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Minimal Deviation Melanoma:
The prime requisite for the diagnosis of minimal deviation melanoma (MDM) was, and is, a vertical growth component. In addition, the cytologic features of MDM often deviate from those of common, fully evolved melanomas; the degree of cytologic atypia often is less than that seen in the vertical growth components of common melanomas. The subjectivity of evaluations of degrees of atypia impacts unfavorably on attempts to promote the concept of MDM.
Degree of atypia is not a significant parameter in the concept of melanoma-in-situ; in this category, emphasis has been placed on the relationship between neoplastic cells, and levels to which the atypical cells have migrated in the epidermis; evaluation of degrees of atypia seem to be somewhat beyond the level of expertise of those who promote the concept of melanoma-in-situ.
In the concept of MDM, variants have been promoted, if not universally accepted:
1. Thin melanoma arising in common premalignant melanocytic dysplasia is the most common variant of MDM. A thin lesion of this type might show variant, arrested variant, typical, or migrant patterns of vertical growth; this is the only variant in which physical dimensions are basic to the definition. As the concept has evolved, a physical dimension has become a part of the criteria; lesions in this category measure less than 1 mm in vertical dimensions; with this limitation, such lesions also qualify as borderline melanocytic neoplasia of indeterminate malignant potential. The cytologic features of the neoplastic cells usually are less atypical than those of a fully evolved melanomas ( a fully evolved melanoma might be characterized as a lesion in vertical growth with a vertical dimension greater than 1.5 mm). Often in evaluations of the degree of atypia in these thin, young lesions, the dermal component is stratified; the cells of nests at the advancing deep margin are less atypical than those of nests at the dermal-epidermal interface. In this variant, the tumor cells generally form rounded nests in the dermis. For these thin lesions, minimal deviation, as a modifier, clearly has application on the basis of both physical dimensions, and degrees of atypia.
2. Minimal deviation melanoma of dermal type and halo nevus-like type (dermal variant) are similar, and closely related. These lesions tend to be dermal nodules without significant lentiginous and junctional components. If such components are represented, the cytologic features of the epidermal component often tend to differ from those of the dermal nodule. In evaluations of cytologic features, the cells of the dermal component cannot be comfortably characterized as direct and immediate progeny of cells at the dermal-epidermal interface. In the dermal nodule, the cells are plump and rounded. They form rounded nests, or are individually isolated in a delicate fibrous matrix - the cells of the dermal nodule cluster in typical or variant vertical growth patterns. A component extending into the reticular dermis in migrant patterns is an acceptable variation. In the dermal variant, a remnant of a common nevus, often in congenital patterns, is present in the adjacent dermis. In the halo nevus-like variant, a remnant of a halo nevus may be preserved in the adjacent dermis. In addition, halo nevus-like phenomena may be represented in the stroma of the tumor, and in nests of tumor cells (so-called tumor infiltrating lymphocytes). For either variant, metastases are uncommon. Generally, in both variants, the tumor cells are plump, round cells with large nuclei; they differ cytologically from the monotonous cells of the atypical, blastoid nodules of giant congenital nevi. The cytologic features often overlap with those of the dermal component of examples of “halo nevi” in which some of the cells of the dermal component show some degree of “epithelioid” atypia.
3. Minimal Deviation Melanoma of halo nevus-like type (ex lentiginous and junctional dysplasia) shows features of halo nevus-like phenomena; most often there is a dermal component with features of a common halo nevus. In addition, the lentiginous and junctional component is atypical; it shows cytologic and histologic features of a common premalignant dysplasia, and spreads in the epidermis beyond the limits of the dermal component. The cells of the junctional component tend to be pigmented, small spindle cells. This variant of MDM enters vertical growth by way of neoplastic progressions in which patterns are basically similar to those encountered in lesions showing a transition from a common premalignant dysplasia to melanoma; nests of cells are delivered into a widened papillary dermis from the population at the dermal-epidermal interface. The newly arrived nests are susceptible to the influence of the host immune response. At the stage when the population of migrant cells acquires the traits which allow it to survive, and accumulate in variant, or typical vertical growth patterns, the degree of atypia is likely to be less than that of a fully evolved common melanoma (this deviation qualifies as minimal cytologic deviation); in such lesions, the vertical dimensions are likely to be thin (minimal deviation as defined by physical dimensions). The subtle cytologic features in the vertical growth component, and a remnant of halo nevus in the adjacent dermis combine to allow for the identification of MDM of halo nevus type. In occasional cases, features of the “ex dysplasia” variant of MDM of halo nevus-like type, and the dermal variant of MDM of halo nevus-like type will be combined in a single lesion.
