Subungual melanoma: Histological examination of 50 cases from early stage to bone invasion

Abstract

Subungual melanoma is a rare form of malignant melanoma. It is extremely difficult to differentiate it histologically from benign melanonychia striata or melanocytic nevus, especially in the early stage. We divided 50 cases of subungual melanoma into four groups according to clinical progress, and examined their histological findings in each respective stage. In the early stage (19 cases), atypical melanocytes were polygonal showing slight nuclear atypia with no mitoses at all. In six out of 19 cases (31.6%), the atypical melanocytes proliferated more in the hyponychium than in the nail matrix, and only very few in the nail bed. Periungual pigmentation (Hutchinson's sign) appeared from the early stage in almost all cases. With stage progression (middle stage, 13 cases; progressive stage, 13 cases; and bone invasive stage, five cases) the number of atypical melanocytes and their degree of nuclear atypia increased, and the ascent of atypical melanocytes and pagetoid spread became conspicuous. Mitoses became apparent only from the progressive stage. From these observations, we would like to propose three new pathological clues of early stage subungual melanoma: (i) “skip lesion”, proliferation of the tumor cells are more prominent in the hyponychium than in the nail bed or nail matrix; (ii) histological confirmation of Hutchinson's sign; and (iii) epithelial thickening and/or compact arrangement of the elongated basal cells.

Histopathological analysis of the progression pattern of subungual melanoma: late tendency of dermal invasion in the nail matrix area

“…2,8,13 In this study, although all cases seemed to originate from the nail matrix, there were no cases with dermal invasion that involved only the nail matrix and four cases with dermal invasion involved areas of the nail unit other than the nail matrix area. When dermal invasion occurred in the nail matrix as well as other areas of the nail unit, the tumor in other areas of the nail unit was thicker than that in the nail matrix area.…”

“…2,3,8,12 Unlike the rest of the skin, the complex anatomy of the nail apparatus poses some challenges in determining the depth of subungual melanoma accurately, which may explain why there is no consensus on the method of determining the thickness of subungual melanoma. 2 Because the thickness of epidermis in each area of normal nail unit varies, dermal invasion may be overestimated depending on where the melanoma is measured.…”

“…[5][6][7] Subungual melanoma usually originates from the nail matrix and may spread to the surrounding nail bed, hyponychium, and proximal nail fold ( Figure 1). 2,8 Little is known about the progression pattern of subungual melanoma and the rate of dermal invasion in each area of the nail unit. Previously, Izumi et al 8 reported that in early subungual melanoma, proliferation of the atypical melanocytes was more prominent in the hyponychium than the nail bed or nail matrix.…”

“…2,8 Little is known about the progression pattern of subungual melanoma and the rate of dermal invasion in each area of the nail unit. Previously, Izumi et al 8 reported that in early subungual melanoma, proliferation of the atypical melanocytes was more prominent in the hyponychium than the nail bed or nail matrix. If dermal invasion occurs more frequently in the nail matrix, an area with the shortest distance to the bone within the nail unit, conservative treatment may not be appropriate for early stages of subungual melanoma.…”

A Hidden Threat: Subungual Melanoma in Hand

“…This is because the atypical melanocytes have subtle degree of atypia and mitosis that cannot be detected due to their small sizes. Features favour subungual melanoma includes "skip lesion" (proliferation of atypical melanocytes in hyponychium is greater than that from nail matrix) and histological confirmed Hutchinson's sign [23].…”