Introduction: When used in a consistent manner, in-vivo dosimetryenhances existing QA programs and helps identify set-up andplanning errors. We have recently commissioned a semiconductordiode system for use in routine photon beam in-vivo dosimetry.Method: The first phase of commissioning included sensitivitycalibration and checks on intrinsic precision, leakage, doselinearity and field perturbation at depth. The second phaseincluded the determination of correction factors to be applied inorder to allow for variations in source-to-skin distance (SSD), fieldsize and the angle of incidence of the beam with respect to thediode.Results: Sensitivity calibration was carried out under referenceconditions (10 � 10 cm2 field size at 100 cm SSD, with the diodedosimetry reference point at dmax). The diodes were found toexhibit negligible leakage and repeatability of measurement withina standard deviation of 0.1%. The diode reading was linear withdose. Field perturbation at 5 cm depth was measured to be 7.3%and 6.0% at 6 MV and 10 MV, respectively. Significant correctionfactors arise from the angle of incidence, particularly in the axialdirection, measuring e6.2% at 6 MV and e4.9% at 10 MV for an angleof 70(. Temperature correction factors are also significant and arebased on the manufacturer’s measurements of 0.4%/(C. Lesssignificant are the SSD and field size correction factors, beingwithin �2% and �1.5%, respectively.The diodes are used clinically for measuring photon entrancedoses. All in-vivo dosimetry results obtained so far have beenwithin �8% of expected entrance doses, without correctionfactors. With correction factors, the noted percentage differenceis significantly smaller, in some cases achieving �0.5% agreement.Conclusion: The diodes can be used in two ways: (1) withoutcorrection, allowing the identification of gross errors (typicallyO10%) early in treatment; (2) with extensive correction, allowing‘near-absolute’ in-vivo dosimetry (typically !2%) for longer-termaudit purposes.Ongoing clinical use will allow assessment of the stability of thesystem and determination of recommendations relating to calibra-tion frequency and overall tolerance margins.

P76 Evaluation of Margining Algorithms in CommercialTreatment Planning Systems

A. M. Pooler*, H. M. Mayles*, O. F. Naismithy, D. P. Dearnaleyon behalf of CHHIP collaboratorsz*Physics Department, Clatterbridge Centre for Oncology NHSFoundation Trust, Bebington, Wirral, UK; yDepartment ofPhysics, Royal Marsden NHS Foundation Trust, Fulham Road,London, UK; zDepartment of Academic Radiotherapy, Instituteof Cancer Research and Royal Marsden NHS Foundation Trust,Sutton, Surrey, UK

Introduction: During commissioning, the margining algorithm of thePhilips Pinnacle TPS was investigated and was found to producelarger volumes than Plato for identical starting volumes. Sub-sequent comparison of the results of a QA outlining exercise for theCHHIP (Conventional or Hypofractionated High Dose IMRT forProstate Cancer) trial confirmed that margining algorithms fordifferent planning systems in the UK produced significantlydifferent results. We investigate the clinical impact of the differingresulting PTVs and suggest a test to establish self-consistency.A recommendation is made to reduce inconsistencies.Methods: Margining algorithms were tested for consistency bydrawing an octahedron on 5 mm CT slices. It was margined once by50 mm and five times by 10 mm.The two GTVs in the CHHIP QA data sets were margined using fiveTPSs to produce PTV3 (prostate + 5 mm except 0 mm post), PTV2(PTV3 + 5 mm) and PTV1 (prostate + SVs + 10 mm). GTVs and PTVswere imported into one system and the volumes measured. Planscreated using the smallest PTVs were recalculated on the largestPTV data set and vice versa.Results: Consistency was best with Nucletron Plato (0.4% difference)and worst for Eclipse (e15%) and Pinnacle (+15%). These incon-sistencies were reduced to 5%byadjusting the supero-infero margins.

The SD of the imported GTV volumes was !0.5%. However, PTV1and PTV3 had a SD of 6% and PTV2 (margined twice) of 11%. TheVarian Eclipse PTV2 was 25% smaller than Pinnacle’s. Whenthe Eclipse plan was recalculated on the Pinnacle structures theminimum PTV2 dose dropped by 5%. When the reverse was donethere was a 6% increase in rectal volume treated to 50 Gy.Conclusion: In the context of a national trial requiring consistentplans, differences in margining algorithms become significant. Asimple consistency check can be applied and margins adjusted toensure volumes are equivalent.

P77 Time for a Change: Is it Time to Consider Job Redesign forTherapy Radiographers?

