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Description
In clinical operation, boron carbide (B4C) wedges are used as energy degraders to vary the proton beam energy between 238 MeV and 70 MeV. Each energy step is completed within 50 ms, corre-sponding to a change in water-equivalent penetration depth of approximately 5 mm. Upstream of the degrader, the beam has a diameter of about 2 mm. Passage through the boron carbide mate-rial introduces scattering, resulting in a beam diameter of roughly 8 mm at the patient.
Boron carbide is one of the hardest known materials, which presents significant challenges in ma-chining and fabrication. Recent advancements in manufacturing techniques have enabled the pro-duction of complex geometries with the precision required for clinical particle therapy. To achieve the necessary dimensional accuracy and surface quality, several design and manufacturing chal-lenges had to be addressed. Computed tomography (CT), typically used for medical diagnostics, was applied to detect internal defects such as cavities, allowing the rejection of faulty components. Heat treatment was implemented to relieve residual stresses that could lead to cracking, and thermal cycling experiments were carried out to optimize cooling strategies, ensuring sufficient heat dissipation during operation. These measures collectively contribute to the reliable perfor-mance of boron carbide wedges in demanding clinical environments.
The implementation of boron carbide (B4C) as an energy degrader material in proton therapy beamlines has been the result of nearly a decade of development and testing. Initial simulations in 2014 and feasibility experiments in 2015 indicated a substantial transmission gain compared to graphite, with early measurements showing improvements of up to 37%. However, subsequent tests in 2019 revealed unexpectedly poor performance at lower energies, raising doubts about the project’s continuation. A detailed analysis later identified design limitations in the wedge geometry as the root cause, necessitating a redesign. Progress was further delayed by global supply chain disruptions, but renewed beamline tests in 2022 demonstrated energy dependent transmission gains of 25–50%, confirming the earlier predictions. After temporary setbacks in 2023, the defini-tive installation of the B4C degrader was successfully completed in June 2023, with all required measurements validating its performance. The new degrader consistently delivers at least 30% higher transmission at 70 MeV to the treatment isocenter while maintaining beam quality parame-ters compatible with clinical operation. This achievement shortens irradiation times at OPTIS2 by approximately 30%, marking a significant advancement in proton therapy beamline technology and concluding a long term collaborative effort.