Abstracts / Clinical Oncology 23 (2011) S1eS58S18

Results: Fifty-five centres across the UK were identified, and overall 91% (50/55) of centres responded.NSCLC: The most common radical fractionation schedule used is 55 Gy in 20fractions over 4 weeks (38 centres). Other frequently used regimens include66 Gy in 33 fractions (nine centres), 64 Gy in 32 fractions (eight centres) andCHART. 14/50 (28%) of UK centres offer CHART. Stereotactic body radio-therapy (SBRT) is offered in 13 (26%) centres. Thirteen centres (26%) useIMRT. Twenty-one centres use IGRT techniques. The majority use cone beamCT to achieve this. All 50 centres offer sequential chemoradiation: 46 givethis routinely and four stated it was used rarely. Regarding concurrentchemoradiation: 36 centres offer this; two of these just rarely (72% in total).The majority of centres use the combination cisplatin and vinorelbinechemotherapy for concurrent chemoradiation (69%).SCLC: 42/50 (84%) centres use concomitant chemoradiotherapy in SCLC; 25Gy in 10 fractions was the regimen offered at themost centres (70%) for PCI inlimited stage disease. The most common schedule was 20 Gy in five fractions(54% of centres) for extensive stage disease.RT planning: 45 (90%) of centres are using PET-CT scans. 27/50 (54%) areusing IV contrast for planning CT scans. Thirty-seven centres (72%) areactively recruiting to at least one of four identified NCRN lung cancer trials.Conclusion: There is significant variation in lung cancer radiotherapyimplementation across the UK. New technologies are being embraced, anduptake to clinical trials needs to be further encouraged.

18 The Relationship between PTV Margin, Imaging Frequency andConfidence of Target Coverage, for Prostate, and Head and NeckRadiotherapy

H.J. Chantler, R.J. Benson, N.G. Burnet, J.C. Dean, S.J. Jefferies,Y.L. Rimmer, S.G. Russell, S.J. Thomas, G.S. TudorAddenbrooke's Hospital, Cambridge, UK

Introduction: The use of online correctional imaging for radiotherapyimproves the accuracy of treatment delivery. A variation in imagingfrequency will affect the necessary PTVmargin for a given confidence level ofCTV coverage.Method: Patient set-up correction data have been collected for both prostateand head and neck patients being treated on our TomoTherapy machines.The relationship between imaging strategies, confidence levels of targetcoverage and CTVePTVmargins (for a 95% dose level) has been analysed. Thepositional correction data for the imaging strategies of first day (I1), average offirst 3 days (I3), average of first 5 days (I5), alternate days (IAD), alternateweeks(IAW) and each day imaging (IED) were evaluated. Prostate intra-fractionmotion data were available from a previous study, and were included in theanalysis; a residual error has also been included for both sets of results.Results:PTVmarginsneed tobe increasedas imaging frequency is reduced fromdaily to less frequent imaging. For the prostate patients analysed, to maintaina confidence level of 90%, the ant/post PTV margin increases from 4.6 mm forIED to 13.0 mm for I1 in incremental steps of 1.0 mm, 0.5 mm, 2.6 mm, 1.6 mmand2.6mmfor the transition in imaging strategies IED to IAW to IAD to I5 to I3 to I1,respectively. For the head and neck, the ant/post PTV margin for IED is 2.8 mm,increasing to 5.8mm for I1 in incremental steps of 0.3mm, 0.1mm,1.0mm, 0.2mm and 1.4 mm, for the same changes in imaging strategy.Conclusion: This analysis provides a quantitative basis from which thedepartment can evaluate the size of the PTVmargin, for a specified confidencelevel, for a given imaging strategy; by imaging less often, either a larger PTVmargin is needed, or a lower probability of treating the CTVmust be accepted.

19 Megavoltage CT from Electronic Portal Imaging of VMAT Deliveries

J.S. Wood, T.E. Marchant, G.J. BudgellNorth Western Medical Physics, The Christie NHS Foundation Trust,Manchester, UK

Introduction: Volumetric modulated arc therapy (VMAT) is becoming popularas a delivery method for IMRT. Continuous radiotherapy delivery duringa 360� gantry rotation has obvious parallels with CT imaging. This workinvestigated whether it is possible to acquire megavoltage cone-beam CT(MV-CBCT) images fromVMAT deliveries. If it is possible, the images could beused to verify patient position in the treatment positionwithout the need foradditional radiation dose to the patient.

