Measurement of the secondary neutron dose generated during proton beam therapy for craniospinal irradiation

We measured the secondary neutron dose from proton craniospinal irradiaton and assessed ways to reduce the secondary neutron dose. A polymethyl methacrylate phantom was irradiated with 159.5-MeV proton beams, and CR-39 etch detectors were used to measure the neutron dose equivalent to the proton absorbed dose. We compared the neutron doses generated in cases with and without the neutron absorbing blocks. The neutron dose equivalent to the proton dose (H(10)/D) absorbed on the surface of the phantom showed a decrease from 7.24 to 4.76 mSv/Gy with a change in the displacement from the primary field edge and a decrease from 7.24 to 1.06 mSv/Gy with a change in the depth at a 5-cm displacement from the field edge. When 4.32-mm-thick neutron absorbing blocks were added, the neutron dose decreased by about 30%. We conclude that ratio of the neutron dose to the proton absorbed dose for a proton craniospinal irradiation treatment beam on the surface is less than 1% of the prescription dose. We also observed a 30% decrease in the neutron dose on adding 4.32-mm-thick neutron-absorbing blocks. The contribution of secondary neutrons to the integral dose was found to be very low, and it can be reduced further by using high hydrogen-boron containing blocks.