• Extremely rare mesenchymal tumor of intermediate malignancy and uncertain line of differentiation; characterized by peripheral metaplastic bone and cords of bland, round cells in a fibromyxoid matrix (Stanford University)
• First described in 1989 (Am J Surg Pathol 1989;13:817), now 300+ cases reported (Am J Surg Pathol 2008;32:996)

• Adults, median age 50 years (range 14-83 years), no sex predilection
• Presents with small painless mass, most commonly lower extremity, but in any soft tissue location including head and neck
• Usually histologically benign with benign clinical course; local recurrences in 17%, malignant behavior in 5%
• Histologically malignant OFMTs are high-grade sarcomas with distant metastases and adverse outcome in up to 60% of patients
• May have neuroectodermal origin due to CD56+, CD99+ (Int J Surg Pathol 2007;15:437, Am J Surg Pathol 2011;35:1615)

• Poor prognostic factors: high cellularity, high nuclear grade, > 2 mitotic figures/50 HPF (Am J Surg Pathol 2003;27:421)

• 21 year old woman with gingival mass (J Periodontol 2009;80:687)
• 73 year old male with left scapular mass (BMJ Case Rep. 2011 Mar 24;2011)
• Invasive spinal tumor (Skeletal Radiol 2008;37:1137)

• Excision

• Well circumscribed, median 4 cm, usually involves deep soft tissue, also cutaneous (Am J Dermatopathol 2007;29:156) or subcutaneous (J Cutan Pathol 2006;33:749) with radiographically apparent shell of bone

• Nests/cords of round/oval cells, primarily epithelioid, with indistinct cytoplasm in fibromyxoid to hyalinized matrix
• Lobulated at low power
• Surrounded by partial shell of lamellar and woven bone and collagenous pseudocapsule
• Minimal atypia and minimal mitotic figures, but may have necrosis, vascular invasion or high nuclear grade
• Prominent spindling or extensive stromal hyalinization may be present

Various images

• Clusters, cords or small aggregates of round, polygonal or spindle cells in myxoid background with osteoid-like material (Diagn Cytopathol 2004;30:41)
• Malignant cases may show significant nuclear pleomorphism with coarse chromatin, irregular contours and 1-2 distinct nucleoli (Acta Cytol 2001;45:745)

• S100 (73%), desmin (38%), cytokeratin (11%), EMA (16%), SMA (6%), INI1 (lost in mosaic pattern in 74%), MUC4 (21%), neurofilament (80%) CD56 (43%) (Am J Surg Pathol 2011;35:1615)

• Alpha smooth muscle actin (usually, may be weak, J Laryngol Otol 1993;107:75)

• Complex cell processes, reduplicated basal lamina (Ultrastruct Pathol 2007;31:233)

• Cytogenetics: PHF1 gene rearrangement (6p21) (Am J Pathol 2012;181:1069); monosomy 22 can be found in malignant form; may also have complex aberrations (Cancer Genet Cytogenet 2007;176:156, Cancer Genet Cytogenet 2002;133:124)
• Molecular: INI1 gene deletion via FISH in 71% (Am J Surg Pathol 2011;35:1615)

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