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A classification of melanoma which gives recognition to patterns of vertical growth would impact on the category of nodular melanoma, and even on some of the typical forms of melanoma, such as acral lentiginous melanoma, and lentigo maligna melanoma. In general, two major categories can be defined:
A) nested, round or polygonal cell melanomas
1) variant vertical growth
a. arrested variant vertical growth
b. progressive variant vertical growth
2) typical vertical growth
B) fascicular, spindle cell melanomas
1) variant vertical growth
2) typical vertical growth
Melanocytic neoplasia in the pattern of arrested variant vertical growth is usually a lesion measuring less than 1 mm in height (a melanocytic lesion in the borderland of neoplasia). It is distinguished by rounded nests of cells in a widened papillary dermis, and by confinement of each nest in concentrically oriented fibrous lamellae. If completely excised, a lesion showing this combination of features is unlikely to be associated with progressive disease. If a lesion showing this pattern measures less than 1 mm in height, it can be assigned to the category of MDM as borderline melanocytic neoplasia of indeterminate malignant potential.
The fascicular spindle cell melanomas are divisible into a variant vertical growth category and a typical vertical growth category. A variety of melanomas including common and uncommon “types” qualify as fascicular, variant vertical growth spindle cell melanomas. The list includes most ALM, most LMM, some metastasizing Spitz nevus-like lesions, and pigmented spindle cell melanomas that cannot be otherwise classified. There are occasional fascicular variant vertical growth round cell melanomas but they are best assigned to the more specific category of “nevoid” melanoma.
In lentiginous melanomas, including the acral variant, and the actinic variant (lentigo maligna melanoma), fascicular patterns and variant vertical growth are common. The cells commonly are spindle shaped. Fascicular melanomas in variant vertical growth commonly show level IV invasion, even in thin examples (migrant or diffuse vertical growth). In addition,such lesions may not be associated with significant markers for host immune response. Such lesions are histologically deceptive. They are organoid or nevoid in character and may even show some evidence of maturation (nests of cells and the cells of the nests are smaller at the deep margin than at the dermal-epidermal interface. They have a rarely expressed propensity to express heteromorphism.
Fascicular typical vertical growth melanomas with no radial growth components have in the past been either misdiagnosed as some variant of nevus (i.e., Spitz-like type or pigmented spindle cell type), or dismissed as a nodular variant. These lesions may not be associated with well developed lentiginous and junctional components in the overlying epidermis; this is particularly true of the pigmented spindle cell variants. They are commonly manifested in the pattern of an expansile nodule that displaces collagen bundles of the reticular dermis. Often the lesion has the configuration of a “keyhole’ with a columnar extension into the deeper of the dermis or the subcutis. On some sections this columnar component is identified as a perifollicular extension. These lesions may be fairly uniform in cytologic features but generally there is nuclear atypia with dense nuclear membranes, prominent central nucleoli, and mitotic activity. Lesions of this type appear to be included in the literature as examples of “metastasizing Spitz nevus.”
Some melanomas, that otherwise might be characterized as nodular, are fascicular and show level IV invasion almost from their inception. Some of these lesions have been characterized as nevoid variants. Some of the “nodular” melanomas could, with attention to cytologic details of vertical growth components, or to the characteristics of host immune response, be assigned to an alternate category such as halo nevus-like variant, dermal variant, or even Spitz-like variant (i.e., components of the category of MDM). One simple approach to a refinement of the category would be to make a distinction between “nodular” melanomas showing typical vertical growth and those with variant vertical growth; such lesions could be subdivided as to cell type, including an indeterminate type, and characterized as showing either typical or variant vertical growth. The indeterminate category would exclude halo nevus-like variants, and Spitz-like variants. In this approach, halo nevus-like variants, and Spitz-like variants become “nevoid” types. Although a category of indeterminate cell type seems to be as ambiguous as the current “nodular” category , there is in the accommodation an admission that the category functions as a default. If special features such as Spitz-like cytology; distinctive, bland “epithelioid” cells of MDM of dermal type, pigmented spindle cells, or halo nevus-like phenomena are not identified in a melanoma, then the cell type might be characterized as non-descript, and lesions lacking specific features might be assigned to an indeterminate category. Superficial spreading melanoma would be exempt on the basis of distinctive radial growth patterns. Other lesions with or without radial growth that are not characteristic examples of SSM, and do not have distinctive cytologic or immunologic markers could then be assigned to a default category of melanoma of indeterminate cell type (i.e., nodular melanoma with the exclusion of MDM of those examples that can be compared to specific variants of “nevi”).
Fascicular melanomas are characterized by:
1) relatively pure fascicular patterns
2) uniform degrees of cytologic atypia (little in the way of nuclea pleomorphism)
3) spindle or polygonal (dendritic) cells
4) level IV invasion common, even in thin lesions
5) minimal markers for host immune response
6) in some examples, maturation is a feature
7) radial growth component may not be a significant feature
There is some utility in recognizing a category of fascicular variant vertical growth melanomas of intermediate grade (a variant of MDM). If such a lesion is composed of round cells in fascicular patterns, it qualifies as a specific round cell variant that by the criteria defined herein is most qualified to be characterized as “nevoid’ melanoma of common nevus-like type. If it is composed of pigmented spindle cells, it qualifies as pigmented spindle cell, fascicular variant. If the cells are polygonal with no other distinguishing characteristics, the lesion is a variant of ambiguous cell type. All these fascicular variant vertical growth melanomas are common in collection of malpractice cases.
For melanomas not of the type (i.e., melanomas of ambiguous type), the lesions might be characterized as:
1) fascicular and nested variant vertical growth melanoma (ambiguous type)
2) fascicular and nested typical vertical growth melanoma (ambiguous cell type)
In the setting of malpractice claims, the cells of these contentious lesions often tend to be short, plump, polygonal or dendritic cells. Often the cells are pigmented, but in some examples, the cells have pale or clear cytoplasm. Many of the lesions with typical vertical growth that would qualify for the default category are configured as an inverted wedge with the base abutting upon the epidermis; closely interwoven fascicles of atypical cells form an expansile tumor that tends to displace the collagen bundles of the reticular dermis (level IV, diffuse or migrant vertical growth). In many examples of both typical and variant vertical vertical growth melanomas of ambiguous cell type, the atypical cells have round or oval nuclei, heavily stained nuclear membranes, and central nucleoli. Mitoses are a feature and their numerical representation provides a measure of grade of neoplasia. In both the variant vertical growth type and the typical vertical growth type, lentiginous and junctional components often are minimal in the overlying epidermis.
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