"The Terrible Ts" That Was Not: Anterior Mediastinal Tuberculosis Mimicking Malignancy in an Immunocompetent Young Adult.

Anterior mediastinal masses are typically approached through the classic "terrible Ts" differential (thymic tumors, lymphoma, germ cell tumors, and thyroid disease). Tuberculosis (TB) rarely presents as an isolated anterior mediastinal mass in an immunocompetent adult and can mimic malignancy. We report the case of a male patient in his late 20s who presented with a two-month persistent dry cough, low-grade fever, and 8 kg of unintentional weight loss. Initial chest radiography demonstrated mediastinal widening, and high-resolution computed tomography (HRCT) revealed a large lobulated anterior mediastinal mass with central necrosis measuring 85 × 67 mm, abutting the pericardium and aortic arch, with subcarinal lymphadenopathy. Serum alpha-fetoprotein (AFP) and beta-human chorionic gonadotropin (β-hCG) were within normal limits (AFP: 3.59 ng/ml (reference range: 0.89-8.78); β-hCG: 1.50 mIU/ml (reference range: <5.00)). As imaging could not reliably distinguish thymic tumors, lymphoma, and necrotizing infectious disease, a CT-guided core biopsy was performed. Histopathology showed caseating granulomas with Ziehl-Neelsen-positive acid-fast bacilli, confirming the diagnosis of TB. The patient improved symptomatically within two weeks of antitubercular therapy (ATT). HRCT at two months showed regression to 70 × 62 × 35 mm. Serial chest radiographs showed continued improvement at six months and complete resolution at eight months, after which ATT was stopped. This case emphasizes that TB should remain in the differential diagnosis for necrotic anterior mediastinal masses in endemic settings and highlights the value of early tissue diagnosis to avoid unnecessary oncologic therapy or surgery.