Revised March 2014
1. Classification Criteria
Melanoma may be classified into the following groups based on clinical and histologic criteria:
Growth Pattern Frequency
a) Superficial spreading melanoma (SSM) 65%
b) Nodular melanoma (NM) 25%
c) Acral-lentiginous melanoma (ALM) 5%
d) Lentigo maligna melanoma (LMM) 5%
SSM may arise in a pre-existing mole with a radial growth phase prior to vertical growth phase and invasion, while NM lacks an identifiable radial growth phase. ALM often masquerades as either subungual hematoma, or hematoma of the sole of the foot. Delay in diagnosis can be avoided by having a high index of suspicion. LMM is a relatively indolent melanoma occurring in chronically sun-damaged skin on the head and neck.
2. Diagnostic Pathology
Biopsy and Pathological Assessment
Pathological assessment of atypical or malignant melanocytic lesions is required to confirm the diagnosis, and if malignancy is present, to provide prognostic information.
An appropriately placed biopsy is important, as melanocytic lesions tend to be polyclonal and quite variable from one area to another. An excisional biopsy, which includes normal tissue around the periphery, is desirable whenever clinically feasible. If it is not possible for cosmetic or functional reasons then a single or multiple incisional or punch biopsies may provide adequate information. Superficial shave biopsy is strongly discouraged. If there is considerable variation in pigmentation, it is recommended that more than one area be sampled. Any palpable tumour with elevation of a portion of the lesion or a nodular component, whether it is pale, pink or pigmented should be sampled.
The best predictor of recurrence from histologic evaluation of the primary lesion is depth of invasion, measured by micrometer (Breslow depth) and histological ulceration. Dermal mitotic activity has been identified as an important prognostic factor in thin melanomas (1 mm. or less in thickness) and is included in the staging system. (Balch CM et al. Final Version of 2009 AJCC Melanoma Staging and Classification. J Clin Oncol. 2009; 27(36):6199-206). Other features such as the Clark’s level of invasion, lymphatic invasion, and lymphocytic response all contribute to the prognosis but are of lesser significance.
A checklist of the details required in a pathology report of melanoma is available from the College of American Pathologists website. (Frishberg D et. al. (2013, October). Protocol for the Examination of Specimens From Patients With Melanoma of the Skin. Retrieved from http://www.cap.org/apps/docs/committees/cancer/cancer_protocols/2013/SkinMelanoma_13protocol_3300.pdf
Fine needle aspiration cytology is not recommended for diagnosis of primary lesions although it may be useful in the assessment of metastatic disease such as satellite skin nodules or in regional lymph nodes.
In some patients, the diagnosis of malignant melanoma, dysplastic nevi or Spitz nevi may pose a very difficult problem. The pathologists in the Skin Tumour Group, who provide a provincial consultative service, may be most helpful with these problems.