Experimental set-up. A 30 x 30 x 30 cm3 RW3 solid water cube was irradiated with a 10 x 10 cm2 field with a nominal energy of 16 MV at a source surface distance of 85 cm. The radiation beam was placed such that the central ray was 5 cm from the phantom border (isocenter was located at 15 cm depth). The PADC detectors were placed horizontally 5 cm from the edge of the phantom. The numbers in the boxes indicate the ratio of neutron dose equivalent from the IMRT field relative to the open field for the same absorbed dose at position D2.