Show Summary Details
Page of

Melanoma Pathology 

Melanoma Pathology
Chapter:
Melanoma Pathology
Author(s):

Carlos Prieto-Granada

, Nicole Howe

, and Timothy McCardle

DOI:
10.1093/med/9780199971015.003.0002
Page of

PRINTED FROM OXFORD MEDICINE ONLINE (www.oxfordmedicine.com). © Oxford University Press, 2016. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Medicine Online for personal use (for details see Privacy Policy and Legal Notice).

date: 19 September 2019

Introduction and Histological Evaluation of Melanocytic Lesions

Melanocytes are neural-crest–derived cells that are vital in the skin homeostasis, being responsible for the protection of the keratinocytes from harmful solar ultraviolet (UV) radiation via the production and transfer of melanin1. Normally, melanocytes are located in the basilar layer of the epidermis, following a melanocyte to keratinocyte ratio of approximately 1:10, with considerable variation; depending on the anatomical site and degree of sun exposure. Melanocytic lesions characteristically demonstrate quite protean clinical and pathological presentations and represent a wide spectrum that encompasses an immense variety of patterns, from low-cellularity benign lesions such as lentigo simplex and benign melanocytic nevi, to atypical (dysplastic) melanocytic nevi, and finally to the malignant counterpart: melanoma.

Melanocytic nevi are defined by the nesting of benign melanocytes, and their classification includes several parameters and criteria. The most basic classification, into junctional, compound, and intradermal nevi, is made taking into consideration the lesion’s location in relationship to the skin’s microanatomy. Nevi are further subclassified into many different variants, each one with its own set of morphological features; Spitz nevi, blue nevi, and congenital nevi, to name a few. Finally, these lesions are classified according to their clinical behavior and/or hypothetical potential to transform into more aggressive lesions; an example of this concept would be the dysplastic nevus (also known as Clark’s nevus) group of lesions.

When evaluating melanocytic lesions under the microscope, one must determine whether it is a benign nevus, an atypical nevus, or a melanoma. This task is not always straightforward, and one should employ stringent and consistent architectural (regarding the overall lesion) and cytological (regarding the individual cells) criteria, some of which criteria are described below. An important factor to consider is clinical information such as the age of the patient and how the lesion presented.

Diagnostic Criteria

Architectural Criteria

  • Symmetry of the lesion from low-power exam: atypical/malignant lesions are often asymmetrical in shape

  • Architectural distortion of the epidermis: atypical/malignant lesions frequently exhibit confluent growth with “consumption” and effacement of the rete ridges2

  • Presence of upward spread of atypical nevomelanocytes: atypical/malignant nevomelanocytes involving the superficial layers of the epidermis (pagetoid spread) also favor an atypical/malignant diagnosis

Cytological Criteria

  • Nuclear to cytoplasmic (N:C) ratio: size of the nuclei when compared to the cytoplasm; increased N:C ratio favors an atypical/malignant diagnosis

  • Nuclear atypia: which includes variation in size of the nuclei (pleomorphism), irregularity of nuclear membranes, dark nuclei (hyperchromasia), and/or prominent nucleoli or macronucleoli

  • Atypical and deep cellular hyperpigmentation: deposition of coarse and/or fine cytoplasmic melanin granules is often seen in atypical/malignant lesions

  • Increased mitotic activity and atypical mitoses

Maturation

Maturation is a particular and important criterion applied to the dermal component, which combines both architectural and cytological features. In benign lesions, the individual nevomelanocytes become smaller, and nests break up into individual cells as they go deeper into the dermis (Figure 2.1, B). This phenomenon is often absent in atypical and malignant lesions.


Figure 2.1 Composite image demonstrating representative Hematoxilin and Eosin (H&E) tissue slides from a wide variety of melanocytic proliferations. Both top and bottom images in A display low-power views (40X) of banal intradermal nevi to highlight the symmetry of these lesions. The phenomenon of maturation is illustrated in B with a medium power view (100X) of an intradermal nevus exhibiting diminution of size of both nests and cells as they progress deep into the lesion. The arrow points out the more mature nevic cells (so-called type C nevus cells). Both top and bottom figures in C demonstrate a nodular melanoma. Note the asymmetrical disposition of the expansile nests in the dermis in the low power view (top image, 40X) as well as the lack of maturation and the pagetoid cells (arrow) in the medium power view (bottom, 100X). A case of lentigo maligna melanoma in situ is shown in D, note the confluence of the intraepidermal melanocytes as well as the adnexal involvement of the sebaceous glands (arrow) (100X). Figure E is showing an example of desmoplastic melanoma, composed of bland spindle cells embedded in a fibrotic/myxoid stroma showing lymphoid aggregates and perineural invasion (arrow) (200X). Finally, F shows an example of a blue nevus-like melanoma (lower left field of the image) which arose from a pre-existing nevus (right upper field of the picture. (200X).

Figure 2.1 Composite image demonstrating representative Hematoxilin and Eosin (H&E) tissue slides from a wide variety of melanocytic proliferations. Both top and bottom images in A display low-power views (40X) of banal intradermal nevi to highlight the symmetry of these lesions. The phenomenon of maturation is illustrated in B with a medium power view (100X) of an intradermal nevus exhibiting diminution of size of both nests and cells as they progress deep into the lesion. The arrow points out the more mature nevic cells (so-called type C nevus cells). Both top and bottom figures in C demonstrate a nodular melanoma. Note the asymmetrical disposition of the expansile nests in the dermis in the low power view (top image, 40X) as well as the lack of maturation and the pagetoid cells (arrow) in the medium power view (bottom, 100X). A case of lentigo maligna melanoma in situ is shown in D, note the confluence of the intraepidermal melanocytes as well as the adnexal involvement of the sebaceous glands (arrow) (100X). Figure E is showing an example of desmoplastic melanoma, composed of bland spindle cells embedded in a fibrotic/myxoid stroma showing lymphoid aggregates and perineural invasion (arrow) (200X). Finally, F shows an example of a blue nevus-like melanoma (lower left field of the image) which arose from a pre-existing nevus (right upper field of the picture. (200X).

