Secondary particles like neutrons, protons and heavier ions produced in light ion therapeutic beams contribute to the dose delivered to tumor and healthy tissues outside the treated volume. These particles are characterized by a wide range of LET (Linear Energy Transfer) and can be a source of significant additional doses to the critical organs of the patient. Production of neutrons and secondary protons in therapeutic ion beams requires special concern since they possess high energies and are easily transported long distances through the patient and can generate damage in healthy tissues. As a consequence, this damage can result in the occurrence of secondary tumors. These issues are especially critical for paediatric patients since their tissues are still in rapid development and a curative treatment may result in very long survival times.
The aim of the present work is to select ion type in order to optimize the therapeutic outcome and minimize secondary doses to normal tissue.
Due to the very complex interaction pathways of high energy and heavy charged ions transported in the patient, 3-D Monte Carlo (MC) particle transport codes provide a unique and very useful tool in the prediction of the physical radiation doses to organs.
In this work calculations of absorbed dose delivered to the treatment volume and to the patient’s organs exposed only to secondary particles, produced in proton and heavy charged ion beams, were performed with the MC code SHIELD-HIT. SHIELD-HIT simulates the interactions of hadrons and atomic nuclei of arbitrary mass number (Z, A) with complex extended targets. The simplified mathematical anthropomorphical phantoms EVA (female), ADAM (male) and a child phantom, based on the MIRD geometry, were applied in the SHIELD-HIT calculations for ion beams from protons up to oxygen. Calculations were also performed for homogeneous water and A-150 cylindrical phantoms. The incident ion beam was simulated as a quasi-monoenergetic beam with an energy spread, E, and a Gaussian spatial distribution. The studies were performed for parallel monoenergetic beams and for a more clinically relevant case with a spread out Bragg peak (SOBP).
For proton beam of energy 200 MeV, the neutron absorbed dose delivered to the water and A-150 phantoms is 0.6 and 0.65 % of the total dose, respectively. For 12C beam, these contributions are 1.0 % and 1.2 %.
The absorbed dose due to secondary neutrons produced by 1H(200 MeV) and 12C (390 MeV/u) beams in tissue-equivalent phantoms is relatively small and in the range 0.6–1.2 % of the total dose.