The concept of melanoma in situ was issued for the first time in 1949, in the first edition of W. F. Lever's book on histopathology of the skin. Such a concept was accepted only slowly, because in that time, and in subsequent years, melanoma was a large pigmented tumor, usually in advanced phases of growth, often ulcerated with stellite nodules or metastases. Early melanoma, when the lesion is flat or only slightly elevated, was termed with other labels as "nevus", "lentigo", "melanosis". Such lesions were also considered as basically different from melanoma. Lever applied the term of melanoma in situ to early melanoma, in which malignant cells appeared confined to the epidermis; he, however, did not indicate it as a specific autonomous lesion. That lesion began autonomous only later, after the studies of A. C. Allen, who studied pigmented skin lesions in the same years. Allen noted the analogy between epithelial squamous lesions and melanocytic lesions (which he considered epithelial in nature). He compared melanoma to squamous cell carcinoma and proposed the analogy between solar keratosis, precursor lesion of squamous cell carcinoma, and junctional nevus, as precursor lesion of melanoma. In fact, Allen distinguished "quiescent" nevus, which had no relationship with malignancy, and "active" nevus, indicated as melanoma precursor. Melanoma in situ, however, was better understood after the general reconstruction of the natural history of melanoma illustrated by Clark at the end of 1960's. Clark ackowledged a distinct phase of development of melanoma, in which malignant cells were limited within epithelia, and called it level I melanoma. He well knew that level I melanoma and melanoma in situ were synonims, but did not consider that lesion a distinc clinicopathologic entity. He considered it only as research concept, important on theoretical ground, because it reprensented the early phase of development of the tumor, but unuseful on practical ground, as a diagnostic category to be included in the histological report. Melanoma in situ began a distinct autonomous lesion, recognizable and diagnosable clinically and histologically only at the end of 1970's, after Ackerman's studies. Ackerman, recalling the analogy between epithelial and melanocytic lesions, already noted by Allen, defined clearly the concept of melanoma in situ, lesion as autonomous as carcinoma in situ, and proposed for the histological diagnosis the same criteria used for unqualified melanoma. Thus, melanoma in situ is the first phase of growth of melanoma, in which it is still confined to the epithelial dominion (epidermis and adnexa) and has not passed through the basal membrane (Figs 1-3). As in epithelia there are not hematic or lymphatic vessels, in this stage, melanoma is not capable of metastasis. It is, therefore, a lesion potentially malignant, whose malignant potential, however, is not still expressed (i.e. it does not infiltrate adjacent tissues and does not give metastases). It is clear that a complete excision of a lesion such this leads to a complete recovery of the patient. Slightly different is the case of melanoma in situ showing signs of histological regression (lymphocytic infiltrate, fibrosis, melanophages) in the underlying dermis. In fact, regression indicates that, before the excision of the lesion, malignant cells infiltrated the dermis, even if subsequently they were destroyed by the tissue reaction. In such a context, it cannot be excluded that some cells have escaped to the desctruction and could reach lymphatic or haematic vessels. The risk is low, but not unreal; this may explain a proportion of sporadic cases of patients with melanoma in situ and metastases. Therefore, patients with melanoma in situ and regression require a follow-up more accurate than that of patients with tha same lesion, but without regression.

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The other pages

Home Page: Dermatopathology - C. Urso, MD

page 1: Melanoma of the skin

page 2: Histologic diagnosis of cutaneous melanoma

page 3: Melanoma arising in a melanocytic nevus

page 4: Infiltration and pseudoinfiltration

page 5: Sweat gland carcinomas

page 6: The sentinel lymph node in cutaneous melanoma

page 7: Bullous cutaneous diseases

page 8: Spitz nevus

page 9: Prognostic factors in cutaneous melanoma

page 10: The dysplastic nevus

page 11: Dermatofibrosarcoma protuberans

page 12: Melanoma in situ
page 13: Merkel cell carcinoma
page 14: Basal cell carcinoma
page 15: Melanoma in paediatric age

last page: Questions, comments and opinions