Endocrine Abstracts

Introduction: Differentiated thyroid cancer (DTC) prognosis is usually favorable, even when metastatic radioiodine avid disease is present. Radioiodine-refractory disease, usually a sign of higher aggressiveness, can be detected by ¹⁸F-FDG-PET/CT. There is a reverse relationship between iodine and FDG accumulation in thyroid cancer lesions, the so-called ‘‘flip-flop’’ phenomenon: when thyroid cancer cells dedifferentiate they tend to lose their radioiodine avidity and start uptaking FDG. We present a case of advanced DTC with heterogeneous uptake of iodine and FDG.

Case presentation: A 69 years-old woman with a past history of right hemithyroidectomy 8 years before, due to a 70 mm hyperplastic nodule, was referred to our center. The patient had a painful lesion in the right acromion whose biopsy suggested papillary thyroid cancer (PTC) metastasis. Histological review of slides of the previous right thyroidectomy revealed PTC (encapsulated follicular variant) with vascular invasion. Neck ultrasound from the remaining thyroid showed irrelevant findings. However, CT scan documented extensive metastatic disease in the upper mediastinum, lungs with micronodules and a large lesion with 49 mm in the left lower lobe, chest wall and right acromion. The patient was submitted to radiotherapy (20Gy) with improvement of the shoulder pain. The histological piece of the remaining thyroidectomy revealed no neoplastic lesions. Then, she was submitted to radioiodine therapy with 150 mCi. Stimulated thyroglobulin was > 300000 ng/ml and the whole-body scintigraphy revealed metastatic disease in the right shoulder, chest wall, pelvis and left femur but no uptake in the lungs and mediastinum. ¹⁸F-FDG-PET/CT was performed and revealed several lesions with elevated metabolism in the mediastinum, pulmonary left hilum and lower left lobe with 61 mm (SUV 22) and contralateral lung, while the lesions referred in the previous 131 I scintigraphy only showed a slight to moderate metabolism (maximum SUV of 7). Due to ¹⁸F-FDG uptake discrepancy within the lungs, the larger left lobe and mediastinic lesions were biopsied by bronchoscopy; the immunohistochemistry was negative for thyroglobulin and PAX 8 and positive for TTF1, suggesting lung adenocarcinoma. The patient was referred to a second radioiodine therapy with 150 mCi and also for lung-directed chemotherapy.

Conclusion: In the presence of flip flop phenomenon, a well-described characteristic of progessive DTC, a high suspicion index must be maintained when the findings are discordant, in order to provide an appropriate differential diagnosis and management of the disease.