WHO classification of skin tumours

The World Health Organizatiοn classification of skin tumours is the most widely used pathologic classification system for skin tumours. The most recent edition is the 4th, which was published in 2018 ¹.  The radiologically relevant and common entities are reflected below.

Classification

 1. Keratinocytic/epidermal tumours

• squamous cell carcinoma
• basal cell carcinoma
• Merkel cell carcinoma
• carcinoma precursors and benign simulants of carcinoma:
  • actinic keratosis
  • verruca
  • seborrhoeic keratosis

2. Melanocytic tumours

• tumours occurring in intermittently sun-exposed skin:
  • melanocytic tumours in intermittently sun-exposed skin
  • low-cumulative sun damage (CSD) melanoma (superficial spreading melanoma)
  • simple lentigo and lentiginous melanocytic naevus
  • junctional, compound, and dermal naevi
  • dysplastic naevus
  • special-site naevi (of the breast, axilla, scalp, and ear)
  • deep penetrating naevus and melanocytoma
  • pigmented epithelioid melanocytoma
  • combined naevus, including combined BAP1-inactivated naevus/melanocytoma
• tumours occurring in chronic sun exposure:
  • lentigo maligna melanoma
  • desmoplastic melanoma
• Spitz tumours, including benign (Spitz naevus) and malignant (Spitz melanoma) 
• acral melanoma, acral naevus
• ocular tumours:
  • uveal melanoma
  • conjunctival melanoma
  • conjunctival naevus
• other malignant melanoma types: nodular melanoma, nevoid melanoma, metastatic melanoma

3. Appendageal tumours

• apocrine and eccrine differentiation​
  • malignant (e.g. porocarcinoma, digital papillary adenocarcinoma)
  • benign (e.g. poroma, cylindroma)
• follicular differentiation
  • malignant (e.g. pilomatrical carcinoma)
  • benign (e.g. pilomatrixoma)
• sebaceous differentiation (Sebaceoma, Sebaceous adenoma, carcinoma)
• other special sites:
  • mammary Paget disease
  • extramammary Paget disease
  • hidradenoma papilliferum

4. Hematolymphoid tumours

• mycosis fungoides
• Sezary syndrome
• Erdheim–Chester disease

5. Soft tissue tumours

• lipoma
• haemangioma
• neurofibroma
• Ewing sarcoma

(for a more complete list see the WHO classification of soft tissue tumours)

6. Inherited tumour syndromes associated with skin malignancies

• familial melanoma
• xeroderma pigmentosum
• naevoid basal cell carcinoma syndrome (Gorlin syndrome)
• Carney complex
• BAP1 tumour predisposition syndrome
• Muir–Torre syndrome