Dr. Pierre Korysko

Abstract:

This article describes the design and implementation of the first known prototype of a dual-scattering system for the delivery of a uniform VHEE beam with transverse dimensions suitable for pre-clinical and potential future clinical use. The results presented in this article are the first experimental characterisation of beam flattening with dual-scattering foils in the VHEE regime, confirmed with both profile measurements with a YAG screen and radiochromic EBT3 films. Polylactic acid and aluminium dual-scattering systems were designed using TOPAS Monte-Carlo simulations and multi-objective minimisation methods. Studies to test the success of the scattering systems in providing beam magnification and uniformity were carried out at the 200 MeV CERN Linear Electron Accelerator for Research (CLEAR) facility. A generalised super-Gaussian function was used to model the final beam, and comparisons were made with the simulations used for the design. Transverse profiles with uniform components were measured with each of the scattering systems and quantified with super-Gaussian fitting. The uniformity of the in-air profiles suggested that the superficial dose contributions from X-rays were low. This study demonstrated that magnified VHEE beam profiles with uniform components could be generated and measured at CLEAR. The results from this study were used as a basis for the design of future experiments. Similar systems and design methods could be employed by future clinical VHEE facilities to provide conformal treatment.

Abstract:

Optical Beam-Loss Monitors (oBLMs) allow for cost-efficient and spatially continuous measurements of beam losses at accelerator facilities. A standard oBLM consists of several tens of metres of optical fibre aligned parallel to a beamline, coupled to photosensors at either or both ends. Using the timing information from loss signals, the loss positions can be reconstructed. This paper presents a novel oBLM system recently deployed at the CERN Linear Electron Accelerator for Research (CLEAR). Multiple methods of extracting timing and position information from measured waveforms with silicon photomultipliers (SiPM) and photomultiplier tubes (PMT) are investigated. For this installation, the optimal approach is determined to be applying a constant fraction discrimination (CFD) on the upstream readout. The position resolution is found to be similar for the tested SiPM and PMT. This work has resulted in the development of a user interface to aid operations by visualising the beam losses and their positions in real time.

Abstract:

Objective. To evaluate spatially fractionated radiation therapy (SFRT) for very-high-energy electrons (VHEEs) delivered with pencil beam scanning. Approach. Radiochromic film was irradiated at the CERN linear electron accelerator for research using 194 MeV electrons with a step-and-shoot technique, moving films within a water tank. Peak-to-valley dose ratios (PVDRs), depths of convergence (PVDR ⩽ 1.1), peak doses, and valley doses assessed SFRT dose distribution quality. A Monte Carlo (MC) model of the pencil beams was developed using TOPAS and applied to a five-beam VHEE SFRT treatment for a canine glioma patient, compared to a clinical 6 MV VMAT plan. The plans were evaluated based on dose-volume histograms, mean dose, and maximum dose to the planning target volume (PTV) and organs at risks (OARs). Main results. Experimental PVDR values were maximized at 15.5 ± 0.1 at 12 mm depth for 5 mm spot spacing. A DOC of 76.5, 70.7, and 56.6 mm was found for 5, 4, and 3 mm beamlet spacings, respectively. MC simulations and experiments showed good agreement, with maximum relative dose differences of 2% in percentage depth dose curves and less than 3% in beam profiles. Simulated PVDR values reached 180 ± 4, potentially achievable with reduced leakage dose. VHEE SFRT plans for the canine glioma patient showed a decrease in mean dose (>16%) to OARs while increasing the PTV mean dose by up to 15%. Lowering beam energy enhanced PTV dose homogeneity and reduced OAR maximum doses. Significance. The presented work demonstrates that pencil beam scanning SFRT with VHEEs could treat deep-seated tumors such as head and neck cancer or lung lesions, though small beam size and leakage dose may limit the achievable PVDR.