Background Oropharyngeal squamous-cell carcinomas caused by human papillomavirus (HPV) are associated with favorable survival, but the independent prognostic significance of tumor HPV status remains unknown.

Methods We performed a retrospective analysis of the association between tumor HPV status and survival among patients with stage III or IV oropharyngeal squamous-cell carcinoma who were enrolled in a randomized trial comparing accelerated-fractionation radiotherapy (with acceleration by means of concomitant boost radiotherapy) with standard-fractionation radiotherapy, each combined with cisplatin therapy, in patients with squamous-cell carcinoma of the head and neck. Proportional-hazards models were used to compare the risk of death among patients with HPV-positive cancer and those with HPV-negative cancer.

Results The median follow-up period was 4.8 years. The 3-year rate of overall survival was similar in the group receiving accelerated-fractionation radiotherapy and the group receiving standard-fractionation radiotherapy (70.3% vs. 64.3%; P=0.18; hazard ratio for death with accelerated-fractionation radiotherapy, 0.90; 95% confidence interval [CI], 0.72 to 1.13), as were the rates of high-grade acute and late toxic events. A total of 63.8% of patients with oropharyngeal cancer (206 of 323) had HPV-positive tumors; these patients had better 3-year rates of overall survival (82.4%, vs. 57.1% among patients with HPV-negative tumors; P<0.001 by the log-rank test) and, after adjustment for age, race, tumor and nodal stage, tobacco exposure, and treatment assignment, had a 58% reduction in the risk of death (hazard ratio, 0.42; 95% CI, 0.27 to 0.66). The risk of death significantly increased with each additional pack-year of tobacco smoking. Using recursive-partitioning analysis, we classified our patients as having a low, intermediate, or high risk of death on the basis of four factors: HPV status, pack-years of tobacco smoking, tumor stage, and nodal stage.

Conclusions Tumor HPV status is a strong and independent prognostic factor for survival among patients with oropharyngeal cancer.

Discussion

Recursive-partitioning analysis was used to identify prognostic factors with the most influential predictive significance in a proportional-hazards model of overall survival and to classify patients into categories of low, intermediate, or high risk of death. The prognostic factors in the analysis were age, tumor stage, nodal stage, race, smoking status, HPV status, anemia status, performance status, treatment assignment, and sex. Panel A shows the resulting classifications. Panel B shows data for overall survival in the classified patients. The Kaplan–Meier curves are shown in black, and the associated 95% confidence intervals in gray.

The 3-year rates of overall survival were 93.0% (95% CI, 88.3 to 97.7) in the low-risk group, 70.8% (95% CI, 60.7 to 80.8) in the intermediate-risk group, and 46.2% (95% CI, 34.7 to 57.7) in the high-risk group. Hazard ratios for death among the 266 patients for whom the risk classification could be made on the basis of the recorded data and among all 433 patients with oropharyngeal cancer (after missing data on HPV status and number of pack-years were estimated with the use of statistical imputation) were as follows: 3.54 (95% CI, 1.91 to 6.57) and 2.67 (95% CI, 1.54 to 4.62), respectively, in the intermediate-risk group versus the low-risk group; and 7.16 (95% CI, 3.97 to 12.93) and 5.23 (95% CI, 3.14 to 8.73), respectively, in the high-risk group versus the low-risk group.

This study provides strong evidence that tumor HPV status is an independent prognostic factor for overall survival and progression-free survival among patients with oropharyngeal squamous-cell carcinomas, which is consistent with the hypothesis that HPV-positive and HPV-negative oropharyngeal squamous-cell carcinomas are distinct and have different causes, risk-factor profiles,and survival outcomes. On the basis of our data, we believe that future clinical trials should be designed specifically for patients with HPV-positive or HPV-negative squamous-cell carcinoma of the head and neck or patients who have been stratified according to HPV status. Moreover, additional information could be gleaned from completed clinical trials, by means of reanalysis, to determine whether imbalances in tumor HPV status between treatment groups affected the outcomes and thus the therapeutic implications.

Our analysis of the association of HPV status with survival was performed in a clinical trial of locally advanced squamous-cell carcinoma of the head and neck that did not show a significant difference in overall survival between a concomitant-boost accelerated-fractionation regimen of radiotherapy and a standard-fractionation regimen, combined with concurrent, high-dose cisplatin. Therefore, either regimen could serve as the comparison for a new therapy being investigated.

