SU-F-T-203: High Dose Definitive Proton Therapy with Integrated Boost to Hypoxic Subvolumes Determined by [18F]-Fluoromisonidazole (FMISO) PET/CT for Patients with Recurrent Chordomas Using Intensity Modulated Proton Therapy (IMPT)

Purpose: Recurrent chordomas are difficult to control locally. This dosimetric study investigates the feasibility of dose escalation to hypoxic regions, visualized on FMISO-PET, while respecting the dose constraints to the neighboring normal tissues/organs. We propose to deliver a higher dose to the areas of hypoxia (84.5Gy) using IMPT with the goal of improving local control. Methods: We currently have four patients with hypoxic subvolumes (HSV) greater than 10cc from the FMISO-PET image. The HSV was delineated based on the standardized uptake values of greater than 1.4 times of the muscle mean. Gross tumor volume (GTV) was delineated using planning CT with the assistance of MRI fusion. The dose scheme is 50.4Gy RBE to CTV in 1.8Gy fractions, followed by an integrated boost of 27.0Gy RBE to GTV in 1.8Gy fractions and 34.5Gy RBE to HSV in 2.3Gy fractions. IMPT integrated boost plans were optimized with multi-criteria optimization (MCO). Posterior-anterior beam angles were used for these plans. We also propose using two posterior oblique fields to boost HSV to spare the skin folding. A medium spot size with 8mm to 15 mm (σ) in air at isocenter with energies from 220 MeV down to 90 MeV was used. Aperture was used for the medium spot size. A small spot size of 2.5 mm to 4.5 mm (σ) in air at isocenter with energies from 240 MeV down to 70 MeV was also proposed. Target coverage and dose to OARs were evaluated. Results: For the sacral chordoma patient that has been planned, the target homogeneity index is 3.2% for HSV, 55.9% for CTV and 11.9% for GTV. The max dose is 77GyRBE to rectum, 86.2GyRBE to sacral nerves and 73.9GyRBE to cauda equina. Conclusion: IMPT with integrated high dose boost to HSV determined from FMISO PET image is feasible. OAR dose constraints were met.