Nevoid Malignant Melanoma (NM)
NM are a subset of melanomas which closely resemble benign (non-malignant) melanocytic nevi (moles), hence raising a significant diagnostic challenge. Fortunately for pathologists, this is a rare form of melanoma, making up less than 1 percent of all melanomas seen in the clinic. Nevoid melanoma (NM) is sometimes defined as a group of lesions that show similar tissue architecture and cellular appearance to benign melanocytic nevi but also show histological and biological behavior (such as recurrence and metastatic spread) similar to malignant melanoma.
An alternative view however, is that there is no scientific basis for classifying NM as a distinct entity. Hence the histologic diagnosis is controversial and nevoid malignant melanoma is a continuing subject for investigation.
Macroscopic Features of NM
Although more research is required, available reports of NM show the following clinical features:
- Most cases have been reported in women, and more frequently seen on the legs and trunk.
- A ‘tan nodule’ of around 1 cm diameter is typical.
- Many are asymmetrical in appearance and a significant number are amelanotic (no pigment).
Histology of Nevoid Malignant Melanoma
Microscopic features seen from biopsy material which may aid a diagnosis of NM include:
- dermal mitoses
- abnormal cellular appearance e.g. enlarged, variable shaped nuclei, irregular nuclear membranes, lack of maturation
- angiotropism (association of tumor cells with the outside of blood vessels)
Prognosis and Management
Studies published to date indicate that the prognosis of NM, if diagnosed as melanoma and treated as such, is comparable to that of conventional melanoma, although this requires more in depth study. Treatment of NM is the same as for conventional melanomas.
Treatment of malignant melanoma generally includes surgical removal (with 1-2 cm margin) and lymph node excision. Post surgical treatment of advanced disease includes chemotherapy, non-specific immunotherapy and interferon treatment. Sadly these yield low rates of survival. Experimental therapeutics such as gene therapy and antigen specific immunotherapy will hopefully provide new, more effective treatment options in the future.
Recent Research on Diagnosis of NM
Distinguishing nevoid melanoma from either benign skin nevi (moles) or common forms of melanoma is a subject for research, and hopefully will enable easier diagnosis of NM in the future.
Digital image analysis suggests that nucleolar organizing regions, nuclear area and perimeter are significantly smaller for nevoid melanoma compared to superficial spreading melanoma.
The expression of immunohistochemical markers HMB-45 and Ki-67: This is characteristic of benign lesions, but is commonly absent in nevoid malignant melanoma.
Unusual variants of malignant melanoma by C. Magro, A.Crowson and M.Mihm, in Modern Pathology, 2006, Vol 19, S41–S70.
Unusual variants of malignant melanoma by R.Barnhill and K.Gupta, in Clinics in Dermatology 2009, Pages 564-587
Diagnosis and treatment protocols of cutaneous melanoma: latest approach 2010, by I.Petrescu,C. Condrea, A.Alexandru, D.Dumitrescu, G.Simion, E.Severin, C.Albu, D.Albu, in Chirurgia (Bucur), 2010, Vol 105, Pages 637-43.