Primary Lymphomas of the Thyroid: Diagnostic Modalities and Challenges

Primary lymphomas of the thyroid (PTLs) are rare entities but pose important diagnostic challenges for the pathologist. Because PTLs tend to develop in patients with a history of Hashimoto thyroiditis (HT), they are frequently misdiagnosed as the more benign and histologically variable HT. Clinical history is by far the most important factor to consider when evaluating a thyroid with a robust lymphocytic infiltrate. Even with a history of HT, in the setting of significant change in clinical status, the pathologist must utilize ancillary testing, including immunohistochemistry and flow cytometry, to thoroughly evaluate for PTLs. Because the principal method of thyroid nodule evaluation is fine-needle aspiration, it may be necessary to obtain additional tissue before a definitive diagnosis can be reached. This is particularly true in cases of marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma), which represent the second most common type of PTLs. While the more common diffuse large B-cell lymphoma often has readily apparent atypia and thus is more easily diagnosed in the absence of ancillary testing, MALT lymphomas are on a histologic continuum with HT. Therefore, this entity is most often missed on fine-needle aspiration specimens and biopsies, particularly if additional testing is not used. In this review, we discuss 1 case of MALT lymphoma and 1 of diffuse large B-cell lymphoma so that the clinical differences between these 2 entities may be appreciated. In addition, the various diagnostic modalities and the inherent challenges to diagnosing PTLs are reviewed.