Acral lentiginous melanoma is a rare subtype that accounts for 1% to 3% of melanoma cases, primarily affecting the acral skin of the palm, nails, and, especially, the sole of the foot. It is not directly related to sun exposure and is the most common subtype among individuals of African and Asian descent.1–3 In Brazil, it has a higher incidence than the global average, attributed to the racial diversity of the population. Some studies suggest that acral melanoma is associated with an unfavorable prognosis when compared to other subtypes.3–5 This may be partly explained by diagnoses at more advanced stages. However, some Asian studies do not confirm these findings and suggest a better prognosis.6–8

Despite its rarity and specific characteristics, the treatment of acral melanoma follows the same principles as other subtypes. Sentinel lymph node (SLN) biopsy is currently recommended for patients with intermediate-thickness melanomas and has been widely used with low morbidity.9–11 Since the publication of The Second Multicenter Selective Lymphadenectomy Trial (MSLTII) and Dermatologic Cooperative Oncology Group Trial (DECOG), patients with a positive SLN have been spared of complete lymph node dissection, and with the introduction of new effective treatments, the role of SLN biopsy has been questioned.12,13 However, its role has proven important in prognosis assessment, staging melanoma more accurately, and defining complementary treatment strategies. Its therapeutic role is still under debate, but data from MSLTI suggest an improvement in prognosis in patients with thick melanoma and better local control with a reduction in nodal recurrence in patients undergoing SLN biopsy.14,15

When analyzing major prospective studies, the majority of patients had the superficial spreading subtype, located on the trunk and extremities, while a smaller number had the acral subtype.12–15 Most studies on acral melanoma are retrospective, involving a limited number of patients, and have been conducted in Asia.7,8,16–22

Due to the lack of data on the use of SLN biopsy in this subgroup of patients and the high proportion of patients with this subtype in Brazil, it is essential to obtain more information about the use of SLN biopsy in this group. Therefore, the aim of this study is to evaluate the results of SLN biopsy in 79 patients with acral melanoma treated at a single oncological center and compare them with the data described in the literature.

In this study, the authors retrospectively analyzed the outcome of 79 patients with acral melanoma who underwent SLN biopsy over a 6-year period at a reference center. The present data reveals a patient profile with thicker melanomas (4.5 mm) and a higher rate of ulceration (67.1%), which is above the averages reported in studies like MSLTI, MSLT II, and DECOG.12–15 It also exceeds the study from SEER involving patients with acral melanoma who underwent regional SLN biopsy (median Breslow thickness of 2 mm and 34% had ulceration) and has similar results of a Brazilian study with 66.7% of ulceration and a median Breslow of 5 mm.16–23

These data may explain the elevated rate of positive SLN at 34.2% in this study. Most studies on melanoma report positive SLN rates around 20%.24–26 Studies specifically involving acral melanoma often show divergent results, often due to a limited number of patients and their retrospective nature, with rates ranging from 11.9% to 42.2%.7,8,16–22 A Korean study with only 34 patients showed a positive SLN rate of 41.2%,17 while another Korean study, with a larger number of patients (107) treated over a 12-year period, reported a positive SLN rate of 20.6.18 A retrospective study using a U.S. database with 753 patients, mostly of Caucasian descent, showed a positive SLN rate of 25.7%,16 and a Brazilian study with 201 acral melanoma patients, showed a positive SLN rate of 29.9%.23

In all patients, SLNs were correctly identified. This is consistent with the findings of Asian studies involving acral melanoma, which can be attributed to the location of acral lesions on extremities with a more predictable drainage chain. This also explains the high rate of SLN identification using the combined technique of blue dye and lymphoscintigraphy.7,8,17–22 Some data suggest that the false-negative rate for SLNs in acral melanoma patients could be higher. However, this data is not consistent with other studies,27–29 and it was not observed in the present study, where the recurrence rate at the drainage site after a negative SLN was only 3.8% (2 cases out of 52), which is lower than that reported in MSLT I 5.5%) and a Korean study (11.8%).14,18

The SLN biopsy has proven to be important in assessing patients' prognosis. Nevertheless, the number of patients with acral melanoma has always been limited in prospective Western studies.12–15 Specific studies on acral melanoma are retrospective and originate from Asia. Despite these limitations, seven studies have shown that the presence of SLN metastasis is a significant risk factor correlated with survival rates, especially in terms of DFS.8,17–21,23

