p16INK4A Immunohistochemical Staining May Be Helpful in Distinguishing Branchial Cleft Cysts From Cystic Squamous Cell Carcinomas Originating in the Oropharynx

A subset of metastatic squamous cell carcinoma (SCC) of the head and neck presents as cystic masses in the neck. Often, distinguishing between these cystic metastases of SCC from benign cystic neck lesions, such as branchial cleft cyst, can be very challenging. Squamous lining cells in benign cystic lesions may, at times, exhibit significant cytologic atypia, raising concern for SCC. Alternatively, metastases from SCC may occasionally be cytologically bland and lack overt features of malignancy. The challenge in distinguishing benign from malignant cystic squamous lesions of the neck is made more difficult when evaluating fine-needle aspiration biopsies (FNABs) given the limited sample size.1 Recent studies have suggested an etiologic relationship between high-risk oncogenic human papillomavirus (HPV) type 16 and some head and neck SCC.2–4 A subset of primary oropharyngeal SCC, including carcinomas of the palatine tonsils and base of tongue, demonstrates a high prevalence of HPV type 16 DNA.2,5–7 This subset of HPV-related SCC typically occurs in younger patients, rather than non-HPV-related SCC of the head and neck, and it usually has a nonkeratinizing “basaloid” morphology. 8–11 In addition, HPV-related oropharyngeal SCC has been reported to have a more favorable prognosis compared with HPV-negative tumors.3,4 As with uterine cervical SCC, HPV-mediated transformation to oropharyngeal SCC is thought to result from integration of the viral genome into the host genome, leading to increased expression of the E6 and E7 oncogenes.12 The E7 gene product of HPV inactivates the retinoblastoma (RB) protein, resulting in increased production of the CDNK2A gene product, p16INK4A, a tumor-suppressor gene involved in cell cyclecontrol.13 Thus, increased protein expression of p16INK4A can act as a surrogate marker for high-risk HPV infection in such squamous cell lesions. A readily available marker that can reliably distinguish between benign and malignant cystic squamous lesions would have tremendous diagnostic utility. In Begum et al’s study of 77 cases with metastatic head and neck SCC diagnosed by FNAB, p16INK4A overexpression was identified in 17 of them (13 oropharyngeal, 1 oral cavity, 3 unknown primary).14 Of note, 2 of the 77 cases represented false-positive diagnoses where branchial cleft cysts were found on resection and were noted to haveas having weak p16INK4A staining, which was interpreted as negative. In situ hybridization (ISH) for HPV16 was also performed, and fewer cases of SCC were positive (13 of 75). The advantage of HPV ISH is direct observation of the infected cell and high specificity. However, p16INK4A is more sensitive, albeit less specific, and more laboratories have the ability to perform immunohistochemistry than ISH.15 Although several studies have looked at p16INK4A expression in head and neck SCC, there is little published on the pattern of p16INK4A expression in branchial cleft cysts. In this study, we further evaluate the diagnostic utility of p16INK4A in distinguishing benign from malignant cystic head and neck squamous lesions by examining p16INK4A expression in branchial cleft cysts and metastatic SCC both in surgical excision specimens and in FNAB cell block samples.