Primary lymphoma of the lung is rare, accounting for ˜3.5% of all extranodal lymphomas and <0.5% of malignant lung tumors.^1,2,3 The most common type is the extranodal marginal zone lymphoma (MZL) of mucosa-associated lymphoid tissue (MALT), accounting for more than 75% of lung lymphomas.^4,5 The terms “bronchus-associated lymphoid tissue (BALT) lymphoma” and “MALT lymphoma” are used interchangeably in the lung for extranodal MZL.

Organized lymphoid tissue is not normally present in the lung. BALT is stimulated by chronic inflammatory conditions such as autoimmune disorders, smoking, and infections.⁶ BALT is distinct from intrapulmonary lymph nodes, which are usually peripheral nodules incidentally found on imaging or in lung resections for unrelated conditions and which are not associated with the development of lymphoma.

MALT lymphoma tends to affect adults with ages ranging from late 20s to late 80s with a mean age of 65. About half are asymptomatic and the lesions are discovered incidentally.⁷ There seems to be no association with tobacco exposure.⁸

When present, symptoms include cough, chest pain, weight loss, dyspnea, or hemoptysis. Fever and night sweats are uncommon, reported in <15% of cases. A serum monoclonal gammopathy is found in up to 43% of patients.⁷ There is a slight female predominance in several studies, and most patients present in clinical stages I and II (>85%).⁷

The staging system most frequently used is a modified Ann Arbor system for extranodal (E) lymphomas. Stage IE is confined to the lung; stage II 1E involves hilar lymph nodes; stage II 2E involves mediastinal lymph nodes; stage II 2EW involves chest wall or diaphragm; stage IIIE involves abdominal lymph nodes; and stage IVE indicates extralymphatic extrapulmonary involvement.²

There is a well-known association between BALT lymphomas and autoimmune disorders, including Sjögren syndrome and rheumatoid arthritis, although the mechanisms underlying the relationship are unclear.⁹ In one of the largest series of BALT lymphomas, only 10% of the patients had an autoimmune disease at presentation.² In other studies, autoimmune diseases have been reported in up to 30% of patients.⁷

The most common presentation on routine chest radiology is a solitary, noncalcified nodule or airspace consolidation with or without air bronchogram. However, many reports of bilateral disease have also occurred, which confirm the heterogeneous presentation of BALT lymphomas. Diffuse infiltrates with a lymphangitic appearance, diffuse interstitial lung disease, nodular patterns, bronchiectasis, lobar pneumonia patterns, and ground-glass opacities have all been reported.^1,10,11 The majority of the lesions have a high standardized uptake value (SUV) on positron emission tomography (PET) scans.^1,10

The diagnosis of BALT lymphoma is made most commonly by VATS (video-assisted thoracoscopy), CT-guided transthoracic biopsy, or transbronchial biopsy. The diagnosis can be suspected, suggested, and rarely confirmed with bronchoalveolar lavage. Cytology specimens procured by fine needle aspiration may yield a diagnosis especially when coupled with flow cytometry and cell block preparations for immunohistochemistry (IHC) so that clonality can be established.¹² In some instances, patients undergo lobectomies, segmentectomies, or even pneumonectomies for the resection of nodular lesions before the diagnosis of BALT lymphoma can be made.^2,8,10,13

BALT lymphomas cause yellow-white nodules with a consistency similar to that of nodal lymphomas. Cystic change is rare and associated with coexistent amyloidosis.³ Mediastinal lymph nodes may be enlarged and are involved histologically in less than half of cases.⁷

BALT lymphomas are distributed in a lymphangitic pattern, typically along bronchovascular bundles, interlobular septa, and visceral pleura. The lymphangitic infiltrates coalesce into masses that extend into the pulmonary parenchyma (Fig. 88.1). Occasionally, the tumor
is a single dominant mass. Prominent collagen sclerosis is common and occasionally obscures the lymphoid infiltrate. Vascular invasion is also often noted, while amyloid deposition occurs in fewer than 10% of tumors.⁷