Study On The Novel Spot-scanning Beam Delivery Method Using Pencil Beams Of Flexibly-regulated Parameters In Heavy Ion Radiotherapy

Compared with conventional photon radiationtherapy,heavy ion radiotherapy has the advantages in both of physics and biology: heavy ion has a unique inversed depth dose distribution(Bragg peak);the energy transfer line density(linear energy transfer,LET)and the relative biological effectiveness(RBE)are higher in Bragg peak region and smaller in plateau region.Therefore,heavy ion beam can protect the normal tissue cells when killing tumor cells to achieve precise radiotherapy.Currently the beam delivery system that used in heavy ion radiationtherapy are passive beam delivery system and active beam delivery system.Based on the active beam delivery,there are spot scanning and raster scanning methods.For the active spot scan beam delivery,laterally-and longitudinally-big spot size pencil beams are unfavorable for three-dimensional conformal of the tumor target volum,whereas laterally-and longitudinally-small spot size pencil beams will greatly increase the number of scanning points and extend the dose optimization time and need more precisely control the heavy ion irradiation parameters.In order to solve the contradiction,this paper proposes a combined spot scanning irradiation method which uses the pencil beams with laterally-and longitudinally-mixed spot size: using laterallyand longitudinally-small spot size pencil beams to irradiate the inner tumor target,and using laterally-and longitudinally-big spot size pencil beams to irradiate to the edge of the tumor target volum.For switching flexibly the above two kind pencil beam,we put forward the passive medthod to adjust pencil beam parameters: titling the mini-ridge filter along the beam direction to adjust the mini-SOBP longitudinally and inserting the different thickness scatterers under the downstream of the vaccum window to change the pencil beam size laterally.Through the SHIELDHIT-HIT12 A Monte Carlo simulation of active spot scanning beam delivery system of the deep tumor treatment terminal at HIRFL-CSR,it is found that the shorter is the beam delivery distance,the smaller is the beam spot FWHM,but the worse is the dose uniformity;mini-ridge filter structure period is a key factor affecting the uniformity of dose and the smaller is the cycle,the better is the dose flatness.Therefore,2 mm structure period for a mini ridge filter statisfies the requirements on the FWHM of beam spot and dose flatness at the isocenter for the active spot scanning beam delivery system at HIRFL when the distance between the vacuum window and the isocenter is set shorter than 125 cm in the nozzle.Based on the optimized heavy ion active spot scanning beam delivery system,a carbon ion pencil beam database of spot scanning irradiation method was established in this paper.The range-energy relationship of carbon ion pencil beam,the depth distribution of physical absorbed dose and the depth distribution of beam spot sigma were obtained by Monte Carlo tool SHIELDHIT12 A simulation and MATLAB fitting.According to the LQ model,the relative biological effects of carbon ion pencil beam were calculated by using the dose averaged LET and LET dependence on ?,? when HSG cell survival rate was 0.1.The depeh distribution of biological effective dose of carbon ion pencil beam was calculated by the distribution of carbon ion pencil beam physical absorption dose and the relative biological effect.In this paper,the heavy ion active spot-scanning radiotherapy with laterally-and longitudinally-mixed spot size pencil beams was established by Monte Carlo simulation and theoretical calculation.In this method,the switching between laterallyand longitudinally-big spot size pencil beams and laterally-and longitudinally-samll spot size pencil beams by tilting the mini-ridge filter 60 degrees and by inserting different scatterers with different thicknesses.Compared with the spot-scanning radiotherapy using a single big size pencil beam spot-scanning radiotherapy and a single size pencil beam,the heavy ion active spot-scanning radiotherapy with laterallyand longitudinally-mixed spot size pencil beams has these following advantages: smaller dose penumbra and smaller dose distal fall-off distance,easy controlling of irradiation parameters,good dose uniformity,short treatment time and so on.