Primary thyroid lymphomas.
Primary thyroid lymphoma is a rare disease that continues to produce diagnostic and therapeutic dilemmas. There was great difficulty in distinguishing thyroid lymphoma from anaplastic thyroid carcinoma but, because of new immunocytochemical staining techniques and increased cytopathologic knowledge, our ability to diagnose thyroid lymphoma has improved drastically over the past decade. Surgery that was once the mainstay of treatment for this disease, now plays a minimal role. Current treatment regimens for primary thyroid lymphoma consist of chemotherapy (cyclophosphamide, doxorubicin, vincristine, and prednisone) and external beam radiation. The overall and distant relapse rates have been shown to be significantly lower in those patients receiving combined modality therapy compared to chemotherapy or radiation alone. Although the role of surgery has changed over time, it continues to play an important role, especially in confirming diagnoses through open biopsies, potentially providing local control in the more indolent subtypes, and may play a role in the palliation of symptoms for large obstructive lymphomas. The evolving classification of extranodal lymphomas has brought about a better understanding of the biologic behavior of these tumors. Most thyroid lymphomas are B-cell origin, with six different histologic subtypes, but there appears to be two distinct clinical and prognostic groupings of these rare tumors. The more indolent lymphomas are the subgroup of mucosa-associated lymphoid tissue (MALT) lymphomas comprising approximately 6% to 27% of thyroid lymphomas. This subgroup, when localized to the thyroid (stage IE), responds well to total thyroidectomy or radiation with a complete response rate of more than 90%, leading some authors to recommend surgery as primary therapy in the treatment of localized MALT lymphomas. Therefore, surgery as a primary treatment for thyroid lymphomas would only be recommended under ideal conditions, such as MALT subtype stage IE only, and completely resectable with minimal morbidity. Unfortunately, this scenario is rarely the case. The more common subtype, comprising up to 70% of cases, is diffuse large B-cell lymphoma. This subtype appears to have the most aggressive clinical course with almost 60% of these tumors diagnosed with disseminated disease. Up to 40% of all diffuse large cell lymphomas appear to have undergone transformation from a MALT lymphoma, but they behave in a similar fashion to diffuse large cell lymphomas. Treatment for these tumors should include chemotherapy and radiation. The overall 5-year survival for this aggressive group is less then 50%. Surgery is rarely beneficial in diffuse large cell lymphoma and the mixed large cell subtypes because the disease is generally disseminated and surgical excision of all disease is not possible or associated with increased morbidity. However, there may be a role for palliative surgical debulking to alleviate obstructive symptoms while the patient is undergoing standard chemotherapy and radiation.