SUN-389 - Metastatic High-Grade Minimally Invasive Follicular Thyroid Carcinoma Masquerading as Ectopic Intrapulmonary Thyroid Tissue

Background
Thyroid tissue in the lung represents metastatic thyroid carcinoma or less commonly ectopic intrapulmonary thyroid tissue. The distinction between these entities presents a significant diagnostic challenge, as both may appear histologically benign yet carry different prognostic implications. We describe a metastatic high- grade follicular thyroid cancer that presented initially as mature thyroid tissue in the lungs.

Clinical Case
An 80-year-old woman, former smoker, presented with incidental bilateral pulmonary nodules demonstrating interval growth (1.5 to 2.2 cm; 1.1 to 1.3 cm), FDG-PET avidity (SUV 3.0), and positive thyroglobulin washout. No abnormal thyroid uptake was identified. Following non-diagnostic biopsies, left upper lobectomy with mediastinal lymphadenectomy revealed mature intrapulmonary thyroid tissue with focal nodular hyperplasia—initially interpreted as ectopic mature thyroid tissue without identified malignancy features.
Two years later, a 1.7 cm right thyroid nodule with macrocalcifications was noted. Fine-needle aspiration demonstrated Bethesda IV cytology. Molecular testing revealed KRAS and TERT promoter mutations, conferring 95% malignancy risk. She underwent total thyroidectomy, and pathology revealed a 2 cm unifocal high-grade minimally invasive FTC (pT1b, pN0a) with 5 mitoses/2 mm², partially disrupted margin, and no vascular invasion.
Postoperative stimulated thyroglobulin was 121 ng/mL and underwent radioactive iodine I-131 ablation (123.3 mCi) and post-therapy imaging revealed radioiodine-avid pulmonary nodules (8 mm and 5 mm right lobe), indicating metastatic disease. Given her age, atrial fibrillation, and osteopenia, she is on levothyroxine with TSH target (0.1–0.5 mIU/L). The patient maintains excellent functional status (IADL 8/8).

Discussion
Ectopic intrapulmonary thyroid is exceedingly rare, and pulmonary thyroid tissue is suspected to be metastatic, thus, should be carefully evaluated including molecular testing when histologic differentiation is non-conclusive. High-grade features alter the risk profile of minimally invasive FTC, which is known to carry excellent prognosis. These lesions might be Bethesda II-IV in FNA cytology and can be initially indolent. This is explained by the stepwise molecular progression of these tumors from well differentiated to high grade and potentially to poorly differentiated or anaplastic thyroid cancer through accumulation of genetic mutations that involve MAPK signaling and TERT promoter, the latter associated with increased metastasis risk (Xu B et al).
Clinicians should maintain high suspicion for metastatic disease when evaluating pulmonary nodules with thyroid pathology and incorporate molecular testing for risk stratification and guided definitive management.

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