Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Melanocytic Nevi of Special Sites
Special site nevi or nevi with site-related atypia are terms used to describe melanocytic nevi located in some anatomic regions that, although benign, show unusual or atypical microscopic findings that may lead to diagnostic confusion with melanoma. A number of special sites have been proposed, including the ear, acral skin, genital region, the breast, and scalp ( Table 7.1 ). One may also include nevi of flexural sites and umbilical melanocytic nevi in this group. While anatomic site likely factors as a parameter for altered growth patterns of melanocytes when the cutaneous microanatomy is different, and/or the site more subject to repetitive trauma, as in the case of acral nevi, the proposed influence of anatomic region to the architecture of a melanocytic nevus is largely speculative and the designation “special site nevus” may be overused by pathologists, as there is significant overlap in the appearance of what nevi can look like at almost any site.
TABLE 7.1
Location and Features of Nevi With Site-Related Atypia
| Anatomic Site | Clinical Features | Atypical Histologic Features | Differentiating Features From Melanoma |
|---|---|---|---|
| Ear |
|
|
|
| Acral |
|
|
|
| Genital |
|
|
|
| Breast |
|
|
|
| Scalp |
|
|
|
Ear Nevi
Clinical Findings
Many melanocytic nevi of the ear found to have unusual or atypical histologic features are often clinically fairly banal, presenting as uniformly pigmented light or dark brown macules or papules. There is a wide range in age and size of ear nevi. The diameter in one study ranged from 3 to 22 mm, in another it was 2 to 9 mm. Clinically atypical pigmented lesions warrant a low threshold for biopsy, since it is estimated that 3% to 16% of all head and neck melanomas occur on the ear.
Histopathologic Findings
Melanocytic nevi of the ear frequently display poor lateral circumscription ( Fig. 7.1A ). Junctional nests of melanocytes vary in size and shape and may not be centered at the tips of rete, but rather be placed along the sides of rete and within inter-rete spaces. Solitary units of melanocytes may sometimes be prominent. Focal suprabasilar melanocytes limited to the lower half of the epidermis are not uncommon (see Fig. 7.1B and C ). One study noted suprabasilar melanocytes in 57% of cases. Melanocytes may also have enlarged nuclei. Reassuring findings in support of a diagnosis of a nevus are overall symmetry, organized growth, and good maturation.
Differential Diagnosis
Melanocytic nevi of the ear with either prominent lentiginous and/or focal suprabasilar growth may be confused with melanoma. The distinction from melanoma is most readily accomplished with an excisional biopsy specimen that permits assessment of symmetry and circumscription. A small symmetric melanocytic proliferation with a predominantly nested growth, relatively bland cytology, and evidence of maturation can readily be diagnosed as a melanocytic nevus. Correlation with the clinical appearance (banal vs. worrisome for melanoma) is also helpful. For the distinction of a nevus of sun-damaged skin of the ear from a melanoma attention to the tumor stroma can be an important clue. The melanocytes of a long-standing nevus are usually dispersed in solar elastotic stroma, while a melanoma is commonly associated with stroma fibrosis that pushes the solar elastotic stroma downward.
Acral Nevi
Clinical Findings
Melanocytic nevi on the acral surfaces are common. The frequency of acral nevi ranges from approximately less than 10% to approximately one-third of the population, depending on the cohort. As with other types of nevi, they are common in lighter-skinned individuals. The number increases with age and tends to peak in the 20s and 30s. Women tend to have more acral nevi than men. Pigmented lesions may involve any aspect of the acral region, including the palms and soles, the nail unit, and the volar, lateral, and dorsal surfaces of the hands, feet, fingers, and toes. Acral nevi are often well-circumscribed, uniformly light or dark brown, flat lesions. Uniformly pigmented papular lesions may also be seen. Size is variable, ranging between 0.5 and 12.0 mm (median 2.0 mm).
Dermoscopic Findings
The majority of benign acral nevi exhibit one of the three major dermoscopic patterns. These include the parallel furrow (45% to 50%), lattice-like (15% to 25%), and fibrillar patterns (10% to 20%; Fig. 7.2 ). Minor dermoscopic patterns of acral nevi include reticular, homogeneous, globular, globule-streak-like, and transition. Occasionally one may see a combination of patterns within a single lesion. The parallel furrow pattern is the most prevalent pattern and is characterized by linear pigmentation along the grooves, which correlates histopathologically with nests of nevomelanocytes underlying the surface sulci with vertically oriented melanin in the stratum corneum. The lattice-like pattern also shows nests of nevomelanocytes underlying the surface sulci, but these are most commonly located on the arches or the peripheral portions of the acral surfaces where skin markings become less parallel and begin to criss-cross, giving rise to the lattice-like pigmentation seen on dermoscopy. The fibrillar pattern is characterized by once again nests of nevomelanocytes underlying the sulci, but in contrast to the vertically oriented parallel furrow pattern, the column of melanin pigment within the stratum corneum is instead oriented in a more angled fashion. This may be due to mechanical stress caused by body weight, as this pattern is most frequently found on the weight-bearing areas. Dermoscopic evaluation plays a key role in the clinical evaluation of acral pigmented lesions and their differentiation from melanoma. Dermoscopic features of acral melanoma include the parallel ridge pattern, as well as other melanoma-specific patterns such as diffuse irregular pigmentation, peripheral dots/globules, abrupt edge, blue-gray veil, and so on. The furrow ink test can be utilized to increase diagnostic accuracy by correctly identifying a furrow versus a ridge. After applying ink to the lesion and subsequently wiping the excess ink away, the only remaining ink will be located within the furrows, which can then be compared with the melanin pigmentation pattern. Some acral nevi lack a clear groove versus ridge pattern and have diffuse melanin pigmentation throughout the epidermis and stratum corneum.
Histopathologic Findings
Most acral nevi with worrisome histopathologic findings are located on the volar surface of acral skin. While any type of melanoma can be seen on acral skin, the most characteristic subtype is an acral lentiginous melanoma characterized by a broad predominantly lentiginous growth of melanocytes with minimal nesting. Therefore, for a small volar melanocytic neoplasm, the predominance of nests, good maturation, and lack of marked nuclear atypia favors a diagnosis of a nevus. Concerning features in acral nevi may include unusually prominent lentiginous or pagetoid growth. Pagetosis has been reported in up to one-third of acral nevi. However, the pagetoid spread usually maintains a sense of organization and is not chaotic ( Figs. 7.3 and 7.4 ). High-grade cytologic atypia is also not a feature of these benign special site nevi. The term MANIAC (melanocytic acral nevi with intraepidermal ascent of cells) has been coined to describe benign acral nevi exhibiting a high degree of pagetosis. Columns of melanin pigment can be identified within the stratum corneum and are typically but not always predominantly located within the surface furrows. Some rare cases may have widespread melanin broadly distributed in the furrows and ridges. The dermal component, when present, should be well nested, exhibit maturation with descent, and also lack other concerning features such as cytologic atypia or mitotic activity. Syringotropism of dermal melanocytes is sometimes observed. There may be a mild lymphohistiocytic infiltrate with scattered melanophages in the dermis.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here
