Experimental validation of coarse ridge filters for FLASH proton therapy

To maximize the potential benefit of the FLASH sparing effect during treatment, normal tissue regions would ideally be irradiated only briefly, typically for a couple of hundred milliseconds. Achieving such fast proton irradiation involves a mono-energetic beam at the highest cyclotron energy and the use of 3D-printed conformal energy modulators (CEM). In ConformalFLASH, a dedicated snout is mounted on the nozzle, containing the CEM, a range shifter, and an aperture.

Demonstrate that ConformalFLASH irradiation using a coarse 3D-printed CEM, defined by a geometry with spike resolution > 0.5 mm in any dimension, is fulfilling existing clinical dose standards. The CEM is robust to printing errors and can be reliably manufactured with unmodified commercially available 3D printers.

Monte-Carlo simulations were conducted to define the 3D-printing specifications of the CEM. A variety of CEMs were then printed according to specifications. CT scans were acquired, and in-beam measurements were performed for each part, using the FLASH beam properties, the FLASH snout, and dosimetry detectors.

Considering the proposed ConformaFLASH setup choice, it was possible to design coarse CEM that are both robust and easily printable using commercial technology. Over several measured cases, the 3D-printed CEM yields clinical-grade proton dose distributions. This confirms the irradiation set-up and the CEM manufacturing specifications as predefined through Monte Carlo simulations. CEM CT scans reinforce further the dosimetric results, to provide additional evidence of 3D printing quality.

The dose distribution obtained through carefully specified CEM proves robust to production errors typically occurring in commercial 3D printing. The robustness opens the way to simplified manufacturing of relatively complex parts. Owing to the beam configuration, the snout, and the range shifter, the CEM was able to generate clinical-quality dose distributions. The integration of the FLASH snout with its elements on the nozzle of the proton therapy system represents an important step forward in comparison to existing state-of-the-art, facilitating easier preclinical and future clinical trial investigations.