ESTRO 38 Abstract book

S352 ESTRO 38

compared to 96.72%/96.13 of the ro-IMPT group (p<0.002). There was no significant difference in the OAR dose between the two planning strategies. One patient did have suboptimal coverage (CTV_low <90%) even with ro- IMPT. Significant weight loss was noted for this patient during the treatment course (>5 lbs). Table 1. Dose comparison between initial plan and dose accumulation from 35 fraction.

Conclusion The irradiated volumes of temporal lobe BSCs were consistently the lowest with PBS, predicting better post- treatment memory in children with centrally located brain tumors. OC-0671 Which planning strategy is better for Head and Neck Cancer: PTV based or CTV based robust IMPT ? W.G. Wang 1 , A. Qin 1 , P. Kabolizadeh 1 , X. Li 1 , D. Yan 1 , G. Liu 1 , R. Deraniyagala 1 , I. Grills 1 , C. Stevens 1 , D. Krauss 1 , X. Ding 1 1 Beaumont Health, Proton Therapy Center, Royal Oak, USA Purpose or Objective A comprehensive plan evaluation platform was established based on the daily Cone-Beam Computed Tomography(CBCT) to assess the treatment robustness quality between PTV based IMPT(PTV-IMPT) and CTV based robust optimized IMPT(ro-IMPT) planning strategies in bilateral Head and Neck Cancer (HNC) treatment. Material and Methods 9 bilateral HNC patients’ CT structure sets were used in this study. Daily CBCT were converted into synthetic-CT (sCT) for dose reconstruction. The accuracy of the proton dose calculation in sCT is cross-validated via the same day’s verification-CT sim (vCT) with 3D gamma index comparison. PTV-IMPT and ro-IMPT were generated on the initial planning CT. CTV high-risk volume(CTV_high) received 70Gy and CTV low/intermediate-risk(CTV_low) received 60Gy. For PTV-IMPT, the PTVs were expanded 3mm from the CTV; For ro-IMPT, robust optimization used 3mm setup and 3.5% range uncertainties. Dose accumulations were then calculated on the 35 sets of daily sCT and the target coverages were compared to the initial plans. Results The 3D gamma index dose comparison (3mm/3%) showed an average pass rate of 98.2%+/-1.5% comparing the same day’s pair of sCT and vCT with both plans (total 38 pairs). Through the dose accumulation of 35 treatment fractions, the PTV-IMPT plan group’s mean V100 of CTV_high/CTV_low coverage degraded to 80.70%/85.73%

Table 2. Average Dose Coverage to the target on the daily basis.

Conclusion A comprehensive plan robustness evaluation platform based on the CBCT is established in our clinical workflow and enables dose accumulation and plan robustness evaluation on a daily basis. ro-IMPT demonstrated an optimal planning strategy over PTV-IMPT for bilateral HNC treatment. However, special cautions are needed for patients with significant weight or geometry changes. OC-0672 Proton radiotherapy for left-sided breast cancer in patients with pectus excavatum anatomy S. Korreman 1 , S. Andreasen 2 , J.B.B. Petersen 1 , B. Offersen 3 1 Aarhus University Hospital, Department of Oncology and Danish Center for Particle Therapy, Aarhus C, Denmark ; 2 Aarhus University, Department of Physics and Astronomy, Aarhus C, Denmark; 3 Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark Purpose or Objective For breast cancer patients with challenging anatomy, such as pectus excavatum, it can be impossible to cover the entire breast and IMN without exceeding dose limits to the organs at risk using standard techniques. Proton therapy