mitoses seen in SCC. All other criteria of SCC can, unfortunately, be seen in pseudoepitheliomatous hyperpasia including irregular inÞltration by variably sized groups of keratinocytes with mitoses and surrounding hostÕs inßammatory reaction. Subepidermal clefting could be due to artifacts or due to dense lichenoid/interface lymphocytic inÞltrate or due to falling of a superÞcial component of BCC. A request for a recut from the Mohs surgeon is warranted in this situation.

13.3.1.2 Melanocytes and the Melanocytic Lesions

Evaluation of melanocytic lesions in frozen sections can be challenging. Mohs sections for melanoma should be thin (2Ð4 mm) and properly stained. The two main challenging questions are, (1) is this a melanocyte and, if so, (2) is it a malignant melanocyte or not. Normally, melanocytes are seen within the basal cell layer of the epidermis, hair bulbs and outer root sheath of hair follicles [9]. In hematoxylinÐeosin-stained sections, the dendrites of epidermal melanocytes are not seen and the cells are usually surrounded by small spaces or a

clear halo, due to the lack of attachment by desmosones and shrinkage during processing. When interpreting Mohs slides for melanoma, the Mohs surgeon should be aware of the normal variation of the number of melanocytes within the body regions. The number of melanocytes increases with exposure to ultraviolet light, but the average number of melanocytes to keratinocytes in the basal epidermal cell layer is approximately 1:10 in normal skin [9, 10]. The highest numbers of melanocytes are seen on the face and the male genitals, and the lowest is on the trunk [11]. There is no signiÞcant variation in the density of distribution of melanocytes for a given region of the skin between Caucasoid and African American skin. Melanocytes tend to be larger and more dendritic in African American skin than in Caucasoid skin [12]. It is expected to see more melanocytes in sections taken from the face and the dorsal hand, as opposed to the back. In long standing sun-exposed skin, there is a moderate increase in the number of melanocytes to approximately 3Ð6 adjacent melanocytes without exhibiting overt conßuence, Pagetoid spread, or nesting [13] (Fig. 13.12). It is also important to know

that the process of melanin transfer from melanocytes to keratinocytes is normal and increases with ultraviolet light exposure. Hence, in sun-damaged skin, melanocytes increase in number and in size. The mere presence of melanin pigmentation in melanoma margins does not indicate a positive margin. The Mohs surgeon should be careful in interpreting sections of melanoma from patients with dark skin color. We found that Mohs sections of acral lentiginous melanoma and melanoma of the genital skin can be challenging. In these circumstances, we found that some Mohs surgeons rely on pigmentation alone to assess the margin if there is no melanocyte. If the next stage does not show any overt melanocytic proliferation and shows only pigmentation, this should be interpreted as a negative margin and sent for permanent section control, provided that an adequate margin was excised depending on the BreslowÕs thickness of the tumor [14]. Some Mohs surgeons take an extra stage, if pigmentation alone is present, to assure complete excision. Again, this last stage should be sent for permanent section control.

Margins of melanoma pose a challenging point not only to the Mohs surgeon but also to some dermatopathologists. Most dermatopathologists comment on the presence of large, somewhat, atypical melanocytes in the margin. The most important point in differentiating positive from negative melanoma margins is the presence of conßuence, Pagetoid spread, or nesting. Sometimes, solitary, large hyperchromatic, and atypical melanocytes can be seen, signifying a positive margin. Unfortunately, there is no immunohistochemical marker that helps in such differentiation. HMB-45 stains both normal and neoplastic epidermal melanocytes. Immunohistochemical staining for Melan-A or Mart-1 is sensitive and speciÞc for melanocytes, but we found that a thin cut and properly stained Mohs section minimizes the need for immunohistochemical staining. The latter should only be used in equivocal cases.

