Cerebellar Metastasis From Radioiodine-Refractory Widely Invasive Oncocytic (Hurthle Cell) Carcinoma of the Thyroid in a Young Adult

Oncocytic (Hurthle cell) carcinoma is an uncommon thyroid malignancy that tends to spread through the bloodstream and often concentrates radioactive iodine poorly.Spread to the brain is rare, and a solitary cerebellar deposit is rarer still.We describe a 44-year-old man who presented with neck swelling and was found to have a widely invasive oncocytic carcinoma with extensive vascular invasion and a minor classic papillary component (5%).He underwent total thyroidectomy with central neck dissection followed by radioactive iodine ablation with 150 mCi of iodine-131.The post-therapy scan showed uptake confined to the thyroid bed.Over the next 18 months, his serum thyroglobulin climbed steadily from 1.5 µg/L to 11.6 µg/L even though the thyroid-stimulating hormone (TSH)-stimulated diagnostic whole-body iodine scan remained negative.Fluorodeoxyglucose positron emission tomography/computed tomography then showed multiple hypermetabolic bilateral pulmonary nodules suspicious for metastases.Roughly 22 months after thyroidectomy, he developed progressive dizziness and headache.Magnetic resonance imaging (MRI) revealed a 3.3 × 3.2 × 3.1 cm mixed cystic and solid left cerebellar mass with surrounding vasogenic edema and mass effect on the brainstem.The mass was resected through a left suboccipital craniotomy.Histology showed a malignant epithelioid neoplasm with nested and trabecular growth, abundant granular eosinophilic cytoplasm, prominent nucleoli, scattered bizarre giant nuclei, and 8 mitoses per 2 mm 2 without necrosis.The tumor cells expressed cytokeratin AE1/AE3, CK7, TTF-1, and PAX-8 and were negative for CK20, CDX-2, synaptophysin, chromogranin, and S100, confirming thyroid origin.The metastasis fulfilled criteria for high-grade differentiated thyroid carcinoma with oncocytic features.His symptoms resolved after surgery, and stereotactic radiosurgery for the resection cavity was recommended.A rising thyroglobulin with negative iodine imaging represents a biochemical incomplete response and should prompt a search for structural disease recurrence.Because the brain lesion in our patient came to light only when symptoms appeared, brain MRI may be worth considering in patients with radiodiodine-refractory distant metastases.