Author: Michael M Folkerts, Miriam Krieger, Pierre Lansonneur, Gerard Paquet, Petar Penchev, Christoph Schuy, Yuri Simeonov, Uli Weber, Klemens Zink 👨🔬
Affiliation: GSI Helmholtzzentrum fuer Schwerionenforschung, Institute of Medical Physics and Radiation Protection (IMPS), University of Applied Sciences, Varian Medical Systems 🌍
Purpose:
Ultra-high dose-rate proton irradiation may spare the healthy tissue (FLASH-effect). The 3D-range-modulator (3D-RM) method reduces the treatment time compared to conventional IMPT and can enable FLASH irradiation, while placing the Bragg-Peak of only one single energy in the target volume (FLASH-IMPT). This work used the Eclipse Treatment Planning System (Varian) to create a multi-field range-shifter (RaShi) IMPT plan and convert it to a modulator-pin geometry. The resulting dose distribution was verified with Monte Carlo simulations and dose measurements.
Methods:
A research version of Eclipse, capable of creating variable-thickness RaShi IMPT plans, was used to optimize a two-field (right and posterior direction) IMPT plan for a liver target retrospectively. The plan utilized the maximum available energy of 250 MeV and a thick 20 cm PMMA pre-absorber. Additionally, weight optimization on an irregular target-specific spot grid was conducted to achieve both optimal dose distribution and the maximum monitor units per scan-spot. An aperture was tailored to the liver contour to limit the extra scattering from the pre-absorber.
The RaShi layers and scan-spot weights of each IMPT field were converted to a 3D-modulator geometry. The dose distribution, resulting from both modulators, was simulated on the patient CT and in a water phantom with the FLUKA Monte Carlo package. The modulators were manufactured on a high-quality 3D-printer and verified with high-resolution dose measurements in a water phantom.
Results:
The cumulative dose distribution from both 3D-RM fields simulated on the patient CT shows good conformity and homogeneity. There is excellent agreement between the measured and simulated dose distribution of field 2 with a 3D Gamma-Index passing-rate (2%/2 mm, local) of 99%.
Conclusion:
A complete workflow was demonstrated, including the design, Monte Carlo simulation and dosimetric verification of a 3D-RM. 3D-RMs can create and reproduce complex IMPT dose distributions with a high degree of conformity.