Introduction

The terms used to denote melanomas from volar surfaces of hands and feet or the nail apparatus, ‘acral melanoma’ and ‘acral lentiginous melanoma’, embody distinct diagnoses. According to the World Health Organization (WHO), acral melanoma (AM) is an anatomical term that refers to melanoma located on glabrous (hairless) skin of the extremities. On the other hand, acral lentiginous melanoma (ALM) represents a histopathological subtype of cutaneous melanoma with characteristic morphological and structural features, recognised alongside others such as lentigo maligna melanoma (LMM), superficial spreading melanoma (SSM), and nodular melanoma (NM) [1].

Almost two decades ago, attention was drawn to the various controversial aspects and unanswered questions about AM, including the misuse of the terms AM and ALM [2]. Apparently, the lack of consensus on how to refer to melanoma in acral skin continues until today: from all original studies retrieved from a search of PubMed using ‘acral melanoma’ as the search term, only 38% specified the histopathological subtype of the lesions, 78% reported the anatomical sites, and 37% reported information on both (Table 1). In 21% of the studies, no specification of the histopathological subtype and anatomical site was reported. The search does not retrieve all studies on AM, but it offers us a picture of what is available in the literature. These studies involve different populations and illustrate the lack of reporting rigour and consensus.

Table 1. Articles focusing on AM according to the histopathological subtype and anatomical site. Publication year Histopathological subtypes Anatomical site Reference 2020 ALM Sole, palm, nail apparatus [3] 2020 Not specified Acral non-nail, nail apparatus* [4] 2020 Not specified Sole, toe, finger, nail apparatus* [5] 2020 Not specified Hand, foot, finger, toe, sole, nail apparatus* [6] 2020 Not specified Sole, palm, nail apparatus [7] 2020 Not specified Not specified [8] 2020 ALM, NM Sole, palm, nail apparatus [9] 2020 ALM, NM, SSM Hand, foot, nail apparatus* [10] 2020 ALM, NM, SSM, LMM Hand, foot* [11] 2020 Not specified Not specified [12] 2020 Not specified Not specified [13] 2020 Not specified Not specified [14] 2019 ALM Sole, palm, nail apparatus [15] 2019 ALM, NM, SSM, LMM Not specified [16] 2019 Not specified Hand, foot* [17] 2019 Not specified Not specified [18] 2019 ALM, NM, SSM Sole, palm, dorsum of hand and foot, nail apparatus [19] 2019 Not specified Not specified [20] 2019 ALM Sole, lateral great toe [21] 2019 ALM, NM, SSM Sole, palm, nail apparatus [22] 2019 Not specified Not specified [23] 2019 Not specified Sole [24] 2019 Not specified Sole, palm, nail apparatus [25] 2018 Not specified Sole, palm, nail apparatus, digit [26] 2018 Not specified Hand, foot* [27] 2018 ALM, NM, SSM Sole, palm, nail apparatus [28] 2018 Not specified Nail apparatus, non-nail apparatus* [29] 2018 Not specified Sole, palm [30] 2018 Not specified Not specified [31] 2018 ALM, SSM, NM Sole, palm, nail apparatus [32] 2018 Not specified Not specified [33] 2018 Not specified Hand, foot* [34] 2018 Not specified Hand, finger, foot and toe* [35] 2018 Not specified Hand, foot, sole, finger* [36] 2018 ALM, NM, SSM Palm, sole, nail apparatus [37] 2018 ALM, NM, SSM, LMM Sole, palm, dorsum of hand and foot, nail apparatus, interdigital [38] 2017 Not specified Not specified [39] 2017 Not specified Not specified [40] 2017 Not specified Foot, palm, nail apparatus* [41] 2017 ALM Sole, nail apparatus [42] 2017 Not specified Foot, hand, nail apparatus* [43] 2017 ALM Sole, palm, nail apparatus [44] 2016 Not specified Hand, foot, nail apparatus* [45] 2016 Not specified Sole, toe, top of the foot, palm, finger, near the fingernail* [46] 2016 ALM, NM Sole, palm, dorsum of hand and foot, nail apparatus [47] 2016 ALM, NM, SSM Sole, toe, dorsum of the foot, finger, nail apparatus [48] 2016 ALM, non-ALM Sole, palm, nail apparatus [49] 2016 ALM, NM, SSM Sole, palm, nail apparatus [50] 2016 ALM, non-ALM Sole, palm, nail apparatus [51] 2015 Not specified Sole, palm [52] 2015 Not specified Not specified [53] 2015 Not specified Sole, palm, nail apparatus [54] 2015 Not specified Hand, foot* [55] 2014 Not specified Foot, sole, toe* [56] 2013 Not specified Not specified [57] 2013 ALM, NM, SSM Sole, palm, nail apparatus [58] 2013 ALM, NM Sole, palm, nail apparatus [59] 2013 Not specified Sole, palm, nail apparatus [60] 2013 ALM, NM, desmoplastic Sole, palm, nail apparatus [61] 2013 Not specified Sole, palm, dorsum of hand and foot [62] 2013 ALM, NM Nail apparatus [63] 2012 Not specified Sole, nail apparatus, web space [64] 2012 Not specified Sole, toe, web space [65] 2012 Not specified Hand, foot* [66] 2011 Not specified Not specified [67] 2011 Not specified Not specified [68] 2011 Not specified Not specified [69] 2009 Not specified Not specified [70] 2005 ALM Sole, palm, finger, toe, nail apparatus* [71] 2004 Not specified Sole, palm, nail apparatus [72] 2004 ALM Sole, palm [73] 2000 ALM, NM, SSM Sole, palm, nail apparatus, dorsum of hand, foot [74] 2000 ALM, SSM Sole, toe, foot* [75] 1998 ALM, NM, SSM Sole, palm, nail apparatus [76] 1993 Not specified Sole, palm, nail apparatus, dorsum of hand and foot [77] 1990 ALM, NM, SSM Sole, palm, nail apparatus [78] 1990 Not specified Sole, palm, nail apparatus [79] 1988 Not specified Sole, palm, nail apparatus [80] 1985 ALM, NM, SSM Sole, palm, nail apparatus [81] 1983 Not specified Sole, palm, nail apparatus, toe, finger, dorsum of foot [82] 1982 Not specified Sole, palm, dorsum of hand and foot [83] A search using the term ‘acral melanoma’ on PubMed in June 2020 (English language and journal article type) retrieved 239 articles (published and ahead of print), of which 154 were original studies. Of those, 94 used the words ‘acral’ and ‘melanoma’ in the title. Four were excluded for using the word ‘lentiginous’ in the title and nine were excluded for other reasons (file not available, only in situ AM, naevi, or cell lines), resulting in 81 articles. Among these articles, 38% reported the histopathological subtypes studied, 78% reported the anatomical sites, and 37% reported information on both. In 21% of the studies, there was no specification of the histopathological subtype and anatomical site. Not specified: information on histopathological subtype or anatomical location of AM cases was not provided. The volar surfaces of the fingers and toes are included as palms and soles, respectively. Specific locations of the soles, such as the heels, are included as soles. * Articles in which it is unclear whether the classification was performed according to the 2018 WHO guidelines.

