MON-409 - When Infarction Misleads: A High-Risk Mutation in A Low-Risk Oncocytic Thyroid Carcinoma

Introduction: Oncocytic thyroid carcinoma (OTC) is a rare subtype of thyroid cancer marked by oncocytes which are large polygonal cells with eosinophilic cytoplasm and prominent nucleoli. In the WHO 5th edition, OTC is recognized as a distinct entity characterized by its unique near-haploid genome and mitochondrial DNA mutations. Trauma can induce infarct-type degenerative changes in oncocytic lesions, reflecting marked sensitivity to hypoxia. We present a case of OTC incidentally discovered following neck trauma, with near-total tumor infarction and discordance between high-risk molecular findings and otherwise low-risk histopathologic features.

Clinical

Case: A 30-year-old female presented for the evaluation of a thyroid nodule incidentally noted on a neck MRI obtained after a motor vehicle accident associated with neck pain. Initial thyroid function tests were within normal limits. Thyroid ultrasound showed a hypervascular, isoechoic solid nodule in the upper pole of the right thyroid measuring 2.3 x 1.7 x 1.8 cm with smooth margins, absence of calcifications, bulging without thyroid capsular disruption (TI-RADS 3 nodule). Multiple lateral compartment cervical lymph nodes were noted with no suspicious features. Fine-needle aspiration showed a Bethesda category IV (oncocytic follicular neoplasm), and molecular testing with ThyroSeq v3 identified a TP53 mutation. Due to the presence of high-risk molecular markers, she underwent a total thyroidectomy with surgical pathology from the right thyroid lobe and isthmus, revealing minimally invasive 1 cm OTC with near-total infarction, confined to the thyroid with no evidence of lymphatic or vascular invasion. TP53 mutation and increased p53 expression by immunohistochemistry were noted. Despite this, the tumor lacked high-grade features and showed no increased mitotic activity. Radioactive iodine (RAI) therapy was not pursued given the absence of high-risk pathologic features and the uncertain benefit of RAI in OTC, with ongoing management by active surveillance. Post-operative thyroglobulin levels were 0.3 and 0.5, with no antibodies detected. Genetic testing confirmed a somatic TP53 mutation confined to the thyroid tumor, excluding germline involvement.

Conclusion: Tumor necrosis is a defining feature of high-grade follicular-cell-derived non-anaplastic carcinoma. However, in our case, the oncocytic tumor was discovered incidentally following neck trauma with near-total infarction, consistent with ischemic necrosis rather than intrinsic tumor aggressiveness. Recognition of this distinction is key to avoiding overestimation of tumor aggressiveness. RAI was appropriately avoided, with excellent prognosis at 12 months, supporting a risk-adapted management approach. This case highlights the challenges posed by trauma-induced infarction in oncocytic thyroid carcinoma with high-risk molecular findings.

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