In summary, with these criteria in mind, when viewing a prototypical benign acquired nevus on low power, a small, symmetrical lesion with sharp, well-defined lateral borders is evident (Figure 2.1, A). The melanocytes are well nested at the dermoepidermal junction, with round- to oval-shaped junctional nests regularly shaped at the tips and sides of the rete ridges. Pagetoid spread is uncommon in benign lesions, unless one is confronted with particular variants such as viewing the center of a Spitz, Reed, or acral nevi, or in cases of traumatized or sunburnt nevi. As one looks toward the base of a benign lesion, the nests become smaller (maturation), and no deep mitoses or pigment are found.

On the other hand, melanomas are quite frequently broad, asymmetrical lesions (Figure 2.1, C, top picture). There are more non-nested than nested melanocytes in melanomas. Those a typical melanocytes that are nested often form irregularly spaced, elongated, and bizarre nests, with involvement of the top of the dermal papillae. As one evaluates the deeper portion of the lesion, there is no evidence of maturation and deep mitoses and pigment may be evident (Figure 2.1C, bottom picture).

One should also be able to recognize and assess the degree of sun damage of the skin surrounding the lesion. An important feature to look for is solar elastosis, which refers to the loss of elastic fibers in the dermis due to sun exposure; which produces thick, wrinkled skin clinically, and appears microscopically as a bluish-tinged dermis with loss of the normal pink collagen bundles. Another feature related to excessive sun exposure is skin atrophy, which microscopically appears as a markedly thinned epidermis. For further in-depth information about distinctions between benign, atypical and malignant melanocytic lesions one could resort to the excellent melanocytic pathology textbook by Drs. LeBoit and Masi3 as well as the melanocytic chapter in Dr. Weedon’s reference skin pathology textbook4.

Melanoma Prognostic Factors

Once the diagnosis of melanoma has been made, the pathologist is called on to evaluate many histological features, some of them carrying vital importance in terms of prognostication.

Established, evidence-based5 prognostic factors include:

  • Tumor thickness (Breslow)

  • Clark’s level

  • Presence of ulceration

  • Mitotic count (per mm2), particularly in “thin” lesions (≤1 mm thick)

  • Presence of satellite lesions

Other features to evaluate in a melanoma include: tumor subtype, growth phase, lymphovascular and perineural invasion, tumor-infiltrating lymphocytes (host response), and regression. The pathologist must also determine if the melanoma involves the excisional margin, see whether in-transit metastases are present, and measure the size of ulceration if present. The main prognostic factors will be discussed below.

Depth and Level of Invasion

Breslow’s Depth/Thickness

Breslow’s depth is of the strongest prognostic predictors in melanoma. It is determined using an ocular micrometer to measure the distance in millimeters between the granular layer of the epidermis and the deepest contiguous portion of the tumor6. The tumor thickness is directly proportional to the chance of developing recurrent or metastatic melanoma, and it defines the T category in the TNM system:

  • Tis: in situ

  • T1: <1.0 mm

  • T2: 1.01–2.0 mm

  • T3: 2.01–4.0 mm

  • T4: >4.0 mm

The other distinctions within the the TNM system refer to reginal lymph node involvement (N), and presence of distant metastasis (M). This is more fully discussed in chapter 4.

Clark’s Levels

Developed by Dr. Wallace Clark7, this early method of assessing the depth of the tumor and thus prognosis relies on microanatomical landmarks. (See Figure 2.2.)

  • Level I: Confined to the epidermis (in situ)

  • Level II: Invasion of papillary dermis

  • Level III: Invasion of papillary/reticular dermis interface

  • Level IV: Invasion of the reticular dermis

  • Level V: Invasion of subcutaneous tissue


Figure. 2.2 Schematic representation of Clark’s anatomical levels:
Level I (1): Confined to the epidermis
Level II (2): Involving the papillary dermis
Level III (3): Filling the papillary dermis and invading deeper into the mid-dermis
Level IV (4): Involving the reticular dermis and the deep vascular plexus
Level V (5): Involving the subcutaneous adipose tissue

Figure. 2.2 Schematic representation of Clark’s anatomical levels:

Level I (1): Confined to the epidermis

Level II (2): Involving the papillary dermis

Level III (3): Filling the papillary dermis and invading deeper into the mid-dermis

Level IV (4): Involving the reticular dermis and the deep vascular plexus

Level V (5): Involving the subcutaneous adipose tissue

After the release of the American Joint Committee on Cancer (AJCC) Seventh Edition Staging Manual, the prognostic impact of Clark’s level has been greatly diminished in favor of Breslow’s tumor thickness5.

Ulceration

The presence of bona fide tumor-induced ulceration is an adverse prognostic factor, and it modifies the T category from a (without ulceration) to b (with ulceration). Tumor-induced ulceration must be distinguished from that product resulting from prior trauma/treatment, and making this distinction is possible via clinical history and specific histopathological changes such as the presence of a fibrinous crust with inflammatory cells in the case of bona fide tumor-induced ulceration. Some pathologists recommend measuring and documenting the ulceration width8.

