Pulmonary Adenocarcinoma Arising From Somatic-type Malignant Transformation of a Nonseminomatous Germ Cell Tumor: A Case Report.

Somatic-type malignant transformation (STM) is a rare but aggressive complication of nonseminomatous germ cell tumors (NSGCTs), most commonly arising from the teratomatous component and frequently occurring in metastatic sites. We report the case of a man in his 20s with a history of mixed NSGCT (50% yolk sac tumor, 40% teratoma, and 10% embryonal carcinoma) treated with left orchiectomy and chemotherapy in 2023, who initially responded to treatment but was subsequently lost to follow-up. He presented with one week of progressive dyspnea and pleuritic left-sided chest pain. Serum alpha-fetoprotein was markedly elevated at 2,966.7 ng/mL. Computed tomography demonstrated a progressive left hilar mass causing near-complete compression of the left mainstem bronchus with postobstructive consolidation, pulmonary vein tumor thrombus extending into the left atrium, a stable subcarinal metastatic mass, and a small left pleural effusion. Brain magnetic resonance imaging and transthoracic echocardiography were unremarkable except for normal cardiac function without intracardiac thrombus. CT-guided biopsy of the lung mass demonstrated adenocarcinoma consistent with a pulmonary phenotype. Following multidisciplinary evaluation, the lesion was determined to represent pulmonary adenocarcinoma arising from somatic-type malignant transformation of the teratomatous component of recurrent NSGCT. The patient remained clinically stable and was managed with close airway surveillance after Pulmonary and Critical Care Medicine consultation, with airway stenting or endobronchial debulking reserved for clinical deterioration. This case highlights the importance of recognizing STM in patients with recurrent NSGCT, as accurate diagnosis requires integration of clinical history, imaging, and pathology and has significant therapeutic and prognostic implications.