Differential response rates to irradiation among patients with human papillomavirus positive and negative oropharyngeal cancer.

Abstract

To evaluate the responsiveness of human papillomavirus (HPV) -positive and HPV-negative oropharyngeal cancer to intensity-modulated radiotherapy (IMRT), using axial imaging obtained daily during the course of image-guided radiotherapy (IGRT). Observational cohort study with matched-pair analysis of patients irradiated for HPV-positive and HPV-negative oropharygeal cancer. Ten patients treated by IMRT to 70 Gy for locally advanced, HPV-positive squamous cell carcinoma of the oropharynx were matched to one HPV-negative control subject by age, gender, performance status, T-category, tumor location, and the use of concurrent chemotherapy. The gross tumor volume (GTV) was delineated on daily IGRT scans obtained via kilovoltage cone-beam computed tomography (CBCT). Mathematical modeling using fitted mixed-effects repeated measure analysis was performed to quantitatively and descriptively assess the trajectory of tumor regression. Patients with HPV-positive tumors experienced a more rapid rate of tumor regression between day 1 of IMRT and the beginning of week 2 (-33% Δ GTV) compared to their counterparts with HPV-negative tumors (-10% Δ GTV), which was statistically significant (p<0.001). During this initial period, the average absolute change in GTV was -22.9 cc/week for HPV-positive tumors and -5.9 cc/week for HPV-negative tumors (p<0.001). After week 2 of IMRT, the rates of GTV regression were comparable between the two groups. HPV-positive oropharyngeal cancers exhibited an enhanced response to radiation, characterized by a dramatically more rapid initial regression than those with HPV-negative tumors. Implications for treatment de-intensification in the context of future clinical trials and the possible mechanisms underlying this increased radiosensitivity will be discussed.