A Silent Intruder: Case Report of Primary Pulmonary Follicular Lymphoma Mimicking Infectious Lung Disease in a Patient with Autoimmune Comorbidities

Primary pulmonary follicular lymphoma (PPFL) is an exceptionally rare subtype of non-Hodgkin lymphoma that arises within the lung parenchyma without nodal or extrapulmonary involvement at diagnosis. Its nonspecific clinical and radiological presentation often mimics more common pulmonary diseases, leading to diagnostic delays. We report the case of a 64-year-old woman with a history of diabetes, hypertension, and cutaneous systemic lupus erythematosus (SLE) who presented with a persistent cough lasting one month. She denied constitutional symptoms, and physical examination as well as routine laboratory investigations were unremarkable. Chest X-ray and computed tomography (CT) of the thorax revealed multiple bilateral pulmonary nodules. Initial bronchoscopy and biopsy were inconclusive. Due to persistent symptoms and radiological findings, a repeat CT-guided lung biopsy was performed, which demonstrated malignant lymphoid infiltration. Immunohistochemical staining was positive for CD20, CD10, B-cell lymphoma 2 (Bcl-2), paired box protein 5 (Pax-5), and leukocyte common antigen (LCA), consistent with grade 2 follicular lymphoma. Based on these findings, a diagnosis of primary pulmonary follicular lymphoma was established. This case highlights the importance of including PPFL in the differential diagnosis of chronic pulmonary nodules, emphasizes the diagnostic value of repeat tissue sampling in inconclusive cases, and underscores the role of multidisciplinary evaluation in achieving an accurate diagnosis.