Paediatric melanoma is a rare entity, accounting for less than 1% of all melanoma diagnoses. The annual incidence is estimated at four cases per million in the USA, with most diagnoses occurring in children aged 10 years or older.1 However, the true incidence is difficult to determine because the diagnosis of paediatric melanoma is challenging and there are many conditions that mimic it, including Spitz nevi, Spitz tumours, and other atypical nevi. Furthermore, diagnostic standards have changed over time, particularly for Spitzoid melanoma. For example, the presence of tumour deposits within a sentinel lymph node was previously considered to be a diagnostic feature. However, in the past decade, several studies2, 3 have described cohorts of patients with Spitzoid melanoma or atypical Spitz tumours with associated sentinel lymph node involvement who remain free of subsequent metastases. Similarly, the presence of any clonal segmental karyotypical or copy number aberration (excluding copying number gains in chromosome 11p, HRAS) detected by fluorescence in-situ hybridisation (FISH) or comparative genomic hybridisation had, in the past, been interpreted as diagnostic of melanoma. However, several studies3, 4 have reported the presence of small numbers of clonal segmental copy number aberrations in benign atypical Spitz tumours, suggesting that chromosomal abnormalities vary in diagnostic and prognostic importance. Therefore, older literature on paediatric melanoma is sometimes problematic to interpret in light of current understanding.
There are three subtypes of melanomas that are prevalent in the paediatric population: Spitzoid melanoma, melanoma arising in congenital melanocytic nevi, and conventional (so-called adult-type) melanoma, which is usually of the superficial spreading or nodular type (table 1). Acral lentiginous melanoma and lentigo maligna types are exceedingly rare, with lentigo maligna reported almost exclusively in patients with deficiencies in DNA repair (ie, xeroderma pigmentosa). The frequency of each subtype varies by age group. In childhood (<11 years), most reported melanomas are Spitzoid, whereas melanoma arising in a congenital nevus occurs much less often.5 Conventioanl melanomas rarely occur, but when they do, most are nodular type. Superficial spreading melanoma is uncommon in childhood. In our experience, many lesions reported as superficial spreading melanomas in childhood are more accurately described as atypical Spitz nevi. In adolescence (11–19 years), the spitzoid subtype still accounts for half of all melanomas, whereas the remainder are considered conventional.5 Among conventional melanomas in adolescents, superficial spreading is the most common pattern, with nodular being less common. Melanoma arising in giant congenital nevi is less common in adolescents than in children.
Trends from the 1970s to early 2000s suggested increasing rates of paediatric melanoma in the USA, with an average annual increase of 2–3% per year.6, 7, 8, 9 However, in the past decade, short-term trends suggest a decreasing incidence.10 This change might reflect a decrease in the number of older adolescents with thin melanomas, and suggests the effectiveness of public health initiatives, increased sunscreen use, decreased time spent outdoors, earlier detection through dermoscopy, or improved access to dermatological services.10, 11 Additionally, some of the decrease could be because of improved histological classification of more atypical nevi that can mimic melanoma (eg, Spitz nevi). For younger patients, who have a higher proportion of tumours with poor prognostic features and different genomic aberrations, the same might not be true.8, 10, 12 In paediatric melanoma, the relationship between patient age, tumour characteristics, and mortality is complex. Some studies have shown decreased mortality in young patients despite having thicker melanomas with ulceration, high mitotic rates, and positive sentinel lymph node biopsies, whereas other studies report the opposite.8, 13, 14, 15, 16 Meaningful conclusions from large databases are limited by several factors, most notably the incomplete recording of important features such as histological subtype or a coexisting congenital melanocytic nevus. As more is known about the genetic and molecular underpinning of melanoma, reporting of various mutations or chromosomal aberrations will be increasingly important (panel).
Key messages
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Paediatric melanoma is rare, with an annual incidence of four per million in individuals younger than 20 years in the USA
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The three most common subtypes are Spitzoid melanoma, melanoma arising in a congenital melanocytic nevus, and conventional (so-called adult-type) melanoma
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In diagnostically challenging cases, an experienced dermatopathologist familiar with ancillary and molecular tests should be consulted
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In childhood, Spitzoid melanoma represents most cases and is considered a low-grade type of melanoma; melanoma arising in a congenital melanocytic nevus is less common, but has a more aggressive disease course
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In adolescence, Spitzoid melanoma still accounts for approximately half of all cases; the remainder are considered conventional, which share many similarities to melanoma diagnosed in adulthood