4. Minimal deviation melanoma of pigmented spindle cell type may be associated with lentiginous and junctional components, or may be dermal in type. Both rounded nests and fascicles are commonly represented in the vertical growth component. These lesions often have a migrant vertical growth component. Metastases seem to be more common with this variant. A dermal lesion of this type is likely to be characterized as malignant blue nevus, even in the absence of a recognizable remnant of a blue nevus. An example with lentiginous and junctional components is likely to be interpreted as a “Spitz” variant; it is likely to be characterized as a “spitzoid” melanoma.
5. Minimal deviation lentigo maligna melanoma is defined by the same criteria as MDM arising in common premalignant dysplasia. In part, it is distinguished by sun-damaged skin. It is also distinguished by a prominence of lentiginous growth, by a tendency for the dermal component to be composed of spindle cells and fascicles, by a tendency to show lower degrees of atypia than the more common variants, and by a tendency for some of the cells in the epidermal component to be multinucleated. In keeping with the spindle cell and fascicular qualities, this lesion often shows patterns of migrant vertical growth, even though the vertical growth component is thin (i.e., less than 1 mm in vertical dimensions).
6. Desmoplastic MDM often is a variant of MDLMM. The characterization of a minimal deviation variant is based more on deceptive, bland cytologic features than physical dimensions, although many examples are relatively thin and may not be pan-dermal.
7. MDM of dermal type in the setting of giant congenital nevus is an expansile lesion, often dermal in type. Distinctions between this variant and so-called hyperplastic nodules are difficult to define; the distinctions are taxonomic.
8. MDM of other variant nevus-like type, such as combined nevus type, or cellular blue nevus type.
9. MDM of Spitz nevus-like type, as initially characterized, was a Spitz nevus-like lesion with a nodule showing a typical vertical growth component. It is important to emphasize that “Spitz nevus” is not a nevus in the same sense as a common nevocytic nevus. “Spitz nevus” is a true neoplasm, generally characterized by a history of rapid onset and growth. Of its many histologic features, the following are relevant to this discussion:
a. Most examples are associated with junctional patterns. The epidermal response in such lesions is hyperplasia and hypertrophy of keratinocytes. Individual cells and clusters of cells may migrate upward into the epidermis; these patterns, if encountered in a thin lesion, may lead the observer to a diagnosis of either melanoma in situ or microinvasive melanoma.
b. Some examples are thin, and show spread of the lentiginous and junctional component in the epidermis away from any dermal component. Such lesions tend to show moderate to moderately severe dysplasia, and usually are associated with markers for host immune response; some examples show marked dysplasia. The epidermal component shares may features (excepting cytologic features) with a common premalignant melanocytic dysplasia. A lesion showing this combination of features, and measuring less than 1 mm in vertical dimensions qualifies as “atypical” in the same manner as a common premalignant melanocytic dysplasia. Such a lesion could be characterized as an “atypical Spitz nevus” but the designation, “nevus,” would be inappropriate. Atypical spindle cell melanocytoma of Spitz nevus-like type (borderline lesion of indeterminate malignant potential) is a suitable designation. If, in the dermis, a nodule is represented in typical vertical growth patterns, then the lesion qualifies as a MDM, even if the nodule measures less than 1 mm in vertical dimensions.
c. Variant vertical growth-like patterns are native to common “Spitz nevi;” the pattern of variant vertical growth characterizes the distribution of nests and fascicles of tumor cells in the widened papillary dermis. The pattern of migrant vertical growth is also common in classical “Spitz nevi.” If emphasis is placed on distribution and spacing of dermal nests of cells, an observer may interpret such a lesion as an unqualified melanoma. The presence of a degree of cytologic atypia beyond that usually seen in classical “Spitz nevi” should be given some attention in the assignment of problematic lesions. Usually, in lesions showing worrisome atypia, there is also some degree of pleomorphism. Mitoses, including atypical mitotic figures, may be an addition feature of problematic lesions. Finally, there may be regional variations in patterns with a localized area in which fascicles of tumor cells are broader than in the remainder of the lesion; in such sites, the degree of cytologic atypia is usually greater than in other sites of the lesion. All these features are worrisome; they may be encountered in atypical “Spitz nevus-like” lesions, which on follow-up prove to be associated with metastases. Such lesions would additionally qualify as melanocytic neoplasia of indeterminate malignant potential.
d. “Spitz nevus-like” lesions in which a nodule shows typical vertical growth patterns should be carefully examined for evidence of worrisome cytologic features as defined above in section “c.” This combination of features is commonly encountered in a review of “metastasizing Spitz nevus-like lesions.”
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