Influence of hormonal replacement with thyroxin in diagnostic evaluation of thyroid carcinoma with FDG using a dual head coincidence camera

Full text: The objective of this study was to evaluate the influence of thyroxin replacement on the results of 18F-fluoro-deoxyglucose (18F-FDG) imaging in patients with thyroid carcinoma. Nineteen patients of differentiated thyroid carcinoma (16 females, age 49.2±17.3 years) were prospectively evaluated using dual-head coincidence camera. Patients were studied twice, first study done during replacement with thyroxin (FDG-T4on) and the second 6 weeks after stopping the thyroxin (FDG-T4off). All patients were also subjected to whole-body scintigraphy (WBS) with 131I and 99mTc-sestamibi. Any abnormal uptake seen in the thyroid bed was interpreted as positive for disease. Thyroglobulin measurement (TG) was done during hormone withdrawal, and a value above 5ng/mL was considered to be due to active disease. Hormonal therapy interruption led to a significant increase in TSH measurement (2.3±4.7 -112.1±77.1, p<0.001). Twelve patients (63%) having TG of 995.7∫1691.0 were considered to have active disease while 7 patients with TG = 0.7±0.4 were labeled as disease free. FDG-T4off was more sensitive than FDG-T4on due to the detection of loco-regional (LLN) and mediastinal lymph nodes (MLN) in one patient. In other two patients FDG-T4off showed few more sites of disease (LLN in one patient and MLN in the other). FDG-T4off detected all the sites identified by 99mTc-sestamibi and I-131 WBS. However, another patient free of disease on WBS was positive on FDGT4off imaging. FDG-T4on showed false-positive results in 2 patients (one in LLN and the other in the lungs). Detection of thyroid remnants was better achieved with WBS (9 patients), followed by FDG-T4on and FDG-T4off (4 patients) and 99mTc-sestamibi (2 patients). FDG-T4off was slightly more sensitive than FDG-T4on in the detection of thyroid carcinoma metastasis. Both methods presented higher sensitivity compared to 99mTc-sestamibi and WBS. In conclusion, though the results of our study are promising but further comparative studies require to be performed in order to define an ideal diagnostic investigation algorithm for patients with thyroid carcinoma. (author)