  In a cyclotron-based proton therapy facility, the energy changes are
performed by means of a degrader of variable thickness. The interaction of the
proton beam with the degrader creates energy tails and increases the beam
emittance. A precise model of the degraded beam properties is important not
only to better understand the performance of a facility already in operation,
but also to support the development of new proton therapy concepts. The exact
knowledge of the degraded beam properties, in terms of energy spectrum and
transverse phase space, depends on the model used to describe the proton
interaction with the degrader material. In this work the model of a graphite
degrader has been developed with four Monte Carlo codes: three conventional
Monte Carlo codes (FLUKA, GEANT4 and MCNPX) and the multi-purpose particle
tracking code OPAL equipped with a simplified Monte Carlo routine. From the
comparison between the different codes, we can deduce how the accuracy of the
degrader model influences the precision of the beam dynamics model of a
possible transport line downstream of the degrader.
