EMZBCL is a low-grade lymphoma arising in MALT, originally described in the gastrointestinal tract, subsequently encountered in a variety of organs, such as lung, breast, salivary and lacrimal glands and thyroid [7–10]. In these sites, MALT appears as result of specific immune reactions or autoimmune disease [7–11].
In fact, lymphocytic thyroiditis or HT are almost certainly a requisite for development of EMZBCL in the thyroid gland . EMZBCL is an indolent tumour which tends to remain localized in the thyroid and the lymphomatous infiltrate is often focal in a background of thyroiditis. In these cases a distinction between EMZBCL and diffuse inflammatory lymphoid infiltrate may be extremely difficult and strict criteria are necessary for the differential diagnosis. Histologically, the presence of a dense lymphoid infiltrate with centrocyte-like cells forming MALT-balls may be suggestive but not specific of lymphoma. The presence of plasmacytoid cells with light chain immunoglobulin restriction is a very important finding for a diagnosis of lymphoma, but represents an inconstant feature. In difficult cases the molecular study for IgH gene rearrangements, demonstrating the B cell monoclonal nature of lymphoid cells, supports a correct diagnosis of lymphoma .
In our case the presence of a minute focus of 4-mm in diameter incidentally discovered after an extensive sampling of the entire thyroidectomy specimen represents an effective potential diagnostic pitfall.
PCR assays revealed the rearrangement of IgH only in the microdissected lymphomatous cells confirming the morphological diagnosis of EMZBCL.
The minute focus of EMZBCL, arising in HT and diagnosed by means of molecular assays, probably represents an early-stage in the development of a low-grade lymphoma strictly correlated to autoimmune thyroiditis. This appears an additional confirmation of the hypotesis that primary thyroid EMZBCL may evolve from HT [3, 12].
Thyroid EMZBCL is an indolent lymphoproliferative disorder and, when other localizations of disease can be excluded, total thyroidectomy, followed by radiotherapy when appropriate, represents a curative surgical treatment with a very good prognosis . Nevertheless, the disclosure of incidental lymphomatous areas is important to assure an accurate follow-up programme for the lymphoproliferative disease, as well as for autoimmune disorders.
The purpose of this report is to underline the difficulties in the formulation of a differential diagnosis, also for an experienced pathologist, in cases of lymphoma arising in a background of HT. Both morphology and immunohistochemistry can result ineffective. Nevertheless, also in presence of a disease incidentally discovered or with an indolent behaviour, the pathologist has the obligation to furnish a correct diagnosis. Then, in difficult cases with doubtful immunohistological findings, ancillary techniques, such as molecular studies, are necessary for a conclusive diagnosis.