Soft Tissue Immunohistochemistry Panels
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Direct answer
An immunohistochemistry panel in soft tissue pathology is a hypothesis test: morphology proposes a short differential, and the panel is built to include and exclude, never to fish. The workhorses are lineage markers — SMA, desmin and h-caldesmon for smooth muscle; myogenin and MyoD1 for skeletal muscle; S100 and SOX10 for Schwannian and melanocytic lines; CD31 and ERG for endothelium — plus signature markers that identify single entities, such as STAT6 for solitary fibrous tumour, TLE1 for synovial sarcoma, nuclear beta-catenin for desmoid fibromatosis and MDM2 for liposarcoma. Translocation-driven tumours are confirmed molecularly when therapy decisions hang on the answer.
What you must remember
- Smooth muscle versus skeletal: SMA is shared with myofibroblasts; the h-caldesmon plus desmin combination supports true smooth muscle; nuclear myogenin/MyoD1 proves rhabdomyoblastic differentiation — diffuse in alveolar, patchy in embryonal rhabdomyosarcoma.
- CD34 is a shape-shifter: positive in GIST, dermatofibrosarcoma protuberans, solitary fibrous tumour and vascular tumours; it excludes almost nothing by itself and must be paired (DOG1/KIT for GIST, STAT6 for SFT).
- Signature nuclear markers: STAT6 (SFT), TLE1 (synovial sarcoma), MDM2 (well-differentiated/dedifferentiated liposarcoma), NKX2.2 with membranous CD99 (Ewing sarcoma), nuclear beta-catenin (deep fibromatosis), brachyury (chordoma).
- Loss stains diagnose: INI1 (SMARCB1) loss — epithelioid sarcoma, rhabdoid tumour; SMARCA4 loss — undifferentiated round cell or small cell carcinoma of the ovaryhypercalcaemic type and thoracic counterparts.
- Neural and melanocytic: S100 and SOX10 are broad (Schwannoma, MPNST variable, melanoma); HMB45 and Melan-A narrow to melanocytic; TFE3 marks alveolar soft part sarcoma and some PEComas.
- Vascular line: CD31 (sensitive) and ERG (nuclear) mark endothelium; CAMTA1 identifies epithelioid haemangioendothelioma and WWTR1 supports epithelioid-type vascular differentials.
- FISH confirmation: SS18 break-apart for synovial sarcoma, FUS (myxoid liposarcoma, low-grade fibromyxoid sarcoma), EWSR1 — immunostains are screening, translocations are diagnosis when morphology is ambiguous.
Building a spindle-cell panel step by step
A deep thigh mass in a 40-year-old yields fascicles of monomorphic spindle cells with delicate collagen. The first tier answers the cheap questions: S100 and SOX10 (schwannoma, MPNST?), SMA and desmin (leiomyoma or leiomyosarcoma?), CD34 (DFSP? SFT? vascular?). Suppose S100 is negative, SMA negative, CD34 diffuse with a patternless architecture — the second tier adds STAT6, and a strong nuclear stain finalises solitary fibrous tumour; if STAT6 were negative, nuclear beta-catenin would raise desmoid fibromatosis and MDM2 would raise a liposarcoma variant. Each round has a purpose: the panel never runs shotgun, because nonspecific cytoplasmic positivity with weak antibodies is how tumours get mislabelled. When the shortlist lands on a fusion tumour — say monophasic synovial sarcoma with patchy TLE1 — a break-apart FISH or next-generation fusion panel converts suspicion into diagnosis, which matters because synovial sarcoma responds to ifosfamide and is a defined molecular entity.
Where students slip
The classic trap is reading CD34 positivity as "vascular tumour" — half the differential that lights up CD34 is not endothelial. Myogenin is patchy by design in embryonal rhabdomyosarcoma, and demanding diffuse staining loses the diagnosis; conversely, weak SMA positivity does not make a tumour a leiomyosarcoma, because virtually every myofibroblastic lesion expresses it. TLE1 over-called at high intensity has produced false synovial sarcomas, which is why examiners ask for molecular confirmation. Finally, INI1 loss must be interpreted with an internal positive control — lymphocytes and endothelial cells on the same slide — a small technical point that decides whether the stain is valid at all, and a favourite viva probe.
Frequently asked questions
Which markers prove skeletal muscle differentiation?
Nuclear myogenin and MyoD1; myogenin is diffuse in alveolar and patchy in embryonal rhabdomyosarcoma, unlike SMA or desmin, which lack specificity.
Why is CD34 alone insufficient in a spindle-cell tumour?
It stains GIST, DFSP, solitary fibrous tumour and vascular lesions alike; pair it with DOG1/KIT, STAT6 or CD31/ERG to reach a lineage.
What does STAT6 nuclear positivity indicate?
The NAB2-STAT6 fusion of solitary fibrous tumour — currently the most discriminating single marker for that entity.
Which tumours show INI1 loss?
Epithelioid sarcoma and malignant rhabdoid tumour classically; loss must be checked against internal controls on the same section.
When is FISH mandatory after immunostaining?
When the differential narrows to a fusion-defined entity with treatment implications — synovial sarcoma, myxoid liposarcoma, Ewing sarcoma — especially where morphology is ambiguous.