Methods: A 20� 20 cm open-field irradiation and two VMAT treatment plans(one prostate and one head and neck) were delivered to a RANDO� phantom,while electronic portal images were acquired in movie mode. Gantry anglesfor each image were derived from data-logging software connected to thelinac. A kV-CBCT image of the phantom was acquired at the same time. Theimages were reconstructed using CBCT software and registered with the kV-CBCT image in Pinnacle3. The spatial and contrast resolutions of theMV-CBCTimages were measured using a Catphan.Results: Measurement of image quality using the Catphan showed that toresolve contrast between different materials requires a 15e20% electrondensity difference; the spatial resolution was measured to be 2.5 mm.Differences in registration parameters for the open-field and prostate VMATMV-CBCT fusions with the kV-CBCT imagewere<1.5mm. These are less thanthe spatial resolution of the MV-CBCT images, suggesting that MV-CBCTimages acquired during VMAT delivery could be used for patient positionverification. However, no anatomical features could be identified in the MV-CBCT reconstruction of the head and neck VMAT delivery and so registrationwith a kV-CBCT image could not be performed.Conclusion: It is possible to reconstructaMV-CBCT image fromprojection imagesacquired during VMAT and to use this to verify patient position. However, thesuccess of the technique is dependent on the individual VMAT plan.

20 Image Guided Radiotherapy: Performance of Cone Beam CT Auto-matic Image Registration of the Prostate

J.R. Sykes *, S.Brettle D. *, D. Magee y, D.I. Thwaites *

*Medical Physics and Engineering, Leeds Teaching Hospitals Trust, Leeds,UKyComputer Vision Group, School of Computing, University of Leeds, Leeds,UK

Introduction: Image registration is a source of uncertainty in image guidedradiotherapy that needs to be quantified. In this work the performance ofmasked, soft tissue, automatic image registration of the prostate with CT andcone beam CT (CBCT) images was measured and its relationship withreduced imaging dose investigated.Methods: An anthropomorphic pelvis phantom (CIRS) was CT scanned andused as a reference for image guided radiotherapy. Seven CBCT scans wereperformed using the Elekta Synergy system with nominal imaging dosesranging between 1 mGy and 40 mGy. Rigid body, image registration wasrepeated 100 times with randomly selected start positions typical of normalprostate set-up errors. Image registration was performed using the ‘ElektaCorrelation Ratio’ algorithmwith the CT data masked to the prostate with a 5mm isotropic margin. Residual error analysis was performed to determinethe accuracy, precision and robustness of image registration. Rigid bodyerrors were analysed in terms of target registration error (TRE), the averageerror between any two corresponding points on the surface of a 5 cm spherecentred on the isocentre. A similar method was performed with 21 CBCTscans from seven patients.Results: The TRE was stable for imaging doses above 6 mGy. Median (TRE)was less than 0.2 mm and the registration failure frequency (RFF, TRE> 3.6mm)was less than 5%. For doses below 6mGy the robustness deteriorated. At1 mGy the median (TRE) was 0.8 mmwith a systematic error of 2.2 mm andRFF¼ 10%. Image registration performance with the patient images washighly variable with four out of 21 CT-CBCT image registrations showingmedian (TRE)< 1 mm and RFF< 20%. For the rest, median (TRE) was as largeas 9 mm and RFF between 20% and 90%.Conclusion: A clear dose response relationship was evident for CT-CBCTimage registration performance of the prostate in phantom measurements.Performance with patient images was highly variable.

21 Late Toxicity and Outcome at 7 Years in Patients Treated within theHIPRO Study

S. Merrick, D. Thomson, R. Swindell, J. Coote, J. Wylie, R. Cowan,T. Elliott, J. Logue, J. Livsey, A. ChoudhuryThe Christie NHS Foundation Trust, Manchester, UK

Introduction: Dose-escalated radiotherapy for localised prostate cancerimproves disease control but at the expense of increased overall treatment