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 mm2 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.