Investigation of megavoltage local tomography for detecting setup errors in radiation therapy

InvestigationofMegaoltageLo calTomographyforDetectingSetupErrorsinRadiationTherapyDaxin Shia,MarkA. AnastasioXiao chuan Panb, CharlesPelizzaricandeterMunrodaDepartmentof Biomedical Engineering, Illinois Instituteof TechnologyChicago, IL60616Departmentof RadiologybandRadiation andCellular Oncologyc,TheUniversityof ChicagoChicago, IL60637dVarian Medical Systems,Ginzton TechnologyCenterMountainView,CA94043ABSTRACTWeinvestigatetheproblemofreconstructinga3DimagetumorolumefromsettruncatedMVcone-beampro jections.Our prop osed approachisdistinctfrompreviously investigated approaches inthatitutilizes alo cal tomography reconstruction algorithm.Using simulated and exp erimental MV pro jection data, we demonstratethat a lo cal cone-b eam tomography algorithm can reconstruct accurate images that contain information regardingb oundaries and edges inside a lo calized region of interest.We also demonstrate that the conventional Feldkamp-Davis-Kress cone-b eam reconstruction algorithm is not well-suited for reconstructing images of low-contrast structures fromtruncated cone-b eam pro jections.Keywords:Megavoltage computed tomography, lo cal tomograph, cone-b eam tomograph, radiation therap1.INTRODUCTIONConformal radiation therapy remains to b e one of the most imp ortant pro cedures for treating cancer.The goal ofconformal radiation therapy istodeliver a high radiation dose to the target volume that contains the cancerous tissue,while minimizing the radiation exp osure of healthy tissues that surround this volume.It has b een demonstrated thatconformal radiation therapy can increase the probability of tumor control, which can increase the chances of survivalfor cancer patients.An optimized conformal radiation treatmentplanisdevelop ed using a three-dimensional (3D)mo del of the patient anatomy that is based up on a pre-treatment computed tomography (CT) scan.When deliveringthetreatment,itiscriticallyimp ortantthatp ositionandorientationofpatien(ormoresp eci cally,tumor volume and radiation sensitive structures that surround it) are consistent with those assumed when designingthe treatment plan.If there are errors in the p ositioning of the patient, the e ectiveness of the treatmentmaybedegraded and a large radiation dose may b e delivered to healthy tissues surrounding the target volume.In practice,cliniciansoftenattempttodetectpatientp ositioningerrorsbyuseofelectronicp ortalimagingdevices(EPIDs)that acquire digital radiographs of the target anatomy during irradiations with the treatment b eam.However, thesedigital radiographs cannot typically b e used to resolve soft tissue structures that have similar densities.A natural and more e ective approach for detecting patient setup errors would b e to use the radiation therapytreatmentmachineitselfasatomographicimagingmo dality.Inmegavoltage(MV)CTthisisaccomplishedbsimultaneously rotating the treatment source and EPID around the patient in analogy to the op eration of a third-generation diagnosticCTscanner.Thisp ermitsfortheacquisitionofasetMVtomographic pro jections fromwhich a transverse slice or volume of the patient anatomy can b e reconstructed.Because the tomographic image isformed using the therapy machine with the patient in the treatment p osition, the reconstructed image is automaticallyregistered withthebeamco ordinate system.Thereconstructedimage canbevisuallyinsp ectedor automaticallyregistered with existing 3D anatomical mo dels to detect alignment or p ositioning errors.Inthevastma jorityofpreviousstudiesMVCT,1{6imagingtaskwastoreconstructanimageentire cross section of the patient anatomy, rather than just a lo calized region containing the tumor volume.Exceptincaseswhereitiscriticaltoverifythep ositionofaradiation sensitivestructurethatresides outsidetumorFurther author information:Send corresp ondence to Mark A. Anastasio, Ph.D.:E-mail:anastasio@iit.edu