A80-4-35 A Waxing and Waning Warning: A Rare Case of Mucosa-Associated Lymphoid Tissue Lymphoma in the Lungs

Abstract Introduction Primary pulmonary mucosa-associated lymphoid tissue (MALT) lymphoma is a rare entity accounting for 0.5-1% of all primary lung tumors. It poses a diagnostic challenge owing to its indolent course and radiological resemblance to infectious or metastatic processes. We report a case of primary MALT lymphoma manifesting as waxing and waning pulmonary nodules. Case Description A 52-year-old man with recurrent nephrolithiasis was admitted for left-sided abdominal pain. A CT scan of the abdomen incidentally revealed bilateral pulmonary nodules. A CT scan of the chest demonstrated multiple bilateral noncalcified nodules, the largest measuring 16 mm in the left lower lobe, with smaller nodules up to 8 mm near the oblique fissure. The patient denied respiratory symptoms but reported intermittent scant morning hemoptysis for four to five months. He was a lifelong nonsmoker with occupational chlorine exposure. Esophagogastroduodenoscopy revealed mild gastritis, and Helicobacter pylori testing was negative. Review of serial imaging from the preceding six years showed multiple pulmonary nodules with a waxing and waning course and eventual resolution of a right lower lobe nodule. The alternating resolution and recurrence suggested an indolent, multifocal process, likely inflammatory or lymphoproliferative in nature. Infectious, autoimmune, and vasculitis workups were unrevealing. PET/CT showed subthreshold fluorodeoxyglucose uptake. Bronchoscopy with bronchoalveolar lavage (BAL) showed predominantly neutrophilic and eosinophilic inflammation. Wedge resection biopsy revealed a lymphoplasmacytic infiltrate with focal lymphoepithelial complexes and monocytoid B cells. Immunohistochemistry showed CD20-positive B cells, CD3-positive T cells, and polytypic plasma cells. Molecular testing demonstrated clonal immunoglobulin kappa light chain (IGK) gene rearrangement, consistent with MALT lymphoma. Bone marrow biopsy was negative, confirming localized pulmonary disease. The patient was initiated on single-agent rituximab therapy. Discussion Pulmonary MALT lymphoma is a rare entity, often detected incidentally or presenting with nonspecific respiratory symptoms. Imaging may reveal nodules, consolidations, or masses, frequently mimicking infections, inflammation, or metastatic disease. Definitive diagnosis requires histopathology, immunohistochemistry, and molecular studies. Typical findings include infiltration of small- to medium-sized lymphocytes with irregular nuclear contours, formation of lymphoepithelial lesions, and colonization of bronchial or alveolar epithelium. Management is individualized based on disease presentation: asymptomatic patients may be observed, whereas symptomatic or progressive cases typically require rituximab, involved-site radiation therapy (ISRT), or surgical resection. Long-term surveillance is warranted to monitor for disease progression or recurrence. This case highlights the importance of considering pulmonary MALT lymphoma in the differential diagnosis of indolent, fluctuating lung nodules, especially when infectious and autoimmune causes are excluded. This abstract is funded by: None