Relationships between tumor thickness and the risk of sentinel node metastasis in acral and non‐acral melanoma

Although there are some reports regarding the relationship between tumor thickness (TT) and sentinel lymph node (SLN) metastatic rates in melanoma patients, there is limited evidence as to whether the risk of SLN metastasis varies by melanoma subtype.1-4 We investigated the differences in TT and SLN metastasis rates between acral melanoma and nonacral melanoma. We retrospectively analyzed the cases of Japanese melanoma patients who underwent an SLN biopsy for melanoma at Sapporo Medical University between January 2006 and December 2021. We compared the TT of the primary tissue in SLN metastasis-positive and metastasis-negative cases divided into acral and nonacral cases. In situ cases and mucosal melanoma cases were excluded. Significant differences between the groups were determined by the Fisher‘s exact test, Chi-squared test, or Mann-Whitney U-test. This study was approved by our Institutional Review Board (IRB No. 292-74). Overall, 161 patients (77 males, 84 females) were examined. The melanoma types were acral (n = 101) and nonacral (n = 60). Sixty-four of the 161 patients (39.8%) had ulcers at the primary sites: 44 of 101 acral (43.6%) and 20 of 60 nonacral (33.3%). There was no significant difference between acral and nonacral cases (P = 0.200). The median TT of all 161 cases was 2.5 mm (2.50 mm in acral cases and 2.55 mm in nonacral cases). TT was not significantly different between acral and nonacral cases (P = 0.854). SLN metastasis was found in 58 of 161 cases (36.0%): 34 of 101 acral (33.7%) and 24 of 60 nonacral (40.0%) (P = 0.418). The median TT of 103 SLN metastasis-negative cases was 1.7 mm, and that of the 58 SLN metastasis-positive cases was 4.15 mm (P < 0.001) (Figure 1a). Of the 58 patients with positive SLN metastasis, 34 were acral and 24 were nonacral melanoma. The median TT of the 34 acral patients was 4.9 mm, and that of the 24 nonacral patients was 3.6 mm (P = 0.144) (Figure 1b). Of the 58 patients with positive SLN metastasis, 29 had ulcers of the primary site and the other 29 had no ulcers. Next, to rule out the ulcer factor, we examined the 97 cases without ulcers of the primary site. The melanoma types were acral (n = 57) and nonacral (n = 40). SLN metastasis was found in 29 of 97 cases (29.9%): 13 of 57 acral (22.8%) and 16 of 40 nonacral (40.0%) (P = 0.077). In the 97 cases without ulcer of the primary site, the presence or absence of SLN metastasis by T classification is shown in Table 1. The risk of SLN was not significantly different between acral and nonacral cases in the T1–T4 group (Table 1). In addition, of the 29 patients with positive SLN metastasis and no ulcer of the primary site, the median TT of the acral cases was 2.8 mm, and that of the nonacral cases was 3.25 mm (P = 0.9397) (Figure 1c). Our results suggest that the relationship between TT and SLN metastatic risk may not be related to the melanoma subtype. However, more recently, Shayan et al. reported acral melanoma was independently associated with the high risk for SLN positivity for stage IB and II melanoma.5 Further examination is required to explore this tendency.