H. Probst*, S. Griffithsy*Sheffield Hallam University, Sheffield, UK; yLeeds TeachingHospitals NHS Trust, Leeds, UK

Background: High therapist vacancy rates and an unsatisfiedworkforce reduces the opportunity to meet waiting time targetsor maintain high standards of care. Current vacancy rates reportedby the Department of Health for therapy radiographers are doublethose for nurses, midwives and other allied health professions.The purpose of this presentation is to propose the case for jobredesign to enhance therapists’ job satisfaction and commitmentto the organisation with the ultimate goal of enhancing perfor-mance and service quality.Method: A prospective case study of job satisfaction utilising aninterpretive research design was conducted across three radio-therapy departments in England. Individual interviews were heldwith a sample of therapy radiographers across a range of grades,including specialist and generalists posts, and a range of time inpost (n¼ 18). Grounded theory was used to analyse the transcribedinterviews and sampling continued until no new themes emergedfrom the interviews.Results: Job design, leadership and burnout were the main themesidentified as significant to job satisfaction. Focussing on the themeof job design, a number of subthemes were evident including:Monotony/repetitiveness of treatment delivery role;Lack of career development opportunities;Opportunity for specialisation;Opportunity for continuing professional development;Changes to the role over time.Conclusions: This presentation will utilise the study findings andevidence from the wider literature on job satisfaction to proposethe case for some consideration of job redesign. Using well-validated models from the literature and the results of this phase Istudy, it will be argued that in terms of job design it is necessary toensure skill variety through regular job rotations in some roles withattention to the range of activities therapy radiographers un-dertake. Allowing sufficient time for education and trainingopportunities should enhance the scope for mental challenge,limiting the chances of stagnation with subsequent effects on jobsatisfaction and retention.

P78 Digitally Reconstructed Radiographs: A New Phantom forVerifying Accuracy and Optimising Quality

A. J. Reilly*, A. S. MacLeod*, D. I. Thwaitesy*Oncology Physics, Edinburgh Cancer Centre (ECC), WesternGeneral Hospital, Edinburgh, UK; yRadiotherapy Physics,Cookridge Hospital, Leeds, UK

Introduction: A new phantom has been developed for verifying thegeometrical accuracy of digitally reconstructed radiographers(DRRs) and performing a fully objective and quantitative assess-ment of their image quality.Methods: The phantom consists of tungsten spheres embedded ina tissue-equivalent resin. For any beam geometry the expectedlocations of the spheres in DRRs are calculated and comparedagainst those in actual DRRs. Any number of scenarios cantherefore be investigated from a single CT scan. For image quality

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assessment, the modulation transfer function (MTF) and normal-ised noise power spectrum (NNPS) are determined by imageprocessing. Software automatically identifies the positions of theball bearings, compares these against those expected and performsthe image quality analysis. The phantom was used to assess the twovirtual-simulation packages used in ECC (Somavision/Eclipse andAdvantage Sim). The influence of slice thickness and low- and high-dose imaging protocols was investigated.Results: In all geometrical scenarios, both packages performedcorrectly. Spatial resolution normal to the scan plane improved asslice thickness decreased. However, for very narrow slices therewas a corresponding degradation in the in-plane direction,suggesting that the narrowest thickness may not be the mostappropriate. For the ECC CT simulator the optimum thickness wasbetween 2 and 3 mm, yielding f50¼ 0.17 cycles/mm for bothsystems in either direction. NNPS results were significantlydifferent between systems, with Advantage Sim DRRs beingconsiderably more susceptible to deterministic artefacts. DRRnoise levels were insensitive to protocol dose level.Conclusion: This phantom provides an efficient, robust means ofrigorously assessing DRR geometrical accuracy and image quality.Both virtual simulation systems performed similarly in terms ofspatial resolution response, but differences in processing algo-rithms were discovered by the NNPS evaluation. DRRs are generallyinsensitive to scanning protocol dose level and the narrowestavailable slice thickness may not always be the most appropriate.

P79 Multiple Method Verification of the Positional Accuracy ofLinac Based Stereotactic Radiosurgery and Radiotherapy

A. Richmond, J. Pearn, A. Horsley, T. Treloar, S. Kelly,S. Pascoe, P. C. Whitfield, J. D. PalmerSouth West Radiosurgery Centre, Plymouth Neurosciences,Derriford Hospital, Plymouth, Devon, UK

