CALCULATED DEPTH-DOSE DISTRIBUTIONS FOR PROTON-BEAMS IN SOME LOW-Z MATERIALS

The extended use of proton beams in clinical radiotherapy has increase d the need to investigate the accuracy of dosimetry for this type of b eam. As for photon and electron beams, Monte Carlo simulations are a u seful tool in the study of proton dosimetry. The existing proton Monte Carlo code PTRAN developed for dosimetry purposes is designed for tra nsport of protons in homogeneous water only. In clinical proton dosime try as well as in treatment conditions several other materials can be present, such as plastic phantoms, plastic modulator wheels and severa l materials in ionization chambers. To investigate the transport of pr otons in other media we started from the PTRAN code, and implemented p roton transport in other materials including heterogeneous systems com posed of different materials. With this extended code, calculations of depth dose distributions for some low-Z materials are performed and c ompared with those obtained for water. The results show that for plast ics (PMMA, polystyrene and A150) the depth dose characteristics are co mparable to those of water. For graphite, air and aluminium larger dif ferences are observed. The differences between water and the low-Z mat erials studied here are small but can be important for accurate dosime try.