Mitotic Count

The mitotic count is performed only in the dermal (invasive) component and is reported in millimeters squared. In the T1 category, mitoses are of vital importance, since the presence of only one mitotic figure changes the category from T1a to T1b (or the presence of ulceration), prompting performance of a sentinel lymph node biopsy5,9.

Presence of Satellite Lesions

The somewhat controversial parameter of satellite lesions includes the concepts of microscopic satellites, clinical satellites and in-transit metastatic lesions. The presence of either of the following features in a otherwise lymph node negative melanoma will upstage the patient to N2c5.

  • Microscopic satellites are defined as discrete tumor nests greater than 0.05 mm in diameter that are separated from the main body of the tumor by normal reticular dermal collagen or subcutaneous fat by a distance of at least 0.3 mm10.

  • Clinical satellites (in-transit metastasis) are lesions that are more than 2 cm from the primary tumor, but not beyond the regional lymph nodes.

Growth Phase

One important concept is that of the growth phases of melanoma, classified into radial and vertical growth phases (RGP and VGP). However, this particular parameter does not carry prognostic relevance, but it is rather utilized to subclassify tumors and understand their biology. Melanoma in situ, micro-invasive, and thin (<1 mm thick) non-mitogenic melanomas are all examples of RGP. These early forms of melanoma are composed of a tumor clone that has not yet acquired the capacity to invade deeply into the dermis and blood vessels. Thus, during RGP, there is typically little or no metastatic potential, with a resulting good prognosis. Tumors that have developed a VPG, on the other hand, commonly are represented by large, pleomorphic, nodule-like nests in the dermis. These are more expansive than the junctional component and differ cytologically, with frequent mitoses. It is important to recognize that VGP can be detected in early, thin lesions (<1 mm thick), by the presence of a single dermal mitosis9. Development of a VGP melanoma clone is linked with a several changes at the molecular level.11,12 (See Figure 2.3 for a representation of RGP and VGP.)


Figure 2.3 Photomicrograph from a H&E slide demonstrating a superficial spreading melanoma with early vertical growth phase (right aspect of the image). The in situ/radial growth phase (RGP) component can be seen on the left side of the photomicrograph. Note the distinct nature of the cells composing the vertical growth phase (VGP) (i.e., different clone). Other features of malignancy can be appreciated in this picture, such as pagetoid spread, confluent growth, and cytological atypia (200X).

Figure 2.3 Photomicrograph from a H&E slide demonstrating a superficial spreading melanoma with early vertical growth phase (right aspect of the image). The in situ/radial growth phase (RGP) component can be seen on the left side of the photomicrograph. Note the distinct nature of the cells composing the vertical growth phase (VGP) (i.e., different clone). Other features of malignancy can be appreciated in this picture, such as pagetoid spread, confluent growth, and cytological atypia (200X).

Melanoma Classification

Currently, melanomas are being classified using a system devised by Dr. Wallace Clark13, which takes into consideration morphological aspects of the early stages of melanoma (RGP) as well as the tumor location on the body. Clark’s classic classification includes four main categories:

  • Superficial spreading melanoma (SSM)

  • Nodular melanoma (NM)

  • Lentigo maligna melanoma (LMM)

  • Acral lentiginous melanoma (ALM)

Since the description of this classification more than 30 years ago, great advances have been made in terms of recognizing the different types of mutations present in distinct melanoma groups14, as well as in understanding the relationship of these melanoma groups to a variety of clinical, pathological, and molecular features. With the advent of revolutionary targeted therapies, mutational characterization of melanomas is vitally important in the modern management of this disease. For these reasons, we favor the new classification proposed by Dr. Boris Bastian, which includes comprehensive clinicopathological and molecular correlations15. These clinicopathological features include anatomical location (epithelium-associated melanomas versus non-epithelium-associated melanomas) and the pattern of sun damage in the skin where the lesions arise from (chronically sun-damaged skin versus non-chronically sun-damaged skin. Interestingly enough, although it has some modifications and overlap between categories, Clark’s original classification remains the foundation and the basis of this new, expanded, and comprehensive melanoma taxonomy.

We will follow this novel classification describing the key clinical, pathological, and molecular findings of each category.

  1. A. Epithelium-associated melanomas:

    • Melanomas arising in non-chronically (intermittently) sun-damaged skin (non-CSD): This category includes the SSM, NM, and Spitzoid melanomas

    • Melanomas arising in chronically sun-damaged skin (CSD): This category includes LMM and desmoplastic melanoma

    • Melanomas arising in glabrous skin: This category includes ALM

    • Melanomas arising in mucosa

  2. B. Non-epithelium-associated melanomas:

    • Cutaneous (dermal): Includes blue nevus–like melanoma and melanoma arising in a congenital nevus

    • Uveal melanoma

    • Melanomas of internal organs (Gastrointestinal, Genitourinary, Central nervous system)

Epithelium-Associated Melanomas

Melanomas Arising in Non-chronically (Intermittently) Sun-Damaged Skin

This group includes SSM, nodular melanoma, and Spitzoid melanoma. It is important to mention that the nodular melanoma variant can also arise in chronically sun-damaged (CSD) skin as well as in acral sites. When lesions of this group metastasize, they usually first involve the lymph node basin.

