Toxoplasmic Lymphadenitis Mimicking a Metastatic Thyroid Carcinoma at 18F-FDG-PET/CT

A 28-year-old woman underwent total thyroidectomy for a papillary thyroid carcinoma in the right thyroid lobe (pTx, pN1b). Subsequently a 131I-ablation (4.4 GBq) was performed. Four years later the patient presented increased thyroglobulin (Tg) serum levels (8.4 μg/l) during thyroxine treatment. Furthermore, enlarged hypoechoic and round-shaped bilateral cervical lymph nodes were detected at cervical ultrasonography (US). Based on laboratory and US findings suspicious for lymph nodal recurrence of thyroid carcinoma, the patient underwent an 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG-PET/CT) to check for distant metastases (Fig. 1). The patient underwent a US-guided fine-needle aspiration cytology on an 18F-FDG-avid cervical lymph-node. The smears were hypercellulated and consisted of numerous small- to medium-sized lymphocytes, macrophages, dendritic cells and tingible body macrophages. The cytological diagnosis was consistent with that of reactive lymphadenitis. Serological test revealed elevated IgM and IgG anti-Toxoplasma antibodies with a very low IgG-avidity, indicating an acute toxoplasmosis. Serum Tg was then measured by using heterophilic antibody blocking tubes, as previously reported, and serum value dropped to <0.2 μg/l. It is well known that antibody interference may falsely increase serum Tg; in particular, increased anti-Toxoplasma antibodies likely interfered to the Tg measurement in our case. Additionally, activated granulocytes and macrophages may display significantly increased glucose consumption, giving false-positive results at 18F-FDG-PET/CT in oncological patients. Few reports have described toxoplasmic infection mimicking malignancy at 18F-FDG-PET/CT; these findings were found mainly in immunodepressive patients or with history of lymphoma. Conversely, we described here a case of toxoplasmosis inducing false-positive Tg measurement, neck US and 18F-FDG-PET/CT findings in a patient with papillary thyroid carcinoma