Thyroid Lymphomas versus Lymphocytic Thyroiditis: A Diagnostic Challenge on Fine Needle Aspiration Cytology

Abstract

Introduction: The diagnosis of Primary Thyroid Lymphoma (PTL) and its distinction from Florid Lymphocytic Thyroiditis (FLT) is often difficult on Fine Needle Aspirate (FNA). The sensitivity is low for Low Grade Lymphoma (LGL) and the distinction of PTL is important as they are treated by chemotherapy rather than surgery. We studied the efficacy of FNA for thyroid lymphoproliferative disorders which includes thyroid lymphoma and lymphocytic thyroditis and detection of cytomorphological features differentiating between thyroiditis and lymphoma. Aim: To find out interobserver variation in cytological diagnosis of thyroid lymphomas and thyroiditis with evaluation of the cytomorphological pointers of lymphoma on thyroid cytology. Materials and Methods: Thirty-three cases of FNA thyroid with lymphoid proliferation in which histological confirmation with Immunohistochemistry (IHC) was available were retrieved and studied for various cytomorphological features by three pathologists. The findings were correlated with the final histopathology diagnosis. Results: Of the total 33 cases that had significant lymphoid infiltrate on thyroid FNA, 28 cases were confirmed as NonHodgkin’s lymphoma, three cases as thyroiditis and two case of carcinoma on histology. All the lymphoma cases were positive for CD20 antibody on histology. The percentage of cases of thyroid lymphoma was high in present study because only selected type of cases were included (FNA thyroid with lymphoid proliferation) and it does not refer to true incidence of thyroid lymphoma. Among three observers, two observers (observer 1&2) diagnosed 18 cases of HGL, two cases of FLT and one observer (observer 3) diagnosed 15 cases of HGL and one cases of FLT correctly on FNA, five cases of LGL were correctly diagnosed by all the three observers. The diagnostic accuracy was 76.1% with high specificity (80%) and sensitivity (94.7%) for HGL and high concordance among the three pathologists (Kappa= 0.8). LGL and FLT had low sensitivity (55.6% and 66.7%) and least interobserver correlation (kappa – 0.21). Monomorphism, large cells, absent plasma cells, macrophages were significantly associated with diagnosis of lymphoma. Conclusion: Cytological features of thyroid lymphoma are diagnostic in HGL, however the features are more non-specific in LGLs. High cellularity, relatively monomorphic population and frequent occurrence of lymphoglandular bodies were common in LGL compared to FLT. It is challenging to diagnose PTLs, especially low grade on FNA. For this reason, it should be kept in mind that patients with chronic lymphocytic thyroiditis should be evaluated carefully and possibility of LGL should not rule out on cytology. The role of FNA is limited but it is simple safe and cost effective for initial workup in centres lacking facility of flow cytometry and immunocytochemistry

Keywords

• interobserver variation
• lymphoid proliferation
• monomorphism
• thyroid cytology