Melanocytic Nevi Workup
- Author: Timothy McCalmont, MD; Chief Editor: Dirk M Elston, MD more...
No laboratory studies are indicated in the evaluation of common acquired melanocytic nevi. As a rule, patients with congenital melanocytic nevi also do not require laboratory evaluation.
Imaging is not warranted in the evaluation of most patients with acquired melanocytic nevi; however, the possibility of neurocutaneous melanosis should be considered in patients (children) with multiple congenital melanocytic nevi involving the skin overlying the spine and the posterior part of the scalp.
Some patients with multiple congenital nevi overlying the CNS warrant radiologic imaging. Such individuals are at risk for leptomeningeal melanosis. Some patients with neurocutaneous melanosis have nests of melanocytes within the CNS at specific sites. These nests of cells can be detected using MRI (especially with T1-weighted images). Although the nests of melanocytes are benign, these cells pose a risk for transformation to CNS melanoma, and follow-up care is indicated.
Simple excisional biopsy is the procedure of choice for removal and diagnosis of a melanocytic nevus. All removed melanocytic nevi should be submitted for microscopic evaluation. Because the interpretation of pigmented lesions may be challenging, many dermatologists prefer to have their specimens read by a qualified dermatopathologist.[10, 11]
Either shave biopsy or punch biopsy is typically used for cosmetic removal of banal melanocytic nevi.
It is optimal to strive for complete excision of a given lesion, if at all possible, when melanoma is considered in the differential diagnosis.
A complete excisional biopsy permits all available histopathological criteria to be applied to a lesion and thus enables a more precise diagnosis.
When a partial punch or shave biopsy sample is taken from a lesion, the interpreting pathologist cannot apply important criteria, such as symmetry and circumscription (lateral demarcation), to the assessment of the lesion. If a partial biopsy specimen of a larger lesion is obtained because of clinical necessity, the fact that the specimen is partial should be clearly indicated on the requisition form.
Partial biopsy samples can sometimes lead to misdiagnosis because of sampling error.
Partial biopsy samples can inflate the number of procedures required for diagnosis because a partial biopsy sample that does not enable a definitive diagnosis to be made necessarily leads to subsequent reexcision of the lesion in question.
The chief histopathological differential diagnosis involves the distinction of melanoma from various forms of melanocytic nevi, including Spitz nevi. Examination via conventional microscopy remains the criterion standard for making this distinction. Genomic analysis via comparative genomic hybridization (CGH) or fluorescence in situ hybridization (FISH) is being used on a limited basis to supplement conventional histopathological interpretation. Genomic analysis permits screening of fixed tumor tissue for cytogenetic aberrations. To date, studies indicate that multiple cytogenetic flaws are typical of melanoma, whereas most melanocytic nevi are cytostructurally normal.
Most melanocytic nevi develop along the dermoepidermal junction, where normal melanocytes are positioned within the lower-most part of the epidermis. Melanocytes are also distributed in smaller numbers within the dermis; therefore, wholly dermal nevi can occur. Melanocytic nevi that are exclusively dermal include the group of lesions referred to as blue nevi.
Melanocytic nevi can be categorized on the basis of the phenotype (cytological appearance) of the melanocytes that make up the lesion. Benign neoplastic melanocytes (sometimes referred to as nevus cells) most commonly have round/ovoid nuclei, scant cytoplasm, and a predilection to form nests and syncytia. However, melanocytes are morphologically nimble cells that can assume a wide variety of appearances, including spindled, epithelioid, and dendritic forms.
Assessing the architecture of any melanocytic neoplasm is imperative as one of the first steps in evaluation. Most melanocytic nevi are relatively small, most are reasonably symmetrical, and most show fairly sharp lateral demarcation or circumscription. Melanomas typically show the converse of these features. Many (but not all) melanocytic nevi are arrayed as nests and fascicles of cells, and the nests and the fascicles of cells are usually relatively uniform in size and shape. In contrast, marked variation in nest size and shape should amplify concern regarding the possibility of melanoma. Lesional melanocytic in a nevus typically appear smaller, microscopically, with descent in the dermis. See the image below. This phenomenon is referred to as maturation.
