Pulmonary nodules are a common imaging finding associated with a broad differential diagnosis. Therefore, each patient's risk factors, the clinical context, and the nodule's characteristics and associated imaging findings are key to determining the etiology.
Rheumatoid lung nodules are generally located in subpleural regions and tend to be asymptomatic, unless an associated complication such as cavitation or pneumothorax is present. Furthermore, rheumatoid lung nodules occur more frequently in patients who have severe or long-standing rheumatoid disease, with concomitant subcutaneous nodules, which this patient does not have.
Patients with methotrexate-induced pneumonitis often have fever, and about one third have peripheral eosinophilia. Imaging findings of methotrexate-induced lung injury vary and include organizing pneumonia, acute interstitial pneumonia with noncardiogenic pulmonary edema, and fibrosis. Ill-defined centrilobular nodules can be present, but solid airway centered nodules such as those seen in this case should point to a different diagnosis. Additionally, this patient was no longer taking methotrexate when her symptoms started; thus, the time course would not fit.
Invasive aspergillosis and Pneumocystis jirovecii pneumonia (PJP) are less likely given the lack of constitutional symptoms, negative results for 1,3-beta-d-glucan and galactomannan on bronchoalveolar lavage, and radiographic findings that are more indicative of Mycobacterium avium complex (MAC).[5,6] Angioinvasive pulmonary aspergillosis generally occurs in immunocompromised patients with neutropenia and presents as solid nodules surrounded by ground-glass attenuation (halo sign), reflecting hemorrhage as a result of vessel invasion. The most common imaging finding in PJP is extensive ground-glass opacity. Nodules are rare in patients with PJP and usually indicate the presence of a superimposed organism.Legionnaires disease is typically accompanied by fever, would be less insidious in onset, and often presents as multilobar or multisegmental consolidation and ground-glass opacities.
Two classic radiographic forms of MAC have been described in the literature: the nodular bronchiectatic form (as seen in this patient) and the fibrocavitary form (as shown in a different patient in Figure 2 and Figure 3). The fibrocavitary form is more difficult to treat. Patients who are receiving tumor necrosis factor (TNF) inhibitors are at high risk for nontuberculous mycobacterial (NTM) infections and complications.
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