Expression of p16^Ink4a in mixed squamous cell and glandular papilloma of the lung

Mixed squamous cell and glandular papilloma (mixed papilloma) of the lung is an extremely rare neoplasm, with only 21 cases reported in the English literature. Although the expression of p16^Ink4a has been analyzed in only two cases of mixed papilloma, they were negative for p16^Ink4a. Therefore, the significance of p16^Ink4a overexpression in mixed papilloma remains unclear. This is the first case of mixed papilloma with positive p16^Ink4a expression in a 72-year-old male smoker. The 20 mm sized tumor was histologically diagnosed as mixed papilloma following right upper lobectomy. Immunohistochemically, cytokeratin 5 and p40 positivity was predominant in basal cells of the glandular component and squamous cells, while thyroid transcription factor-1, p53, and Ki-67 were focally positive. Both glandular and squamous components were diffusely positive for p16^Ink4a. This finding could be important to clarify the pathogenesis and biology of mixed papilloma.