Thyroid Infiltration by Chronic Lymphocytic Leukemia: A Rare Case Report With Cytological and Flow Cytometry Findings

ABSTRACT Primary and secondary involvement of the thyroid gland by CLL has been reported as extremely rare in the literature. Fine‐needle aspiration cytology is one of the primary diagnostic procedures for determining the nature of suspicious thyroid nodules. Clinical information is particularly important during cytomorphological evaluation, especially in patients with a history of hematological malignancy. We report a case of a 58‐year‐old woman diagnosed with CLL, presenting with thyroid gland involvement 8 years after the initial diagnosis. Fine‐needle aspiration cytology revealed an infiltrative cellular smear with coarse chromatin and monomorphic cells displaying a tortoise shell pattern within a colloid‐containing background. Diagnostic confirmation was achieved through flow cytometric analysis of the aspirated fluid material. In patients with a history of hematologic malignancy, careful evaluation is required when a lymphocytic background is observed in aspirations, and additional methods should be employed if necessary. Distinguishing between primary and secondary lymphomas of the thyroid gland is clinically significant, as their treatment and prognosis differ substantially.