Intraosseous carcinoma of the anterior maxilla identified as the occult primary tumour of carcinoma of unknown primary.

Carcinoma of unknown primary (CUP) is defined as lymph node metastasis without a detectable origin until after the initial treatment has been performed. The most common occult primary site in the head and neck, as revealed by a review of the published literature, is the oropharynx. An occult primary site in the oral region is extremely rare. We report a rare case of head and neck CUP (HNCUP) in a 69-year-old female patient, wherein the occult primary lesion was a primary intraosseous carcinoma (PIOC) invading the anterior maxilla. During the course of the initial diagnostic workup, no primary lesion could be identified; however, cervical lymph node metastasis to left levels IB and IIA were observed in the patient. A neck dissection followed by adjuvant radiotherapy was performed. However, the PIOC of the anterior maxilla was identified 6 months after neck treatment and was confirmed as the occult primary tumour of the HNCUP. This case is quite rare and required a comprehensive workup to guide optimal treatment. Careful follow-up or active biopsy should be considered if osteolytic changes are observed in the jaw.