25Other skin cancers the oncologist treats — Merkel cell · adnexal · angiosarcoma · DFSP
Snapshot. Beyond melanoma [§13], BCC/cSCC [§21] and mycosis fungoides [§24], four rarer skin cancers land on the medical oncologist. MERKEL CELL CARCINOMA (MCC) — an aggressive neuroendocrine skin cancer of the elderly/immunosuppressed (Merkel-cell polyomavirus (MCPyV) drives ~80%; the rest pure UV) — behaves like "skin small-cell" but is exquisitely immunotherapy-responsive. SKIN ADNEXAL CARCINOMAS (sweat-gland/eccrine, porocarcinoma, sebaceous, microcystic) — rare, surgery-led, no established systemic standard. CUTANEOUS ANGIOSARCOMA — bruise-like violaceous patch on the elderly scalp/face, or post-RT breast / chronic lymphoedema (Stewart–Treves) — aggressive, infiltrates far beyond what the eye sees. DFSP (dermatofibrosarcoma protuberans) — indolent plaque-sarcoma of skin, COL1A1–PDGFB fusion, locally relentless, almost never metastasises.
Workup
Biopsy decides everything — and the IHC panel separates the mimics: MCC = CK20-positive (dot-like paranuclear) + TTF-1 NEGATIVE (⚠ the reverse excludes a small-cell lung metastasis to skin — always the DDx) · adnexal = adnexal-lineage markers, and sebaceous carcinoma → MMR IHC (Muir–Torre / Lynch screen) · angiosarcoma = CD31/ERG vascular markers · DFSP = CD34+ ± COL1A1–PDGFB by FISH. MCC staging: sentinel lymph-node biopsy (SLNB) is MANDATORY even with a clinically negative neck/axilla (a third are node-positive) + PET/CT · adnexal/angiosarcoma: examine + image the draining nodes; angiosarcoma of scalp → image the FIELD (multifocality is the rule). Advanced adnexal disease → send the IO hooks up front: PD-L1 IHC + MMR/MSI testing ± TMB on NGS — without one of them immunotherapy has no documented basis.
Treatment by setting
| Setting | Treatment |
|---|
| MERKEL CELL — localized | Wide local excision + SLNB → adjuvant RT to the primary site
MCC is highly radiosensitive and recurs locally — the RT is not optional in most; node-positive → nodal dissection and/or nodal RT |
| MERKEL CELL — advanced / metastatic | IMMUNOTHERAPY FIRST LINE — avelumab (JAVELIN Merkel 200) or pembrolizumab (KEYNOTE-017)
durable responses in a cancer chemo cannot hold — the virus/UV biology is why IO works so well · chemo (platinum–etoposide, SCLC-style) ONLY when IO is contraindicated: responses are real but SHORT Surveillance is intense — recurrence risk is front-loaded |
ADNEXAL CARCINOMAS
sweat-gland / porocarcinoma / sebaceous / microcystic | Surgery-led: wide excision (or Mohs) ± node evaluation ± adjuvant RT for high-risk features
margins · perineural invasion · nodes Advanced/metastatic → NO established standard — platinum + taxane is the accepted approach; IO only with a documented hook (PD-L1 / MSI-H / high TMB)
the honest footing advanced adnexal disease is treated on ⚠ SEBACEOUS carcinoma → screen for Muir–Torre (Lynch)
MMR IHC on the tumour + family history — a skin diagnosis that can unmask a colon-cancer syndrome |
CUTANEOUS ANGIOSARCOMA
elderly scalp/face · post-RT breast · lymphoedema (Stewart–Treves) | Resectable → surgery + wide-field RT — accepting that margins are usually an illusion
it spreads through the dermis far beyond the visible edge — multidisciplinary from the start Advanced → PACLITAXEL — notably active in this one sarcoma
the taxane-sensitive vascular tumour; anthracycline-based chemo is the alternative · bleeding fungating lesions → haemostatic RT |
| DFSP | Wide excision (or Mohs) with generous margins — the whole game is local control
metastasis is rare; incomplete excision guarantees recurrence Unresectable / recurrent → IMATINIB
the COL1A1–PDGFB fusion puts the tumour under PDGF-receptor drive — imatinib blocks it; also used neoadjuvantly to shrink before surgery |
Watch
MCC on immunotherapy → the standard irAE families (colitis, pneumonitis, endocrine — baseline TFTs/glucose/cortisol). Immunosuppressed MCC patients (transplant, CLL) — IO risks the graft; individualise. Angiosarcoma → bleeding from fungating disease; paclitaxel → neuropathy each dose. Imatinib (DFSP) → oedema, cytopenias, GI upset [same profile as §23 GIST]. Sebaceous carcinoma → the Lynch/Muir–Torre workup is itself the "watch" — colonoscopy screening for the family if positive.