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(American Journal of Pathology. 2002;160:1435-1443.)
© 2002 American Society for Investigative Pathology

Regular Articles

Primary Thymic Extranodal Marginal-Zone B-Cell Lymphoma of Mucosa-Associated Lymphoid Tissue Type Exhibits Distinctive Clinicopathological and Molecular Features

Hiroshi Inagaki^*, John K. C. Chan, Josephine W. M. Ng, Mitsukuni Okabe^*, Tadashi Yoshino, Masataka Okamoto^¶, Hiroshi Ogawa^||, Hiroshi Matsushita^**, Tomoyuki Yokose, Yoshihiro Matsuno, Naoya Nakamura, Tetsuro Nagasaka^¶,¶, Ryuzo Ueda^||||, Tadaaki Eimoto^* and Shigeo Nakamura^**

From the Departments of Pathology^* andMedicine,^|||| Nagoya City University Medical School,Nagoya, Japan; the Department ofPathology, Okayama University Medical School,Okayama, Japan; the Department of Hematology,^¶ Fujita University Medical School, Toyoake, Japan; the Department ofPathology,^|| Seirei Mikatagahara Hospital, Hamamatsu,Japan; the Department of Pathology,^** ToranomonHospital, Tokyo, Japan; the PathologyDivision, National Cancer CenterResearch Institute East, Kashiwa, Japan; the Clinical LaboratoryDivision, National CancerCenter Hospital, Tokyo, Japan; the Department ofPathology, Fukushima MedicalUniversity Medical School, Fukushima, Japan; the PathologyDivision,^{¶ ¶} Nagoya University Hospital,Nagoya, Japan; the Department ofPathology,^*** Aichi Cancer CenterHospital, Nagoya, Japan; and the Departments ofPathology and ClinicalOncology, Queen Elizabeth Hospital, HongKong

Extranodal marginal-zone B-cell lymphoma (MZBL) of mucosa-associated lymphoid tissue (MALT) arising in the thymus is rare, with the largest series in the literature including only three cases. In the present study, we investigated 15 cases of thymic MALT lymphoma to systematically characterize its clinical, histopathological, and molecular features. There was a marked female predilection (male:female = 1:4), with a mean age of 55 years at diagnosis. There was a strong association with autoimmune disease, especially Sjögren’s syndrome. Histologically, the thymic lymphoma showed the characteristic morphological features of extranodal MZBL of MALT type. Cysts were common. Prominent lymphoepithelial lesions were formed by centrocyte-like cells infiltrating and expanding the Hassall’s corpuscles and epithelium lining the cysts. Plasmacytic differentiation was apparent in all cases. Notably, 13 of 15 cases expressed immunoglobulin (Ig) A phenotype; IgA expression in thymic MALT lymphoma was in striking contrast with the IgM phenotype observed in most of the Sjögren’s syndrome-associated MZBLs and MALT lymphomas at other sites. Epstein-Barr virus was absent, and API2-MALT1 gene fusion, a recently reported MALT lymphoma-specific gene abnormality, was not detected in any case. Although one patient died of disease 85 months after the diagnosis, other patients were alive with overall 3-year and 5-year survival rates being 89% and 83%, respectively. Among the 22 patients reported previously and in the present series, at least 17 patients (77%) were Asians. These data indicate that thymic MALT lymphoma may represent a distinct subgroup of MALT lymphoma characterized by apparent predilection for Asians, a strong association with autoimmune disease, frequent presence of cysts, consistent plasma cell differentiation, tumor cells expressing IgA phenotype, and consistent lack of API2-MALT1 gene fusion.

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