SAT-359 - In Plain Sight: Papillary Thyroid Carcinoma Masquerading as Paraganglioma

Background:
Classic variant papillary thyroid carcinoma (PTC) is the most common form of differentiated thyroid cancer. It typically demonstrates low to moderate fluorodeoxyglucose (FDG) uptake, while more aggressive variants and dedifferentiated thyroid cancer show increased FDG avidity. Meanwhile, somatostatin receptor targeted imaging such as DOTATATE PET/CT is used for the detection of neuroendocrine tumors and demonstrates variable uptake in PTC. We present a unique case of intense FDG uptake and high somatostatin receptor avidity in a patient with PTC that closely mimicked paraganglioma.

Case Presentation:
A 63 year old female with a history of PTC, classic variant, status post thyroidectomy and radioactive iodine (I-131) therapy, with no evidence of iodine avid metastases on post treatment scan, presented with an enlarging mediastinal mass. An 18-FDG PET/CT scan had been obtained due to rising thyroglobulin levels post I-131 treatment and revealed a 3.8 x 3.6 x 3.6 cm hypermetabolic mediastinal mass with significant FDG activity, thought to be recurrent thyroid disease versus paraganglioma, as well as an FDG avid 1.5 cm right carotid bulb mass suspicious for paraganglioma. A subsequent Octreoscan (somatostatin-receptor imaging) demonstrated robust, diffusely homogenous somatostatin receptor expression in the mediastinal mass and the right carotid bulb mass, also supporting the diagnosis of paraganglioma. Biochemical evaluation was negative for a functioning paraganglioma. She was then lost to follow up until she presented two years later with enlargement of the mediastinal mass to 5.7 x 7.3 x 6.1 cm and an increased thyroglobulin level of 254.8 ng/mL (1.5 – 38.5 ng/mL). 64Cu DOTATATE PET/CT scan showed mild to moderate somatostatin receptor expression now only in the periphery, and a photopenic center due to suspected dedifferentiation. She underwent partial resection of the mass with further debulking not feasible due to invasion of the mass into surrounding vasculature. Pathology was consistent with metastatic PTC.

Conclusion:
This case highlights an unusual presentation of metastatic PTC that demonstrated marked FDG avidity and 64Cu DOTATATE uptake, imaging characteristics that initially suggested paraganglioma, particularly in the setting of a concurrent carotid bulb mass. While neuroendocrine tumors are typically associated with dual-tracer positivity, this case highlights that differentiated thyroid cancer can infrequently exhibit a similar pattern. This reflects tumor heterogeneity and underscores the importance of maintaining differentiated thyroid cancer in the differential diagnosis. Prompt histopathologic confirmation is necessary for the diagnosis and timely management of metastatic PTC presenting with atypical imaging findings.

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