WHO classification of skin tumors

The World Health Organizatiοn classification of skin tumors is the most widely used pathologic classification system for skin tumors. 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 tumors

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

2. Melanocytic tumors

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

3. Appendageal tumors

• 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 tumors

• mycosis fungoides
• Sezary syndrome
• Erdheim–Chester disease

5. Soft tissue tumors

• lipoma
• hemangioma
• neurofibroma
• Ewing sarcoma

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

6. Inherited tumor syndromes associated with skin malignancies

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