Malignant melanoma presenting as a thyroid mass with synchronous retroperitoneal metastases in a 55-year-old male: a case report.

BACKGROUND: Malignant melanoma (MM) is an aggressive tumor characterized by a high tendency for early metastasis. Although most cases have identifiable primary sites, melanoma of unknown primary (MUP) presents as metastatic disease without detectable primary lesions. MM presenting as a thyroid mass with concurrent metastasis to the spleen-kidney interspace is extremely rare, and the occult nature of the primary lesion also poses a significant diagnostic challenge, often leading to confusion with aggressive primary thyroid malignancies. CASE DESCRIPTION: A 55-year-old male with an unremarkable medical history presented with a 3-month history of a rapidly enlarging, painless left neck mass. Physical examination revealed a firm 5-cm mass in the left thyroid lobe and a similarly sized palpable mass in the right supraclavicular region. No suspicious lesions were noted on the skin or mucous membranes. 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) identified multifocal hypermetabolic lesions involving the left thyroid lobe, right supraclavicular fossa, and splenorenal recess. To improve surgical precision, patient-specific three-dimensional (3D) printing technology was used preoperatively to depict the spatial relationship between the cervical masses and adjacent neurovascular structures. The patient subsequently underwent total thyroidectomy, cervical lymph node dissection, and resection of the retroperitoneal lesion. Postoperative histopathological examination confirmed metastatic melanoma at all sites, and molecular analysis identified the BRAF V600E mutation. The patient successfully initiated targeted therapy with dabrafenib and trametinib and remains stable during follow-up. CONCLUSIONS: Advanced melanoma should be included in the differential diagnosis of rapidly expanding, atypical thyroid masses, even in the complete absence of a known primary lesion. Clinicians must utilize fine-needle aspiration cytology and a targeted immunohistochemical panel to differentiate metastatic melanoma from primary anaplastic or medullary thyroid carcinomas early. Whole-body 18F-FDG PET/CT is crucial for detecting occult systemic metastases, while patient-specific 3D printing provides substantial practical value for complex head-and-neck operative planning.