This post is about cutaneous spindle cell tumours and their immunohistochemistry (IHC) profiles. I get many questions from laboratory staff about why we perform certain IHC and in what situations are they used. Since spindle cell tumours require IHC for their correct specific diagnosis I thought I would do a post on them (plus I was also involved in a recent published journal article about this very subject).
Cutaneous spindle cell tumours include atypical fibroxanthoma (AFX), spindle cell melanoma, leiomyosarcoma and spindle squamous cell carcinoma. Why can’t the pathologist diagnose these tumours simply on H+E without the need for IHC? Well, cutaneous spindle cell tumours look extremely similar on H+E and the effects of misdiagnosing a spindle cell melanoma (which obviously is extremely serious) as, for example, an AFX (which has a rather benign, indolent clinical course despite it’s alarming histopathology appearance) can be diastrous for the patient.
Below is a table which provides an example panel leading to the diagnosis of AFX and the reasons for the use of each particular antibody.
S100 – Negative. Essential to exclude melanoma. Also highlights Langerhans cells, which can be prominent in some AFX.
Melan A - Negative Optional, if S100 is negative.
HMB45 – Negative Optional, if S100 is negative.
Broad spectrum cytokeratin (e.g. MNF116, AE1 & 3) – Negative Beware of included normal adnexal structures or hyperplastic epidermal downgrowths.
34betaE12 – Negative Highlights some squamous cell carcinomas more prominently than broad spectrum cytokeratins.
Smooth muscle actin – Positive ~75% of AFX tumors are positive.
Desmin – Negative Useful to exclude leiomyosarcoma in actin positive cases.
CD68 – Positive ~90% of AFX tumors are positive.
CD10 – Positive Although most AFX are positive, the specificity of this antibody is low, making it of limited discriminatory value.
At a minimum an S100 ( to exclude melanoma), CD68 (positive for AFX), a keratin (preferably 34betaE12 as this stains most spindle squamous cell carcinomas) and desmin (positive for leiomyosarcoma), should be performed on all cutaneous spindle cell tumours.
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