Mid-Treatment PET During Radiation Therapy in HPV-Positive Oropharyngeal Cancer Patients Demonstrates Variable Response: Implications for Treatment De-escalation: Imaging

Abstract

HPV associated oropharyngeal squamous cell carcinomas are known to have a good prognosis and treatment de-escalation is an area of active study. Other disease sites, such as lung cancer (ie, RTOG 11-06) are attempting to use mid-treatment PET/CT scans to try and guide decisions on treatment intensification. We sought to determine patterns of response from a mid-treatment PET/CT in HPV positive or pharyngeal cancer patients to see if this may serve as a biomarker to better determine appropriate candidates for treatment de-escalation. Between 4/2011 and 5/2013, 53 patients had a mid-treatment PET/CT during their course of head and neck radiation therapy at Memorial-Sloan Kettering Cancer Center. All patients were treated with IMRT with or without concurrent cisplatin-based chemotherapy. HPV status was determined by p16 IHC. The highest standardized uptake value (SUV) of both the gross primary and gross nodal disease was recorded before, during and after radiation therapy. The percentage difference of the SUV values on the early response PET and pre radiation PET was used determine the early PET response. A CR was determined to be complete resolution of FDG activity, any decrease in FDG activity was considered a PR, and no response was determined to be no change or an increase in FDG activity. Of the 53 patients, 23 had oropharyngeal primaries. 5 patients were excluded because of induction chemotherapy (n=2), HPV status unavailable (n=2), and HPV negative status (n=1). Of the remaining 18 HPV+ patients, 13 had a BOT primary and 5 had a tonsil primary. 17 patients were stage IVA and 1 patient was stage III. Mid-treatment PET/CT was performed at a median of 43 days after start of treatment (range 34-52). At the primary site, 5 patients had a complete response, 10 patients had a partial response, and 2 patients had no response. No patient with >10 pack -yr smoking history had a CR at the primary site on early response PET. In the neck, 4 patients had a CR and 14 patients had a PR. Individually, 51 nodes were evaluable in these 18 patients. 23 nodes had a CR, 25 nodes had a PR, while 3 nodes had no response. Preliminary results from our study indicate that mid-treatment response PET scans demonstrated variability in response of HPV+ tumors near doses of 60Gy. Given the variability of PET response, caution should be exercised to routinely implement dose de-escalation to all HPV+ oropharyngeal cancer. This information may have implications for treatment de-escalation strategies and further study is warranted to confirm these results.