Study On Direct Aperture Optimization Algorithm In Heavy Ion Radiotherapy

Comparing with photons or protons in radiation therapy,heavy ion beams have advantages mainly in two aspects.One is physical aspect,the inverse depth dose distribution i.e.Bragg curve.Heavy ion beam can deliver the high dose precisely to the tumor target by modulating the beam energy,besides ensuring less dose to the surrounding normal tissues.The other one is biological aspect,the high relative biological effectiveness(RBE)due to its high linear energy transfer(LET)in Bragg peak region,moreover,heavy ion beams have the similar biological effect to photons in the plateau region for low LET by contrast,resulting it can effective to kill the tumor cells while sparing the normal cells further.Because of the two properties,heavy ion beam is considered as the optimal irradiation in radiotherapy.In convention photon radiotherapy,the inverse planning based intensity modulated radiation therapy(IMRT)is developing rapidly and widely applied in major radiotherapy centers around the world.IMRT can deliver non-uniform irradiation to the tumor target by modulating the beam intensity in different beam directions,in contrast to convention radiotherapy,it has great improvement to plan target volume(PTV)and organ at risk(OAR)and eventually get the ideal dose distribution.In some recent approaches,direct aperture optimization(DAO)algorithm was developed by Professor Shepard,the intensity maps was abandoned and instead directly optimize the shapes and the weights of the MLC segment shapes(apertures)in each beam's eye view(BEV)direction.DAO produced highly efficient treatment deliveries that maintain the full dosimetric benefits of IMRT.However,the genuine IMRT has not yet been achieved in heavy ion beam therapy.Therefore,this study improved DAO model based on passive beam delivery system to develop the genuine IMRT technology in heavy ion radiotherapy.Due to the difference between the photon and the heavy ion beam model,the concept of layer-stacking in each BEV direction and the lateral scattering Gaussian distribution model are proposed.Based on the dose optimizations,the results demonstrate that DAO in heavy ion radiotherapy can produce highly conformal treatment plans using few apertures per beam direction,optimizing the shapes and the weights of the apertures directly.For the ideal concave target(has similar shape to TG119 standard model),we simulated irradiation method in different BEV directions,the result preliminary demonstrated that IMRT technology can be applied in heavy ion radiotherapy based on passive beam delivery system.In the study of spread out Bragg peak(SOBP),RBE value changes with depth based on the radiobiological model.We confirm mini-SOBPs based combinatorial irradiation method to in layer-stacking conformal heavy-ion therapy based on passive beam delivery system.Gaussian shaped mini spread-out Bragg peaks(mini-SOBPs)with different full width at half maximum(FWHM)were generated by different mini ridge filters(mini-RFs).Based on the dose optimizations,the Bragg peak is spread out by both physical absorbed dose and biological effective dose for different monoenergetic heavy ion beams.The mini-SOBPs based combinatorial irradiation method was confirmed to reduce the distal dose fall-off distances of SOBPs while minimizing the layer numbers for layer-stacking conformal heavy-ion therapy,therefore the normal tissues or OAR behind the tumor target was protected effectively.