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Rare skin tumours — Merkel cell carcinoma, Kaposi sarcoma and DFSP
Three rare tumours that appear in exams because of one "clue" each: Merkel — a fast-growing red-purple nodule in an older immunosuppressed adult; Kaposi — purple patches linked to HHV-8; DFSP — a firm plaque on the trunk that recurs after excision and is linked to the COL1A1-PDGFB translocation.
Why it matters in the exam
Questions show a painless red-bluish nodule that has grown over weeks on the face of an older person — and ask about Merkel, polyomavirus and sentinel node biopsy. Alternatively they show purple lesions on the feet of an older man of Mediterranean or Ashkenazi Jewish origin, or of a patient with HIV. The response of DFSP to imatinib is also asked. In all three tumours the first step is a biopsy, and pathology with the right stains decides. It is also important to look for the context: immunosuppression, HIV, organ transplant, prior radiotherapy and chronic lymphoedema. Many questions are built on recognising the epidemiological clue — age, origin, immune status and site — before even reaching pathology, and on the link between the tumour and the virus or translocation that drives it.
Key points
- MCC is an aggressive neuroendocrine skin tumour, linked to Merkel cell polyomavirus and sun, and presents as a fast-growing red-purple nodule.
- In MCC, sentinel node biopsy, excision and radiotherapy are done, with immunotherapy in advanced disease.
- Kaposi sarcoma is caused by HHV-8 and occurs in classic, endemic, HIV-related or transplant-related forms.
- In HIV-related Kaposi the first step is antiretroviral therapy; after transplant, reduce immunosuppression and switch to sirolimus.
- DFSP results from the COL1A1-PDGFB translocation, recurs locally and responds to imatinib when unresectable.
Summary table
| Tumour | Clinical clues | Pathogenesis | Treatment |
|---|---|---|---|
| Merkel cell carcinoma (MCC) | Painless, red-purple, fast-growing nodule, older adult, exposed skin, immunosuppression | Merkel cell polyomavirus or UV damage; dot-like cytokeratin 20 | Excision with 1–2 cm margins, sentinel node, radiotherapy; immunotherapy (avelumab, pembrolizumab) in advanced disease |
| Classic Kaposi | Purple patches and plaques on the lower limbs, older Mediterranean or Ashkenazi man | HHV-8 | Observation, local radiotherapy, excision, chemotherapy when widespread |
| HIV- or transplant-related Kaposi | Widespread lesions, mucosa, internal organs | HHV-8 with immunosuppression | Antiretroviral therapy, reduced immunosuppression, switch to sirolimus; liposomal doxorubicin |
| DFSP | Firm pink-brown plaque on the trunk, slow-growing, recurs locally | t(17;22) COL1A1-PDGFB translocation | Mohs surgery or wide excision; imatinib for unresectable disease |
| Cutaneous angiosarcoma | Purple bruise-like patch or plaque on the scalp of an older adult, or on a lymphoedematous arm | Chronic lymphoedema (Stewart-Treves) or radiotherapy | Surgery, radiotherapy, taxanes |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
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Sources
- NCCN Guidelines Merkel Cell Carcinoma, 2025 — Workup and treatment
- European guideline for Kaposi sarcoma (EADV/EDF/EORTC), 2019 — Classification and treatment
- European consensus on dermatofibrosarcoma protuberans (EDF/EADO/EORTC), 2022 — Surgery and imatinib
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.