Primary malignant melanoma of the lung with C-KIT mutation and SRD5A3-KIT fusion.

Background: Primary pulmonary malignant melanoma (PMML), an exceedingly rare aggressive neoplasm originating from bronchial mucosal melanocytes, is characterized by early metastatic dissemination and high mortality. While over 95% of malignant melanomas are cutaneous in origin, fewer than 80 PMML cases have been documented globally. The molecular pathogenesis of PMML remains poorly defined, with less prior genomic studies utilizing Next-generation sequencing (NGS) reported to date.

Case presentation: A 68-year-old asymptomatic woman was referred to our institution in June 2022 after a routine health screening revealed a solitary pulmonary nodule. Chest CT demonstrated a 1.2 cm × 0.8 cm hypodense nodular opacity nodule in the posterior segment of the left upper lobe. The lesion remained stable during a 2-month observation period. Despite the absence of respiratory symptoms (e.g., cough, hemoptysis) or constitutional signs (e.g., weight loss), the patient elected surgical resection due to persistent malignancy concerns.

Conclusion: Histopathological examination revealed tumor cells exhibiting epithelioid to spindle-shaped morphology, characterized by prominent nucleoli and intracytoplasmic melanin deposition (hematoxylin and eosin staining). Immunohistochemical analysis demonstrated diffuse and strong positivity for S-100, HMB-45, and Melan-A. Based on the histomorphological features and immunohistochemical profile, a diagnosis of malignant melanoma was established. NGS detected a somatic KIT exon 11 mutation (c.1727 T > C, p. Leu576Pro; variant allele frequency: 20.1%) and identified an SRD5A3-KIT gene fusion involving transcript variants NM_024592.4 (SRD5A3) and NM_000222.2 (KIT), with breakpoints in Exon 5 of SRD5A3 and Exon 6 of KIT. The fusion variant showed a somatic mutation frequency of 24.8%. These findings not only expand the molecular landscape of PMML but also suggest therapeutic opportunities through targeted kinase inhibition. This case underscores the critical role of integrated multimodal analysis (radiological-pathological-molecular) in characterizing rare malignancies.