6955 A Rare Case of Rapidly Enlarging High Grade Follicular-Derived Carcinoma in a Young Patient - Formerly Known as Insular Thyroid Carcinoma with good response to Radioactive iodine ablation- A New Name, Same Aggressive Features

Abstract Disclosure: R. Abdelmasih: None. N. Prasad: None. N. Shah: None. Introduction: Follicular-derived thyroid carcinomas, formerly known as insular thyroid carcinomas, are an albeit rare but highly lethal thyroid carcinoma subtype with an incidence of 1-10% worldwide. It is known for its aggressive nature and comprises the majority of deaths from non-anaplastic follicular thyroid cancers. Case Presentation: A 30-year-old male presented with 2-year history of progressively enlarging painless right neck mass. Physical exam revealed 6x3 cm right well-circumscribed mobile mass not displacing the trachea. Head and neck CT showed 6.5 cm right thyroid mass with level 2A lymph node. Neck Ultrasound showed a 6.4 cm right thyroid nodule. Fine needle aspiration of the mass showed atypia of undetermined significance. Patient returned to clinic 1 year later with progressively enlarging right neck mass, now associated with dysphagia measuring 10 cm in physical exam deviating the trachea to the left. Repeat CT H&N showed 10 cm right thyroid mass with moderate tracheal deviation and US showed 11 cm right heterogenous thyroid mass with regular margins and negative LNs. Patient underwent right thyroid lobectomy with surgical pathology showed 11.4 cm tumor with vascular invasion and no extrathyroidal extension. Pathology showed high grade follicular-derived Thyroid Carcinoma. Patient underwent completion thyroidectomy with benign pathology followed by radioactive iodine ablation 107.8 mCi I131 with successful targeting of residual thyroid tissue and no evidence of iodine avid distant metastasis. Patient has been followed closely for 2 years now with regular US, thyroglobulin level with recurrence. Discussion: Insular thyroid carcinomas are aggressive malignancies predominantly seen in females within the age of 55-65 though cases of young patients and children have been reported. Patients often present with enlarging goiter, dysphagia, and dyspnea however, metastatic disease to lymph nodes, lung and bone has also been reported as primary presentation in 20% of cases. Per literature review, recurrence and mortality rates have very wide range of 36-83% and 9-75% respectively which could be explained by a variety of prognostic factors including Patient age, tumor size, Macroscopic evident extrathyroidal extension, distant metastasis, or tumor necrosis. Often times insular thyroid carcinomas are mistaken for anaplastic carcinoma and thus fine needle aspiration is a helpful diagnostic tool to differentiate insular carcinoma. Pathology typically demonstrates well-defined insulae of thyroglobulin-filled follicles, necrotic foci, and high random mitotic activity. Treatment includes total thyroidectomy followed by high-dose radiotherapy. We Present this case to shed light on such a rare aggressive entity that requires early detection, belligerent management, and close follow-up for promising outcomes. Presentation: 6/3/2024