Adenocarcinoma-Papillary Cystic Pattern Arising in a Mixed Squamous and Glandular Papilloma of the Lung.

Mixed squamous and glandular papilloma (mixed papilloma) of the lung has been reported in fewer than 25 cases in the English literature. Although it is known as a benign tumor, malignant transformation has been reported. Papillary cystic carcinoma is characterized by papillary and cystic growth patterns and has been reported as a subtype of adenocarcinoma, mainly in the salivary glands, breast, and pancreas. In this article, we report a case of adenocarcinoma-papillary cystic pattern arising from mixed papilloma of the lung in a 76-year-old male patient. Chest computed tomography scan revealed an endobronchial mass growing at the right medial segmental bronchus. Middle lobe lobectomy was performed, revealing a 4.9 × 1.9 cm-sized mass that protruded into the bronchus. Microscopically, the tumor showed numerous cysts lined by micropapillary projections. The tumor cells had round and vesicular nuclei with prominent nucleoli, and mitosis was frequent. A limited portion of the tumor consisted of benign mixed papilloma. The tumor showed diffuse immunoreactivity for thyroid transcription factor-1 and strong expression of p16. We investigated the mutational status of cancer-related genes using targeted next-generation sequencing and identified a genetic alteration in the BRAF gene. This is the first case report of papillary cystic carcinoma arising in mixed papilloma of the lung.