Abstract
HER2 overexpression is also seen in cases of adenocarcinoma, breast cancer, gastric cancer, etc. Amongst head and neck cancers, salivary duct carcinoma is the most well-known histological type. However, reports of HER2 positive primary unknown adenocarcinoma with cervical lymph node metastasis are extremely rare. We experienced a case of HER2 positive primary unknown adenocarcinoma with cervical lymph node metastasis. A 56-year-old male, who complained of a neck mass, had multiple lymph node enlargement, a tumor in the left lobe of the thyroid and multiple pulmonary nodules. Lymphadenectomy showed GCDFP-15 positive adenocarcinoma with apocrine differentiation. Primary tumor was not indicated by various examinations, and no result suggested a diagnosis of thyroid cancer or salivary duct cancer. While starting to treat the patient as primary unknown cancer, we performed comprehensive genome profiling. ERBB2 was highly amplified, and immunostaining showed HER2（3＋）. Following treatment as HER2 positive breast cancer, the patient has shown a partial response for 3 years until now.
