ESTRO 2023 - Abstract Book

S511

Sunday 14 May 2023

ESTRO 2023

Conclusion We introduced a dosimetrically-motivated approach for beam angle optimization using iterative FMO and beam elimination combined with HDAO to generate deliverable colli-DTRT plans. colli-DTRT was investigated for 4 clinically motivated cases, demonstrating potential to improve organ-at-risk sparing compared to VMAT. This work was partially supported by Varian Medical Systems and grant 20021_185366 of Swiss National Science Foundation. OC-0622 Evaluation of the dosimetric impact of PET-based adaptive radiotherapy in the PEARL Clinical Trial M. Evans 1 , T. Rackley 2 , S. Berenato 3 , O. Woodley 3 , R. Windle 3 , R. Maggs 3 , J. Staffurth 1 , S. Hargreaves 4 , P. Wheeler 3 1 Cardiff University, Division of Cancer and Genetics, Cardiff, United Kingdom; 2 Velindre University NHS Trust, Clinical Oncology, Cardiff, United Kingdom; 3 Velindre University NHS Trust, Radiotherapy Physics, Cardiff, United Kingdom; 4 Bristol Haematology and Oncology Centre, Clinical Oncology, Cardiff, United Kingdom Purpose or Objective PEARL is an ongoing, UK multi-centre clinical trial of adaptive, toxicity-sparing radiotherapy (RT) in patients with Stage I/II Human Papillomavirus (HPV)-positive oropharyngeal cancer. Patients receive a total dose of 66Gy in 33 fractions over 6 weeks, delivered in 2 phases using Volumetric Modulated Arc Therapy (VMAT). Adaptation is based on metabolic primary tumour response, visualised on a planning 18-FDG-PET CT scan carried out after 2 weeks chemoRT (10 fractions RT, 1 cycle Cisplatin). The adapted plan is implemented in Phase II, for the final 18 fractions of RT. This sub-study was conducted to evaluate the dosimetric impact of adaptation. Materials and Methods For the first 10 recruited patients, automated planning was used to generate i) phase II PET-adapted RT plans as per the PEARL protocol and ii) standard, non-adapted plans for comparison. Both were planned to 38.7Gy in 18 fractions. Dosimetric comparison of Adapted vs Non-Adapted plans was performed for Swallowing Organs at Risk (SWOARS) and parotid glands; analysis was performed using Wilcoxon analysis. EdgeVCC, an automated planning system developed at Velindre Cancer Centre, Cardiff, was used to generate Adapted and Non-Adapted plans for each patient. Results Adapted, PET-based RT significantly reduced absorbed doses to the Superior and Middle Pharyngeal Constrictor Muscles (PCM_Superior, PCM_Middle) and Supraglottic Larynx (Larynx_SG), with no detriment to other OARs. Adaptation led to minor reductions in mean dose metrics to these organs overall (<1.2 Gy), but substantial reductions in higher Dose Volume Histogram (DVH) metrics (V30Gy, V35Gy, V38.7Gy), most notably V35Gy and V38.7Gy, with:

• V30Gy reduced by 4-7%

• V35Gy reduced by 11.8%, 11.1% and 12.1% for PCM_Superior, PCM_Middle and Larynx_SG respectively

• V38.7Gy reduced by 5.5%, 6.1% and 6.6% for PCM_Superior, PCM_Middle and Larynx_SG respectively