Superficial Spreading Melanoma (SSM)

Superficial spreading melanoma is the most common melanoma subtype and usually affects individuals in the third and sixth decades of life; it tends to present clinically as irregularly pigmented asymmetrical patches or nodules involving the trunk and extremities. The affected patients often bear multiple acquired melanocytic nevi. Since this group of lesions arises in intermittently damaged skin, the features of chronically sun-damaged skin are not encountered.

Histopathologically, SSM are commonly broad, asymmetrical, and fail to mature from top to bottom. Irregular thickening of the epidermis (acanthosis) and effacement of the dermoepidermal junction are evident. A lymphoid and plasma cell infiltrate walls off the base of the lesion. The radial growth phase is characterized by atypical melanocytes with buckshot scatter and pagetoid spread in the epidermis. The melanocytes are large, with abundant pale cytoplasm and vesicular pleomorphic nuclei. They commonly exhibit poorly formed and unevenly spaced nests throughout the epidermis, with a fine dusty pigmentation. The nests may vary in size and shape, becoming elongated, bizarre, or confluent with mitoses. Single melanocytes outnumber the nested ones. See Figure 2.3 for a representation of a SSM.

Key Histopathological Findings of Superficial Spreading Melanoma

  • Radial growth phase is present

  • “Buckshot” pattern

  • Pagetoid spread

  • Atypical melanoctyes and irregular nests throughout the epidermis

  • Prominent dermoepidermal junction activity

  • Lichenoid inflammatory infiltrate is common

  • Non-nested melanocytes outnumber nested

  • Asymmetrical (left to right) with failure to mature (top to bottom)

  • Cytologically atypical and mitoses present

  • No solar elastosis

Nodular Melanoma (NM)

Nodular melanoma is most often found on the trunk and extremities of males in their fifth or sixth decades. It is characterized by a vertical growth phase. This type of melanoma does not arise in a precursor lesion; therefore, one must ensure there is no evidence of radial growth beyond three rete ridges. On low power, a symmetrical dermal nodule is evident. There is failure to mature from top to bottom, and deep pigment may be found in the melanocytic nests. Commonly, necrosis is found, as well as a lymphoid-predominant infiltrate walling off the lesion at the base. See Figure 2.1, C for an example of a nodular melanoma.

Key Histopathological Features of Nodular Melanoma

  • Dermal nodule with cytological atypia

  • Vertical growth phase only

  • Junctional activity does not go beyond three rete ridges

  • Symmetrical

  • Does not mature or disperse towards base of the lesion

  • Necrosis common

  • Deep pigment in nests possible

  • Lymphoid infiltrate frequent at the base

  • Plasma cells common in the infiltrate

  • Deep mitoses are often present

Molecular Aspects of Superficial Spreading Melanoma and Nodular Melanoma

Importantly, the majority of the lesions classified in this group are characterized by mutations on either BRAF (70%) or NRAS (15%) genes, with BRAF mutations overwhelmingly present in the SSM variant. Over 30 different types of BRAF mutations have been described, but BRAF V600E seems to be the most prevalent. This mutation is thought to be triggered by UV damage and originates when thymine is substituted with adenine at nucleotide 1799, which results in valine (V) being substituted for by glutamate (E) at codon 600. Secondary genetic alterations in this group include TERT mutations as well as CDKN2A and PTEN deletions. BRAFV600 mutations lead to uncontrolled tumor cell growth and are the basis for targeted therapy using small molecules with tyrosine kinase inhibition activity, the most well-known one being vemurafenib15.

Spitzoid Melanoma (SM)

Spitzoid lesions are characteristically very rare, diagnostically challenging, and predominantly encountered in children. They preferentially arise in the head and neck and lower extremities (around the knee), but they can appear anywhere, even in glabrous skin. Clinically, they appear as rapidly growing unpigmented nodules, and there is less of a link with sun exposure. Spitz nevi and spitzoid melanomas can closely resemble each another histologically. Spitzoid melanoma illustrates many of the features of Spitz nevus, including Kamino bodies (eosinophilic globules that are the product of basement membrane degradation), vascular ectasia, and a “raining-down” appearance. However, a diagnosis of melanoma is favored when asymmetry, conspicuous deep mitosis, necrosis, and failure to mature are evident.

Key Histopathological Features of Spitzoid Melanomas

  • Similar to Spitz nevus

  • Ulceration

  • Asymmetry

  • Poorly circumscribed

  • Pagetoid spread

  • Expansile nodule

  • Failure to mature

  • Impaired maturation

  • Deep pigmentation

  • Deep “marginal” mitoses

Molecular Aspects of Spitzoid Melanomas

Lesions from this group are characterized by alterations involving a multitude of genes, including HRAS mutations and gene fusions involving ROS1, NTRK1, ALK, RET, BRAF, and NTRK316,17.

Melanomas Arising in Chronically Sun-Damaged Skin (CSD)

Lentigo Maligna Melanoma (LMM)

Lentigo maligna melanoma has a predilection for arising in sun-damaged skin of the elderly, typically patients in the seventh decade of life or later, with a clinical history of non-melanoma skin cancer. Clinically, it typically presents with a slow-growing, irregularly pigmented facial patch in the chronically sun-exposed areas of the head, neck, lower arms, and lower legs. Frequently there are areas of regression that appear as white scars or hypopigmented macules. On histological examination, the lesions arise in a background of marked solar elastosis and epidermal atrophy. They are asymmetrical with poorly defined lateral borders, with atypical melanocytes only one cell thick at the dermoepidermal junction—consistent with the fact that the lesions typically extend further than the apparent clinical margin. The rete-ridge pattern is commonly effaced, with poorly nested and confluent multinucleated melanocytes with prominent dendritic processes at the dermoepidermal junction. Although effacement of the rete is common, lentingous epidermal hyperplasia may occur, and lentigo maligna may closely resemble a junctional lentigious nevus or dysplastic nevus. However, a broad junctional lesion on sun-damaged skin is most likely melanoma in situ.

