Anatomic Pathology / GRADING ATYPICAL MELANOCYTIC NEVI

Low concordance in grading atypical (dysplastic) melanocytic nevi (AMN) has been reported, and no systematic evaluation is available. We studied 123 AMN with architectural and cytologic atypia (40 associated with atypical-mole syndrome), classified according to standard criteria by 3 independent observers. Histologic variables included junctional and dermal symmetry, lateral extension, cohesion and migration of epidermal melanocytes, maturation, regression, nuclear features, nuclear grade, melanin, inflammatory infiltrate location, and fibroplasia. AMN (43 junctional and 80 compound) were graded mild (31), moderate (61), and severe (31). AMN-severe correlated with 3 or more nuclear abnormalities (especially pleomorphism, heterogeneous chromatin, and prominent nucleolus) and absence of regression, mixed junctional pattern, and suprabasilar melanocytes on top of lentiginous hyperplasia. AMN-severe diagnostic accuracy was 99.5% using these criteria, but only the absence of nuclear pleomorphism differentiated AMN-mild from AMN-moderate. No architectural features distinguishing AMN-mild from AMN-moderate were selected as significant by the discriminant analysis. AMN from atypical-mole syndrome revealed subtle architectural differences, but none were statistically significant in the discriminant analysis. Histologic criteria can reliably distinguish AMN-severe but fail to differentiate AMN-mild from AMN-moderate. AMN from atypical-mole syndrome cannot be diagnosed using pathologic criteria alone. Atypical (dysplastic) melanocytic nevi (AMN) are an important marker for increased melanoma risk, demonstrating an 8-fold or greater association with malignant melanoma.1,2 One clinically atypical nevus has been associated with a 2-fold risk, while 10 or more confer a 12-fold increased risk.3 AMN histology is well characterized, allowing a reliable distinction from common acquired nevi and malignant melanoma,4,5 but the reported agreement on the degree of atypia is normally low.5,6 In addition, the cytologic atypia seen in such lesions has been suggested to be incidental to a proliferative radial growth rather than indicative of dysplasia.7 AMN have been defined mainly by architectural and cytologic criteria that include basilar proliferation of atypical melanocytes in a lentiginous or nested pattern and an association with the following: (1) the presence of lamellar fibrosis or concentric eosinophilic fibrosis, (2) neovascularization, (3) inflammatory response, and (4) fusion of rete ridges.8 These histopathologic characteristics have correlated objectively with nuclear atypia,1,9 improving the consensus and reproducibility of AMN diagnosis.10 Therefore, the minimal essential histologic criteria proposed for AMN must be based on both nuclear atypia and abnormal patterns of intraepidermal melanocytic proliferation.1,11 Any grading system must be able to consistently recognize 2 or more grades of atypia with biologic relevance.12 The subjective definition for grading of dysplasia in AMN has resulted in no consensus on that matter, resulting in a concordance ranging from 35% to 58% (kappa value, 0.380.47) for experienced dermatopathologists and from 16% to 65% (kappa value, 0.05-0.24) for those with less experience. However, a statistically significant relationship has been described between melanocytic dysplasia and the person’s Anatomic Pathology / ORIGINAL ARTICLE Am J Clin Pathol 2001;115:194-204 195 © American Society of Clinical Pathologists total number of melanocytic lesions, indicating that the pathologic grading scheme may be useful.13 Further refinement of the criteria for grading melanocytic dysplasia and experience in grading are critical for accuracy in subcategorization of AMN.5 No systematic evaluation of the histologic criteria for grading AMN is available to date. This article describes a statistically validated system for grading melanocytic dysplasia in a series of AMN, a third associated with atypical-mole syndrome, prioritizing the histologic features. We also performed a detailed comparison of histologic features in sporadic AMN with those associated with the atypical-mole syndrome. Materials and Methods We retrospectively studied 123 consecutive lesions clinically diagnosed as atypical melanocytic nevi from 111 patients biopsied between January 1, 1989, and December 31, 1998, in St Bartholomew’s and the Royal London Hospitals, London, England. All nevi fulfilled the standard definition of AMN3,14; the clinical records were reviewed for patient age, nevus location, and familial or personal history of atypical-mole syndrome or malignant melanoma. All specimens were fixed in 10% buffered formalin, routinely processed, serially sectioned, and stained with H&E. All nevi showed architectural and cytologic atypia and were evaluated histologically and graded by 3 independent observers (R.C., A.B., and S.J.D.