Acral lentiginous melanoma: indolent subtype with long radial growth phase.

Acral lentiginous melanoma (ALM) is the most common type of cutaneous melanoma in Asian populations. Traditionally, ALM was believed to have a poorer prognosis than other subtypes of cutaneous melanoma because of its aggressive behavior. However, in Asians, there have been several unusual case reports that have shown only subtle melanocytic proliferation despite clinically malignant manifestations. We performed a retrospective study of 13 patients with ALM. We reviewed the clinical histories, histopathologies, and immunohistochemical staining from these patients. Clinically, the lesions were characterized by a brown to black pigmented patch with irregular borders and variegated pigmentation on the sole, heel, or finger. Histopathologically, most specimens revealed only bland scattered proliferation of atypical melanocytes without marked cellular atypia or dermal invasion. However, some of the lesions were suspected to progress to the invasive stage, into the papillary dermis after a long period of time. Although cytological atypia of the melanocytes is not sufficient to ALM in situ, the melanocytic proliferation pattern, dermal inflammation, and correlation with clinical presentation would suffice in making the correct diagnosis of ALM in situ. In these cases, more biopsies of other areas should be recommended for the precise diagnosis and early complete excision in accordance with ALM in situ.