Lentigo maligna melanoma lesions often show spindle-shaped atypical melanocytes with hyperchromatic irregular pleomorphic nuclei. Non-nested melanocytes outnumber those that are nested. Nests are unevenly spaced, not limited to the rete tips, and can extend down adnexal structures. Of importance, small biopsies often result in misdiagnosis, as skip areas are common. The false-negative rate of small biopsies has been found to be up to 80%. Therefore, broad thin-shave biopsies are best.

Pigmented actinic keratoses (squamous pre-cancerous lesions) often collide with lentigo maligna melanoma, reiterating the importance of sufficient biopsy size. Both develop in a background of marked solar elastosis with retraction artifact. Actinic melanocytic hyperplasia has an increased number of melanocytes, with mild cytological atypia and hyperchromatic irregular nuclei making margin determination more difficult for surgeons. However, there are no nests, giant cells, or evidence of severe cytological atypia, deep follicular involvement, or pagetoid spread in lesions of actinic melanocytic hyperplasia.

After long-term growth of lentigo maligna, invasive/vertical growth-phase lentigo maligna melanoma may develop. This is marked clinically by a nodule on physical exam. It shares all the histological features of lentigo maligna, with the addition of a vertical growth phase. This can be classified as epithelioid, spindled, or desmoplastic.

Distant spread in this group of lesions presents in equal proportions as satellite, in-transit, lymph node, and distant metastases. See Figure 1D for an example of LMM.

Key Histopathological Features of Lentigo Maligna Melanoma

  • Solar elastosis

  • Epidermal atrophy

  • Broad, asymmetrical lesion

  • Skip areas are common

  • Spindle-shaped melanocytes

  • Predominately junctional growth of atypical melanoctyes

  • Cytological atypia

  • Dermal mitoses present

  • Extends down adnexal structures

  • Rete-ridge pattern effaced with giant cells

Molecular Aspects of Lentigo Maligna Melanoma

LMM lesions predominantly exhibit NRAS (15%) and KIT (10%) mutations, with secondary mutations involving TERT and TP53. Patients harboring tumors with mutations/amplifications affecting areas from the KIT gene that code tyrosine kinase domains (chiefly exons 9, 11, 13, and 17) of the C-KIT/CD117 receptor have shown response to tyrosine kinase inhibitor therapy with Imatinib in recently completed clinical trials18. This also applies to the other two melanomas with KIT mutations: acral lentiginous melanoma and mucosal melanoma (see ALM and MM discussions below).

Desmoplastic Melanoma (DM)

This group of lesions is encountered in individuals in their seventh decade of life and later, mostly involving the head and neck (35%), trunk (21%), and extremities (27%). Desmoplastic melanomas commonly present as amelanotic nodules or plaques, making diagnosis difficult clinically. As a result, the majority of these lesions are advanced at the time of biopsy.

There are multiple histological variants, and they can be classified as pure DM or mixed DM—the former category applies when the desmoplastic spindle cell component represents more than 90% of the lesion19. One variant has abundant pleomorphic cells and hyperchromatic bizarre nuclei throughout sheets of atypical spindle cells. A second form is similar to a scar or dermatofibroma, with thick collagen strands and admixed spindle cells. The last subtype, referred to as neurotropic melanoma, often resembles a neurofibroma or neuroma (benign neural lesions). Here, myxoid stroma and cells lacking in melanin pigment are evident. Delicate, spindle-shaped melanocytes are arranged in fascicles in the upper dermis, referred to as neural transformation. They illustrate neurotropism, demonstrated by their circumferentially arrangement around nerves. Dense groups of lymphocytes are commonly found scattered in the dermis, with an overlying lentigo maligna melanoma in situ in the epidermis in 80% of the cases.

Involvement of lymph nodes in pure DMs is extremely rare (7%). Although distant metastases are rare (usually to lungs), desmoplastic melanomas have a higher rate of recurrence—attributed to perineural invasion and the infiltrative nature of the lesion. Mixed DMs have a higher rate of lymph node involvement. The diagnosis often requires immunohistochemistry, as they closely resemble other spindle cell lesions. Please refer to Figure 1, D for an example of DMM.

Key Histopathological Features of Desmoplastic Melanoma

  • Spindle cells

  • Lack pigment

  • Overlying melanoma in situ common

  • Perineural involvement

  • High rate of recurrence

  • Nodular lymphocytic infiltrate

  • Neurotropism

  • Neural, myxoid, and storiform variants

Molecular Aspects of Desmoplastic Melanoma

The most frequently gene found to be affected in this group of lesions is NF1 (25-90%)15,20. Recently, the frequency of a polymorphism of the RET gene at codon G691S has been shown to be significantly increased in desmoplastic versus conventional melanoma21.

Melanomas Arising in Glabrous Skin

Acral Lentiginous Melanoma (ALM)

Acral lentiginous melanoma is also more frequently encountered in older patients, in their sixth decade of life or later. It involves the plantar (most commonly the heel) and palmar surfaces and the nail apparatus. This melanoma presents as an irregularly pigmented macule, with one or more nodules if an invasive component is present. Epidermal melanocytes can appear to be benign, as they have only slight upward spread. The rete ridges illustrate lentiginous elongation with atypical melanocytes and mitoses in the basal layer. Marked hyperkeratosis and parakeratosis are evident, with occasional appendageal involvement. There is often conspicuous cytoplasmic retraction. The dermoepidermal junction typically has poorly nested and confluent melanoctyes. Nest formation is a late finding, and invasive tumors more commonly have a spindled vertical growth phase and are less frequently epithelioid.

