MON-332 Extended Survival in Stage IV Papillary Thyroid Carcinoma presenting with Anaplastic Transformation

Abstract Disclosure: A. Dowlatshahi: None. U. Khan: None. L.M. Cobos: None. Background: Anaplastic thyroid cancer is an aggressive thyroid malignancy, with 5-year survival rate of <10% and mean survival time of 6 months after diagnosis with roughly 40% of patients having distant metastases at time of diagnosis. Surgery and radiation typically improve survival though prognosis is poor.1 We discuss a rare case of papillary thyroid carcinoma with anaplastic transformation with roughly 6 year survival.Clinical Case: A 62 year-old Hispanic female came to the ED after 10-months of left neck mass and dysphagia. Biopsy revealed 6 positive lymph nodes with imaging revealing a 4cm mass, a 1.7cm left thyroid nodule, and 1cm centrally calcified nodule concerning for malignancy. FNA Biopsy initially showed atypia of unknown significance however, repeated FNA revealed a benign left thyroid nodule, papillary thyroid cancer at 80% of tumor volume with an additional 20% attributed to anaplastic thyroid; she was diagnosed with papillary thyroid cancer with focal anaplastic transformation arising from ectopic thyroid tissue with positive BRAF mutation.PET CT in September 2018 demonstrated focal sigmoid colon uptake along with a 3mm RUL lung nodule. Repeat CT scans did not redemonstrate any colonic findings. The patient underwent bilateral neck dissection, total thyroidectomy, and parathyroid autotransplant in September 2018 followed by radiation therapy in October 2018. She briefly received chemotherapy with paclitaxel and carboplatin in November 2018 with RAI and anti-BRAF regimen of dabrafenib and trametinib from January to April 2019 which was discontinued for intolerance. Regular imaging and lab studies showed no recurrence of disease. The patient continues with follow-up almost 6 years from diagnosis on thyroid hormone suppression with levothyroxine.Conclusion: This case demonstrates an atypical course of anaplastic thyroid cancer. The patient had good response to surgery and chemoradiation and continues to fare well on thyroid hormone suppression almost 6 years from diagnosis, a very rare prognosis.Of note, the sigmoid colon uptake positive on initial PET CT disappeared on repeat imaging studies. It is unclear if the sigmoid colon uptake had been a false positive result or had resolved with treatment. It is important to note that the atypia of unknown significance on initial biopsy with benign features may have obscured the true diagnosis of papillary and anaplastic thyroid cancers in the absence of further investigation. This represents a case of significant deviation from present understanding of thyroid malignancy.References: (1) Solomon JP, Wen F, Jih LJ. Anaplastic Transformation of Papillary Thyroid Cancer in the Retroperitoneum. Case Rep Pathol. 2015;2015:241308. doi: 10.1155/2015/241308. Epub 2015 Aug 16. PMID: 26351607; PMCID: PMC4553202. Presentation: Monday, July 14, 2025