Monte Carlo based implementation of an energy modulation system for proton therapy

A certain numbers of radiation therapy techniques, requires that the treatment setup, varies in time during the dose delivery. This can be the case of the proton therapy where an uniform dose can be obtained using the movement of special element during the patient irradiation. Hadron therapy is a special radiotherapeutic treatment based on the use of high energy proton beams. Thanks to their specific interaction with matter, hadrons release all their energy at the end of their paths (Bragg peak). The Bragg peak permits to irradiate selectively tumors sparing the surrounding normal tissues. The full width half maximum of a typical Bragg peak is usually of the order of 1 mm: completely insufficiently to cover a tumor region (of the order of 20 mm for the case of the eye melanoma). In order to form an homogeneous dose distribution within the tumor volume, individual pristine beams have to be added up. Most proton therapy facilities are currently using a broad beam modulation technique where a spread-out Bragg peak (SOBP) is generated by a set of absorbers having different thickness. Technical solutions are ridge filters or modulator wheels ensuring temporal variation of the beam energy. In our previous work, using the GEANT4 Monte Carlo package, we simulated all the elements of the CATANA proton therapy facility (G.A.P. Cirrone et al.). CATANA is the first and actually unique proton therapy center in Italy. Now we show the possibility to use the GEANT4 toolkit also to design and simulate the time-dependent geometry of the modulator wheel. Simulated results are reported and compared with the experimental ones. The implemented solution for the time-dependent geometry, can represents an useful example for the GEANT4 users that want use this feasibility in a different environment.