Purpose: The Mevion S250i proton system utilizes for energy selection a technique of binary insertion of Lexan plates of various thicknesses, resulting in an energy dependence of virtual source distance (VSD). We present our study on the VSD determination.
Methods: Two methods were used to determine the VSDs: 1) EBT3 film and XRV3000 scintillation camera were used to measure spot size in air of different energy beams at points of -20, -10, 0, 10, 16.5 cm distance from ISO along the beam axis; 2) A Markus chamber was used to measure dose of single spot at the same points. Corrections to the measured doses were made to factors introduced by chamber size and beam shape. A linear fit to spot sigma vs position yields both VSD and beam divergence, and an inverse square law fit to dose also yields the VSD.
Results: For the 11 energies selected from 227 to 28.7 MeV, the spot sigma at ISO and beam divergence increase linearly from 4.0 to 25.6 mm and 0.15 to 3.17 degree, respectively. The VSD measured with the two methods agree within 5%, demonstrating a rapid decrease from 154 to 52 cm when the energy changes from 227 to 184 MeV, followed by only a slight further decrease from 52 to 47 cm when the energy is reduced to below 184 MeV. Incidentally, this seemly transitioning point of 184 MeV coincides with insertion in the beam path of the two most downstream and thickest plates of 3.324 and 6.641 cm WET.
Conclusion: The unique design of the energy selection system of the Mevion S250i has resulted in an unique energy dependence of the VSD, exhibited by a rapid decrease in the higher energy range of 227-184 MeV, and remains virtually unchanged in the lower energies of 184 to 28.7 MeV.