Long-Term Results for MC1273, A Phase II Evaluation of De-Escalated Adjuvant Radiation Therapy for Human Papillomavirus Associated Oropharyngeal Squamous Cell Carcinoma (HPV+ OPSCC)

Purpose/Objective(s) We previously reported the two-year results of MC1273, a phase II trial evaluating 30-36 Gy of adjuvant radiation therapy (RT) for selected patients (pts) with HPV+ OPSCC. Herein we report the long-term disease control and toxicity rates for this de-escalated regimen. Materials/Methods Details for the MC1273 regimen have previously been published. All pts received surgery & neck dissection for a margin negative resection. Eligibility criteria included pts with HPV+OPSCC, ≤10 pack-year smoking history, and negative margins. Cohort A (≥T3, ≥N2, lymphovascular invasion, or perineural invasion) received 30 Gy delivered in 1.5 Gy b.i.d. over 12 days along with weekly docetaxel (15 mg/m2, days 1 & 8). Pts with +ECE were enrolled in Cohort B and received the same treatment plus a simultaneous integrated boost to nodal levels with ECE to 36 Gy in 1.8 Gy b.i.d. The primary endpoint was local/regional control (LRC) at 2 years with each cohort powered to detect a 10% locoregional failure rate with 85% confidence. Secondary endpoints included 2-yr progression free survival (PFS), overall survival (OS), toxicity, swallow function, and pt reported QOL. Follow-up was q3 mos for the first 2 years, q6 mo for year 3, and yearly until year 5. Based upon the 2-year MC1273 data, pts with pT4 disease were excluded from MC1675, the follow-up phase III trial randomizing pts to the 30-36 Gy regimen versus a standard 60 Gy regimen. Results Accrual was from September 2013 to June 2016 (n = 80, 1 ineligible, Cohort A: 37, Cohort B: 43, median (range) age 61 years (25 - 77), male n = 71 (89.9%). Median follow-up for alive pts as of 2/22/21 was 52 mos (30 – 67). Late grade 2 or higher toxicity rates at 2, 3, and 5 years were 6.7%, 6.8%, and 5.0%, respectively, with the most common being dry mouth (1.7%, 3.4%, and 0.0% at years 2, 3, and 5) and dysphagia (3.3%, 0.0%, and 5.0% at years 2, 3, and 5). Grade 3 rates were 0.0%, 0.0%, and 1.2% at years 2, 3, and 5, respectively, with one pt who developed a malignant peripheral nerve sheath tumor in the brachial plexus at the edge of the neck volume 51 mo after RT. The 2, 3, and 5 year disease metrics for the entire study population, Cohort A, and Cohort B can be found in the Table, along with an unplanned subset analysis for pts qualifying for MC1675 (n = 72). Conclusion MC1273 maintained excellent long-term disease control rates comparable to historical controls, particularly in the intermediate risk cohort and pts qualifying for MC1675. Toxicity rates remained low long-term.