SAT576 Change Is Not Always For The Best! A Transformation of Papillary Into Metastatic Anaplastic Thyroid Carcinoma

Disclosure: J. Syeda: None. R. Zafar: None. K. Rajamani: None. INTRODUCTION: Anaplastic thyroid carcinoma (ATC) can occur de-novo or from preexisting well-differentiated thyroid carcinoma (1). The presence or absence of certain driver mutations can help predict the likelihood of this transformation process. We present a case of an elderly female with an initial diagnosis of well-differentiated papillary thyroid cancer (PTC) with a transformation into metastatic anaplastic thyroid carcinoma (ATC). CASE PRESENTATION: An 81-year-old female patient was initially seen 8 years ago with a 35-year history of goiter. At the initial presentation, she felt well and did not have any symptoms. Physical examination revealed an enlarged thyroid gland with left lobe nodularity. A thyroid ultrasound showed a multinodular thyroid gland including a 2x1.7x1.2 cm solid nodule with macro calcifications in the right lobe, 2.8x2.6x2.5 cm solid nodule, and 1.6x1.3x1.6 cm heterogeneous solid nodule with calcifications and irregular margins in the left lobe. She had a fine needle aspiration of these three nodules. The right nodule was non-diagnostic and both left nodules were positive for PTC. She underwent a total thyroidectomy and central lymph node dissection. Radioactive iodine treatment with 163mCi was administered after surgery. One and a half years after surgery, her thyroglobulin levels started trending upward. This led to a further workup including a PET/CT scan, which showed a hyper metabolic 6 mm left supraclavicular lymph node. Needle biopsy of this lymph node disclosed metastatic PTC. The pathologic diagnosis following surgical resection was positive for metastatic poorly differentiated thyroid carcinoma. A subsequent PET/CT scan and a thyroid ultrasound showed an abnormal uptake of the left neck lymph node and additional metastatic nodes in the left neck respectively. A core needle biopsy of these lesions was positive for ATC. Foundation One CDx testing was positive for BRAF V600E, TP53, and TERT promoter mutations. Cellblock from the initial thyroidectomy was reevaluated by pathology and confirmed no evidence of ATC. Foundation One CDx testing of the thyroidectomy specimen was positive for BRAF V600E and TERT promoter mutations. Surveillance imaging shows mediastinal adenopathy, left pleural thickening, and bone and liver metastases. DISCUSSION: Anaplastic transformation is not uncommon (1). This case supports the evidence that TERT promoter mutations may not trigger anaplastic transformation but may predispose to the aggressive PTC phenotype prone to conversion to ATC when one of the ATC-related “second hit” genetic events occurs (2). This case suggests that the “second hit” event is a TP53 mutation. The detection of a TP53 mutation in PTC with an aggressive phenotype may help predict anaplastic transformation. REFERENCES: 1-Endocrine Pathology. 2021 Mar; 32(3):347-356. 2- Semin Cancer Biol. 2020 Apr; 61: 23-29. Presentation: Saturday, June 17, 2023