SAT-LB102 When the Answer Is Not in the First Look: An Unusual Presentation of Follicular Thyroid Carcinoma

Follicular Thyroid Carcinoma (FTC) accounts for 12% of thyroid cancers. It is most common in women between the ages of 40-60 years old. Although typically diagnosed after incidental palpation or via imaging obtained for other purposes, up to 15% of FTC may exhibit distant metastasis at the time of diagnosis. We present a case which required an interdisciplinary approach to lead to the correct diagnosis. A 49 year-old male presented with an orange-sized lump on his head. He was referred to a tertiary hospital for further evaluation. Brain MRI showed an infiltrative mass within the right parietal bone with extracalvarial, epidural and intraparenchymal extension. He underwent craniotomy for skull tumor resection and pathology revelead a metastatic adenocarcinoma of unknown origin. Pan-CT scanning revealed a 5 cm hypervascular complex nodule in the right thyroid lobe. PET showed uptake in this mass. Fine Needle Aspiration was consistent with FTC. Clusters of tumor cells from the bone surgical sample were positive for CK7, PAX8 and negative for PSA, CK20, PSAP, TTF1, CDX2 and Hepar. After comparison between brain and thyroid samples, a diagnosis of metastatic FTC was corroborated. Foundation testing was sent to guide further therapy. Programmed death-ligand 1 (PD-L1) results were discordant between samples, with the metastatic tissue showing frank 60% positivity, compared to the scant 5% expression at the primary tumor. The most common molecular alterations in FTC are RAS and PAX-8-PPAR gamma1 point mutations. When suspecting a primary tumor of thyroid origin, in the setting of an adenocarcinoma of unknown origin, immunohistochemistry (IHC) is crucial for accurate diagnosis. FTC typically exhibits distinct IHC patterns for markers such as TGB, HBME-1, PAX8, CD-56 and galectin-3. Several characteristics such as degree of angioinvasion and TERT mutations have been linked to greater risks of metastasis and poor prognosis. More recently, PD-L1 was found to be highly expressed in a subset of patients with advanced thyroid cancer, such as follicular and anaplastic thyroid carcinoma. The presence of PD-L1 expression may guide our therapeutic approach towards FTC in the future. Unless otherwise noted, all abstracts presented at ENDO are embargoed until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. Abstracts presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.