Differentiated thyroid cancer: Case report of false positive 131I uptake on whole body scintigraphy in a patient with bilateral femur osteonecrosis.

Objective: Differentiated thyroid cancer (DTC) is one of the fastest growing cancers worldwide. Despite the generally good prognosis of thyroid carcinoma, about 5% of patients will develop metastatic disease, exhibiting a more aggressive behavior. Radioiodine whole-body scintigraphy (WBS) has been used in the detection of DTC. Radioiodine is a sensitive marker for detection of thyroid cancer; however, radioiodine uptake is not specific for thyroid tissue. It can also be seen in healthy tissue as well as in inflammation, or in a variety of benign and malignant non-thyroidal entities.

Subject and methods: The subject of the present case report is a 52 years old man with brain metastatic DTC who received radioiodine therapy and corticosteroids as palliative therapy. Whole-body scintigraphy revealed bilateral iodine uptake of the femur. Corticosteroid therapy is among the most widely recognized risk factor for osteonecrosis, which at the present case had to be recognized as a false positive (iodine-131) ¹³¹I uptake in order to avoid diagnostic error.

Results: Post therapeutic whole body scintigraphy revealed no uptake in the thyroid bed as well as pathologic uptake of radioiodine in both femurs. The magnetic resonance imaging (MRI) of the femurs combined with the history of long term exposition on high doses of corticosteroids evidenced diagnosis of steroid-induced osteonecrosis of the femurs.

Conclusion: Radioiodine WBS plays an important role in clinical decision making for the evaluation and the management of patients with DTC. Despite its high range of sensitivity and specificity, a variety of reports of false positive whole body scans has demonstrated a diversity of causes. Comprehension of the physiology of iodine uptake and of the pathophysiology of clinical entities which end up giving false positives scans, provides clinicians a useful tool in order to avoid diagnostic and therapeutic errors as far as DTC is concerned.