Abdominal cyst of unclear aetiology: gastrointestinal stromal tumour or reactivation of abdominal tuberculosis

Differential diagnosis of a new abdominal mass is broad and includes infection, malignancy and other inflammatory processes. Definitive diagnosis may be challenging without invasive biopsy, as history, physical exam and imaging may be non-specific. A 69-year-old man with a history of abdominal tuberculosis presented with a new painful abdominal cyst consistent with reactivation of tuberculosis versus new malignancy. Investigations revealed 4+ acid-fast bacilli from the aspirate suggestive of tuberculosis, but no improvement was noted on antituberculous therapy. Core needle biopsy noted c-KIT-positive spindle cells, diagnostic for a gastrointestinal stromal tumour, while cultures grew non-tuberculous mycobacteria.