MINIMAL DEVIATION MELANOMA OF HALO NEVUS-LIKE TYPE:
Halo nevus is a well established clinicopathologic entity. In it, "nevus" cells and a lymphoid infiltrate are combined. If a dermal component of nevus cells is represented, lymphocytes intermingle among some or all of the dermal nevus cells to form lymphomelanocytic islands. The dermal nevus cells often show cytologic atypia with dense nuclear chromatin. In some nests, the small nevus cells may be pigmented in areas that are rarely pigmented in common nevi. A more significant degree of atypia may be a feature with variations in nuclear size and staining; often the large atypical nuclei show uniformly dense chromatin. One additional cytologic type is occasionally encountered; it is a plump, rounded cell with an eccentric nucleus showing dense nuclear chromatin. This type of epithelioid cell may be mistaken as a marker for a Spitz nevus variant.
In halo nevus, focal or complete regression of the nevus cell component is the anticipated clinical outcome. Histologically, a marked reduction in, or disappearance of, both "nevus" cells and lymphocytes leaves behind a vascularized, fibrous matrix. An explanation for the sequence leading to this alteration is usually expressed in most general terms.
The lesion, halo nevus, derives its name from the clinical appearance of a classic example; at the periphery of a pigmented punta, in a classic example, there is a symmetrical zone of depigmentation. Histologically, in the depigmented zone, spotty lichenoid patterns are associated with a loss of pigment in the basal layer of the epidermis. In the lesion proper, a lytic process affects "nevus" cells. In many examples, it is a dermal component that is most affected. Often, a lentiginous and junctional component is also represented in the overlying epidermis. When the lymphoid infiltrates have invaded the lentiginous and junctional components of the epidermis, the patterns of the halo phenomena acquire a "lichenoid" quality. If halo nevus-like reaction is acceptable as a variation of a "lichenoid" reaction, but distinguished from classic lichenoid reactions by the character of the target cells (i.e., melanocytic cells rather than keratinocytes), then a characterization of the halo nevus phenomena as a cytolytic process wouldalso be in keeping with the notion that the basic process has “lichenoid qualities”(the process is cell mediated cytolysis; an intimate relationship between target cells and T lymphocytes is basic to cell mediated reactions).
Fig. text1 (below): The pattern of a classic halo nevus is represented. A population of dermal "nevus" cells is obscured by a band-like infiltrate of lymphocytes. Nuclear atypia is not a significant feature. There are no patterns at the dermal-epidermal interface which might be mistaken for the patterns of a premalignant melanocytic dysplasia.
Fig. text2 (below, right): At higher magnification, a junctional nest is represented to the right (blue arrows). Lymphocytes have collected in the lytic defects of the junctional nest; the cytolysis of the halo nevus-like (i.e., modified "lichenoid") reaction is represented. The infiltrates hug the epidermis to the left of the center of the field and focally have migrated into the epidermal domain. The red arrows identify areas in which basement membrane material is duplicated (an additional "lichenoid-like" quality). The nevus cells of the dermal component, particularly near the areas of the duplicated basement membranes, show the minimal nuclear atypia usually found in the dermal component of classic halo nevus. There are variations in both nuclear size and nuclear staining. In the deep portion of the field to the right and below, lymphocytes are loosely spaced in a delicate, fibrous matrix. The nests of nevus cells, and individual nevus cells are small in this area. The pale, sparsely cellular matrix also might be interpreted as histologic evidence of the cytolytic effects of halo nevus phenomena .
DISCUSSION: The patterns as illustrated in the pictorials at TIER3are all representative of varying patterns in a single specimen (all representative of a consultation case submitted by: Dr. James Bootle, Atlanta Georgia). The lesion is remarkably variable from field to field. A good beginning would be to approach the variations as if they were representative of inobligate steps of neoplasia.
The steps selected for this discussion represent attempts to correlate patterns with proposed stages of neoplasia. The steps include: 1.) halo nevus-like lesions, 2.) atypical halo nevus (junctional type), 3.) non-inflamed dermal dysplasia of halo nevus-like type, 4.) near-neoplasia of halo nevus-like type, dermal variant (dermal lymphomelanocytic islands), and 5.) MDM of halo nevus-like type.
Figures 3-5 , as limited, selected fields, are of a type which might be seen in the dermal component of a classic halo nevus. The degree of atypia infigures 3 & 4 is a prominent feature but is acceptable in the setting of halo nevus. In figure 5, a pattern of regression is represented and might be accepted as within the spectrum of the variations in patterns encountered during the evolution of a halo nevus. On the other hand, the pattern in figure 5 might be cited as an area of regression in a premalignant dysplasia (as commonly encountered in the dysplastic nevus syndrome). In areas of early regression, lymphomelanocytic islands may be surrounded by duplicated hyaline membranes.
The patterns infigures 1, 2, & 5 might be characterized as being in keeping with those of an atypical halo nevus (i.e., lentiginous and junctional patterns of a premalignant dysplasia in combination with halo nevus-like phenomena). This component, if allowed to progress, might evolve into a lesion resembling a common melanoma. Only in the presence of a remnant of halo nevus-like phenomena is it possible to identify the lesion as "halo nevus-like variant."
The patterns infigures 6 & 7 are those of near-neoplasia of halo nevus-like type (representation of a portion of a nearby vertical growth component is insufficient to influence the interpretation of these 2 fields). The nests in which plump, rounded melanocytic cells are loosely spaced with lymphocytes among the "epithelioid" cells are lymphomelanocytic islands of the of halo nevus-like type (see CD on Halo Nevus Variants). In this example, the islands may be altered nests which have recently relinquished a junctional position; they may not (and need not) have evolved from cells of a preexisting dermal component. In neoplastic progressions, either the dermal or the epidermal component may take precedence; in some examples, perhaps both components contribute. The plump, epithelioid cell variant is most characteristic of progressions in the dermal component.
Figures 8, 9, & 15a are representative of "nevoid" patterns. It would be easy to dismiss these patterns as those of common nevus and simply ignore the cytologic atypia, and the minimal deviations in aggregate patterns. On the other hand, the lesion may well represent a non-inflamed, dermal dysplasia of halo nevus-like type. It may be a lesion within the span of neoplastic progressions of halo nevus-like type but one devoid of markers for host immune response. Lastly, some observers might accept the patterns as those of a peculiar, thin, minimal deviation, nevoid melanoma. To be most accurate, the patterns might qualify as those of a borderline melanocytic neoplasm of indeterminate malignant potential. The latter characterization has applicability for a variety of thin melanocytic neoplasms, including both precursor lesions, and small, young "melanomas."
Figures 10-14 are representative of portions of vertical growth-like patterns of spindle cell type. In addition some of the features of the precursor stages are preserved in the underlying dermis. The solid plaque is prototypic of vertical growth, or vertical growth-like patterns. Such patterns, if nevoid and cytologically monotonous, qualify the respective lesion as nevoid minimal deviation melanoma.
In figures 15b&c, the patterns of a lymphomelanocytic island are contrasted with those of a portion of the vertical growth-like component (the latter probably representing progression in the component derived from the epidermis rather than a dermal component. The round, "epithelioid" cells are easily dismissed as Spitz nevus-like and, in turn, a lesion in which nests of such cells are represented is easily assigned to the category of "Spitz nevus." The latter has become a default category to which a variety of benign and malignant lesions can be assigned, often with embarrassing consequences.