-Spindle Cell Sarcoma, suggestive of dedifferentiated Liposarcoma.
This is a spindle cell neoplasm. It looks morphologically very bland with relative low cellularity and little nuclear atypia. Mitosis or obvious pleomoprhism are not easily appreciated.
Positive immunostains: Only vimentin and focal MDM2 (not shown in pictures).
Negative stains: SMA, CAM5.2, S100, AE1AE3, CD117, CK7, CD34, DOG1, CK7 and CK20.
Given the location and big size of this tumor, the most probable diagnosis is an area of dedifferentiated liposarcoma. CD34 negativity helps to rule out the possibility of Malignant Solitary fibrous Tumor. Negativity of CD34, CD117 and DOG1 argue against GIST. A good way to confirm the possibility of dedifferentiated liposarcoma is to request MDM2 by FISH.