In this study, an OS of 67.7% was observed in the total group, which is lower than what is typically seen in melanoma studies, ranging around 91.7%, and lower than other retrospective studies involving acral melanoma, which typically report survival rates around 80%.3,4 The reasons for this unfavorable prognosis can be explained by the more advanced diagnosis in the patients as the authors can observe in a Brazilian study that has similar characteristics and an even worse prognosis (OS of 44.6% and 38.6% DFS).23

When analyzing SLN results, the authors observed a clear trend toward improved survival in patients with negative SLNs (OS 70.9% vs. 53.2% with p = 0.081). However, statistical significance was only achieved in terms of DFS (46.7% vs. 33.9% with p = 0.001). The authors believe that, due to the retrospective nature of the study and the limited number of patients, statistical significance was not reached. Other studies focusing exclusively on acral melanoma have also shown similar results. In a study involving acral melanoma patients in the United States, melanoma-specific survival was 88.5% in the negative SLN group, compared to 58.9% in the positive SLN group.16 In six Asian studies, including three Japanese, one Chinese, and two Korean studies, positive SLN correlated with worst survival, especially in terms of DFS.8,17–21

Since the publication of DECOG and MSLT II, conservative treatment after a positive SLN has been recommended.12,13 In the present study, only half of the patients (48%) followed this recommendation, largely because data supporting this approach emerged during the course of the study. Additionally, the authors are treating a population with more advanced lesions, with substantial tumor deposits in the SLN, far beyond that was described in patients in the DECOG and MSLT II studies. This also justifies why Only 4 out of 14 patients who underwent elective lymphadenectomy after a positive SLN did not have any other positive lymph nodes besides the SLN.

Nevertheless, despite all these data, in the analysis of subgroups with positive SLNs, OS, and DFS were worse in the group that underwent elective lymphadenectomy after a positive SLN. However, this difference did not reach statistical significance (OS 51.5% vs. 56.5% with p = 0.659; DFS 23.8% vs. 46.9% with p = 0.147). In this group, where the conservative approach was adopted, Only 1 patient underwent lymphadenectomy due to local recurrence. The mortality rate was 9.3% in the group that underwent conservative treatment and 20% in the group that underwent lymphadenectomy after a positive SLN. These data may suggest that lymphadenectomy was indicated in patients with a larger tumor volume, but it also suggests that conservative treatment after a positive SLN could be offered in acral melanoma patients.

In this study, the SLN biopsy in acral melanoma patients demonstrated high accuracy in identifying sentinel lymph nodes and had significant implications for determining prognosis. Patients with acral melanoma and positive sentinel lymph nodes had a worse prognosis compared to those with negative sentinel lymph nodes. Patients with positive SLN biopsy who underwent lymphadenectomy showed a prognosis like those treated expectantly, suggesting that this approach could also be recommended for this subgroup of patients. However, due to the retrospective nature of the study and the limited number of patients, each patient should be assessed individually.

None declared.

Thiago Francischetto: Conceptualization and design of the study; original draft; critical review of the literature; final approval of the final version of the manuscript.

Ana Clara Falcão: Data collection; statistical analysis.

Adson Santos Neves: Original draft; obtaining, analyzing and data interpretation.

Ana Beatriz Lira: Data collection; statistical analysis.

Robson Freitas de Moura: Obtaining, analyzing and data interpretation.

Thiago Souza Oliveira Freitas de Moura: Original draft; obtaining, analyzing and data interpretation.

Juvandy Antonio Inacio Santos: Original draft; obtaining, analyzing and data interpretation.

André Bacellar Costa Lima: Original draft; obtaining, analyzing and data interpretation.

Marco Antonio Oliveira Lessa: Original draft; obtaining, analyzing and data interpretation.

Jussamara Britos Santos: Original draft; obtaining, analyzing and data interpretation.

Alexandre Farias de Albuquerque: Original draft; obtaining, analyzing and data interpretation.

Vaner Paulo da Silva Fonseca Pinheiro: Statistical analysis; obtaining, analyzing and data interpretation; Final approval of the final version of the manuscript.