MON-337 Co-existence of Mixed Medullary-Follicular Thyroid Carcinoma, Atypical Oncocytic Thyroid Tumor and Classical Papillary Thyroid Carcinoma in a Patient

Abstract Disclosure: B. Wright: None. S. Gundlapally: None. S. Patel: None. M. Laura: None. A. Smoleva: None. M. Antony: None. Introduction: Co-existence of more than one type of thyroid cancer in a patient is of rare occurrence. Most of the cases which have been reported describe the presence of mixed medullary-follicular thyroid carcinoma in patients. We present a thyroid case in which the final pathology not only revealed mixed medullary-follicular thyroid carcinoma but also an atypical oncocytic thyroid tumor and classical papillary thyroid carcinoma.Case Description: A 78 years-old Caucasian male was clinically diagnosed with goiter and underwent further evaluation. Patient denied local neck symptoms, denied family history of thyroid cancer, and denied prior neck radiation exposure. Neck ultrasound revealed multinodular goiter with a solid and heterogenous right lobe nodule measuring at least 6.2 x 6 x 4 cm and a solid/hyperechoic left lobe nodule measuring up to 1.1 cm. FNA of the dominant right lobe nodule revealed Bethesda category 4 with ThyroSeq V3 GC positive with cancer probability 80%. CT soft tissue neck with contrast revealed a large heterogeneous well-circumscribed enhancing right thyroid mass measuring 7.2 x 5.1 x 4.6 cm with evidence of mass effect and no lymphadenopathy. Patient underwent right hemithyroidectomy and final pathology with immunohistochemistry [IHC] revealed the following findings: (1) A 0.7 cm unifocal mixed medullary-follicular cell thyroid carcinoma with focal invasion into adjacent parenchyma and tumor approaching 0.1 cm from inked tissue edges. A group of cells were strongly positive for calcitonin and chromogranin. Few cells positive for INSM1 and PAX8, P53 showed wild-type pattern, Ki-67 proliferative index up to 2%. Negative RAS p.Q61R. (2) A 6.3 cm well-circumscribed oncocytic, follicular patterned neoplasm with extensive degenerative changes, fibrosis, edema and cystic changes most consistent with an atypical oncocytic thyroid tumor. P53 showed wild-type pattern, Ki-67 proliferative index up to 1%, negative CD34 and 31 suggestive of negative vascular invasion and positive RAS p.Q61R in tumor cells. (3) A 0.1 cm classical papillary thyroid carcinoma with no concerning features. Post-op work up revealed undetectable serum calcitonin <2.0 [0-8.4pg/ml], normal serum calcium 9.4 [8.6-10 mg/dl], normal plasma metanephrine <25 [0-88 pg/ml], normal plasma normetanephrine 87.6 [0-285.2 pg/ml], unremarkable neck ultrasound with bilateral morphologically normal appearing lymph nodes. Patient is scheduled to undergo completion thyroidectomy in February 2025. Conclusion: Although thyroid cancers comprising of mixed pathology of cancer cells are of rare occurrence, its incidence may increase in the future due to increased case reporting which can pave the way for research to understand the clinical behavior, prognosis of such cancers. Presentation: Monday, July 14, 2025