Measurement of neutron ambient dose equivalent in carbon-ion radiotherapy with active scanned beam

In ion beam radiotherapy, out-of-field dose due to secondary neutrons as well as leakage charged particles is considerably lower than in-field dose, but it may be not negligible to estimate the secondary cancer risk especially for the young patients. In National Institute of Radiological Sciences (NIRS), out-of-field dose in carbon-ion radiotherapy with passive beam is continuously investigated. Also, in the previous paper, we showed that active scanned beam can reduce the out-of-field dose compared to passive beam by Monte Carlo simulation.[1] In this study, neutron ambient dose equivalents in carbon-ion radiotherapy with active beam were measured with a rem meter at the new treatment research facility in NIRS where the treatment with active beam was started in May 2011, and then were compared to those with passive beam.[2] Measurements were performed with WENDI-II for 290 and 400 MeV/u carbon beams. A water phantom was placed to simulate a patient at the isocenter (I.C.). The center of WENDI-II was set to be at the I.C. height and on the I.C. line orthogonal to the beam axis. The measured position represented by the distance between the I.C. and the center of WENDI-II, d, was varied from 50 to 200 cm. When the target volume is the same, the neutron dose with active beam was about 15% of that with passive beam at a maximum. This decrease was larger as the position became farther from the I.C. The neutron dose at d=200 cm with active beam was only 4% of that with passive beam. In conclusion, we experimentally demonstrated that the use of active scanning beam in carbon ion radiotherapy can greatly reduce the secondary neutron dose which contributes most to out-of-field dose.NEUDOS1