6847 Poorly Differentiated Thyroid Cancer: A Rare Entity

Abstract Disclosure: S. Fatima: None. S. Ward: None. A. Champion: None. Introduction: Poorly differentiated thyroid carcinoma (PDTC) is a rare thyroid carcinoma originating from follicular epithelial cells. Regarding the degree of differentiation, PDTC ranks between differentiated thyroid cancer and anaplastic thyroid cancer. We hereby present a rare case of PDTC. Case: 29-year-old female with a history of metastatic papillary thyroid cancer (PTC) presented to the emergency department with shortness of breath. She initially noticed a left thyroid mass in 2021. CT neck in 1/2022 showed a 7 cm left thyroid mass with substernal extension, small right thyroid mass with tracheal compression, left bulky cystic nodal masses extending anterior and posterior to carotid and internal jugular vein. PET scan in 3/2022 showed left 7.6 cm thyroid mass SUV 12.1 with tracheal compression, left level II-IV adenopathy with level IV nodal masses 5.1 cm SUV 4.5. Ultrasound-guided FNA in 4/2022 showed PTC in left 8.7cm thyroid nodule; right 2.8 cm nodule showed Atypia of Undetermined Significance; Left 5.5 cm neck level III lymph node (LN) showed PTC metastasis; right 5.1 cm neck level III LN showed PTC metastasis (+ thyroglobulin and TTF-1). CT chest in 11/2022 showed additional involvement of manubrium. Repeat core needle biopsy of left thyroid mass showed left invasive thyroid carcinoma. She was started on neoadjuvant Lenvatinib in 1/2023. Follow-up CT neck in 5/2023 showed a reduction in the size of the left thyroid mass to 5.5cm x 5.8cm x 6.5cm. At this time, she had symptoms of dysphagia, orthopnea, weight loss of about 60 lbs and neck pain. In 11/2023 at time of current presentation she underwent total thyroidectomy, bilateral modified radical neck dissection and manubriectomy. Surgical pathology showed PDTC in left thyroid mass, PTC in right thyroid mass with metastatic PDTC in left proximal sternocleidomastoid, manubrium and LN involvement in neck and mediastinum. OmniSeq Insight testing showed APC and BRCA1 gene mutations, negative BRAF V600E, RET fusion, RET mutation, NTRK1/2/3 fusion and PD L1 immunohistochemistry score is <1%. The patient is planned to receive [1]3[1]I ablation therapy. Conclusion: PDTC accounts for 3–5% of all thyroid carcinomas. The 5, 10-, and 15-year survival rates of patients are 50–85%, 34–50%, and 0%, respectively. RAS gene alterations are found in 25–35%, BRAF mutation in 15–27%, TERT promoter mutation in 40% and mutant TP53 in 16–28% of PDTCs. TERT promoter gene and TP53 mutation usually indicate an aggressive phenotype and a dismal prognosis. Surgery, [1]3[1]I therapy, and external beam radiotherapy constitute the mainstay of traditional treatment for patients with advanced disease; however, treatment is challenging. Tyrosine kinase inhibitors i.e, sorafenib and lenvatinib, have been used in cases of progressive, recurrent, or metastatic disease not responsive to [1]3[1]I therapy. Immunotherapy may be an option in PD-L1-positive tumors. Presentation: 6/1/2024