A reassessment of primary thyroid lymphoma: high-grade MALT-type lymphoma as a distinct subtype of diffuse large B-cell lymphoma

Aims: Primary lymphoma of the thyroid gland (PTL) is a relatively rare disease. During an 18-year period, 53 cases of primary non-Hodgkin's lymphoma involving this extranodal site were seen at our institutions. The aims of this study were to evaluate the spectrum of PTLs using current lymphoma classification concepts and immunocytochemical markers, determine whether features of MALT-type lymphoma were evident in PTL, and if there was any clinical significance of such a finding. Methods and results: The cases were retrospectively studied clinically, histologically and immunohistochemically. The tumours were classified according to the Revised European-American Lymphoma Classification of lymphoid malignancies (REAL classification). Thirty-eight patients were females, 15 were males and mean age at diagnosis was 66.3 years (range 38-90). Three cases were low-grade marginal zone lymphomas (low-grade MALT-type lymphomas). There were 15 diffuse large B-cell lymphomas (DLBCL) of which there were 27 DLBCL-NOS and 18 high-grade MALT-type lymphomas. Within the diffuse large B-cell lymphoma (DLBCL) category, cases were subdivided into those without (DLBCL-NOS) and those with features of 'high-grade' MALT-type lymphoma based on presence of a low-grade component or large cell lymphoepithelial lesions (HG MALT-type lymphoma). In addition there were three follicle centre lymphomas, one anaplastic large cell lymphoma and one peripheral T-cell lymphoma, Twenty cases were stage I-E, 18 Stage IIE, and four stage IV. All patients with low-grade MALT-type lymphoma are alive without disease. The 5-year survivals for DLBCL-NOS and HG MALT-type lymphoma were 75% and 25%, respectively, Univariate analysis (log rank) among the DLBCLs showed stage (P<0.001) and subtype (P = 0.005) were associated with survival, Stage was associated with type of DLBCL, 65% of DLBCL-NOS being stage I-E compared to 20% of HG MALT-type lymphomas, Conclusions: We conclude that primary thyroid lymphomas occur most commonly in elderly women and are frequently present in clinical stage I-E and IIE. Low-grade MALT-type lymphomas are relatively uncommon but appear to have a favourable prognosis, DLBCL is the most common lymphoma and features of MALT can be seen in over one-third of cases. As a group, HG MALT-type lymphomas had a worse outcome than DLBCL- NOS, primarily due to higher clinical stage at diagnosis. These two subtypes of DLBCL appear to be distinct clinical and histological entities.