Correlation of dermoscopic structures of melanocytic neoplasms in special sites
Facial Skin
Facial skin is characterized by a thin epidermis with a thin stratum corneum, a flat DEJ and multiple pilosebaceous units. These unique features imply that the pigment network is replaced by a pseudonetwork pattern instead of a pigment network.
Pseudonetwork
Corresponds to structureless brown pigmentation interrupted by hypopigmented holes (Kittler et al., 2016a). Histologically, the pseudonetwork pattern corresponds to pigmented cells located in the epidermis and in a flattened DEJ interrupted by follicular openings, sebaceous or sweat glands (Marghoob and Braun, 2012; Schiffner et al., 2000).
Diagnosis of tumoral lesions occurring on the face can be challenging and generally imply a disruption of a regular pseudonetwork. Diverse dermoscopic structures have been associated with LM and LM melanoma: black blotches with obliteration of the follicles and disruption of the pseudonetwork, dark (brown or black) rhomboidal structures and zig-zag pattern (angulated lines in the interfollicular space), asymmetric follicular openings (incomplete circles), gray circles, concentric circles (circles within circles), target-like pattern (dot within a circle) and annular-granular pattern (aggregated gray dots or globules in the interfollicular area) (table 5) (Lallas et al., 2016; Schiffner et al., 2000; Tschandl et al., 2015). Histologically, these dermoscopic features correspond to a proliferation of atypical melanocytes along the DEJ and variable follicular and dermal invasion (Gómez-Martín et al., 2017; Schiffner et al., 2000)
Volar skin (Palms and soles)
The palmoplantar skin is characterized by a thick cornified layer, the presence of dermatoglyphics and absence of hair follicles. In volar skin, pigment predominates in the furrows or the ridges of the dermatoplyphics. Thus, two main patterns have been described for melanocytic volar lesions: the parallel furrow pattern and the parallel ridge pattern.
Parallel furrow pattern
is characterized by thin parallel pigmented lines in the furrows and is generally associated with benign melanocytic lesions. Histologically, it corresponds to melanocytes transferring pigment to keratinocytes located on the crista limitans (furrows) (Ishihara et al., 2006).
Parallel ridge pattern
is characterized by thick parallel pigmented lines on the ridges and is associated with acral melanoma. Histologically, it reveals melanocyte proliferations around the rete ridges associated with the acrosyringia (crista intermedia) (Ishihara et al., 2006). The presence of stem cells and the microenvironment associated with the acrosyringia could explain the proliferation of malignant cells that proliferate in this location (Okamoto et al., 2014).
Other patterns that can be observed in volar skin are fibrillar pattern and lattice-like pattern.
Fibrillar pattern
(fine or fibrillar pigmentation arranged in the direction crossing the parallel skin markings) is secondary to the mechanical pressure and it is an artifactual modification of the furrow pattern (Miyazaki et al., 2005).
Lattice-like pattern
(pigmented lines across the furrows) the pigmentation is seen along the sulci of the skin markings, therefore the lattice-like pattern can be considered an anatomical modification of the parallel furrow pattern (Saida and Koga, 2007).
Mucosa
Studies about correlation between dermoscopy and histology of mucosal lesions are scarce. In addition, the histological diagnosis of melanocytic lesions in mucosal sites can be challenging since benign and malignant lesions can present with atypical findings (Yélamos et al., 2016). The pigment network is usually absent in mucosal lesions, due to the nearly complete absence of rete ridges. However, with the exception of the pigment network, melanocytic lesions of the mucosa can present with dermoscopic structures similar to those seen in non-mucosal locations such as lines, dots/globules, circles and structureless areas.
Multiple patterns can be observed in benign melanocytic lesions of mucosae.
The homogenous pattern
is composed of structureless brown areas on dermoscopy. Histologically, it reveals flattened rete ridges along with acanthosis (Lin et al., 2009).
The ring-like pattern
presents with pigmented circles, and the fish scale-like pattern shows half circles. They both share the same histopathological correlations revealing hyperpigmented rete ridges with skip areas of pigmentation at the top of the dermal papillae (Ferrari et al., 2014; Lin et al., 2009).