-C.), according to standard criteria (see “Degree of atypia,” below).1,5,8 In case of grading disagreement, the lesions were discussed during simultaneous inspection before final categorization. Reproducibility data were not recorded. No case had evidence of scarring, incomplete resection precluding adequate evaluation, or features suggesting congenital onset. A standardized protocol was developed for reviewing each lesion, including the following histologic features: 1. Degree of atypia. The grading was evaluated on multiple H&E-stained routinely processed sections of the whole lesion. Standard criteria were used for that purpose.5,11 Those criteria included the presence of lentiginous melanocytic hyperplasia (from discontinuous to confluent), upward melanocyte migration (little or no to fully pagetoid spread), nesting variation and bridging (few nests to confluent nests), and nuclear features comprising enlargement (compared with basal keratinocyte nuclei), pleomorphism, presence of nucleoli, and hyperchromatism. All these variables were evaluated independently and considered positive if present in at least 50% of the sample. All AMN were required to show architectural and cytologic atypia (at least 1 criterion from each group) and were scored according to the number of criteria fulfilled as mild (2-3 criteria), moderate (4-5 criteria), or severe (6-7 criteria). Processing artifacts were found to influence the cohesion of cells in nests (from preserved to diminished); therefore, this feature was excluded from the final classification. 2. Nevus type was registered according to standard criteria as junctional or compound. 3. Lateral extension, studied by the presence and pattern of melanocyte proliferation extending at least 3 rete ridges or “pegs” beyond any dermal component. The pattern at this level was classified with the same criteria used for the general junctional pattern. 4. General junctional pattern. The pattern was classified according to the predominant pattern as lentiginous (basilar melanocytic hyperplasia mainly), nested (melanocyte nest predominating), or mixed lentiginous-nested. 5. Junctional symmetry was judged by 2 separate issues: (1) symmetry evaluated by the number of involved rete ridges beyond the dermal ridge at both ends (rete ridge differences >3 were considered evidence of asymmetry) and (2) symmetry judged by the junctional pattern at both ends of the nevus (lentiginous, nested, and mixed). The presence of the same pattern at both sides was considered evidence of “side-to-side symmetry.” Otherwise the lesion was registered as asymmetric. 6. Junctional nest features comprised orientation evaluated by the predominant pattern (>50% of the lesion) and cohesion. Only sharply demarcated melanocyte nests were considered cohesive, regardless of the cellular cohesion within the nests. 7. Presence of suprabasal melanocytes migrating upward beyond the epidermal basal layer in at least 2 highpower fields. The location of suprabasal melanocytes in relation to intraepidermal theques and lentiginous hyperplasia also was recorded. 8. Melanocyte maturation and nest size in the dermal component. A progressively decreased melanocyte and dermal nest size was considered evidence of maturation. Cell type maturation (round or spindle cell) also was considered. 9. Predominant cell type for junctional and dermal components (if applicable) was evaluated as described in No. 8. 10. Nuclear features. The following variables were analyzed independently and registered as positive if present in more than 50% of melanocytes: • Size. The absolute assessment of nuclear size is reliable only by morphometric evaluation. Therefore, only anisokaryosis, which needs no external control, was studied for a more accurate analysis. Nuclear size variation more than 3:1 was considered evidence of anisokaryosis. • Pleomorphism was defined as irregularity in nuclear contour associated with nuclear grooves. The presence of nuclear pseudoinclusions also was recorded. Pozo et al / GRADING ATYPICAL MELANOCYTIC NEVI 196 Am J Clin Pathol 2001;115:194-204 © American Society of Clinical Pathologists • Chromatin distribution in fine and homogeneous (euchromatic) or gross and heterogeneous (hyperchromatism) patterns was registered. • Presence of prominent nucleolus. 11. Nuclear grade was coded as high if at least 3 of the aforementioned nuclear features were suggestive of malignancy. 12. The presence of melanin granules in the cytoplasm of melanocytes was recorded along with granule size in finely divided granules (dusty cytoplasm) or large granules at least one third the size of an erythrocyte. 13. Location of inflammatory infiltrate. Lymphocytic infiltrate was coded by its predominant location in epidermal, perivascular, or interstitial locations. Lesions with no preferential location were registered as nonspecific. 14. Both presence/absence and degree of fibroplasia were used to evaluate the stromal changes. Fibroplasia was registered as concentric eosinophilic fibrous tissue or lamellar (delicately layered fibrous tissue with entrapped spindle-shaped cells). 15. Regression was identified histologically by melanocyte dropout, dermal fibrosis different from lamellar fibroplasia, increased vascularity in the dermis, and a variable degree of lymphocytic infiltrate with melanophages. All variables were analyzed statistically using the Fisher exact test for qualitative variables and analysis of variance and the Student t test for quantitative variables. The data were compared for the degree of atypia, the specific distinction between AMN-mild and AMN-moderate, and the differentiation of AMN associated with atypical-mole syndrome. Differences were considered statistically significant if P < .05, and significant variables were used for a stepwise multivariate analysis.15