Abstract
Background: Primary thyroid lymphoma (PTL) is an uncommon malignancy that predominantly affects women in their sixth or seventh decade. It is strongly associated with chronic lymphocytic thyroiditis (Hashimoto's thyroiditis) and other autoimmune conditions. The hallmark clinical feature is a rapidly enlarging thyroid mass, which can quickly cause compressive symptoms such as dysphagia, hoarseness, and dyspnoea. Timely recognition and treatment are essential. [(18)F]fluoro-2-deoxy-D-glucose positron emission tomography/computed tomography ([(18)F]FDG-PET/CT) plays a central role in the diagnosis, staging, response assessment, prognostication, and surveillance of high-grade lymphomas, significantly influencing clinical management. Case presentation: We report the case of a woman in her sixties with a history of multinodular goitre but without an autoimmune background, who presented with a large left-sided cervical mass that had rapidly enlarged over approximately two months. Laboratory tests, fine-needle aspiration (FNA), and [(18)F]FDG-PET/CT revealed abnormal cytology and a highly hypermetabolic necrotic left thyroid mass, without extra-thyroidal disease, suggestive of lymphoma. Definitive biopsy with immunohistochemistry confirmed a high-grade B-cell lymphoma, positive for CD5 and demonstrating triple expression of Bcl2, Bcl6, and c-Myc. The patient underwent chemotherapy, achieving a marked morphometabolic response after two cycles, consolidated after four cycles. Conclusions: This rare case highlights the importance of considering PTL in the differential diagnosis of an isolated, rapidly enlarging thyroid mass, regardless of prior Hashimoto's thyroiditis. Early diagnosis and timely treatment are crucial to improve patient outcomes.