D110-23 A Lonely Spot: Primary Malignant Melanoma Arising in the Lung

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Abstract Introduction Primary malignant melanoma of the lung (PMML) is extremely rare, accounting for only 0.01% of all lung tumors. [1] Most melanomas found in the lungs represent metastases from other primary sites. Clinical presentation may include cough, hemoptysis, dyspnea, chest pain, and fever, with asymptomatic cases being uncommon. [2] We report a case of PMML that presented as an incidental solitary pulmonary nodule (SPN) detected during preoperative evaluation for coronary artery bypass grafting (CABG). Case Presentation A 77-year-old male, nonsmoker, with significant left main coronary artery stenosis was scheduled for CABG due to non-ST elevation myocardial infarction. A preoperative chest CT revealed an incidental 14 mm SPN in the left lung lingula with mediastinal lymphadenopathy. This prompted navigational bronchoscopy with transbronchial cryobiopsy of the lingular nodule and endobronchial ultrasound-guided fine-needle aspiration (EBUS-FNA) of the lymph node. Histopathology revealed a malignant neoplasm with epithelioid and spindle cell features. Immunohistochemical staining demonstrated tumor cell positivity for S-100, SOX-10, PRAME, HMB-45, and MART-1, and negativity for TTF-1, p40, Napsin A, CK5, CK7, and CK20, supporting the diagnosis of PMML. A brain MRI and whole-body PET/CT were performed for staging and were negative for any metastasis, effectively ruling out cutaneous, mucosal, and ocular melanoma. The patient denied any personal or family history of melanoma or significant weight loss. Percutaneous coronary intervention (PCI) was favored over CABG due to the aggressive nature of melanoma with nodal involvement. The patient is currently under outpatient oncology follow-up and receiving dual immunotherapy with nivolumab and ipilimumab. Discussion Only about 75 cases of primary malignant melanoma of the lung have been reported in the literature.[2] The clinical and radiologic appearance of PMML closely resembles that of primary lung cancer. Definitive diagnosis requires integration of clinical, radiologic, histopathologic, and immunohistochemical findings, along with exclusion of extrapulmonary primary sites.[3] Although regression of a primary melanoma may occur by the time metastasis is detected in cases of melanoma of unknown primary (MUP), complete regression is exceedingly rare—occurring in only 0.22-0.27% of all melanomas.[4] This diagnostic ambiguity makes differentiation between PMML and MUP particularly challenging, though in this case, PMML remains the most likely diagnosis. Conclusion An asymptomatic SPN in an apparently low-risk patient can, on rare occasions, represent an aggressive malignant lesion. Therefore, differential diagnoses should remain broad when evaluating SPNs, especially in the elderly population. This abstract is funded by: None

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