Solitary Melanoma Lesion in a Pulmonary Nodule: A Case Report

Most melanomas in the respiratory system present as metastatic tumors, with primary lung melanoma nodules being incredibly rare. Melanoma metastasis to the lung is often the first clinically apparent site of visceral metastasis. Any pulmonary lesion confirmed to be melanoma must be assumed to represent metastatic disease until proven otherwise, since it is much more common than a primary lesion. This case describes a 63-year-old female with an incidental 12 x 11 mm nodule found on chest computed tomography (CT) in the upper lobe of the left lung. This nodule appeared to be new, as evidenced by a previous cardiac CT performed one year prior that should have had the nodule in the field of view. Positron emission tomography/CT (PET/CT) demonstrated a fluorodeoxyglucose (FDG)-avid solid left upper lobe nodule with no evidence of regional lymph node involvement or distant metastatic disease, concluding the nodule was most consistent with a primary pulmonary neoplasm. Magnetic resonance imaging (MRI) of the brain confirmed no evidence of brain metastatic disease. CT-guided fine-needle biopsy was performed, with histology consistent with malignant melanoma. The tumor board agreed that, despite the lack of evidence of melanoma on skin exam, fundus exam, or imaging of distant sites, the patient's melanoma should be treated as metastatic melanoma of unknown primary site (cTx, cN0, cM1b, stage IV). Treatment with nivolumab-relatlimab 280 mg intravenously every 28 days was initiated, with a significant treatment response within two infusions. This case report discusses the diagnosis and approach to a solitary lung melanoma suspicious for primary pulmonary malignant melanoma, with no clear evidence of metastasis, with current therapeutic guidelines for metastatic melanoma cases.