In addition, subungual melanomas may also occur. This is a rare variant of ALM, most common in Caucasians, with a predilection for the fingers over the toes, especially the thumbs. Clinically, it presents with loss of the nails, melanonychia, or a subungual ulcer or nodule. Histologically, atypical melanocytes are found proliferating from the nail bed. It is most similar to an acral lentiginous melanoma, but desmoplastic, metaplastic, nodular, and superficial spreading variants are possible.

This group of tumors tends to develop satellite, in-transit, and lymph node metastases as the first manifestation of spread.

Key Histopathological Features of Acral Lentiginous Melanoma

  • Elongation of rete ridges

  • Mild atypia of epidermal melanocytes

  • Retraction artifact

  • Nuclear atypia

  • Involvement of appendages

  • Nest formation is late finding

  • In situ lesions with irregular acanthosis

  • Hyperkeratosis and parakeratosis

  • Invasive tumors often spindled

  • Superficial spreading and nodular variants possible

Molecular Aspects of Acral Lentiginous Melanoma

Mutations involving the KIT (15%), BRAF (15%), and NRAS (15%) genes have been described in this group of melanomas. Amplifications of TERT are found as secondary mutations. Interestingly enough, in ALM, molecular alterations such as mutations in the KIT gene can be found forming a “field effect,” which involves melanocytes away from the tumor that appear morphologically benign15. Clinical trials with tyrosine kinase inhibitors were performed in a subset of these tumors18 (see the preceding discussion on the molecular aspects of LMM).

Melanomas Arising in Mucosa

Melanomas involving the mucosae, tarsal conjunctiva, upper respiratory tract, esophagus, lower gynecological (GYN) tract, and rectum are considered to be mucosal melanomas (MM). These most often arise in the sixth to seventh decades of life and in a precursor lesion with a radial growth phase which closely resembles acral lentiginous melanoma. However, mucosal melanomas can also show nodular, lentigo maligna, and superficial spreading patterns. The most common locations in the head and neck are the sinonasal tract, followed by the oral cavity—more specifically the hard palate, maxillary gingiva, and lips. They present clinically as a pigmented mucosal macule with ulterior nodular growth. Regarding staging and prognosis, Clark and Breslow staging systems are of little utility, particularly in head and neck mucosal melanomas. In 2004, Prasad et al. proposed a classification system that was based on the level of invasion by the melanoma, comprising level 1 (in situ), level 2 (superficially invasive), and level 3 melanomas (deeply invasive)22. In melanomas of the lower GYN tract, a modified version of the Clark system—the Chung system—was developed (Level 1: Intraepithelial/melanoma in situ; Level 2: invasion of <1 mm into dermis/lamina propria; Level 3: invasion 1–2 mm into subepithelial tissue; Level 4: invasion >2 mm into fibrous or fibromuscular tissue; and Level 5: extension into subcutaneous fat)23. In GYN tract melanomas, Breslow thickness retains its relevance in terms of prognostication.

Mucosal melanomas have an overall propensity for local recurrence and distant metastases over lymph node metastases.

Key Histopathological Features of Mucosal Melanoma

  • On oral, genital, or conjunctival mucosa

  • Various growth patterns

  • Atypical spindled melanocytes in lamina propria

  • Adjacent atypical melanocytic hyperplasia is common

  • Variable pigmentation

  • Variable desmoplasia

Molecular Aspects of Mucosal Melanoma

Like acral lentiginous melanomas, mucosal melanomas exhibit KIT (15%) and NRAS (15%) mutations, with TERT amplifications being the secondary molecular alterations. Patients harboring tumors with mutations/amplifications affecting areas from the KIT gene that code tyrosine kinase domains (chiefly exons 9, 11, 13, and 17) of the C-KIT/CD117 receptor have showed response to tyrosine kinase inhibitor therapy with Imatinib and Sumatinib in recently completed clinical trials. Clinical trials with tyrosine kinase inhibitors were performed in a subset of these tumors (see preceding discussion on molecular aspects of LMM).

Non-Epithelium-Associated Melanomas

Cutaneous (Dermal) Melanomas

Blue Nevus–Like Melanoma (Malignant Blue Nevus)

Blue nevus–like melanoma/malignant blue nevus (BNM) is a de novo melanoma that resembles a cellular blue nevus or a melanoma that arises in association with a blue nevus. This rare melanoma variant has been described in all age groups. They commonly present as a blue or black polypoid nodule on the scalp or trunk. Malignant blue nevi have a high rate of mortality, as they are aggressive tumors with a high rate of metastasis, often to lymph nodes, bones, and visceral organs such as lungs and liver. See Figure 1F for an example of a blue nevus-like melanoma (lower left field of the image).

Key Histopathological Features of Blue Nevus–Like Melanoma/Malignant Blue Nevus

Arises from precursor:

  • Nodular growth pattern

  • Necrosis

  • Pleomorphism

  • Hyperchromasia

  • Nuclear atypia

  • Atypical mitoses

Mimicking blue nevus:

  • Absence of precursor

  • Low power resembles common blue nevus

  • Cytological features of melanoma

  • Admixed pigment-laden dendritic cells

Molecular Aspects of Blue Nevus–Like Melanoma

These lesions are characterized by mutations involving the GNAQ and GNA11 genes, which are also encountered in uveal melanomas15.

