ALM can occur on the palms, soles, digits or beneath the nail plate. The lesion is characterized clinically by a tan, brown to black flat lesion with variations in color, and irregular borders. Papules or nodules are often present5
. ALM is traditionally believed to have a less favorable prognosis than other forms of melanoma. However, this may be due to the delayed diagnosis of more advanced cases; and ALM and other types of melanoma with same tumor thickness have been shown to have an equal prognosis. The early stages of ALM usually show a biphasic growth pattern, demonstrating only a proliferation of atypical melanocytes within the epidermis, termed ALM in situ6
Several articles have addressed the clinicopathological characteristics of early lesions of melanoma that affect the acral areas. Saida4
proposed different phases of tumor growth corresponding to the histopathological features of pigmented macules of melanoma in situ
on acral skin. His phase 1 features both cytologically normal and abnormal melanocytes in increased numbers, disposed singly, largely in the basal layer of epidermis, with larger numbers of cells, abnormal cells and cells above the junction in phase 2, and almost exclusively abnormal cells, with many above the junction in phase 3. Frankel7
suggested a similar classification for intraepithelial melanocytic proliferation in the spectrum of melanoma in situ
, in part to circumvent that term. Frankel's concept involves the progression of intraepithelial melanocytic proliferation (IMN-I through-III). Our case largely corresponds to Saida's phase 1 or Frankel's IMN-I lesions.
Cho et al2
reported two cases showing black discoloration of the thumb nail which were histologically found to be ALM in situ
. Their cases were particularly interesting because atypical melanocytic hyperplasia was confined to the epidermis despite the lesion being present for a long time: 12 years and 30 years, respectively. Nogita et al3
reported a series of cases with only a few scattered foci of single cell melanocytic proliferation with minimal atypia, and they designated their cases as atypical melanosis of the foot.
Recently, Kwon et al5
identified 9 patients who had ALM clinically, but their lesions showed melanocytes with minimal cytologic atypia confined to the epidermis, and this was regarded as ALM in situ
. The duration of time starting from when these patients first noticed the pigmented lesions up to when they first visited the clinic ranged from 5 to 30 years (average duration: 13.3 years).
Our case had also shown a few hyperplastic, atypical melanocytes, which were scattered in a single fashion. These melanocytes had remained confined within the epidermis over a 7-year period; the lesion developed into an invasive ALM during an overall 12-year period. We overlooked the malignant potential of the initial lesion, and it had unfortunately progressed. Our case supports the suggestion that ALM in situ may exist before the evolution of invasive ALM, and it can present with a minimal number of atypical melanocytes.
In conclusion, the separation between ALM in situ and atypical melanosis of the foot may be quite difficult; therefore, we suggest that ALM in situ be preferred to other rather obscure diagnoses such as atypical melanotic hyperplasia and atypical melanosis of the foot. Any atypical melanocytic lesion on the foot should be considered as ALM in situ, and must be totally excised, along with a thorough pathological examination of the specimen and close follow-up of the patient.