We observed strong agreement between tumor HPV status, as determined by in situ hybridization, and expression of p16, an established biomarker for the function of the HPV E7 oncoprotein. Our HPV-16 in situ hybridization assay has sensitivity for single viral copies, and a positive result is strongly correlated with expression of the HPV E6 and E7 oncogenes — the standard for defining a tumor as being associated with HPV. A limitation of our method is the unknown sensitivity of the probe cocktail for non–HPV-16 types, which account for an estimated 5 to 10% of HPV-positive oropharyngeal squamous-cell carcinomas.Thus, the misclassification of HPV-positive tumors as HPV-negative tumors probably explains the slightly larger reduction in the risk of death when the analysis was based on status with respect to p16 expression rather than HPV presence. A strength of the p16-expression assay is that it is not specific for HPV type, unlike the in situ hybridization assays; therefore, p16-expression status is a very good surrogate for tumor HPV status.

The superior prognosis for HPV-positive oropharyngeal squamous-cell carcinoma, as compared with that for the HPV-negative cancer, appears to have multifactorial underpinnings. Known favorable prognostic factors associated with the HPV-positive subgroup account for approximately 10% of the detected difference in outcome. The higher survival rate among patients with HPV-positive cancer is due in part to greater local–regional control, reflecting higher intrinsic sensitivity to radiation or better radiosensitization with the use of cisplatin. Although rates of response to induction chemotherapy are higher among patients with HPV-positive tumors than among those with HPV-negative tumors,⁴ single-agent cisplatin therapy did not appear to differentially affect the elimination of occult distant metastases. Second primary tumors, which are largely related to smoking, were less frequent among patients with HPV-positive tumors, a finding that is consistent with the lower exposure to tobacco in this subgroup. However, the rates of death from second primary tumors were similar in the HPV-positive and HPV-negative subgroups and therefore do not account for the overall differences in survival rates.

Our data clearly indicate that HPV status and status with respect to tobacco smoking are major independent prognostic factors for patients with oropharyngeal squamous-cell carcinoma, probably because they determine the molecular profile of the cancer and thus the response to therapy. Although HPV-positive oropharyngeal squamous-cell carcinoma is genetically distinct from the HPV-negative cancer with respect to patterns of loss of heterozygosity, chromosomal abnormalities, and gene-expression profiles and is inversely correlated with biomarkers for a poor prognosis in squamous-cell carcinoma of the head and neck (e.g., p53 mutations or expression of epidermal growth factor receptor), no specific mechanism has been shown to explain the higher rates of response to radiation therapy and chemotherapy among patients with HPV-positive cancer.Epidemiologic data indicate that tobacco smoking is not a strong cofactor for the development of HPV-positive oropharyngeal squamous-cell carcinoma.Nevertheless, our data reveal that the biologic behavior of an HPV-positive tumor may be altered by tobacco use. Genetic alterations induced by tobacco-associated carcinogens may render HPV-positive tumors less responsive to therapy. The likelihood of such genetic alterations appears to increase as the number of pack-years of tobacco smoking increases. The cutoff point of 10 pack-years, which was the best predictor of survival in our recursive-partitioning analysis, may be more useful than a continuum for the design of future risk-based clinical trials but will require further validation.

The extent to which the superior survival for a patient with HPV-positive oropharyngeal squamous-cell carcinoma depends on the administered therapy is unclear. Published data indicate that tumor HPV status is a strong and consistent determinant of superior survival, regardless of treatment strategy (e.g., surgery, radiation therapy, concurrent chemoradiation therapy [in this study], or induction chemotherapy plus concurrent chemoradiation therapy), with 5-year survival rates among patients with HPV-positive tumors of approximately 75 to 80%, versus 45 to 50% among patients with HPV-negative tumors.

Though no direct evidence from formal clinical trials exists to guide treatment decisions for the individual patient on the basis of tumor HPV status, this study provides a direction for future clinical research. A combination of tumor HPV status, pack-years of tobacco smoking, and cancer stage may be used to classify patients as having a low, intermediate, or high risk of death. Whether patients with HPV-positive tumors who are considered to be in the low-risk category can be spared the long-term complications of intensive, multimodal therapy without compromising their survival is now a highly relevant clinical question. In contrast, such a strategy would be inappropriate for the 36% of patients with HPV-positive tumors who are in the intermediate-risk group, for whom the 3-year rate of overall survival (71%, with an even lower rate of progression-free survival) is unacceptable. Unfortunately, patients in the high-risk group have an extremely poor prognosis and thus should be offered enrollment in trials testing more intensive investigational therapies. Should our risk model be validated in other cohorts, it will be important to incorporate tumor HPV status and tobacco exposure as nonanatomical determinants of risk classification and therapy selection for patients with oropharyngeal squamous-cell carcinoma.