Clinical: Pharyngeal mass
Histology:
Approximately 20% of the tumor looks like this.
Approximately 80% of the tumor looks like this. The mitotic rate is less than 10/10 high-power fields (at most 5 figures/1mm^2^). Necrosis was not seen.
Special/ancillary testing:
Testing performed on a region like image #2 above:
- Smooth muscle actin: scattered positive
- Desmin: scattered positive
- CD34: positive (non-specific)
- S100: negative
- SOX10: negative
- AE1/AE3: negative
- STAT6: negative
- ALK1: negative
- beta-catenin: negative
- MUC4: negative
- MDM2: strongly positive in most tumor cells
- CDK4: strongly positive in most tumor cells
Diagnosis:
Following immunohistochemical testing, we had a differential diagnosis of cellular well-differentiated liposarcoma (WDLPS) and a low-grade dedifferentiated liposarcoma (DDLPS). In the 5th edition of the WHO classification of soft tissue and bone tumors, they note that "low-grade DDLPS is virtually indistinguishable from cellular WDLPS". Which is fun to read when you're trying to sign out a case like this.
We identified a helpful publication (Graham et al., 2023) that stratified tumors like this into 1) classic WDLPS without the non-lipocytic components like we see in this case, 2) tumors with non-lipocytic areas and < 5 mitotic figures/10 HPF, and 3) tumors with non-lipocytic areas and 5 or more mitotic figures/10 HPF. They found that groups 1 and 2 performed similarly while group 3 tended to have a worse disease-specific survival compared to groups 1 and 2. Since we had at least 5 mitotic figures/10 HPF, after consultation with multiple pathologists, we felt that this was most in keeping with an FNCLCC intermediate-grade dedifferentiated liposarcoma (DDLPS).
