Histopathology of melanoma.

The accurate histopathological evaluation, the diagnosis, and the assessment of level of invasion of malignant melanoma have assumed a role of paramount importance in defining need for further surgery, a chemotherapeutic or immunotherapeutic approach to management, and prognosis. The classification of melanoma into lentigo maligna melanoma, superficial spreading melanoma, and nodular melanoma and further breakdown of those invasive melanomas into levels of invasion (Levels I-V) is the current practice and reflects the fact that the overall classification is clinocopathological rather than defined strictly by clinical or pathological criteria alone. Since any of the various types of melanoma may be invasive, the current terminology for determining invasion may be briefly reviewed. The definitions are set in anatomic terms so that there would seem to be little room for subjective variation in pathologic staging. By and large, this method has proven useful with a high percentage of unanimity of opinion among pathologists and dermopathologists (1) (Fig. 1). Level I. In Level I melanoma, all of the tumor cells are above the basement membrane. This, by definition, is in situ melanoma. Level II. This anatomic level denotes involvement of the papillary dermis by malignant cells. The papillary dermis is composed of delicate collagen and ground substance and lies immediately deep to the basement membrane. It is most easily recognizable on low power scan of normal skin as that homogeneous eosinophilic area undulating in apposition to the rete pegs of the epidermis. An important additional consideration is that the papillary dermis surrounds skin appendages that may themselves plunge into the deeper portions of the skin. Hence, periappendigeal tumor invasion may still be classified as Level II if the reticular dermis is not violated. Level III. Here one sees invasion at the interface between papillary and reticular dermis. In this category there is some room for subjective variation in interpretation as this level is the least well defined anatomically. The reticular dermis is that portion of the dermis where the collagen is organized into readily distinguishable bundles. The interface between papillary and reticular dermis is Level III, although the anatomic demarcation may be indistinct due to a gradual transition from what is clearly papillary to what is clearly reticular dermis. There is additionally some variation in organization of the various layers of the dermis depending on the site of the sample. Other problems include distortion of the anatomy at this level due to scarring or inflammation together with the inevitable problem of deciding on a cell by cell basis which cells are benign, malignant, inflammatory, reactive fibroblasts, etc. Previous usage allowed Level III to be defined as widening and filling of the papillary dermis with impingement on the reticular dermis. It should be mentioned that the presence of single cells, strands of cells, or small nests of cells in the transition zone between papillary and reticular dermis or between collagen bundles of