Background Acral lentiginous melanoma (ALM) is the fourth histopathological subtype of malignant melanoma, accounting for < 10% of all melanomas in white-skinned populations. It is characterized by a lentiginous pattern of proliferation of the intraepidermal component of the tumour. Its individualization is still controversial, especially in regard of its prognostic value.
Objectives To characterize better ALM from a pathological point of view and to assess the prognostic value of all histopathological features of ALM.
Methods We performed a review of all cases of ALM followed from 1996 to 2004 at the University Hospital Department of Dermatology, Lyon, France. We examined all haematoxylin, eosin and saffron-stained tissue sections of the primary lesions. Several pathological parameters of interest in melanoma were evaluated for disease-free and specific survival with the Kaplan–Meier method and the Cox proportional hazards regression model.
Results Representative histological material was available for 121 patients. The mean Breslow thickness was 2·5 mm (in situ–20 mm). Fifteen lesions (12%) were in situ, nine (7%) were at Clark level II, 35 (29%) at III, 40 (33%) IV and 22 (18%) V. Extension along adnexal structures was found in almost half of the ALMs (46%), without prognostic significance. Seventeen (14%) lesions showed no microscopic pigmentation. Remnants of pre-existing naevus were found in four (3%) melanomas. The width of the 36 (30%) ulcerated lesions ranged from 1 to 20 mm (mean 7·6). Ulceration and its width were both associated with a large tumour thickness (P < 0·01), a high level of invasion (P < 0·01), the presence of vascular invasion (P < 0·01) and the lack of pigment production (P < 0·01). Among the 99 ALMs which were in the vertical growth phase (VGP), 21 showed a high mitotic rate (> 6 mitoses mm−2). A high mitotic rate was found to be significantly associated with the presence of ulceration (P < 0·01). The presence of microscopic satellites was noted in 10 (10%) lesions. The uncommon presence of small cells (8%) in the VGP was statistically significantly (P < 0·01) associated with a worse prognosis compared with other cell types. Multivariate analysis identified mitotic rate (P < 0·01), microsatellites (P = 0·05), Clark level (P = 0·01) and gender (P = 0·03) as independent prognostic factors for disease-free survival. Only the presence of microsatellites (P = 0·02) and a high mitotic rate (P < 0·01) were independently correlated with specific survival in ALM.
Conclusions This is a detailed pathological study of a large cohort with ALM, an uncommon subtype of melanoma. Mitotic activity appears to be of particular importance in predicting the outcome of ALM.