Special Stains & Immunohistochemistry
- CD34: Strong positivity in spindle cells (diffuse membranous).
- MAST cells: Increased (tryptase+).
- Reticular fibres: Surround individual cells.
- Negative: S100 (adipocytes only), desmin, actin, ALK1, MDM2.
CD34 distinguishes from solitary fibrous tumour (STAT6+).
Cytogenetics
Characteristic abnormality: Translocation t(13;16)(q13;q13) or 13q14 deletion, involving RB1 and FOX01 genes. Identical to spindle cell lipoma, supporting spectrum concept. MDM2/FH amplification absent (vs. liposarcoma).
Differential Diagnosis
| Feature | Pleomorphic Lipoma | Spindle Cell Lipoma | Pleomorphic Liposarcoma | Well-Differentiated Liposarcoma |
|---|---|---|---|---|
| Age/Sex | 55-65M | 55-65M | 60+ M=F | 60+ M=F |
| Location | Neck/shoulder SC | Neck/shoulder SC | Deep extremities | Deep retroperitoneum |
| Fat | Variable | Variable | Scant | Abundant |
| Cells | Floret giants, spindles | Spindles only | High-grade pleomorphic + lipoblasts | Atypical adipocytes + lipoblasts |
| Mitoses/Necrosis | Absent | Absent | Present | Rare |
| Collagen | Ropey thick | Ropey thick | Absent | Variable |
| MDM2 | Neg | Neg | Neg | Pos |
| Cytogenetics | 13q14 del | 13q14 del | Complex | 12q13-15 amp |
Key: Subcutis location, ropey collagen, floret cells without lipoblasts favour pleomorphic lipoma.
Clinicopathologic Variants
- Pseudoangiomatous: Slit-like spaces.
- Fat-poor: Mimics solitary fibrous tumour.
- MYXOID: Abundant mucin.
- Intramuscular: Rare, deeper location.
Prognosis & Treatment
Benign; simple excision curative. Recurrence <5% if incomplete. No metastasis.
Frequently Asked Questions
Q: Is pleomorphic lipoma malignant?
A: No, despite bizarre cells, it is benign with no metastatic potential.
Q: How to distinguish from liposarcoma?
A: Subcutaneous site, ropey collagen, no lipoblasts/mitoses, 13q deletion, MDM2 negative.
Q: Is FNAC reliable?
A: Often suspicious; excision biopsy required for definitive diagnosis.
Q: Common sites?
A: Posterior neck/shoulder in elderly males.
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References
- Spindle cell lipoma pathology — DermNet NZ. 2023. https://dermnetnz.org/topics/spindle-cell-lipoma-pathology
- Pleomorphic lipoma: A gentle giant of pathology — PMC/NIH (J Cytol). 2015-10-01. https://pmc.ncbi.nlm.nih.gov/articles/PMC4687215/
- Liposarcoma pathology — DermNet NZ. 2023. https://dermnetnz.org/topics/liposarcoma-pathology
- Pleomorphic lipoma pathology image — DermNet NZ. 2023. https://dermnetnz.org/imagedetail/18716-pleomorphic-lipoma-pathology
- Basic soft tissue pathology (dermpath) — YouTube (Pathology lecture). 2023. https://www.youtube.com/watch?v=h7oxx9g7DH4