Ocular Melanomas

Uveal Melanoma

These tumors affect patients in a wide variety of age groups, with the average age being 60 years old. They arise in the posterior and anterior choroid, the ciliary body, and the iris. Clinically, they elicit symptoms such as visual field disturbances and blurred vision. Histopathologically, they present as nodules of plump, spindled and/or epithelioid melanoma cells with macronucleoli. These lesions are notorious for their late metastasis to the liver. They can also metastasize to bones.

In terms of molecular alterations, they share the same mutations present in blue nevus–like melanomas involving the GNAQ and GNA11 genes, with predominance of GNA11 (90%). Alterations in the BAP1 gene have been described as secondary mutations15.

Melanomas of Internal Organs (GI, GU, CNS)

These tumors are extremely rare, and consequently, very little information on them is available. One example is the melanocytoma/melanoma spectrum arising in the central nervous system. This category of tumors is thought to arise from meningeal melanocytes and share many features with uveal and blue nevus–like melanomas, including the most prevalent mutations (GNAQ and GNA11)15.

Ancillary Techniques

Histochemistry and Immunohistochemistry

Special stains (histochemical stains) may be used and are exceptionally helpful in certain circumstances, such as when examining an amelanotic or oligomelanotic lesion. Fontanta Masson, a silver histochemical stain, results in a black precipitate with melanin.

Of more importance, immunohistochemical stains utilize antibodies directed toward the antigen of interest. Some immunostains that commonly assist in the diagnosis of melanoma are reviewed in Table 2.1.

Table 2.1 Immunostains That Commonly Assist in the Diagnosis of Melanoma

Name

Target

Normal staining

Neoplastic staining

Staining pattern in cells

Staining pattern in lesions

Observations

S-100

Ca2+– binding proteins

  • Melanocytes

  • Langerhans cells/dendritic cells

  • Sweat glands

  • Myoepithelial cells

  • Chondrocytes

  • Adipose tissue

  • Nerves

  • Melanoma

  • Nevi

  • Some adnexal tumors

  • Neural tumors

Nuclear and cytoplasmic

Strong and diffuse

  • Useful in spindle cell lesions, particularly desmoplastic melanomas

  • Sensitive but less specific

SOX-10

SRY-related HMG-box 10 (SOX10) protein (transcription factor)

  • Melanocytes

  • Nerves

  • Melanoma

  • Nevi

  • Neural tumors

Nuclear

Diffuse with variable intensity from cell to cell

  • Useful in spindle cell lesions, particularly desmoplastic melanoma versus scar25

  • Sensitive and specific stain

HMB-45

Melanosomal glycoprotein gp100 (Pmel17)

Melanocytes

  • Most melanomas with the exception of desmoplastic melanomas

  • Nevi

  • Perivascular epithelioid cell tumor (PEComas)

Cytoplasmic granular

In nevi, the positivity is present in the superficial portion; the opposite occurs in malignant lesions

Desmoplastic melanomas are negative with HMB-45

Melan-A/Mart-1

Cytoplasmic protein sensitive and specific for melanoma and melanocytic lesions

Melanocytes

  • Most melanomas with exception of desmoplastic melanomas

  • Nevi

  • PEComas

  • Adrenal neoplasms

  • Ovarian tumors

Cytoplasmic

Strong and diffuse

More sensitive than HMB45

Tyrosinase

Enzyme involved in melanin synthesis and melanosome formation

Melanocytes

  • Most melanomas with the exception of desmoplastic melanomas

  • Nevi

Cytoplasmic

Strong and diffuse

p16Ink4A

Cyclin-dependent kinase inhibitor 2A, multiple tumor suppressor 1, tumor suppressor protein coded by the CDKN2A gene

None

  • 50% of sporadic melanomas show alterations

  • Nevi

  • High grade squamous lesions of the GYN tract, among others

Cytoplasmic and nuclear

Labeling tends to be lost with tumor progression12

MIB-1 (Ki-67)

Labile, non-histone nuclear protein expressed in G1, S, G2, and M phases of cell cycle

All proliferating cells

Increased staining in malignant neoplasms like melanoma

Nuclear

Deep nuclei positive

More sensitive than mitoses

Phosphohistone protein 3 (PHP3)

Core histone protein that is a major constituent of chromatin; marker of cells in late G2 and M phases

All dividing cells

Increased staining in malignant neoplasms like melanoma

Nuclear and mitotic figures

Mitotic figures positive

BRAF/VE1

Abnormal protein product of the BRAF V600 mutations

None

  • BRAF V600 mutated melanomas and nevi

  • CNS pediatric tumors

  • Papillary carcinoma of thyroid

Cytoplasmic

Strong and diffuse with good correlation with lesions harboring the mutation26

Molecular Techniques

A multitude of ancillary molecular techniques are available nowadays that aid with diagnosis and also by directing therapy. The molecular methods that are utilized as diagnostic tools include fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH). With regard to FISH, there is a commercially available panel of four to five probes that are particularly used in ambiguous melanocytic tumors, whereas CGH provides the pathologist with a panoramic view of the copy gains and losses and also helps define difficult tumors. In terms of guiding the therapeutic options with newly developed targeted therapies in patients with melanomas, tumors now are routinely subjected to mutation analysis for the BRAF V600 group of mutations. Other loci that are explored less frequently include specific exons of the KIT gene. These mutation analyses are carried out in a variety of platforms, including sequencing (prominently pyrosequencing) and matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF) types of platforms (SEQUENOM)24. With the advent of next-generation sequencing (NGS) techniques, a good number of these tests will be performed in one single platform.

