35 y/o female athlete present with a growing mass in her right proximal thigh. An MRI with contrast was performed that showed a 9.2 x 7.0 x 8.2 cm right gluteal soft tissue mass with intermediate signal on T1 sequences isointense to muscle, heterogeneously bright signal intensity on T2 and avid contrast enhancement. Some areas of central nonenhancement are present in the anterior half of the mass as may be seen with necrosis.
The lesion involves the right gluteus maximus, right gluteus medius and subcutaneous tissues with signal abnormality along the posterior right iliac wing suspicious for involvement by the mass and extension into the bone.
A biopsy of the mass was performed (see H&E images). A panel of immunohistochemistry showed only CD34 and CD99 positivity; Desmin, S-100, ALK-1, pancytokeratin, EMA, SMA, MyoD1, CD117, STAT6, INI-1, SOX10 and MDM2 are negative. A sarcoma fusion NGS panel was ordered. A fusion was found, and the patient began treatment with Larotrectinib with good clinical response.
Which gene fusion is likely to have been found?
a. ESWR1-WT1
b. NAB2-STAT6
c. FUS-DDIT3
d. STRN-NTRK2
e. MYH9-USP6
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