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Melanoma Arising in Association With and/or Simulating a Blue Nevus
Melanoma may develop in association with a blue nevus (BN), in particular a cellular blue nevus (CBN), or closely simulate the appearance of a BN (BN-like melanoma). Such tumors have historically been referred to as malignant blue nevus . Malignant blue tumor would be a preferable term to avoid the oxymoron of “malignant nevus.” Melanomas arising in association with a blue nevus may also be termed BLUE (BLue nevus-derived Uveal melanoma Equivalent melanomas to reflect a shared origin and pathway. Like uveal melanomas, BLUE melanomas tend are not associated with epithelia, they share similar cytologic features, and tend to be commonly associated with GNAQ/GNA11 mutations and loss of BAP1. The spectrum of BN-like melanomas also includes metastatic melanoma simulating BN. Those lesions are discussed in more detail in the chapter of blue nevi and metastatic melanoma.
Clinical Findings
Melanomas arising in association with a BN are rare. They tend to affect middle-aged or elderly individuals, predominantly whites, with a median age at presentation close to 50 years. Slightly more than half of the reported cases occurred in the head and neck region, followed by the trunk and buttocks. There may or may not be a history of a known BN with recent growth or change. Most of the lesions are larger than 1 cm in diameter at presentation.
The clinical features of the subset of BN-like melanomas (without an associated BN) are difficult to define because of the inherent subjectivity of what is BN-like. Some pathologists may annotate a melanoma as BN-like based on the heavy pigmentation, whereas others may restrict the term to tumors closely resembling melanomas arising in association with a BN except for the failure to identify a benign precursor lesion. If the common denominator is that of a darkly pigmented primary tumor, and heavily pigmented nodular melanomas or “pigment-synthesizing melanomas” are included, the clinical spectrum is accordingly broad.
Histopathologic Findings
Melanoma Associated With a Common Blue Nevus or Melanocytosis (Nevus of Ota or Ito)
In one recent series of melanomas arising in association with a BN, in 6 of 23 cases, the melanoma was associated with a common BN. In this scenario, the melanoma is usually dominant and readily recognized as malignant neoplasm. Without ancillary studies and/or clinical correlation, it may be impossible to determine whether the melanoma is simply colliding with an unrelated BN or whether it arose from it. Most melanomas, which can be confirmed by mutation analysis to be related to a BN, are nodular melanomas. The detection of a conventional superficial spreading, lentigo maligna, or acral melanoma associated with a BN usually represents a collision scenario, such as when the excision of a melanoma in situ reveals residual melanoma in situ as well as an incidental BN.
Because metastatic melanoma can simulate the appearance of a common BN, careful attention to cytologic findings (epithelioid atypia, mitoses) and at times ancillary studies are needed to distinguish a melanoma associated with a BN from a melanoma with a satellite simulating the appearance of a common BN.
Melanoma may also arise in a larger area with bland BN-like features (i.e., dermal melanocytosis, such as a nevus of Ota [ Fig. 19.1 ] or Ito). The melanoma presents as a hypercellular expansile nodular growth distinctly different from the bland background.
Melanoma Associated With a Cellular Blue Nevus
Most melanomas associated with a BN are seen together with a CBN ( Figs. 19.2–19.5 ). This may occur in the setting of a solitary lesion of CBN ( Fig. 19.2 ) or in context with one or possibly multiple cellular nodules within a small or large plaque type BN (see Figs. 19.3–19.5 ). Usually, the melanoma nodule stands out at low magnification as an expansile nodule of high cellular density. The malignant tumor nodule often lacks intervening stroma (stromal overgrowth) and may be composed of sheets of enlarged epithelioid cells, dense fascicles of spindle cells, or a combination thereof ( Box 19.1 ). Mitotic figures are usually present. There may or may not be associated necrosis. Although necrosis is more commonly seen with melanoma, it is per se insufficient for melanoma. On occasion, a cellular BN may undergo ischemic or traumatic necrosis.
Box 19.1
Melanoma Associated With a Blue Nevus (Blue Melanoma)
Clinical Findings
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Most often located on scalp, followed by trunk and buttocks
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History of longstanding blue nevus with change
Histopathologic Findings
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Dual population of malignant and benign melanocytes
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Dense fascicles of spindle cells with stromal overgrowth
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Sheets of atypical epithelioid cells
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Nuclear pleomorphism
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Mitoses
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Necrosis (variable)
Differential Diagnosis
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Cellular blue nevus
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Pigmented nodular conventional melanoma
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Pigment-synthesizing melanoma
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Deep-penetrating/plexiform spindle cell melanoma
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Malignant melanotic schwannian neoplasm
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Metastatic melanoma
Molecular Findings
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Genomic aberrations common
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Loss of BAP1 common
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GNAQ or GNA11 mutations common
In the absence of tumor necrosis, the presence of expansile tumor cell aggregates without intervening fibrous stroma (see Figs. 19.3 and 19.4 ) and/or aggregates of atypical epithelioid tumor cells ( Figs. 19.4–19.6 ) are clues for the recognition of melanoma. In difficult or controversial cases, cytogenetic studies may be helpful for diagnosis (see Fig. 19.5 ).
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