SAT557 Unexpected Whole Body Scan Finding

Disclosure: A. Arce Gastelum: None. C.Y. Liu: None. K. McCowen: None. A 41-year-old man presented with a neck mass. Following total thyroidectomy with central and bilateral lateral neck node dissection, he received a diagnosis of stage I T2N1bMx multifocal papillary thyroid cancer with extensive extrathyroidal extension, angioinvasion and 14/65 positive lymph nodes. Surgery was complicated by temporary post operative hypoparathyroidism. Lab testing one week post thyroidectomy, before radioiodine ablation showed a serum TSH 23 uIU/mL, thyroglobulin: 34.1 ng/mL, with thyroglobulin antibody: <0.9 [IU]/mL.Post RAIA Whole Body Scan revealed increased uptake in the right orbit, concerning for metastatic disease. Confirmatory MRI of the brain showed a right superior intraorbital cystic lesion. Although orbital metastases are infrequent, a metastatic lesion was suspected due to the aggressiveness of his thyroid cancer. The patient was referred to ophthalmology several months after surgery and underwent right orbitotomy with excision and biopsy of the mass. The pathology was consistent with a multilocular cystic tumor lined by bland cuboidal epithelium, containing proteinaceous concretions and entirely negative for metastasis. Extensive thyroid cancer is treated with surgery followed by radioactive iodine (I-131) in case of tissue remnants or extra nodal metastases. Although unusual, false-positive scans exhibiting the presence of 131I uptake in the absence of residual thyroid tissue can occur.Fibrotic lung conditions, pleural effusions and bronchiectatic lesions have been reported to concentrate iodine in the absence of thyroid cancer metastasis. One prior report of a conjunctival inclusion cyst also resulted in radioiodine uptake in the orbit. Unless identified as a false positive, radioiodine uptake may result in a diagnostic fallacy. The correlation of clinical signs and serum thyroglobulin trends with imaging techniques should be emphasized to prevent unnecessary additional therapy. Presentation Date: Saturday, June 17, 2023