The parallel pattern
is composed of brown, linear parallel streaks (Ferrari et al., 2014) and the hyphal pattern consists of lines resembling fungal hyphae. Histologically, they corresponds to hyperpigmentation at the tip of the obliquely elongated rete ridge (Lin et al., 2009).
The dotted-globular pattern
presents with multiple dots/globules with similar sizes and shapes and corresponds to locally aggregated melanin in the upper dermis (Lin et al., 2009).
A multicomponent pattern
consisting of blue, gray and white colors, regression structures, blue-whitish veil and structureless areas can be seen in mucosal melanomas (Blum et al., 2011; Lin et al., 2009). Histologically, the correlation is similar to that described in non-mucosal locations.
Nails
Diagnosis of pigmentation of the nail unit is challenging as the differential diagnosis is broad. It includes trauma, infection, exogenous pigmentation and melanocytic activation (lentigo, drug-induced, post inflammatory pigmentation) or proliferation (nevus, melanocytic hyperplasia or melanoma (Alessandrini et al., 2017; Braun et al., 2007; “dermoscopedia,” n.d.).
Gray band
In pigmented nail bands (longitudinal melanonychia), the presence of gray granules within a gray band correlates with epithelial hyperpigmentation due to melanocytic activation.
Brown band
Brown/black granules within a brown or black band reveals on the histopathologic study a melanocytic proliferation (Braun et al., 2007).
Irregular brown band
In melanocytic proliferations, an irregular pigmented band originating from the proximal nail fold with multiple colors, variable thickness of the lines and loss of parallelism is suggestive of nail apparatus melanoma (NAM) (Braun et al., 2007).
Micro-Hutchison’s sign
is the periungual skin pigmentation of hyponychium (pigmentation of the cuticle) evident only with dermoscopy (Braun et al., 2007). It is associated with NAM in adults and corresponds to the initial radial growth of melanoma into adjacent tissue. Histologically, it may reflect the Breslow index, as micro-Hutchinson sign has been seen associated with early melanoma (Starace et al., 2018).
Vascular polymorphism
showing different types of vessels on dermoscopy, reflects chaotic neoangiogenesis on histology and has been associated with the invasive stage of NAM (Starace et al., 2018)
Free edge dermoscopy
Dermoscopy can also guide biopsy by evaluating the free edge of the nail, avoiding inadequate sampling and delayed diagnosis.
Pigmentation of the upper part of the nail´s free edge
correlates histologically with melanocytic proliferations located in the proximal portion of the nail matrix. P
Pigmentation of the lower part of the nail´s free edge
correlates histologically with melanocytes proliferating distally in the nail matrix (Braun et al., 2006).
Intra operative in vivo dermoscopy of the nail matrix
Dermoscopy can be used as an intraoperative procedure to see the pattern of pigmentation of nail matrix and to select the most appropriate area to biopsy. Dermoscopy of the nail matrix reveals dermoscopic features not seen in the nail plate (Hirata et al., 2006, 2011). There are 4 dermoscopic patterns:
Regular gray pattern (fine regular grayish lines)
corresponding with pigmentation in the basal layer (melanocytic activation) and absence of melanocytic hyperplasia or nests.
Regular brown pattern (regular brown lines)
that histologically corresponds to benign melanocytic hyperplasia with larger amount of melanin and absence of melanocytic nests.
Regular brown pattern with globules or blotches
(regular longitudinal brown lines and presence of globules or blotches of regular size and distribution) is associated with melanocytic nevi. Histologically, the globules correlate with the presence of nests. The blotches correlate with the presence of large amounts of melanin.
Irregular pattern
(longitudinal lines of irregular color and thickness, with or without irregular globules or blotches), histologically correspond to a disorganized proliferation of atypical melanocytes with atypical cell nests. The irregular lines reveal atypical melanocyte proliferation producing thick longitudinal lines with spread of the pigmentation to adjacent areas. The irregular pattern shows a high sensitivity and specificity for melanoma (Hirata et al., 2011).
