By Gary M. White, MD
A slowly-growing, darker brown or black lesion with variegated color and/or shape is most typical.
Melanoma in situ (MIS) refers to a nevomelanocytic lesion with neoplastic melanocytes limited to the epidermis. Once cells penetrate into the dermis, the lesion becomes a melanoma. The term lentigo maligna (LM) refers to a subset of MIS where a slowly growing pigmented lesion occurs in the sun-exposed areas of an older person. The incidence of MIS has been greatly increasing. For example, in one study, the ratio of MIS:Melanoma was 0.09:1 in 1977 but was 1:1 in 2002.
The usual ABCDE criteria apply here. The MIS most closely resembles the superficial spreading melanoma. Irregular shape, color, dark black areas, etc. are key diagnostic signs. Amelanotic melanoma in situ may occur.
In one study in Connecticut [JAAD Sept 2012] that focused on MIS (either MIS or LM), the MIS to invasive melanoma case ratio during the 5-year period was 257/190 (1.35:1). In that same study, MIS occurred more frequently on the head-neck (119/257) than the trunk (67/257), a reversal of invasive melanoma distribution.
Complete surgical excision is the preferred therapy. In one study of 882 cases of MIS of the trunk and proximal extremities excised with Mohs micrographic surgery (MMS) aided by MART 1+ immunohistochemical staining [JAAD 2016;75;1015], there was only on local recurrence (0.1%). Analysis of their data showed that only 83% of MISs were excised with a 6 mm margin. Margins of 9 mm were needed to excise 97% of MIS of the trunk and proximal extremities. These authors recommend that MMS is the treatment of choice for MIS where available. However, if standard surgical excision is to be performed, a margin of 9 mm of normal-appearing skin should be used, similar to that recommended for early invasive melanoma. Because lentigo maligna histologically has atypical melanocytes trailing off at the edge vs a more discrete cut off for non-LM MIS (e.g. of the trunk and proximal extremities) a 9 mm margin would be recommended for LM treated with standard excision as well.
For further discussion of treatment options, e.g. imiquimod, XRT, CO2 laser, see lentigo maligna.
In general, patients should be reassured that although they must be screened yearly, their life expectancy is near normal. The trouble is that rarely, patients will die of "metastasizing MIS". Prompted by the death of a patient with apparent “metastasizing melanoma in situ”, one study reassessed 104 cases of MIS whose diagnosis had been determined with H&E. Immunohistochemical analysis using Mela-A/Mart-1 was employed and showed 30 of 104 (29%) had invasive melanomas. (positive staining cells in the dermis). Most (26/30) had a depth of < 0.5 mm. [The Lancet 2002; 359, 9321;1921]. Another more recent study confirmed this. It found occult invasive melanoma in 33% of previously diagnosed MIS [JAMA Derm 2016;152;1201].
Patients with MIS need annual surveillance for the rest of their life as the risk of developing a second melanoma is elevated and on par with (or in some studies even higher than [JAAD 2015;72;794]) patients with melanoma. For example, Youlden et al found that patients with a history of MIS are 4.6 times more likely to develop a subsequent primary melanoma compared to the general population [JAMA Dermatol. 2014;150:526-34].
MIS on the upper back.
Pink and brown together in the same lesion is a worrisome sign.
Rarely, MIS/lentigo maligna may be completely amelanotic.
This lesions turned out to be MIS of the nail plate apparatus.
MIS eminating at the edges from a compound nevus.
This was a recurrent lentigo maligna in the scar after previous excision.
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