March 2013 - Institute of Physics and Engineering in Medicine

IGRT QA DEVELOPMENTS FEATUREIGRT QA: current andfuture developmentsDavid Eaton, James Earley, Daniel Emmens, Becky Lindsay, Tom Marchantand Jonathan Sykes update us with new developments in IGRT QA‘ UPDATESSome newdevelopments inIGRT QA havetaken place.A clinicalmanufacturerhas withdrawnfrom the linacmarket, andthere are newreports out andin progress.In a previous article we described the rapidclinical implementation of in-room x-ray basedimage guidance systems, and the wide variationfound in a UK survey of commissioning andquality assurance (QA). 1 Since then there havebeen a number of developments.Firstly, the only clinical manufacturer of MV-CBCTand kV-CT (CT-on-rails) systems has withdrawn fromthe linac market, so these devices are unlikely to gainwidespread use. Conversely, it seems likely that kV-CBCT (e.g. Varian OBI and Elekta Synergy) and MV-CT (e.g. Tomotherapy) systems will achieve almostcomplete market penetration by the end of the currentlinac replacement cycle.New reportsSecondly, the AAPM published Report TG-179 on QAfor IGRT using CT-based technologies. 2 This containedanother extensive description of the available systemsand expert literature review of clinical applications,followed by general recommendations on areas ofgeometric accuracy, image quality, dose, registrationand operational accuracy. These guidelines are avaluable addition to the literature and we recommendtheir use to the medical physics community in the UKand further afield. However, we believe that morepractical and specific guidance would be beneficial, inparticular to establish consistent reporting of imagequality and dose measurements. Protocols shouldideally be specific enough to allow comparison ofdifferent centres, but general enough to be applied todifferent types of IGRT equipment.Thirdly, NCAT/NRIG produced a report on clinicalimplementation of IGRT, 3 reiterating and updating the2008 report, On Target. The report forms part of aconcerted effort to support all radiotherapy centres todeliver appropriate levels of IGRT for every patientwithin 12 months. National radiography leads havebeen appointed to assist and advise each centre, alongwith a remote physics support service, and threedepartments have been established as training hubs.Finally, QA resources are limited in mostdepartments and the advent of various newtechnologies has increased the quantity of tests to apotentially unmanageable level. It is thereforeimperative that tests performed reflect the clinical useof the system. Appropriate tolerances and frequencieswill also depend on the impact of potential errors, andbalance mitigation of risk with the practical resourcesavailable. Approaches based on local implementationof national benchmarks may be more sustainable andeffective in preventing errors. 4Guidance from IPEMTwo IPEM reports are currently in preparation, on thedosimetry of wide beam CT scanners and an update toReport 81 on QA in radiotherapy, with a new chapteron IGRT systems. These will provide further practicalguidance on individual tests, frequencies andtolerances.Collaboration between diagnostic andtherapeutic physicists is invaluable in IGRT, inparticular in the areas of dose measurement andoptimisation of imaging protocols. Questions remainon whether it is useful to set diagnostic reference levels(DRLs) with typical doses or worst case limits, or tocombine concomitant and therapeutic doses togetheron an organ-by-organ basis.Approaches based on localimplementation of nationalbenchmarks may be more“sustainable and effective“All users can contribute to future developments byaccruing data on long-term stability of equipment, byparticipating in inter-centre audits and credentialingfor new trials, and by sharing experience across the fullrange of available systems and practices. Only togethercan we ensure that accurate, focussed and safe deliveryof technologically advanced radiotherapy is given toevery patient, every day.Further comments from the working partyGeometric accuracyThe three-dimensional relationship between imagingand treatment isocentres is the foundation on which allimage matching and correction depends. Differentmethods to account for component sag during gantryrotation are available, such as ‘flexmap’ generation, buta quick daily check should be performed in all centresto identify gross errors in alignment (>2 mm). ‰26 | MARCH 2013 | SCOPE