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Abstract Disclosure: R.A. Zielinski: None. M. Khalifa: None. M. Dillon: None. M. Kaur: None. Background: Hyaline trabecular tumor (HTT) is a rare benign follicular thyroid neoplasm characterized by extensive intra-trabecular hyaline stromal material. Resemblances to other follicular neoplasms create difficulty in distinguishing HTT from its more nefarious counterparts such as medullary thyroid cancer (MTC) and papillary thyroid cancer (PTC).Clinical Case:Our patient is a 56-year-old female with Hashimoto’s thyroiditis diagnosed in her twenties who has been on different strengths of levothyroxine based on her fluctuating TSH. A thyroid ultrasound done to evaluate left-sided neck fullness on physical exam showed a solid, hypoechoic 2.3 cm x 1.4 x 0.9 cm left middle thyroid nodule without calcifications (TI-RADS 4) and benign-appearing lymph nodes, the largest of which was 1.5 cm at level 1B. Cytology from a thyroid FNA biopsy was suspicious for PTC, Bethesda category IV. A thyrosure genetic panel was negative for genetic mutations. FNA biopsy of the 1.5 cm lymph node was negative for malignancy. The patient was referred for thyroid lobectomy but instead elected to undergo total thyroidectomy with level VI lymph node dissection. Pathology revealed a 2.2 cm HTT in the left lobe, scattered psammoma bodies in adjacent soft tissue, and a 0.2 cm multifocal papillary microcarcinoma in the right lobe. The microcarcinoma had negative margins, no lymphatic or vascular invasion, and 7/7 benign level VI lymph nodes; staging it as pT1aN0. Immunohistochemistry staining was positive for TTF-1 and thyroglobulin and negative for calcitonin, chromogranin, and synaptophysin. Post-thyroidectomy she was started on Tirosint 175 mcg daily with good response. No I-131 treatment was pursued due to the microcarcinoma being categorized as low risk.Discussion and Conclusion: HTT is a benign neoplasm and management is usually conservative with surgical excision. The diagnosis of HTT by cytology alone is challenging due to the presence of nuclear clearing, grooves and intranuclear pseudo-inclusions that may be mistaken for PTC or MTC. Cell membrane immunoreactivity of MIB1 monoclonal to Ki67 and the molecular identification of the novel PAX8-GLIS1 or PAX8-GLIS3 fusions may aid in making the correct diagnosis.References:Rossi ED, et al. The Diagnosis of Hyalinizing Trabecular Tumor: A Difficult and Controversial Thyroid Entity. Head Neck Pathol. 2020 Sep;14(3):778-784. doi: 10.1007/s12105-019-01083-5. Epub 2019 Sep 30. PMID: 31571046; PMCID: PMC7413943. Presentation: 6/2/2024
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