References

1. Lin JY, Fisher DE. Melanocyte biology and skin pigmentation. Nature. 2007;445:843–850.Find this resource:

2. Walters RF, Groben PA, Busam K, et al. Consumption of the epidermis: a criterion in the differential diagnosis of melanoma and dysplastic nevi that is associated with increasing breslow depth and ulceration. Am J Dermatopathol. 2007;29:527–533.Find this resource:

3. Massi G, LeBoit PE. Histological diagnosis of nevi and melanoma. 2nd ed. New York: Springer; 2013.Find this resource:

4. Weedon D. Lentigines, nevi and melanoma. In: Weedon D, Strutton G, Rubin AI. Weedon’s Skin Pathology. 3rd ed. Churchill Livingstone Elsevier; 2010: 705–756.Find this resource:

5. Balch CM, Gershenwald JE, Soong SJ, et al. Final version of 2009 AJCC melanoma staging and classification. J Clin Oncol. 2009;27:6199–6206.Find this resource:

6. Breslow A. Thickness, cross-sectional areas and depth of invasion in the prognosis of cutaneous melanoma. Ann Surg. 1970;172:902–908.Find this resource:

7. Clark WH, Jr., From L, Bernardino EA, et al. The histogenesis and biologic behavior of primary human malignant melanomas of the skin. Cancer Res. 1969;29:705–727.Find this resource:

8. In ‘t Hout FE, Haydu LE, Murali R, et al. Prognostic importance of the extent of ulceration in patients with clinically localized cutaneous melanoma. Ann Surg. 2012;255:1165–1170.Find this resource:

9. Gimotty PA, Van Belle P, Elder DE, et al. Biologic and prognostic significance of dermal Ki67 expression, mitoses, and tumorigenicity in thin invasive cutaneous melanoma. J Clin Oncol. 2005;23:8048–8056.Find this resource:

10. Balch CM. Microscopic satellites around a primary melanoma: another piece of the puzzle in melanoma staging. Ann Surg Oncol. 2009;16:1092–1094.Find this resource:

11. Etoh T, Byers HR, Mihm MC, Jr. Integrin expression in malignant melanoma and their role in cell attachment and migration on extracellular matrix proteins. J Dermatol. 1992;19:841–846.Find this resource:

12. Strickler AG, Schaefer JT, Slingluff CL, Jr., et al. Immunolabeling for p16, WT1, and Fli-1 in the assignment of growth phase for cutaneous melanomas. Am J Dermatopathol. 2014;36:718–722.Find this resource:

13. Clark WH, Jr., Elder DE, Van Horn M. The biologic forms of malignant melanoma. Hum Pathol. 1986;17:443–450.Find this resource:

14. Genomic Classification of Cutaneous Melanoma. Cell. 2015;161:1681–1696.Find this resource:

15. Bastian BC. The molecular pathology of melanoma: an integrated taxonomy of melanocytic neoplasia. Annu Rev Pathol. 2014;9:239–271.Find this resource:

16. Busam KJ, Kutzner H, Cerroni L, et al. Clinical and pathologic findings of Spitz nevi and atypical Spitz tumors with ALK fusions. Am J Surg Pathol. 2014;38:925–933.Find this resource:

17. Wiesner T, He J, Yelensky R, et al. Kinase fusions are frequent in Spitz tumours and spitzoid melanomas. Nat Commun. 2014;5:3116.Find this resource:

18. Hodi FS, Corless CL, Giobbie-Hurder A, et al. Imatinib for melanomas harboring mutationally activated or amplified KIT arising on mucosal, acral, and chronically sun-damaged skin. J Clin Oncol. 2013;31:3182–3190.Find this resource:

19. Chen LL, Jaimes N, Barker CA, et al. Desmoplastic melanoma: a review. J Am Acad Dermatol. 2013;68:825–833.Find this resource:

20. Wiesner T, Kiuru M, Scott SN, et al. NF1 Mutations are common in desmoplastic melanoma. Am J Surg Pathol. 2015.Find this resource:

21. Narita N, Tanemura A, Murali R, et al. Functional RET G691S polymorphism in cutaneous malignant melanoma. Oncogene. 2009;28:3058–3068.Find this resource:

22. Prasad ML, Patel SG, Huvos AG, et al. Primary mucosal melanoma of the head and neck: a proposal for microstaging localized, Stage I (lymph node-negative) tumors. Cancer. 2004;100:1657–1664.Find this resource:

23. Chung AF, Woodruff JM, Lewis JL, Jr. Malignant melanoma of the vulva: A report of 44 cases. Obstet Gynecol. 1975;45:638–646.Find this resource:

24. Curry JL, Torres-Cabala CA, Tetzlaff MT, et al. Molecular platforms utilized to detect BRAF V600E mutation in melanoma. Semin Cutan Med Surg. 2012;31:267–273.Find this resource:

25. Ramos-Herberth FI, Karamchandani J, Kim J, et al. SOX10 immunostaining distinguishes desmoplastic melanoma from excision scar. J Cutan Pathol. 2010;37:944–952.Find this resource:

26. Busam KJ, Hedvat C, Pulitzer M, et al. Immunohistochemical analysis of BRAF(V600E) expression of primary and metastatic melanoma and comparison with mutation status and melanocyte differentiation antigens of metastatic lesions. Am J Surg Pathol. 2013;37:413–420.Find this resource: