The geometry of the Scanditronix MM50 (see fig.8) at the Memorial Sloan Kettering Cancer Center consists of an exit vacuum window, a thin scattering foil, a beam monitor ion chamber, a mirror, a primary collimator and a multileaf collimator which is set to a square for these calculations. The accelerator head is filled with helium gas. The MM50 uses both beam scanning and a very thin scattering foil to give a uniform beam distribution. The beam sizes simulated are larger than for the lower-energy beams from other accelerators to ensure broad-beam conditions. Unlike other accelerators, this machine does not have electron applicators at these beam energies. The beam is shaped by the primary collimator and the multileaf collimator. The scan pattern was set for a circle of 18 cm radius according to the machine specifications.
Figures 36, 37 and 38 show the energy spectra of 25, 40 and 50 MeV beams incident on the phantom surface and inside the treatment field.
The energy spectra from this accelerator are very narrow and close to monoenergetic because of two factors: the scanned beam and the large air space between last part of the beam defining system and the phantom surface. This air gap means many of the electrons scattered from the beam defining system leave the field.
Note that in the central-axis depth-dose curves the contaminant photons in the beam have a negligible contribution to the bremsstrahlung tails.
Figure 39 presents calculated dose profiles at various depths in a water phantom for the 25 MeV and the 50 MeV beams from the Scanditronix Medical Microtron 50. The beams are very flat across the field at various depths in a water phantom.