Naevoid Melanoma

The lesions are called 'naevoid' because the cytological and architectural features resemble features of benign intradermal or compound melanocytic naevi.

Recent studies have shown that there is a significant local recurrence and mortality rate in patients diagnosed with naevoid melanoma.

Histological features : Images

To make a diagnosis of naevoid melanoma a high index of suspicion is essential.

An intraepidermal component is recognized at low power.

The intradermal component is composed of tightly packed nests of small dark stained cells resembling naevus cells .

The cells display hyperchromatic mildly pleomorphic nuclei , scanty cytoplasm and occasional prominent nucleoli.

Dermal mitosis may or may not be present.

A reactive stromal response is evident characterized by proliferation of blood vessels and patchy lymphocytic infiltrate.

Immunohistochemistry:

S100 protein & HMB45 are usually positive.

The cells also express proliferating cell nuclear antigen (PCNA) and Ki67 (using MIB-1).

HMB45 may be negative in some cases.

Note: In Spitz naevus there is stratification of staining .In the base of the lesion cellular staining is less intense.

[Some clues to diagnosis of Nevoid melanoma:

Hypercellularity ; Loss of orderly arrangement of nests ; Cells present in strands and cords in some cases; Presence of deep dermal mitosis ; Lack of maturation with depth; Infiltrative pattern at the base of the lesion ]

( My approach to atypical melanocytic lesions: Article link - In the hands of the experienced pathologist, the histological features are usually sufficient to classify a lesion as naevoid melanoma; however, immunohistochemistry may be a valuable adjunct in difficult cases. In naevi, fewer than 5% of cells express Ki-67 (MIB-1) and most of the reactive cells are present in the superficial dermis. In melanoma, MIB-1 reactive cells are more numerous and are distributed at all levels of the dermal component. An important caveat is that lymphocytes, histiocytes, and sometimes endothelial cells may also be MIB-1 positive, and therefore cell morphology should be taken into account to determine whether the immunoreactive cells are actually melanocytes. Differential staining may also be seen with HMB-45 and cyclin D1; banal naevi exhibit reactivity for these immunomarkers in the superficial dermal component. HMB-45 and cyclin-1 staining in melanoma is seen throughout the dermal component (to varying degrees). It should be noted, however, that not all naevi or melanomas stain with HMB-45. In addition, it must be remembered that cyclin D1 is a nuclear antigen; therefore, cytoplasmic reactivity is not informative.)

Case link & discussion- (Dr Reed&Mihm)-CLICK HERE

ARTICLE LINK (full text): CLICK HERE

Similarities and differences between Nevoid melanoma and compound naevus:

Nevoid malignant melanoma small cell type:

Similarities: i) Cells are usually small ; ii) Pagetoid cells are rare ; iii) Melanocytes are nested iv) Cells may disperse at the base.

Differences: Compound Naevus: Mitosis is absent; Nucleoli inconspicuous at base of the lesion; Nuclei not hyperchromatic ; HMB45 negative at base; MIB1 negative at base.

Naevoid malignant melanoma (small cell type): Mitosis present in dermis ; Nucleoli visible at the base; Nuclei hyperchromatic ; HMB45- positive at base; MIB-1 positive at base.

Similarities and differences between Nevoid melanoma and Spitz's naevus:

Nevoid malignant melanoma large cell type:

Similarities: i) Cells tend to be large; ii) Pagetoid cells may be present; iii) Melanocytes are nested iv) Cells may disperse at the base.

Differences: Spitz's naevus : Mitoses present in dermis, but rarely at the base; cell size small at the base; Kamino bodies are common; HMB-45 patchy positivity, may be at base; MIB-1 diminishes at base.

Naevoid malignant melanoma (large cell type): Mitoses present in dermis even at base; Cell size large at base; Few Kamino bodies; HMB-45 positive at base; MIB-1 undiminished at base.

Verrucous Naevoid Melanoma

Melanoma is characterized by epidermal hyperplasia , elongation of rete ridges and overlying hyperkeratosis.

Clinically these lesions may be mistaken for seborrheic keratosis.
(At low power some these lesions may resemble papillomatous benign intradermal naevi.)

The tumour consists of small naevus like cells with no evidence of maturation with depth.

The lesion may demonstrate atypical junctional component, asymmetric lateral growth pattern and area of regression.

Image1 ; Image2 ; Image3 ; Image4 ; Image5 ; Image 6

CASE LINK: CLICK HERE

Verrucous naevoid and keratotic malignant melanoma: a clinico-pathological study of 20 cases. Histopathology 1993;23: 453-458

Verrucous-keratotic variations of malignant melanoma. A clinicopathological study. Am J. Dermatopathol 1982; 4: 403-10

Verrucous pseudonaevoid melanoma. J Surg Oncol 1987;36: 134-7