Comparative simulations of neutron dose in soft tissue and phantom materials for proton and carbon ion therapy with actively scanned beams

Purpose: In the clinical environment phantom materials are usually used to simulate the patient for neutron dosimetric measurements. It is not clear that the results of such phantom measurements represent the actual neutron dose in the patient. The aim of this study was to compare the difference in secondary neutron equivalent dose for different phantom materials to that in human tissue, for both proton and carbon ion radiation therapy. Methods: In order to compare the neutron equivalent dose induced by primary particles in different materials, Monte Carlo simulations were performed using the FLUKA Monte Carlo package. The scored dosimetric quantities were absorbed dose and neutron ambient dose equivalent for monoenergetic proton and carbon ion beams of clinically relevant energies. It was shown that neutron equivalent dose, for which no scoring routine exists in the current FLUKA release, can be approximated by neutron ambient dose equivalent within 4% for the investigated energies of proton and carbon ion beams. Results: The Monte Carlo simulations performed in this work showed differences in neutron ambient dose equivalent in radiation therapy phantom materials compared to ICRP soft tissue for primary proton and carbon ion beams. For Alderson soft tissue the maximum deviation was 11% for protons and 8% for carbon ions. For water the maximum deviation was 10% for protons and 9% for carbon ions. In the case of RW3 solid water, the maximum deviation compared to ICRP soft tissue was as large as 28% and 21% for protons and carbon ions, respectively. Conclusions: Alderson soft tissue and water are suitable phantom materials for neutron dosimetry for the accuracy which is achievable. When using solid water phantoms, the chemical and therefore nuclear composition of the phantom material has to be accounted for. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3584196]