Macromelanosomes: their significantly greater presence in the margins of a lentigo maligna versus solar lentigo.

Background: Macromelanosomes are melanin-containing granules characterized by their large size and spherical or ellipsoidal morphology. They are reported to be present in a variety of pigmented skin lesions, including lentigines. However, there is limited information on macromelanosomes in malignant melanocytic proliferations. The margins of a lentigo maligna/malignant melanoma in situ (LM/MMIS) and solar lentigo share morphological similarities, including lentiginous proliferation of melanocytes, increased melanin in basal cell layer keratinocytes, and solar elastosis. This may represent a potential diagnostic pitfall, particularly in small biopsies. We sought to identify whether the presence of macromelanosomes at the lesional margins of LM/MMIS may represent a morphological clue in distinguishing between these 2 entities.

Methods: Data were obtained from 2 different institutions between 2001 and 2010. 619 cases of solar lentigo and 117 cases of LM/MMIS were screened for the presence and distribution of macromelanosomes.

Results: 25 of 117 cases (21%) of LM/MMIS had macromelanosomes identified at the lesional margins, compared with 7 of 619 (1%) of solar lentigo cases (P < 0.0001).

Conclusions: Our data demonstrate that the identification of macromelanosomes within a lentiginous melanocytic proliferation should prompt further evaluation to rule out the possibility of a contiguous LM/MMIS, particularly in a small biopsy.