  To expedite the commissioning process of the proton therapy system at Samsung
Medical Center (SMC), we have developed a Monte Carlo simulation model of the
proton therapy nozzles using TOPAS. At SMC proton therapy center, we have two
gantry rooms with different types of nozzles; a multi-purpose nozzle and a
dedicated scanning nozzle. Each nozzle has been modeled in detail following the
geometry information provided by the manufacturer, Sumitomo Heavy Industries,
Ltd. For this purpose, novel features of TOPAS, such as the time feature or the
ridge filter class, have been used. And the appropriate physics models for
proton nozzle simulation were defined. Dosimetric properties, like percent
depth dose curve, spread-out Bragg peak (SOBP), beam spot size, have been
simulated and verified against measured beam data. Beyond the Monte Carlo
nozzle modeling, we have developed an interface between TOPAS and the treatment
planning system (TPS), RayStation. An exported RT plan data from the TPS has
been interpreted by the interface and then translated into the TOPAS input
text. The developed Monte Carlo nozzle model can be used to estimate non-beam
performance of the nozzles such as the neutron background. Furthermore, the
nozzle model can be used to study mechanical optimization in the design of the
nozzle.
