Actin(SM) expression. In addition, all the above tumors were negative for TTF-1, which is a useful marker for differential diagnosis.