[Unusual and unknown aspect of cutaneous malignant melanoma: minimal deviation malignant melanoma. Retrospective study of 45 cases].

Minimal deviation malignant melanoma (MDMM) is a rare melanocytic tumor of the skin that shares both malignant and benign histologic features: 1) a vertical growth phase similar to that of malignant melanomas with expansile nodules invading throughout the reticular dermis (Clark's level IV), and even the subcutaneous fat (Clark's level V); 2) but a monotonous and uniform proliferation of melanocytic cells that are only moderately atypical. A very slight cellular maturation from the top to bottom of the lesions may be observed, but usually there is no maturation at all. By themselves, the cells do not appear as malignant. A borderline variant of MDMM shows identical cytological and growth pattern features, but remains confined to a Clark's level III of invasion. Over the past 14 years, 45 cases (female to male ratio: 1.5:1.0) of MDMM, 8 of which were of the borderline variant, were observed. The lesions appeared as acquired pigmented tumors, 0.5 to 1.0 cm in size which predominated on the trunk. Young adults (mean age: 34 years) were most frequently involved. Based on the cytological characteristics of the cellular proliferation, they could be subdivided in 4 groups: 1) epithelioid and spindle cell type (15 cases); 2) epithelioid type (14 cases); 3) spindle cell type (7 cases); and 4) pigmented spindle cell type (9 cases). The mean thickness of the 45 cases was 3.06 mm (1.24 mm for the borderline lesions; 3.40 mm for the MDMM). Tumors with a spindle cell component appeared thicker than those without. Mitoses were numerous (mean: 3.2/10 high power fields). Each tumor but one showed a junctional component. This appeared as melanocytic hyperplasia or melanocytic nests at the dermoepidermal interface. Moreover, 10 cases (7 MDMM and 3 borderline tumors) disclosed intraepidermal spread of melanocytes. However, as observed with the dermal component of the lesions, these intraepidermal melanocytes never appeared cytologically malignant, although moderate atypism could be observed. At last, an association with a compound or dermal nevus was seen in 8 cases. MDMM appears as a particular subgroup of cutaneous malignant melanomas with distinct and characteristic histologic features. Its differential diagnosis includes blue nevus, especially the cellular variant, combined nevus, spindle and epithelioid cell nevus (Spitz-Allen's nevus) and cutaneous metastasis of malignant melanoma. The most important features for the differential diagnosis are the growth pattern, the absence of cellular maturation, the absence of real malignant cells and moderate cellular atypism.(ABSTRACT TRUNCATED AT 400 WORDS)