Melanocytic Tumors

2021-02-24 12:00 AM

Congenital nevi (birthmarks) are present at birth; giant congenital nevi have an increased risk of developing melanoma.

MELANOCYTIC TUMORS

Congenital nevi (birthmarks) are present at birth; giant congenital nevi have an increased risk of developing melanoma.

Nevocellular nevus (mole) is a benign tumour of melanocytes (melanocytic nevus cells) that is clearly related to sun exposure. Types of nevi include junctional, compound, and intradermal. Nevi have uniform tan to brown colour with sharp, well-circumscribed borders and tend to be stable in shape and size. Malignant transformation is uncommon.

Dysplastic nevi (BK moles) are larger and more irregular than common nevi, and they may have pigment variation. Microscopically, the nevus exhibits cytological and architectural atypia. Dysplastic nevus syndrome is autosomal dominant (CMM1 locus on chromosome 1); patients often have multiple dysplastic nevi, and there is an increased risk of developing melanoma.

Malignant melanoma is a malignancy of melanocytes whose incidence is increasing at a rapid rate, with a peak in ages 40–70. Risk factors include chronic sun exposure, sunburn, fair skin, dysplastic nevus syndrome, and familial melanoma (associated with loss of function mutation of the p16 tumour suppressor gene, CDKN2A, on chromosome 9; somatic mutations of NRAS and BRAF also occur). Melanomas characteristically form skin lesions of large diameter with asymmetric and irregular borders and variegated colour; the lesions may be macules, papules, or nodules. Melanomas on males have increased frequency on the upper back; females have increased frequency on the back and legs.

Several types of melanomas occur:

  • Lentigo malignant melanoma is usually located on the face or neck of older individuals and has the best prognosis.
  •  Superficial spreading melanoma is the most common type of melanoma and has a primarily horizontal growth pattern.
  •  Acral lentiginous melanoma is the most common melanoma in dark-skinned individuals; it affects palms, soles, and subungual area.
  •  Nodular melanoma is a nodular tumour with a vertical growth pattern that has the worst prognosis of the melanomas.

The prognosis of melanomas is determined by TNM staging; T status is based on the depth of invasion (Breslow thickness measured histologically in millimetres).

The local disease is treated with wide surgical excision and sometimes sentinel node biopsy. Systemic disease is treated with chemotherapy or immunotherapy. Metastases may occur after years of dormancy.