13.3.1.3 Interpretation of Melanoma

in Frozen Sections

Generally speaking, the histologic criteria for melanoma in situ include (Fig. 13.13): (1) conßuence of

atypical melanocytes along the dermo-epidermal junction with extension along adnexal structures with or without nesting. (2) Extension of atypical melanocytes at the upper reaches of the epidermis (Pagetoid spread) and (3) presence of large starburst/giant cells. The latter should be differentiated from large Bowen cells in casesofPagetoidBowendisease.Immunohistochemical staining is greatly helpful where Melan-A or Mart-1 stains melanoma cells, but not Bowen cells [15].

Invasive melanoma shows inÞltration of the dermis by individual cells or atypical, variably sized groups or sheats of melanocytes. These invasive melanocytes can exhibit different cytologic features: epithelioid, nevoid, or even spindle shape. They do not express the normal maturation with descent seen in banal nevi. Melanoma cells show large/open-face nuclei with prominent nucleoli. A variable degree of mitotic activity can be seen. Destruction or effacement of adnexal structures can be seen and sometimes angiovascular invasion can be noted.

MMS for melanoma should only be performed by an experienced Mohs surgeon who is well adept and is very familiar with the histologic changes of different melanocytic lesions. The most common histologic challenges in MMS for melanoma are (1) the melanocytic hyperplasia seen in sun-damaged skin, actinic keratosis, and lentigines. These can mimic melanoma in situ, especially when they are tangentially cut. They should be differentiated from melanoma in situ by the absence of melanocytic atypia and the absence of conßuence or nesting of melanocytes. (2) The presence of an associated benign melanocytic nevus. The latter is identiÞed by its benign cytology, maturation with descent, and the symmetry of the lesion. (3) Pagetoid Bowen disease, which can be differentiated from Pagetiod melanoma, as discussed earlier. (4) Collections of Langerhans cells at the upper reaches of the epidermis can be seen as an incidental Þnding and can mimic melanoma in situ (Fig. 13.14). S-100 is not helpful since it stains both melanocytes and Langerhans cells. CD-la is helpful

because it decorates Langerhans cells speciÞcally. Immunohistochemical staining for Melan-A and Mart-1 is very helpful in this situation [16]. (5) Dermal reactive Þbrohistiocytic inÞltrates can mimic invasive melanoma. The latter shows large atypical melanocytes arranged in groups, sheets, or fascicles and show the classic melanocytic cytology. In cases of desmoplastic melanoma, the differentiation can sometimes be difÞcult and immunohistochemical staining for Melan-A is usually helpful.

(6) Tangentially cut reactive endothelial cells can mimic epithelioid melanocyte. Deeper levels would help in identifying the anatomic location of these cells in relation to a vessel, and again Melan-A is a helpful tool in such differentiation, CD31 and Factor VIIIÐrelated antigen stain endothelial cells but not melanocytes. (7) A tangentially cut intraepidermal sweat gland duct can sometimes mimic a melanocytic nest. Deeper section showing the duct or a cuticular material can be helpful in such differentiation. Immunohistochemical staining for cytokeratin (positive duct and negative melanoma) can be helpful.

13.3.1.4 The Pilosebaceous Unit

The hair follicle is divided into four zones from bottom to top: the hair bulb, the suprabulbar zone (where different layers of the hair follicle starts to appear), the isthmus (between the insertion of the arrector pili muscle and the entrance of the sebaceous duct), and the infundibulum (uppermost part of the hair follicle and greatly resembles the epidermis histologically). The bulb of terminal hairs lies deep in the dermis at the junction with or within the upper subcutaneous adipose tissue. If one sees a bulbar structure at the upper dermis, it is either a vellus hair or a telgen hair (the resting stage of the hair follicle). A cross section of the latter shows irregular island of basaloid cells with a variable degree of peripheral palisading. This can be easily differentiated from basal cell carcinoma by the irregular outline, the surrounding mature perifollicular Þbrous sheath, and sometimes, if the Þbrous follicular stella or part of it is seen in the section. The follicular stella is an angioÞbrotic streamer extending from the telogen hair follicle down to the subcutaneous fat (to the former position of the bulb) [17].