History
    A 46-year-old male patient with chest abnormality for 2 years.

Findings
    Chest PA reveals ill-defined consolidation in right upper lobe and middle lobe. Follow-up chest PA shows consolidation in the same regions, which is slightly decreased in volume. CT scan reveals ill-defined consolidation with air-bronchogram in right upper and middle lobes.

Discussion
    The differential diagnosis of non-resolving consolidation include bronchiolitis obliterans with organizing pneumonia, eosinophilic pulmonary syndromes, pulmonary alveolar proteinosis, drug-induced lung disease, and neoplastic disorders. Bronchoalveolar cell carcinoma and primary pulmonary lymphoma are classic neoplastic disorders that may present as focal, alveolar opacities on chest radiograph without evidence of volume loss.
    Primary pulmonary lymphomas are uncommon. Classification of these pulmonary lymphomas is surrounded with controversy. The attempt to classify primary pulmonary lymphomas has additionally been difficult acknowledging the recognition in the 1980s that lymphoma may arise from MALT. Although this rare lesion principally arises in the stomach, occasionally it has been identified in bowel, salivary gland, thyroid gland and lung. Accordingly, when located in the lung, this lymphoma appears to arise from bronchus-associated lymphoid tissue (BALT).
    BALT represents a low-grade B lymphocyte lymphoma that is composed of monotonous small lymphocytes with focal plasmacytoid features. The origin of the cell is thought to be a B lymphocyte from the marginal zone of the lymphoid follicle. Pulmonary MALT lymphoma comprise a distinct group of extranodal lymphomas. These low-grade lymphomas are usually localized and frequently do not appear to progress for periods of 10 years or more. Radiographic findings are areas of dense, persistent consolidation. Air bronchograms were present in the vast majority. CT may be required to demonstrate the air bronchograms, give further information on the distribution and morphology of the lesions.

References
1. O’Donnel, PG, Jackson SA, Tung KT, Hassan B, Wilkins B, Mead GM. Radiological appreance of lymphomas arising from Mucosa-associated lymphoid tissue (MALT) in the lung. Clinical Radiology 1998;53:258-263.
2. Chow WH, Ducheine Y, Hilfer J, Brandstetter RD. Chronic pneumonia: Primary malignant non-Hodgkin’s lymphoma of the lung arising in mucosa-associated lymphoid tissue. Chest 1996;110:838-840.