Calculated depth dose distributions for proton beams in some low-Z materials.

The extended use of proton beams in clinical radiotherapy has increased the need to investigate the accuracy of dosimetry for this type of beam. As for photon and electron beams, Monte Carlo simulations are a useful tool in the study of proton dosimetry. The existing proton Monte Carlo code PTRAN developed for dosimetry purposes is designed for transport of protons in homogeneous water only. In clinical proton dosimetry as well as in treatment conditions several other materials can be present, such as plastic phantoms, plastic modulator wheels and several materials in ionization chambers. To investigate the transport of protons in other media we started from the PTRAN code, and implemented proton transport in other materials including heterogeneous systems composed of different materials. With this extended code, calculations of depth dose distributions for some low-Z materials are performed and compared with those obtained for water. The results show that for plastics (PMMA, polystyrene and A150) the depth dose characteristics are comparable to those of water. For graphite, air and aluminium larger differences are observed. The differences between water and the low-Z materials studied here are small but can be important for accurate dosimetry.