TOPIC: Lung Cancer TYPE: Fellow Case Reports INTRODUCTION: Cystic lung airspaces can be seen in a multitude of diseases including lymphangioleiomyomatosis, Langerhan cell histiocytosis, Birt-Hogg-Dube syndrome, cystic metastases, or even postinfectious pneumatoceles. Lymphoma, however, is not often included in the differential diagnosis. We describe a rare presentation of B cell lymphoma presenting as a cystic lung disease. CASE PRESENTATION: Forty five year old female with a past medical history of diabetes mellitus and hypertension was originally referred to us for a lung biopsy. Patient had worsening shortness of breath for 1 year. She had a computed tomography of the chest at another facility which showed extensive cystic spaces in the lungs, suspicious for lymphangiomyomatosis. Patient was evaluated by a pulmonologist as an outpatient and had a bronchoscopy with biopsy. After receiving inconclusive results, she was given a course of antibiotics and had relief of her symptoms for some time. Months later the patient had an episode of severe coughing which prompted a visit to a hospital within our system where she had another computed tomography of the chest with contrast revealing the same diffuse cystic disease that was worse at the bases. She was subsequently referred to us and underwent video-assisted thoracic surgery with right upper, middle, and lower lobe biopsies. Pathology of the right middle lobe was revealed diffuse large B-cell lymphoma. Cystic change was seen in all of the biopsies. DISCUSSION: Lymphoma is one of the great masqueraders in medicine. Because of this, it should be included in the differential diagnosis for patients such as this with multiple cystic airspaces. Primary pulmonary extranodal lymphomas are rare, accounting for 0.4% of all lymphomas, and only 15-20% of these are high grade. These lymphomas are usually either mucosa-associated (MALT) or bronchus-associated lymphoid tissue (BALT) lymphomas. Presentations vary but are most often found as solitary nodules or masses and may be found incidentally. With more diffuse disease, other symptoms like shortness of breath, cough, chest pain, or even hemoptysis may arise. There is an association between immunosuppression and pulmonary lymphomas, as well with human immunodeficiency virus (HIV) infection and inflammatory conditions like rheumatoid arthritis and Sjogren disease. Diagnosis requires surgical biopsy because a bronchoalveolar lavage or fine needle biopsy may show lymphoma cell infiltration but cannot exclude other differential diagnoses such as reactive lymphoid proliferation, or lymphoid interstitial pneumonia. CONCLUSIONS: Lymphoma is one of the great masqueraders in medicine and can present in many different ways. Because of this, it should be included in the differential diagnosis in patients such as this with multiple cystic airspaces of the lungs. Diagnosis requires a surgical biopsy which should be included in these cases. REFERENCE #1: Girard, N., & Cadranel, J. (n.d.). Rare Primary Lung Tumors. In Murray and Nadel's Textbook of Respiratory Medicine (6th ed., pp. 965-980). Elsevier. REFERENCE #2: Hagga, John. "Nonneoplastic Parenchymal Lung Disease." CT and MRI of the whole body, 6th ed., Elsevier, 2017, pp. 928–983. REFERENCE #3: Myers, J. L. (2018). Lung. In Rosai and Ackerman's Surgical Pathology (11th ed., pp. 372-443). Elsevier. DISCLOSURES: no disclosure on file for Bishoy Hanna; No relevant relationships by Diego Marin, source=Web Response no disclosure on file for Christina Migliore Patel; no disclosure on file for Xinlai Sun