ESTRO 2023 - Abstract Book

S67

Saturday 13 May

ESTRO 2023

development in 5-year OS over time. For p16+ tumors the survival probability increased from 55% in the beginning of the period to 81% at present (Figure 2A). This development is probably linked up with the intensification of treatment introduced over time, including the use of chemoradiotherapy. Figure 2B illustrates the corresponding development for patients with p16-/p16-unknown tumors. Although improvement in outcome over time has been achieved also for this group of patients, the benefit from intensification of treatment is much less obvious than for p16+ tumors. Conclusion Based on this nationwide cohort of 8595 patients prospectively collected over a period of 35-years, we demonstrate that outcome for Danish patients with OPC has improved significantly from 1986-2020 alongside intensification of treatment. This development is predominantly ascribable to a marked increase in the incidence of HPV-related p16+ tumors combined with their pronounced sensitivity to treatment. However, HPV-negative disease continues to be a therapeutic challenge.

OC-0106 Prospective Trial of Personalized Fractionation in Low-risk HPV Positive Oropharyngeal Cancerroph J. Caudell 1 , M. Echevarria 1 , G. Yang 1 , Y. Kim 2 , K. Kirtane 3 , J. Kish 4 , J. Muzaffar 3 , C. Chung 3 , H. Enderling 5 1 Moffitt Cancer Center, Radiation Oncology, Tampa, USA; 2 Moffitt Cancer Center, Biostatistics, Tampa, USA; 3 Moffitt Cancer Center, Head and Neck/Endocrine, Tampa, USA; 4 Moffitt Cancer Center, Personalized Medicine, Tampa, USA; 5 Moffitt Cancer Center, Mathematical Oncology, Tampa, USA Purpose or Objective An early imaging response during radiotherapy for head and neck cancer is associated with progression free survival (PFS). We developed a mathematical model of pre-treatment growth and radiation response dynamics, with the hypothesis that the model output, proliferation saturation index (PSI), could select the most favorable fractionation [hyperfractionation (HFx) or conventional (QD)] to elicit a rapid imaging response at four weeks during treatment. Materials and Methods Patients with T0-2 N0-1 M0 p16-positive low-risk OPC were enrolled (NCT03656133). PSI was calculated from pre-treatment diagnostic and simulation CT scans, with patients > 0.75 assigned to HFx. This protocol was a single arm, phase II design with a primary endpoint of improving ≥ 32% reduction in tumor volume by week 4 to 63%, compared to a historical control of 49%. Secondary endpoints included acute toxicity per CTCAE v5, imaging response at 2-3 months post treatment, and PFS and overall survival (OS). Results From 10/2018 – 2/2022, 55 patients were enrolled with select pretreatment characteristics listed in Table 1. Based on PSI, HFx was assigned in 42 patients (76.4%) and QD in 13 patients (23.6%). Response at the target lesion at 4 weeks was a median 40.1% reduction (range 100% reduction – 80.4% growth). 32 of 55 (61.2%) patients met the criteria ≥ 32% reduction in tumor volume by week 4, meeting criteria for efficacy. Only 5 patients (9.1%) developed acute CTCAE v5 grade 3 toxicity attributable to RT; there were no grade 4-5 adverse events (AEs). All grade 3 acute AEs had resolved at last follow-up, with no late grade 3-4 AEs reported. Of the 53 evaluable patients at 2-3 months post-treatment, 52 (98.1%) had a complete anatomic and/or metabolic response, and one (2.1%) had a partial anatomic/metabolic response. Median follow-up was 20 months at time of analysis. For the entire cohort, actuarial PFS at 2 years was 91.3%. Patients with a rapid imaging response had 2-year PFS 91.6%, compared with slow responders of 90.9% (p=0.22).

Patient Characteristics N (%) Median Age

64 (range 30-82)