Differential diagnosis between rheumatoid nodules and fibrocaseous tubercles in the lung: A case series

The histological similarities between rheumatoid nodules and post-primary fibrocaseous tubercles in the lung may present a significant challenge in differential diagnosis. Differential diagnosis of fibrocaseous tubercles and rheumatoid nodules is crucial in clinical practice, particularly due to the high risk of co-existing fibrocaseous tuberculosis in rheumatoid arthritis. This condition increases the likelihood of military dissemination and mortality, especially in elderly patients, with elderly women being at the greatest risk. This case series aims to differentiate these two pathological entities based on the clinical histories and histological analyses of surgical specimens from two patients. The main histological marker for RhNod is the presence of vascular remnants within the fibrinoid necrotic area, reflecting the vascular origin of RhNods. Additional supporting evidence for the rheumatoid nature of the process includes the presence of inflamed blood vessels elsewhere in the lung (such as non-specific, fibrinoid necrotic, and/or granulomatous autoimmune vasculitis), as well as the potential presence of co-existent interstitial pneumonitis, with or without pleuritis. In contrast, the necrotic tuberculous process is characterized by coalescent necrosis that does not respect anatomical borders and lacks structural remnants of lung tissue, which is a hallmark of tuberculosis pathology.