Peritoneal tuberculosis mimicking malignancy on FDG PET/CT in a patient with ankylosing spondylitis on adalimumab: a diagnostic challenge.

Tumor necrosis factor-alpha (TNF-α) inhibitors, such as adalimumab, are integral in the managing refractory ankylosing spondylitis (AS). However, their immunosuppressive effects elevate the risk of reactivation of latent tuberculosis (TB), especially extrapulmonary TB (EPTB), which can present with non-specific symptoms and mimic malignancy. We report the case of a 40-year-old man with ankylosing spondylitis who had been on long-term adalimumab therapy. He presented with a low-grade fever and weight loss. An 18F-FDG PET/CT scan revealed intense FDG uptake in the peritoneum, omentum, and mesentery. There was no ascites or visceral involvement; however, a few FDG-avid retroperitoneal, mediastinal, and right cervical lymph nodes were noted, along with a right pleural effusion and no lesions in the lung parenchyma. Despite a negative microbiological workup, empirical anti-tubercular therapy (ATT) was initiated due to strong clinical and imaging suspicion of tuberculosis. The patient showed significant clinical improvement, and a follow-up PET/CT scan six months later indicated complete metabolic resolution of the lesions. This case underscores the diagnostic challenge of peritoneal TB in immunosuppressed individuals. It highlights the supportive role of FDG PET/CT in guiding empirical therapy and monitoring treatment response without microbiological confirmation.