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15 July 2015 - Case #358

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Case #358

Clinical history:
A 85 year old woman presented with a 5.6 cm left pleural mass. The lesion later metastasized to the patient's right lung.

Microscopic images:

What is your diagnosis?

Click here for diagnosis and discussion:

Diagnosis: Dedifferentiated solitary fibrous tumor

Solitary fibrous tumor is a fibroblastic proliferation, arising from the mesenchymal cells of the pleura, usually due to recurrent NAB2::STAT6 gene fusions (Nat Genet 2013;45:180, Nat Genet 2013;45:131, Genes Chromosomes Cancer 2013;52:873, Diagn Pathol 2014;9:224). It typically is confined to the surface of the lung, arising from the visceral pleura (Surg Today 2016;46:335). Clinically, it is associated with pulmonary osteoarthropathy, digital clubbing and hypoglycemia, which regress after tumor resection. The hypoglycemia appears to be mediated by IGF2 production (Doege-Potter syndrome), causing these tumors to also be designated IGF2omas (J Clin Endocrinol Metab 2015;100:2519, Pneumologie 2013;67:340).

Grossly, it arises from the pleural surface by a pedicle, and appears as a solitary, well circumscribed and possibly encapsulated lesion. It is composed of dense, gray-white fibrous tissue with a firm, whorled cut surface, resembling uterine leiomyoma. Large tumors may be cystic and hemorrhagic. Microscopically, it contains fibroblast-like cells with variable cellularity in collagenous and keloid-like stroma, reticulin fibers and hemangiopericytoma-like vessels. Typically, there is no / rare mitotic activity and no atypia. Tumor cells are immunoreactive for CD34 (strong), CD99 and vimentin, and negative for cytokeratin, EMA, S100, smooth muscle actin and desmin.

Dedifferentiation has been described with an abrupt transition between low grade and high grade areas and loss of CD34 positivity (Am J Surg Pathol 2009;33:1314). Features of overt malignancy include increased cellularity, marked nuclear atypia, prominent necrosis and high mitotic activity (J Cardiothorac Surg 2014;9:113).

Benign appearing solitary fibrous tumors may recur locally, necessitating long term follow up, and rarely cause death due to extensive intrathoracic growth (Ann Thorac Cardiovasc Surg 2014;20 Suppl:441, Asian Cardiovasc Thorac Ann 2014;22:981). Malignant appearing tumors are associated with an aggressive clinical course and death in most cases (Am J Surg Pathol 2012;36:1202, Intern Med 2014;53:617).

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