Introduction: Certainty about positional accuracy is of paramountimportance when delivering stereotactic radiosurgery and radio-therapy. Movement of the BrainLabTM stereotactic frame orthermoplastic mask between placement or moulding and treat-ment needs to be quantified. Our objective was to assess accuracyusing CT verification, portal imaging and a physical measurementtechnique employing a depth helmet, RadionicsTM.Methods: Thirty-one patients were studied. 16 underwent SRS witha stereotactic frame localisation, 6 underwent SRS with thermo-plastic mask localisation and 9 had several fractions of SRT usinga thermoplastic mask for localisation. CT verification was assessedby means of a repeat CT on the first day of treatment anda comparison of anatomical and artificial markers. Daily portalimages were compared to digitally reconstructed radiographsgenerated from the localising/planning CT. Depth helmet measure-ments were performed immediately after placement of the frame/mask and again immediately before each fraction of treatment. Alltreatments were delivered using the BrainLabTM m3 microleafcollimator system. In all cases paired or multiple measurements ofthe displacements were performed and 3D vectors were calculatedfor all three techniques.Results: From all methods of measurement the frame had anoverall 3D displacement vector of 0.5 mm with a standard deviationof 0.1 mm. The mask system was 1.3 mm with a standard deviationof 0.6 mm.Conclusions: Each of the measurement techniques has limitations.The invasive headframe provides a more accurate method offixation than the thermoplastic mask system and should be usedwhen dealing with tumour volumes close to eloquent structures.

P80 Image Guided Radiotherapy (IGRT) in Prostate Carcinoma:Visualisation of Implanted Fiducial Markers

Y. L. Rimmer*, R. J. Benson*, D. S. Routsis*, J. Fairfouly,A. C. F. Hooley, M. Akhtarz, N. G. Burnetx*Oncology Centre, Addenbrooke’s Hospital, Cambridge, UK;yDepartment of Medical Physics, Addenbrooke’s Hospital,

Cambridge, UK; zDepartment of Urology, Addenbrooke’sHospital, Cambridge, UK; xDepartment of Oncology,University of Cambridge, Cambridge, UK

Introduction: IGRT using the Acculoc� localisation system for thetreatment of prostate carcinoma has been implemented in ourcentre. Three gold fiducial markers are implanted into the prostategland, the localisation software compares the planning CT co-ordinates of the markers and the isocentre with the daily ontreatment marker co-ordinates, calculating the couch translationvector for patient realignment. This requires no linac modification,can be used with standard electronic portal imaging devices andthe imaging exposure is incorporated into the treatment plan. Thesuccess of this technique is, however, absolutely dependent uponthe ease of visualisation of the fiducial markers.Methods: 12 patients have been recruited to date. A dedicatedstudy urologist implants the intra-prostatic markers under localanaesthesia as an out-patient procedure. At planning the isocentreis placed to achieve good spatial separation from the fiducialmarkers. A reticule projects a ball and cross image indicative of thecentral beam axis for isocentre localisation. An orthogonal pair ofportal images (PI) is acquired before and after couch adjustmentusing amorphous silicon flat panels.Results: Colour coding of individual markers on the referencedigitally reconstructed radiographs aids speed of identification fordaily matching. Initially recognition of 3 markers was possible in allanterior images but only in 25% of lateral images. Improved markervisualisation was achieved by both altering the reticule design andincreasing the lateral PI exposure. The central ball on the reticulewas removed to project only the lower density crosswire, and thelateral PI exposure was increased from 3 MU to 5 MU andsubsequently 10 MU, resulting in acceptable image quality in allcases.Conclusion: Increasing the imaging exposure results in improvedvisibility of the fiducial markers, integral to the success of this IGRTsystem. Investigation of image quality and PI dose in individualpatients is in progress.

P81 Practical Issues in Implementing IGRT using ImplantedMarkers in Prostate Radiotherapy

D. S. Routsis*, Y. L. Rimmer*, R. J. Benson*, J. Fairfouly,A. C. F. Hooley, J. Treeby*, J. Dean*, N. Muskett*, M. Akhtarz,N. G. Burnetx*Oncology Centre, Addenbrooke’s Hospital, Cambridge, UK;yDepartment of Medical Physics, Addenbrooke’s Hospital,Cambridge, UK; zDepartment of Urology, Addenbrooke’sHospital, Cambridge, UK; xDepartment of Oncology,University of Cambridge, Cambridge, UK

Introduction: Image guided radiotherapy (IGRT) aims to removeuncertainties in radiotherapy delivery but can be a complex andcostly system to implement. One simple method of IGRT that canbe implemented in all UK departments uses existing portal imagingsystems in conjunction with implanted markers. This paperdiscusses the practical issues raised when implementing thissystem into a busy clinical department.Methods: 3 gold markers are implanted into patients requiringprostatic radiotherapy. Immediately prior to each treatment,images are obtained using treatment portals and exposures only.The Acculoc� software matches marker locations to those plannedand performs an automated on-line 3D analysis, determining fieldplacement errors and corrective movements required. The feasi-bility of this system relies on the cost, acceptability and accuracyof marker insertion, the accuracy and reliability of identifying andmatching markers and the speed of the image matching andanalysis process.Results: Many issues were highlighted that hindered easy assimi-lation into clinical practice, but solutions were identified:Implantation in theatre would increase cost and complexity andneeded to be avoided. Effective co-ordination and collaborationbetween urologists, oncologist, radiographers and nurses resulted

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