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8 April 2015 - Case #348

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Case #348

Clinical history:
A 56 year old man presented with a 5 cm painless mass in his thigh, which was surgically excised. Grossly, the mass was well circumscribed with an osseous rim.

Microscopic images:



What is your diagnosis?

Click here for diagnosis and discussion:


Diagnosis: Ossifying fibromyxoid tumor of soft parts

Discussion:
Immunohistochemistry on the lesional cells showed focal S100, neurofilament and desmin positivity. IHC and molecular studies for INI1 were not performed (unfortunately).

Ossifying fibromyxoid tumor of soft tissue is a rare mesenchymal tumor of intermediate malignancy characterized by peripheral metaplastic bone and cords of bland, round cells in a fibromyxoid matrix. It was first described in 1989 by Enzinger, Weiss and Liang as a tumor of primarily adults with a male predominance, presenting as a small, painless, well circumscribed mass in subcutis or muscle, typically in the extremities, trunk or head and neck region (Am J Surg Pathol 1989;13:817).

Grossly, the tumors have a median size of 3 - 5 cm (range 1 to 21 cm). They classically have lobular architecture, corded and trabecular growth patterns, moderate amounts of myxocollagenous matrix and often focal peripheral metaplastic bone formation (Am J Surg Pathol 2008;32:996). The tumor cells usually have indistinct cytoplasm, vesicular nuclei, no / mild atypia and no / minimal mitotic activity.

Tumors are typically S100+, with occasional focal positivity for desmin, GFAP and AE1 / AE3. INI1 is lost in a mosaic pattern in 74% and FISH shows INI1 deletion in 71% (Am J Surg Pathol 2011;35:1615). Up to 85% demonstrate recurrent gene rearrangements, typically PHF1 (49 - 80%) (Am J Surg Pathol 2013;37:1751, Genes Chromosomes Cancer 2014;53:183). Similar gene fusions have been found in endometrial stromal sarcoma.

Cases have been divided into typical, atypical and malignant, using the Folpe and Weiss system, based on high cellularity, high nuclear grade or > 2 MF/50 HPF (Am J Surg Pathol 2003;27:421). Excision of typical and atypical cases is usually curative. Malignant cases are associated with local recurrence, metastases or death (Am J Surg Pathol 2011;35:1615).


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