Case 747 -- A 42 year old woman with a posterior knee mass

Contributed by Christopher C. Griffith, MD, PhD and Karen Schoedel, MD


A 42 year old previously healthy woman presented with an enlarging soft tissue mass in the popliteal fossa of the left posterior knee. The mass had been noticed 4 months prior and recently began to interfere with flexion of the joint. No preceding trauma was noted.

On physical examination a 3.5 x 3 cm mass was noted in the popliteal space which extended to but was not fixed to the skin. There was fixation to deeper structures. The mass was firm but ballotable.

An MRI showed what appeared to be a fluid-filled mass in the subcutaneous or fascial area. There was no connection to the joint space. A schwannoma was included in the differential diagnosis.


At operation for excision of the mass an intraoperative consultation was requested for further histological characterization of the lesion, specifically the question of a sarcoma was the primary concern.

A 4.0 x 3.0 x 3.0 cm firm, somewhat gelatinous, multilobulated mass was received in the frozen section room. Sectioning revealed two components to the mass with a larger portion of the mass composed of brown-red gelatinous tissue and the smaller portion composed of firm, tan-white tissue (Image 1, mass after formalin fixation).

Frozen sections showed a spindle cell lesion with the more gelatinous area showing moderate cellularity with mild nuclear atypia in a prominent myxoid background (Image 2). The more fibrous area showed increased cellularity with pleomorphism and mitotic activity (Image 3). A sarcoma was favored with areas of at least moderate grade. Based on this diagnosis additional margins were taken.


Permanent sections showed similar morphology to that which was seen at frozen section. There were lower grade myxoid areas showing moderate cellularity with mild to moderate nuclear atypia in a myxoid background (Image 4). The nuclei were mildly pleomorphic with hyperchromasia and the cells had scant eosinophilic cytoplasm. There were prominent thin walled blood vessels having a curvilinear pattern.

There were also areas of more intermediate grade with increased cellularity that showed many multinucleated tumor giant cells and increased mitotic activity, including some atypical mitotic figures (Image 5). The multinucleated giant tumor cells showed abundant eosinophilic cytoplasm and many had irregularly shaped nuclei. High grade areas were also present showing much higher cellularity with a nearly solid growth of tumor cells.

There was a loss of the myxoid background and a loss of the prominent background vasculature in these areas. Thin fibrous strands were present amongst the cells in these areas. Mitotic activity was high up to 32 mitoses per 50 high power fields (Images 6 and 7). Focal areas of necrosis were present (Image 8).


To differentiate this tumor from some of the other sarcomas included in the differential, dual color break apart fluorescence in situ hybridization (FISH) was performed for DDIT3 (CHOP) gene at chromosome location 12q13. This was negative for a translocation of the DDIT3 (CHOP) gene.


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