S4327 Rare Tumor, Rare Route: Diagnosing a Mediastinal Yolk Sac Tumor via Endoscopy

Introduction: Primary mediastinal germ cell tumors (PMGCTs) are rare, accounting for only 1%–4% of all mediastinal masses. Yolk sac tumors, a subtype of non-seminomatous germ cell tumors, most commonly arise in the gonads, with extragonadal occurrences primarily seen in the mediastinum or retroperitoneum. Direct esophageal invasion by such tumors is extremely uncommon, and diagnosis via esophagogastroduodenoscopy (EGD) is exceedingly rare. We present a unique case of a primary mediastinal yolk sac tumor with direct esophageal involvement diagnosed by upper endoscopy in a young adult man presenting with progressive dysphagia. Case Description/Methods: A 34-year-old man warehouse worker with a history of obesity and prior tobacco use presented with a 4-month history of progressively worsening dysphagia, initially to solids and eventually to liquids. Computed tomography angiography of the chest demonstrated a large heterogeneous mass in the subcarinal and middle mediastinum with internal air, concerning for communication with the esophagus or superimposed infection. EGD was performed and revealed a single, fungating, ulcerated, and invasive mass measuring approximately 9 mm in the middle third of the esophagus. Biopsies of the lesion were obtained. Histologic examination revealed malignant neoplastic cells consistent with a germ cell tumor showing yolk sac (endodermal sinus tumor) differentiation. Serum AFP level was markedly elevated at 797.86 ng/mL, further supporting the diagnosis. Testicular ultrasound showed no evidence of a gonadal primary, confirming the mediastinum as the primary site of disease. Discussion: Primary mediastinal yolk sac tumors are rare and typically affect young adult men. They most often present with nonspecific symptoms such as chest pain, cough, dyspnea, or superior vena cava syndrome. Gastrointestinal symptoms, particularly dysphagia, are highly atypical and usually occur only in cases with direct tumor invasion into the esophagus—a rare phenomenon. In this case, progressive dysphagia was the key symptom leading to diagnostic evaluation. While computed tomography imaging raised suspicion for malignancy, the diagnosis was confirmed via endoscopic biopsy—an uncommon route for identifying PMGCTs, as they typically do not involve the esophageal mucosa. This case highlights the importance of considering mediastinal tumors in the differential diagnosis of unexplained dysphagia, especially in young male patients. It also underscores the diagnostic value of EGD in atypical cases and expands the clinical spectrum of PMGCTs with esophageal involvement.