ESTRO 2023 - Abstract Book

S1545

Digital Posters

ESTRO 2023

Longitudinal and lateral profiles for three single-spot central proton beams (62.4, 148.2, and 215.7 MeV) were measured using a custom-designed water phantom. The range (R80), defined at 80% fall-off of the maximum dose, and lateral profiles at 20%, 50%, and 80% of R80 were evaluated. Doses in water were calculated by the TPS and measured using a 12 PinPoint detector set-up (PTW; Freiburg, Germany). All measurements were performed with and without an applied magnetic field and compared to the respective output of the TPS. Results Range reduction due to the trajectory change by the magnetic field was between 0.0 and 2.2 mm for the TPS and 0.1 to 2.1 mm for the measurements, with increasing range reduction for higher proton energies. Absolute range comparisons with and without magnetic field agreed between -0.1 and 0.6 mm, respectively. Proton profiles in water showed also good agreement, with lateral offsets due to the magnetic field agreeing at all positions within 0.5 mm.

Figure 1: Normalized lateral beam profiles for a 148.2 MeV proton beam with and without an applied magnetic field of 1 T. Profiles taken at 20%, 50%, and 80% of the total range (R80). Conclusion The dosimetric validation of the prototype MC-based treatment planning system indicates that the effects of the magnetic field is modelled with high and sufficient accuracy. Work in progress is the evaluation of more complex heterogeneous scenarios as well as the investigation of optimization effectiveness.

PO-1817 Feasibility of Xoft Axxent Electronic IORT for intracranial tumors in terms of radiation dosimetry

S. Can 1 , D. Karaçetin 1 , P. Silli 2 , C. Perez 3 , Ö. Atilla 1 , İ . Harmankaya 1

1 Basaksehir Cam and Sakura City Hospital, Radiation Oncology, Istanbul, Turkey; 2 Xoft Inc., A Subsidiary of iCad. Inc., Business Development, Florence, Italy; 3 Xoft Inc., a Subsidiary of iCad. Inc., Business Development, Madrid, Spain Purpose or Objective The study aimed to analyze IORT plans utilized with Xoft Axxent eBx which is a 50 kVp low energy IORT system comparing it with non-coplanar VMAT SRS treatment technique using Elekta Versa HD™ in terms of radiation dosimetry for single target high grade primary CNS tumors and intracranial metastatic disease. The patients who have non-operated tumor and treated with SRS were considered, however, it was aimed to show the benefit of IORT might present even better advantages to protect OAR’s and to reduce possible radiation necrosis. Materials and Methods CT images were acquired with a 1.0 mm slice thickness and fused with a T1 weighted gradient-echo MRI sequence with contrast media for target delineation. Non-coplanar VMAT plans were devised in the Monaco® TPS and all patients were treated with VMAT SRS. The treatment plan prescription was 20 Gy in single fraction to PTV with the coverage rate of 95%. All CT data sets were imported to BrachyCare TPS to visualize the dose distribution of IORT plans to deliver 20 Gy at the applicator surface. PTV was assumed as the resection cavity which is filled spherical balloon applicator volume at the time of surgery. Gradient index (GI) which was calculated based on Paddick formula and NTCP values calculated based on Lyman Kutcher-Burman method. Results The 23 patients were retrospectively analyzed. The mean Dmax of Brainstem was (4.43 ± 3.13) Gy in VMAT plans, and it was (3.27 ± 2.80) Gy in IORT plans. IORT has great advantageous to reduce dose even if the tumor location is close to critical structures. IORT dose was approximately 65% and 68% less than VMAT dose for eyes and optic nerves respectively. Additionally, V2Gy, V10Gy, V12Gy of Brain-PTV were also analyzed for radiation necrosis. IORT was crucial to provide better V2Gy result for Brain-PTV due to the nature of source energy. However, VMAT plan was superior to control high dose region outside of PTV. Dose distribution in both approaches was shown in Figure-1. The GI value was 2.65 and 2.24 in VMAT plan and IORT respectively. IORT was superior to provide low-dose spillage. In both approaches, NTCP value