Melanocytomas comprise <0.1% of all tumors in the central nervous system (CNS) [1]; meningeal melanocytoma is a rare benign tumor derived from leptomeningeal melanocytes [2]. Almost all of these tumors are found at the posterior fossa or upper spinal level [3],[4]. The most common image appearance is as a well-defined intradural extramedullary mass; other differential diagnoses are meningioma or schwannoma [3],[5]. Typical magnetic resonance imaging (MRI) findings are hyperintense on T1-weighted images (T1WI) and hypointense on T2-weighted images (T2WI), with unclear contrast enhancement [3]. In pathology, melanoma should be excluded. Relatively good prognosis can be expected after total resection. However, some cases report an aggressive course of rapid growth, recurrence, and transformation to malignant melanoma [6],[7]. Accordingly, careful observation is essential. We report a rare case of meningeal melanocytoma with direct bone invasion at the C2 level that was completely removed by surgery.

A 52-year-old woman with no previous medical history was accidentally diagnosed with a cervical spinal tumor at C2 level when injured in a traffic accident (Figure 1A). Since then, she had been complaining of numbness of the left limbs but there was no clear association with the lesion. The tumor kept gradually growing and the spinal cord became compressed over a 6-year follow-up, without symptom exacerbation. She planned to undergo tumor removal in August 2022. Preoperatively, the mass was well-circumscribed, and no calcification was observed by computed tomography (CT). A cervical spinal CT revealed that it was intradural extramedullary, hyperintense on T1WI, hypointense on T2WI, with unclear contrast enhancement (Figure 1B, Figure 1C, Figure 1D, Figure 1E, Figure 1F, Figure 1G, Figure 1H). There was no lesion on head MRI.

A posterolateral approach was performed to resect the tumor. We approached the dura matter by C2 partial laminectomy and performed an incision. Intraoperatively, we found that the tumor was dark brown, and the lamina of the C2, dura, and pia nearby were partially pigmented. We removed the tumor piece by piece due to its softness and adhesion to dura, until achieving gross total resection (Figure 2A, Figure 2B, Figure 2C, Figure 2D, Figure 2E). After resection, the dural attachment was coagulated and the dura was sutured water-tight.

In pathological examination, proliferation of tumor cells with high melanin deposition was found in the tumor and in the pigmented lesion of the vertebral arch and dura (Figure 3A). There was slight cellular atypism but no atypical mitotic figure (0/10 high power field) but no clear necrosis. Immunohistochemistry analysis showed positive results for SRY-related high-mobility-group/HMG box 10 (SOX-10), S-100 protein, and human melanoma black-45 (HMB-45) (Figure 3B, Figure 3C, Figure 3D, Figure 3E). The Ki-67 index was <2% (Figure 3F). The tumor was diagnosed as meningeal melanocytoma as there were insufficient findings to consider it as malignant melanoma. The patient is followed up regularly but not receiving radiation or chemotherapy. An MRI at three months after surgery showed no evidence of tumor recurrence (Figure 4A and Figure 4B).

In 1972, Limas and Tio first reported a melanocytic tumor in the craniocervical junction [8]. According to the 2016 World Health Organization classification of CNS tumors, primary melanocytic tumors of the CNS are classified as: (1) meningeal melanocytosis (Grade 0), (2) meningeal melanocytoma (Grade 1), (3) meningeal melanoma (Grade 3), and (4) meningeal melanomatosis (Grade 3). Surgical removal is recommended as primary treatment [9]. Gross total removal is not difficult for intradural extramedullary tumors. The radiographic features and treatment of primary CNS melanocytoma from 10 case reports are summarized in Table 1. Most cases (9/10) achieved gross total resection or subtotal/near-total resection. In one case of partial resection, the patient was treated by radiation, immunotherapy, and second surgery, but he finally expired three years after diagnosis [15]. Two cases suffered recurrence and malignant transformation despite gross total resection and underwent chemotherapy. However, they showed resistance to the treatment [6],[16]. To the best of our knowledge, there are few reports of a good course with radiation and chemotherapy. We highly recommend total resection as first treatment of meningeal melanocytoma before tumor growth to prevent complications. Radiation and chemotherapy should be recognized as adjunctive treatment when total resection is not achieved or in case of tumor recurrence. Recently, immunotherapy with ipilimumab (anti-cytotoxic T1-Lymphocyte-assosiated protein drug) or nivolumab (anti-programmed cell death protein-1 drug) showed good prognosis in melanoma [18]. The suggested effectiveness of those treatments for meningeal melanocytoma or melanoma warrants further research [16],[18]. Histopathologic and immunohistochemical examination is essential to diagnose meningeal melanocytoma. Immunohistochemical staining indicates that melanocytoma is positive for S-100 and HMB-45 but negative for epithelial membrane antigen [3],[19],[20]. In addition, the Ki-67/MIB-1 index is generally low (<5%), in contrast with the high score found in melanoma [2],[3].

In our case, the tumor was well-circumscribed, hyperintense, on T1WI and hypointense on T2WI, with unclear contrast enhancement. Its slow growth as well as the histopathologic and immunohistochemical findings of low Ki-67 index and no abnormal mitosis/necrosis suggested meningeal melanocytoma. In contrast, the intraoperative findings of direct bone metastasis cannot exclude melanoma. Previous research indicated a transformation of melanocytoma into melanoma [6],[7],[16], so we should consider radiotherapy depending on the disease course. Despite some reports of dural invasion in meningeal melanocytoma, we found no report of direct bone invasion; thus, the present case is rare.

We report a rare case of meningeal melanocytoma in the upper cervical spine. Before surgery, we considered the tumor as a meningioma. It was difficult to identify as meningeal melanocytoma from images and clinical symptoms, as intraoperative findings, pathological examination, and immunostaining are essential to diagnose it. We recommend complete excision at an early stage as first choice treatment to avoid complications and transformation to malignant melanoma. In the present case, the gradual growth and pathological appearance indicates benign and good prognosis from surgical total resection. However, the direct bone invasion makes local recurrence possible, so regular follow-up is mandatory.