Purpose: A compact Radiotherapy fixed Proton PBS system is utilized using an upright or “seated” robotic patient positioning system with fan-beam CT image guidance and synchrotron as the Proton source. The usage of fixed beam with image and positioning infrastructure at the primary beam path, with low neutrons generation, introduces the possibility to accommodate the total system in a typical Linac vault. We conducted a Monte Carlo simulation to evaluate the annual doses for this setup.
Methods: P-Cure (P-Cure, Shilat, Israel) offers a gantry-less system, for Proton PBS using patient seated position. We retrieved the sketches of a Linac vault in a Rad Onc department with a maze designed to accommodate an 18 MV Linac. The fixed beam was directed in the opposite direction to the door and simulated a 250 MeV Proton beam using Fluka, a Monte Carlo simulation package. Workload was estimated with 30 patients treated daily and number of Protons corresponding of 2Gy per liter delivery. The wall material was concrete with density of 2.35 g/cm3 and the door was Lead with 80 cm width. We simulated a cubic water phantom at isocenter and calculated spatial effective dose distribution using dose conversion coefficients from ICRP Publication 116. A 40X40 cm2 field size at isocenter was conducted using spots having gaussian radial distribution of 0.3 cm FWHM. Total histories allowed a statistical uncertainties of mesh calculated doses below 3% for the primary beam path and door area.
Results: The annual primary dose was found to be less than 2 mSv per year for the primary beam shielding, and less than 1 mSv per year adjacent to the outer vault door.
Conclusion: Preliminary shielding simulation results show feasibility to accommodate a fixed Proton PBS beam with patient upright positioning system in a common photon Linac Vault.