Conventional (ordinary or common acquired) melanocytic nevi develop as a proliferation of single melanocytes along the dermoepidermal junction. As the melanocytes proliferate, small nests of cells develop in the lower-most part of the epidermis, and the resultant lesion is termed a junctional melanocytic nevus. With continued proliferation, nests persevere along the junction, but they can also be found within the superficial dermis, a configuration termed compound melanocytic nevus. As a nevus ages, the junctional component often diminishes or entirely involutes. The resultant nevus is termed an intradermal melanocytic nevus, depicted below.
Congenital melanocytic nevi are similar to their acquired counterparts in that junctional, compound, and intradermal patterns can be seen. Most congenital nevi extend well into the dermis, with melanocytes positioned in the interstitial dermis between collagen bundles. The depth of extension into the dermis is variable. Some large congenital nevi exhibit cells that extend into subcutaneous septa. Congenital melanocytic nevi with melanocytes confined to the upper half of the reticular dermis have been termed superficial congenital nevi. Lesions of this type are typically smaller than 2 cm in overall diameter.
Spitz nevi, shown below, are virtually always acquired melanocytic nevi, and they can exhibit a microscopic pattern that is junctional, compound, or wholly intradermal. Like all benign nevi, Spitz nevi tend to be relatively small and symmetrical and laterally demarcated, but Spitz nevi differ from conventional nevi in that nucleomegalic cells are common and predominate in some lesions. Such nucleomegalic Spitz nevus cells may be aneuploid, tetraploid, or octoploid. The explanation as to why Spitz nevi are commonly nondiploid while lacking other attributes of malignancy has not yet been forthcoming. Clearly, anomalies in ploidy alone are not sufficient for full malignant transformation.
Overlap in the morphology of melanocytic nevi can occur. For example, occasional melanocytic nevi can display overlap between the morphology of Spitz nevus and dysplastic nevus. Overlap between blue and Spitz nevi can also be seen. Such lesions have been humorously referred to as “Sparks” (Spitz + Clarks) or “blitz” (blue + Spitz) nevi.
When more than one type of cellular morphology occurs in a melanocytic nevus, the lesion is referred to as a combined melanocytic nevus. Conventional melanocytic nevi are commonly combined with Spitz or blue nevi.
Blue nevi, shown below, are typically largely or entirely dermal melanocytic neoplasms composed of spindled and/or dendritic melanocytes with heavy cytoplasmic pigmentation. Some blue nevi are composed of epithelioid melanocytes, especially the deeply extending variant designated deep penetrating nevus, and many exhibit considerable associated sclerosis. The designation blue is far from the truth in many, if not most, instances because melanocytic nevi with spindled and dendritic melanocytes can be tan, brown, black, gray, or even skin-colored; however, despite its inaccuracy, the designation blue nevus remains the universal standard for this category of lesions.
Some authorities have promoted the use an eponymic approach to the classification of melanocytic nevi. The merit of this proposal derives from the fact that eponymic naming avoids the semantic misdeeds of the past. For example, the benign entity formerly (and incorrectly) known as juvenile melanoma becomes a Spitz nevus in the eponymic system. Dysplastic nevi are eponymically known as Clark nevi, in memory of Wallace Clark. The designation Miescher nevus can be used to designate dome-shaped, superficial congenital nevi that are commonly expressed on the face and the upper part of the trunk, whereas the designation Unna nevus can be used to refer to acrochordonlike lesions that commonly develop near skin folds.[13, 14]
Despite the enthusiasm for eponymic naming in some areas, the usage of such designations can impede communication with the uninitiated. The author sees no difficulty in the usage of eponyms (and uses eponyms sporadically in his own practice), as long as the exact nature of the lesion in question is clearly stated in the language of the pathology report.
Melanocytic nevi are benign lesions. No staging is required, with the possible exception of atypical benign lesions for which uncertainty exists regarding the diagnosis and melanoma cannot be entirely eliminated from the differential diagnosis.
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