A 59-year-old woman presented with a 2-month history of daily pain and swelling in the left submandibular region. The swelling occurred with meals and would last 30 minutes before gradually resolving. Over the previous 3 weeks, the pain and swelling had steadily increased. The patient's history was remarkable for papillary thyroid carcinoma that had been treated with a lobectomy and a completion thyroidectomy followed by iodine 131 (131I) ablation (145.9 mCi), which had ended 1 year earlier.