We have herein reported a patient who underwent liver resection for a solitary metastasis from a clitoral malignant melanoma, a particularly rare condition. The decision to perform a hepatic resection was based on the likelihood that it would improve the patient’s prognosis.

Cutaneous malignant melanomas account for most malignant melanomas, and mucosal malignant melanomas account for 0.8% to 3.0% of such tumors [5, 6]. Female genital tract melanomas are classified as mucosal melanomas and account for 20% of all mucosal malignant melanomas [7], thus constituting a particularly rare form of female genital tract melanomas. A search of PubMed from 2000 to 2024 for the terms “clitoral malignant melanoma”, “malignant melanoma of clitoris”, and “vulvar melanoma” yielded three reports [8, 9], including our case (Table 1). To the best of our knowledge, this is the first case of treatment of a distant metastasis from a clitoral malignant melanoma.

Malignant melanomas can exhibit simultaneous hematogenous and lymphogenous spread and can metastasize to any organ [10]. Mucosal malignant melanomas are reportedly more likely to give rise to distant metastases than are cutaneous malignant melanomas. The most common sites of distant melanoma metastases are skin/subcutaneous tissue [11], whereas liver, lung, and extra-regional lymph node metastases are more common sites of metastases from mucosal malignant melanomas [12]. The three reported patients who we identified had short follow-up and no recurrence, whereas our patient had recurrences in the lungs and liver, which is consistent with the pattern of recurrence of mucosal melanomas previously reported. Malignant melanoma is known to have the poorest prognosis of all skin cancers. The 5 year survival rate for early-stage (Stage I) malignant melanoma is 91.4%, whereas at 24.6%, the prognosis is relatively poor for patients with distant metastases (Stage IV) [1]. The prognosis of vulvar malignant melanomas is even worse, with 5 year survival rates of 73.6% for Stage I and 3.9% for Stage IV [13]. In this case, there is potential for a favorable prognosis through the resection of oligometastasis and adjuvant chemotherapy.

Molecularly targeted agents or immune checkpoint inhibitors are generally recommended for Stage IV malignant melanoma. However, surgery is also considered for patients with oligometastasis, defined as fewer than five metastases that can be completely resected [2,3,4]. The recurrence rate of malignant melanoma is as high as 75% and generally requires long-term follow-up for 5–10 years after surgery and short-term follow-up every 3 months for the first 3 years after surgery [14,15,16]. Despite the high recurrence rate, resection of metastases has been reported to improve overall survival (OS) [2,3,4]. In a study of resectable Stage IV malignant melanoma, not limited to liver metastases, the surgery group had a significantly greater 4 year survival rate than did the conventional chemotherapy group (20.8% vs. 7.0%, p < 0.01) [17]. A meta-analysis of 22 studies involving 579 patients who had undergone resection of malignant melanoma metastases in the liver revealed that OS was significantly longer in the surgery than non-surgery group (14–41 vs. 4–12 months; hazard ratio, 0.32; 95% confidence interval, 0.22–0.46) [2]. Additionally, the interval between resection of the primary tumor and recurrence may be a prognostic indicator. Adam et al. [18] reported that the interval (< 12 vs. > 12 months) between liver metastases and resection of the primary tumor has an impact on 5 year survival in patients with non-colorectal nonendocrine liver metastases (including malignant melanoma) (risk rate, 1.82; confidence interval, 1.47–2.26; p = 0.0001). In addition to achieving R0 resection and considering disease-free interval, it is imperative to comprehensively evaluate the sufficient remaining liver volume and the patient’s surgical tolerance to determine the overall surgical candidacy [2]. In recent years, the minimally invasive procedure of robot-assisted surgery has also attracted attention [19,20,21,22]. Compared with laparoscopic surgery, robot-assisted surgery is reportedly associated with lower rates of open conversion and shorter postoperative hospital stays [20]. This factor is likely to contribute to the patient’s surgical tolerance as well. Our patient achieved long-term recurrence-free survival (RFS) of 4 years after excision of a lung metastasis. We therefore considered that a favorable outcome would also likely be achieved by resection of a solitary liver metastasis and thus performed robot-assisted excision of the lesion.

In recent years, immune checkpoint inhibitors have been found to be effective when administered as postoperative adjuvant chemotherapy [23, 24]. Consequently, nivolumab and pembrolizumab, human IgG4 monoclonal antibodies against PD-1, were listed for adjuvant chemotherapy of malignant melanoma from 2018 in Japan. To our knowledge, no reports have indicated that such adjuvant chemotherapy is effective specifically for mucosal malignant melanoma; however, pembrolizumab reportedly achieved a significantly longer RFS than ipilimumab and high-dose alpha-interferon (hazard ratio, 0.77; confidence interval, 0.59–0.99; p = 0.002), as well as superior safety, with rates of Grade ≥ 3 adverse effects at 19.5%, 49.2%, and 71.2%, respectively [25]. We prescribed pembrolizumab as adjuvant chemotherapy after resection of lung and liver metastases and there has been no recurrence yet. However, clearly superior OS has not yet been reported; future results are awaited.