Thyroid Lymphoma

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Primary thyroid non-Hodgkin's lymphoma (NHL) is uncommon, representing 4% of all thyroid cancers [1]. Only 2% of extranodal lymphomas arise within the thyroid. Secondary involvement is more frequent as a manifestation of generalized disease, which occurs in 10% of all lymphomas and leukemias [2]. The mean age at presentation is 60-70 years with a female: male predominance of 3:1. Presentation before age 40 is rare. Preexisting Hashimoto's thyroiditis is a significant risk factor [3] and may be the result of chronic antigen stimulation. Radiation exposure is not associated with an increased risk.

Primary thyroid lymphoma is usually of B-cell type and CD20 positive [4]; T-cell lym phoma is very rare. Approximately 80% of tumors are of the diffuse large-cell type (histiocytic) and the majority of these are high grade. The remainder comprise follicular (nodular), mixed, plasmacytoid, and lymphocytic subtypes. NHL of the thyroid is classified into MALT (mucosa-associated lymphoid tissue) positive and MALT negative characterized by the presence of lymphoepithelial lesions with cytokeratin staining. The frequency of MALT-positive thyroid lymphoma varies between 10% and 80% and is associated with improved prognosis [5]. The REAL (Revised European-American Lymphoma) and WHO (World Health Organization) classifications designate these as extranodal marginal zone B-cell lymphomas of MALT type.

Typical presentation is with a rapidly enlarging thyroid mass; associated compressive symptoms comprise dysphagia, dyspnea, stridor, hoarseness, and superior vena caval obstruction. A preceding history of goiter in association with hypothyroidism is characteristic. The thyroid mass is often fixed to underlying structures, with confluent cervical lymphadenopathy occurring in up to 50% of cases. Only 10% of patients report B symptoms of fever, night sweats, or weight loss.

Lymphoma should enter the differential diagnosis of a solitary thyroid nodule, a dominant nodule in a multinodular goiter, or any patient with rapid enlargement of a goiter associated with Hashimoto's thyroiditis. Diagnosis may be made from fine-needle aspiration cytology

(FNAC) but immunocytochemistry is necessary to demonstrate the type of lymphocyte mono-clonality, achieved by core biopsy. Large B-cell lymphoma has to be distinguished from undif-ferentiated carcinoma and MALT lymphoma from Hashimoto's thyroiditis.

The Ann Arbor classification is used to stage lymphoma. Over 90% of patients with primary lymphoma present with disease confined to the thyroid (stage IE) or limited to the thyroid and regional neck nodes (stage IIE). More widespread disease (stages III and IV) makes up the remainder, with affected sites including the gastrointestinal tract, bone marrow, lungs, or liver. Initial staging investigations (Figure 30.1) should therefore comprise a full blood count, serum biochemistry including thyroid function, lactate dehydrogenase and uric acid, CT scan of the neck thorax abdomen plus pelvis, and bone marrow aspirate plus trephine. Lactate dehydro-genase, uric acid, and antithyroid antibody titers (antimicrosomal) are often raised. CT imaging often demonstrates invasion of the trachea, retrosternal extension, or involvement of mediastinal lymph nodes.

As patients are often elderly and require urgent treatment to relieve airway obstruction, full staging may be impracticable until later. Aggressive surgery to debulk the tumor is neither feasible nor necessary [6]. For localized disease, external beam radiotherapy has been the standard practice for several decades, resulting in 5-year survival rates of approximately 35%. Local bulky disease and gross mediastinal involvement are significantly associated with failure distant from the irradiated volume [4]. Chemotherapy for high grade lymphoma has demonstrated better local and distant disease control with overall long-term disease-free survival of about 50%. The combination of radiotherapy preceded by chemotherapy has become standard practice in most institutions, improving 5-year survival rates to 65-90% [7]. Six cycles of cyclophosphamide, doxorubicin, vin-cristine, and prednisolone (CHOP) given over 4 months is usually recommended. If complete response has been achieved with drugs, external beam radiotherapy should follow with 35 Gy mid-plane dose given in 20 fractions, confined to the initial extent of disease.

Rituximab, an unconjugated anti-CD20 monoclonal antibody, has been designed to recognize cancer-forming B-cell lymphocytes [8].

The drug binds to CD20 receptors on the surface of B cells and initiates a series of reactions that result in cell death. It also appears to sensitize cells to conventional chemotherapy. It has been extensively evaluated and is now an integral component of chemotherapy [9]. The National Institute for Clinical Excellence (NICE) recommends that rituximab should be included (with CHOP) in the treatment of patients with moderate to advanced CD20-positive B-cell NHL stages II, III, and IV. Patients with stage I disease should be given the drug only as part of a clinical trial.

Lymphomas showing MALT characteristics present as localized extranodal tumor with favorable prognostic factors and follow a more indolent course. Radiotherapy as single modality treatment resulted in a complete response rate approaching 100%, a relapse rate of around 30%, a salvage rate of over 50%, and an overall cause-specific survival of almost 90% at 5 and 10 years [5]. Our policy is to treat stage IEA MALT-positive lymphoma with radiotherapy only but to use initial chemotherapy for all other tumors. The radiotherapy volume includes both sides of the neck from the mastoid tips to the carina treated by opposed anterior and under-couched fields to a dose of 40 Gy in 20 fractions over 4 weeks. In addition to infraclavicular lung shielding, the mandible and submandibular salivary glands are protected, although there is no midline lead. Primary Hodgkin's disease of the thyroid is exceedingly rare and should be separated from primary Hodgkin's disease of the mediastinum or neck, involving the thyroid by direct spread. Treatment is similar to extran-odal Hodgkin's disease at any other site [10].

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