Patterns of local horizontal spread of melanomas: consequences for surgery and histopathologic investigation.

Understanding local spreading patterns of melanomas is a precondition for the localized surgical treatment and histopathologic investigation. We used hematoxylin and eosin-stained paraffin sections for a two-phase, cellular and microscopic study of patterns of lateral spread in superficial spreading melanomas (SSMs), nodular melanomas (NMs), lentigo maligna melanomas (LMMs), and acral lentiginous melanomas (ALMs). Complete histologic examination of vertical excisional margins was carried out with paraffin sections 5 mm beyond the clinical tumor border of 1395 SSMs, 376 NMs, 179 LMMs, 46 ALMs, and 37 acrally located SSMs or NMs. Further sections of embedded material were analyzed when tumor-positive margins were found. In case of continuous tumor spread, reoperations were continued until the tissue was free of tumor cells. In case of noncontinuity, a final excision was made to a minimum safety margin of 10 to 20 mm. Concentrically consecutive, 5-microm thick hematoxylin and eosin-stained sections were taken from the outside of a 10-mm safety margin inward to the clinical borders of 34 SSMs, five NMs, 10 LMMs, and five ALMs. Noncontinuous subclinical spread was found in all SSMs and NMs in the form of few isolated cell nests at the epidermis-dermis junction. Ninety-two percent of these were located within 6 mm of the central tumor. All LMMs and ALMs showed a clearly demonstrable, uninterrupted spread into the periphery at the epidermis-dermis junction, too, usually in groups of outgrowths. The probability of finding these outgrowths 5 mm beyond the clinical tumor border was 54% in LMM and ALM. Complete histologic examination of vertical excisional margins (micrographic surgery) is therefore the therapy of choice only for LMM and ALM and is inefficient for SSM and NM.