These inconsistencies regarding the reporting of histopathological subtypes have a major impact in the interpretation of data derived from studies aimed at understanding the epidemiological, clinical, and biological characteristics of AM. In the following sections, we discuss different aspects of AM, highlighting the differences observed when the histopathological subtypes are considered, and in doing so we aim to draw attention to the importance of an accurate anatomical and histopathological classification. As an important note, we clarify that we only reviewed articles that followed the WHO guidelines for diagnosing AM [1].

AM epidemiology

According to several population-based epidemiology studies, AM represents only about 3% of melanomas that occur in European-descent individuals [84-86], whereas it represents a higher proportion of cases in Asian, Hispanic, and African populations [1]. For example, AM accounts for more than 40% of cutaneous melanoma in Asia [87, 88] and 20.1% in Latin American countries [89]. Furthermore, the relative frequency of AM is higher in genetically heterogeneous or Amerindian-descent populations (indigenous Latin Americans) than in European-descent individuals from Latin American countries [90, 91].

In the case of ALM, its absolute global incidence is similar across populations and is much lower than that of SSM or LMM. However, it also accounts for a higher percentage of total melanoma cases in non-European descent populations compared to populations of European descent [92].

These contrasts in the relative incidence of AM and ALM may be ascribed to marked differences in sun-induced melanoma incidence among these populations; for example, the incidence of cutaneous melanoma is 49 per 100,000 individuals in Australia, which has a predominantly European-descent population [93], but is typically estimated to be less than 5 per 100,000 inhabitants in several countries in Latin America [91, 94, 95]. As AM and ALM have similar incidences around the globe and are not thought to be ultraviolet (UV)-related, they constitute a higher proportion of cases in countries with a lower incidence of melanoma.

Among studies that have reported the histopathological subtypes of AM, a high relative frequency of ALM is consistently reported, but the frequencies of AM histopathological subtypes vary considerably. ALM has been reported to represent 44–90% of AM, while acral NM (2.5–41%) and acral SSM (0.7–28%) are relatively less frequent subtypes of AM [16, 22, 28, 38, 50, 58, 76, 96]. When studying specific populations, these variations are noteworthy. In European-descent individuals, ALM repr