8112 Hyalinizing Trabecular Tumor of the Thyroid in a Patient with Familial Adenomatous Polyposis

Abstract Disclosure: C. Kelly: None. B.M. O'Donnell: None. A 33 year old female presented to the Endocrinology clinic for evaluation of a 1.4 cm thyroid nodule. She has a relevant past medical history of Familial Adenomatous Polyposis (FAP). She was diagnosed with FAP in her early 20s after she had developed hematochezia. When she underwent her first endoscopy she reports there “were hundreds of polyps” as well as multiple stomach ulcers/polyps. In 2012 she underwent a total proctocolectomy with ileal pouch-anal anastomosis (IPAA). Then in August 2023 the patient underwent a Whipple procedure due to the discovery of duodenal polyps on endoscopy that demonstrated low grade dysplasia. On pathology the duodenal adenoma was significant for adenocarcinoma invading the lamina propria, no lymphovascular invasion, negative margins, and 0/20 involved lymph nodes. No chemotherapy indicated per Medical Oncology. Thyroid ultrasound was performed as a screening assessment, and given the presence of a nodule she was referred to Endocrinology. On repeat ultrasound approximately 1 year after her initial ultrasound, the nodules measured 15.5. mm x 15.9 mm x 16.9 mm; increased in size from 14 mm x 14 mm x 12 mm. Due to the increase in size and the nodule characteristics of hypoechogenecity and taller-than-wide shape we recommended the patient undergo fine needle aspiration (FNA) biopsy of the nodule. Cytology from the FNA was notable for Papillary Thyroid Carcinoma. In December 2023 the patient underwent a total thyroidectomy, and pathology showed bilateral foci of hyalinizing trabecular tumor. Hyalinizing trabecular tumors (HTT) of the thyroid are follicular neoplasms with benign behavior. On sonographic imaging they tend to present as round to oval hypoechoic, well-defined lesions. On cytology these lesions can be mistaken for papillary thyroid carcinoma (PTC), medullary thyroid carcinoma, non-invasive follicular thyroid neoplasm with papillary like features, paraganglioma, and follicular neoplasm. The incidence of HTT varies but is approximately 1.5% of benign thyroid tumors. While FAP has been linked to thyroid tumors, in 70-90% of cases the pathology is consistent with cribriform-morular variant of PTC (CMV-PTC). CMV-PTC is in and of itself a rare variant accounting for less than 1% of all PTCs in the general population. Historically, the lifetime risk of thyroid cancer development in FAP patients is 2%, but this number has been climbing recently with a prevalence as high as 12% in patients screened with ultrasonography. It is important that this patient’s case be added to the body of medical literature as HTT is not currently known to be associated with FAP, but as more lesions are found with increased ultrasound surveillance, it may be an emerging extraintestinal manifestation of the disease. Presentation: 6/1/2024