Mixed squamous cell and glandular papilloma of the lung: A case report of a novel mutation in the BRAF gene and coexistent HPV infection, possible relationship to ciliated muconodular papillary tumor

Mixed squamous cell and glandular papilloma (mixed papilloma) is a very rare tumor, with fewer than 25 cases having been reported in the literature. Although a scattering of cases of p16Ink4a overexpression have been described to date, no human papillomavirus (HPV) DNA has been detected in these tumors, either by in situ hybridization (ISH) or polymerase chain reaction (PCR). This is the first case of mixed papilloma with PCR-confirmed HPV genotype 16, 35, 51 infections in an 18-year-old non-smoking male, coexisting with multiple atypical adenomatous hyperplasias (AAHs). Histologically, this tumor shows a predominant papillary architecture, covered by a mixture of stratified squamous cells, ciliated or non-ciliated cuboidal to columnar cells, mucous cells, and scattered goblet cells. Immunohistochemically, the squamous component was positive for p40, and the glandular cells were focally positive for TTF-1. Both components were diffusely immunoreactive to CK7. In addition, BRAF V600E mutation was also first demonstrated in mixed papilloma, but not in the AAHs. These findings suggest that HPV infection and the BRAF mutation may be important in the pathogenetic role in young non-smoking patients.