Revisit of Histopathology in Rheumatoid Lung Nodules.

Diagnosing pulmonary rheumatoid nodules (RN) is challenging because they are rare and share clinical, radiologic, and histopathologic features with other necrotizing granulomatous diseases such as infectious granulomas (IG) and granulomatosis with polyangiitis (GPA). Herein, we revisit the histopathologic features of RN and the findings in the adjacent lung tissue, in comparison with IG and GPA, to identify distinguishing features. Twenty-eight cases with surgically resected RN evaluated at our institution (1991 to 2024), 33 IG (10 mycobacterial infection, 10 histoplasmosis, 13 coccidiomycosis), and 10 GPA cases were included in the study. All available slides with H&E and special stains were reviewed for various histologic parameters within the nodules and in surrounding lung tissue. Many histopathologic features of RN overlap with the necrotizing granulomas seen in infections and GPA. However, some findings in the adjacent lung tissue showed significant differences. The lack of airspace granulomas/non-necrotizing granuloma and the absence of necrotizing vasculitis in the setting of RN can help to differentiate between infection cases and GPA, respectively. Given their considerable overlap, correlation with clinical context, serologic and microbiologic studies, as well as careful evaluation of special stains is crucial in the diagnosis of RN.