Objective To study the dose distribution characteristics formobile intra-operative radiotherapy accelerator (Mobetron) in an operating room,and to provide basic data for developing appropriate radiation protection measures and protection standard.Methods For most commonly used electron energy 9 MeV,TLD dosimeters were placed at 50,100,150 and 200 cm high plane,respectively.For each plane,the measurement points were selected at every 50 cm from the central axis at every 45° at eight different directions.Also different electron energies,such as 4,6,9 and 12 MeV,were taken into consideration at the plane at 100 cm height.After 10 Gy with a dose rate of 10 Gy/min were delivered,the TLD dosimeters were used to read out the data.Results For 9 MeV,at the phantom plane(100 cm high plane),the average doses were 169,756,395 and 241 μSv at 50,100,150 and 200 cm from the central axis,respectively.Themaximum deviation between the doses at 50 cm from the central axis in different angles and their average values were 9.1％.In the identical angle,the average doses of 50,100,150 and 200 cm high planes at the distance of 100 cm from central axis were 527,756,570 and 141 μSv,respecitvely.For the energies of 4,6,9 and 12 MeV,the average doses were 573,486,689 and 781 μSv at 100 cm from the central axis at 90° of 100 cm high plane.Conclusions For the same energy,the dose values at different directions were decreased by the minus exponential function law with the distance.The doses were uniformly distributed at different directions at the same distance from the central axis.The doses on the plane of 100 cm height were much higher than those at other heights,and the dose values were increased with the election energy.

Objective To study the calculation of the room shielding thickness of tomotherapy accelerator,a new type of radiotherapy facility,especially the impact of the beam block on the shielding design.Methods According to the relevant standards,combined with the room geometry,the shielding thickness was calculated without the presence of the beam block,considering the primary beam,the scattered beam and leakage.Meanwhile,the shielding thickness was also calculated as comparison with the presence of the beam block,based on the characteristics of tomotherapy facility and its radiation field.Results There was statistical difference between the shielding thicknesses calculated with the presence of the beam block and those without the beam block,to the primary beam direction including the south wall,north wall,the roof and the floor,the shielding thickness were decreased by 95.59%,63.63% ,80.73%and 51.30% ,respectively.Conclusions For the tomotherapy accelerator,the beam block could be of great help to minify the shielding thickness of the room.The radiation field of the tomotherapy facility could be used for the calculation to improve accuracy,and the shielding thickness can also be estimated by subtracting the initial shielding thickness without beam block of the beam block equivalent thickness in the primary beam direction alternatively.

Objective To provide evidence for a more reasonable method of determining red bone marrow dose by analyzing and comparing existing simulation methods.Methods By utilizing Monte Carlo simulation software MCNPX,the absorbed doses of red hone marrow of Rensselaer Polytechnic Institute (RPI)adult female voxel phantom were calculated throush 4 different methods:direct energy deposition.dose response function(DRF),King-Spiers factor method and mass-energy absorption coefficient (MEAC).The radiation sources were defined as infinite plate.sources with the energy ranging from 20 keV to 10 MeV.and 23 sources with different energies were simulated in total.The source was placed right next to the front of the RPI model to achieve a homogeneous anteroposterior radiation scenario.The results of different simulated photon energy sources through different methods were compared.Results When the photon energy was lower than 100 key,the direct energy deposition method gave the highest result while the MEAC and King-Spiers factor methods showed more reasonable results.When the photon energy was higher than 150 keV taking into account of the higher absorption ability of red bone marrow at highcr photon energy,the result of the King-Spiers factor method was larger than those of other methods.Conclusions The King-Spiers factor method might be the most reasonable method to estimate the red bone marrow dose from external radiation.