The anatomical distribution of lentiginous melanoma (lentigo maligna and lentigo maligna melanoma): Differences according to sex

The clinical diagnosis of lentiginous melanoma (lentigo maligna/LM and lentigo maligna melanoma/LMM) is challenging due to its poorly demarcated borders and tendency to occur on sun-damaged skin with background melanocytic hyperplasia.1 This challenge is compounded by the necessity to distinguish lentiginous melanomas from similarly appearing benign lentiginous naevi and dysplastic naevi.2 Detailed information regarding the anatomical distribution of these melanomas may guide clinicians, leading to earlier diagnosis and improved outcomes. Existing studies have reported on the incidence of lentiginous melanomas using broad anatomic regions such as the head and trunk, precluding more comprehensive analysis. The objective of this study was to describe the anatomic distribution of lentiginous melanomas in detail using high-resolution subsite data and assess differences according to sex. A cross-sectional study of all lentiginous melanomas diagnosed at the Victorian Melanoma Service between January 1994 and December 2021 was performed. Tumour location was prospectively recorded using detailed coding across 232 anatomic sites. Patients were excluded from the study if the information on tumour location was not recorded. Institutional ethics approval for the study was obtained. For statistical analysis, tumour location was categorized into 50 anatomic subsites. The incidence rate per unit area of lentiginous melanoma by subsite was analysed using Poisson regression. To compare the distribution between males and females, the χ2 test was used to select a reference subsite that had similar proportions of melanoma. Results were considered statistically significant if p < 0.05. All statistical analyses were performed in Stata (Version 14, StataCorp, College Station, TX). A total of 1782 cases of lentiginous melanoma were included for analysis. The baseline characteristics of the study participants are reported in Table 1. Overall, central facial subsites had the highest incidence rate of lentiginous melanoma per unit area (Figure 1). The nose (incidence rate ratio (IRR), 15.7; 95% CI 11.8–20.7, p < 0.001), cheek (IRR 11.4; 95% CI 8.9–14.6, p < 0.001) and pre-auricular (IRR 8.2; 95% CI 5.7–11.8, p < 0.001) areas had the highest incidence rate per unit area relative to the posterior forearm. The lower limb contained the subsites with the lowest incidence of lentiginous melanomas. For comparisons of lesion distribution between males (n = 1002) and females (n = 780), the posterior forearm was chosen as the reference site; it contained similar proportions of lentiginous melanoma amongst men (3.9%) and women (4.1%) (χ2 test; p = 0.91). Relative to the posterior forearm, a number of sites had differing incidence rate ratios between the sexes, as indicated by an interaction (p < 0.05) between subsite and sex (Table S1). Compared to women, men had a significantly higher incidence of lentiginous melanomas per unit surface area in the anterior and posterior scalp, ears, upper back and trapezius. Compared to men, women had a significantly higher incidence of lentiginous melanomas per unit surface area in the cheek and anterior arm. The incidence of lentiginous melanomas for the head and neck by sex is displayed in Figure 2. In addition to providing high-resolution information regarding the anatomical distribution of lentiginous melanomas, our findings highlight differences according to sex. Males had a higher incidence of lentiginous melanomas on peripheral subsites of the head such as the ears and scalp, while females had a higher incidence on the central face subsites such as the cheeks with similar findings from previous reports.3-5 This distinction may be due to males being more likely to develop androgenic alopecia leading to loss of hair coverage of the peripheral head when compared to females. This hypothesis is supported by an experimental study in which hair coverage down to the ear was found to reduce UV exposure by up to 81%.6 Males having a higher incidence of lentiginous melanomas on the upper back may be due to their increased likelihood of performing outdoor occupations such as gardening and construction where bent-over positions maximize sun exposure to this area. Regardless of sex, the highest incidence was demonstrated at the head and neck sites which is consistent with previous reports.4, 7-9 This supports the theory that accumulated sun damage is the key driver of lentiginous melanoma formation.10 Limitations of our study included referral bias of complex melanomas to our tertiary centre, which may artificially inflate the proportion of facial lesions, and retrospective nature of the study design. The anatomic subsite distribution of lentiginous melanomas has a distinct profile by sex which may assist clinicians in their diagnosis and surveillance of these skin cancers. The high incidence of lentiginous melanoma observed on the scalp, ears and cheeks also highlights the importance of appropriate photoprotection such as using wide-brimmed hats. Future research evaluating the proportion of LM to LMM at each anatomic subsite may further assist clinicians in their diagnosis and management of this melanoma subtype. The authors did not receive financial support for the authorship/publication of this work. Dr. Yan Pan is a current AJD Associate Editor. Ethics approval from the Alfred Hospital Ethics Committee (Project No: 357/22). Open access publishing facilitated by The University of Melbourne, as part of the Wiley - The University of Melbourne agreement via the Council of Australian University Librarians. 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