Biomarker-driven radiation therapy dose reduction after transoral robotic surgery for the treatment of HPV-positive oropharyngeal cancer.

TPS6119 Background: HPV-related oropharyngeal squamous cell carcinoma (OPSCC) is a highly curable cancer with a considerable burden of treatment-related toxicities. Transoral surgery (TOS) is a treatment option for patients with early-stage disease; however, adjuvant radiotherapy (RT) is often recommended and can add to these late toxicities. Circulating tumor HPV DNA (ctHPVDNA) is a sensitive marker of tumor recurrence, and undetectable post-operative ctHPVDNA is associated with improved recurrence-free survival. We hypothesize that patients with undetectable post-TOS ctHPVDNA without high risk pathologic factors can safely undergo de-escalation of adjuvant RT to 36 Gy, and this reduction will improve long-term swallowing function. Methods: We opened a multi-institutional phase II study (NCT05387915) including patients with pT1-2, N0-1, M0 HPV-related OPSCC who had TOS resection of their primary site and at least ipsilateral neck dissection. Pertinent inclusion criteria include a 10 years since last tobacco consumption, and pathologic findings of one or more intermediate risk factors: close margin (1 – 4 mm), perineural invasion, 2 – 4 positive lymph nodes or a single lymph node >3 cm. Patients with high risk factors (positive margin, extranodal extension, 5 or more positive lymph nodes) are excluded. Patients must have a detectable pre-operative ctHPVDNA and a post-operative ctHPVDNA drawn 7-14 days after TOS; if negative, patients receive 36 Gy of adjuvant RT in 15 fractions of 2.4 Gy delivered daily; if positive, they receive care per the discretion of the treating radiation oncologist. If the final surgical margins at the site of the primary tumor are >2mm, patients are eligible for omission of the primary tumor post-operative bed from the radiation volume. Our primary endpoint is 1-year swallowing function as measured by the composite MD Anderson Dysphagia Index (MDADI) score. Secondary endpoints include overall/progression-free survival and other quality of life metrics. The study is powered to detect an 8.5-point improvement in MDADI composite score at one year over the historical control from ECOG 3311 Arm B. With an alpha of 0.05 and a power of 0.90 we require 27 evaluable patients. Accounting for dropout and patients lost to follow-up, we aim to enroll 35 total patients. Since accrual began in June 2022, we have enrolled 18 patients as of January 2024. We recently opened the study at a second institution and are in the process of opening at a third institution. Clinical trial information: NCT05387915 .