p16 Immunohistochemistry as a standalone test for risk stratification in oropharyngeal squamous cell carcinoma

It is widely acknowledged that human papillomavirus (HPV)-related oropharyngeal carcinoma is a biologically unique form of head and neck cancer that should be singled out and treated differently. It is now incumbent to find a test (or combination of tests) that accurately identifies cancers with the associated favorable prognosis for proper patient counseling and management and for placing patients in the correct treatment arms in the emerging clinical trials that are attempting to establish unique treatment types and approaches. The test (or combination of tests) that are utilized must be widely available, reliable, easy to interpret, and well-validated. While HPV-specific testing seems completely logical to use as a single test or one of a combination of tests, it turns out to be quite complicated in practice. Because of the different forms of the virus, the differing types of HPV-specific tests can give different information. HPV DNA, RNA, and protein assays have varying sensitivities for virus detection and also varying availability from formalin-fixed, paraffin-embedded tissue. Since p16 protein over expression is very sensitive for the presence of transcriptionally-active HPV and since it correlates strongly with patient outcomes, is widely available, and easy to interpret, it appears to currently be the single test that combines all of the desired attributes in a risk stratification marker for widespread implementation in clinical and research study settings. This article will review the literature on p16 immunohistochemistry and its relation to HPV-specific testing, discuss some practical issues related to its implementation, and present the case for why it should be the single test used for this purpose.