Progressive Pulmonary Lesions and Cervical Lymphadenopathy Revealing Large B-cell Lymphoma: A Rare Case Report.

Primary pulmonary and thyroid diffuse large B-cell lymphoma (DLBCL) is an exceptionally rare extranodal manifestation of non-Hodgkin lymphoma and often presents with nonspecific clinical and radiographic features, leading to significant diagnostic challenges. We report a case of a 70-year-old woman with extensive past medical history who was incidentally found to have pulmonary abnormalities during routine abdominal magnetic resonance imaging (MRI) surveillance for hepatocellular carcinoma. Subsequent non-contrast computed tomography of the chest revealed multiple spiculated pulmonary masses involving the left upper and lower lobes and a nodular lesion at the right lung base. Positron emission tomography/computed tomography demonstrated marked fluorodeoxyglucose avidity within these pulmonary lesions, as well as focal uptake in the left thyroid lobe. Despite the absence of constitutional "B" symptoms, malignancy was strongly suspected. The initial pathology from the left lower lobe lung biopsy showed only benign features consistent with organizing pneumonia. Given the possibility of false negative results, further evaluation with ultrasound-guided fine-needle aspiration of a left cervical mass adjacent to the thyroid gland revealed an atypical lymphoid population suspicious for large B-cell lymphoma. A repeat lung biopsy ultimately confirmed DLBCL, with immunohistochemistry positive for CD20 and negative for CD3 and cytokeratin. This case underscores the diagnostic complexity of primary extranodal DLBCL involving the lung and thyroid, highlights the limitations of initial tissue sampling, and emphasizes the importance of persistent diagnostic evaluation and multidisciplinary collaboration when clinical suspicion remains high.