Results: For phase 1, the IMRT techniques reduced the mean doseto normal supratentorial and infratentorial brain (STB and ITB) by 2and 4 Gy, respectively. Greater high-dose sparing of normal brainwas achieved by increasing treatment complexity with 20% (8e25%)of STB-PTV1 and 34% (30e40%) of ITB-PTV1 spared at 20 Gy withIMRT plans. Mean conformity indices were 2.76 (PP), 2.13 (3F), 2.05(4F), 1.58 (4FIMRT) and 1.38 (7FIMRT) for phase 1. Replacing WBRTby WVRT in phase 1 reduced the total mean dose to STB-PTV2 andITB-PTV2 by 8 Gy but no significant reduction for the pituitary andparotid glands.Conclusions: Replacing WBRT with WVRT has a larger effect onreducing normal-brain dose than WVRT technique selection.7FIMRT offered the best phase 1 high-dose sparing. However,whether this leads to a clinically meaningful reduction in latesequelae remains unverified.

P68 Waiting Times for Radical Radiotherapy in NSCLCR. Muirhead, N. O’RourkeBeatson Oncology Centre, Glasgow, UK

Introduction: In 2000, O’Rourke et al. presented a single-centreprospective audit of 29 NSCLC patients, investigating waiting timesand tumour growth. It demonstrated that of patients who weredeemed suitable for curative treatment, 21% became incurablewhile awaiting radical radiotherapy. In the past five years there hasbeen substantial investment in UK radiotherapy resources andincreasing political emphasis on cancer waiting times. Simulta-neously the evolutionofMDTworkinghas facilitated patient journeysthrough investigation to treatment. We repeated this audit to assessthe improvement over five years in the light of these changes.Method: All NSCLC patients presenting to NOR within 2005, judgedfit enough for radical radiotherapy treatment, were identified.Using the case notes, CT scans and planning CT scans, data werecorrelated.Results: 29 patients were found to be appropriate for radicaloncological treatment. Twenty required immediate radical radio-therapy with either CHART, SOCCAR, or standard radical radio-therapy. The median wait from the first oncology appointment tocommencing radiotherapy was 28.5 days (range 18e45 days). Fourpatients from this group progressed while on the waiting list, 20% ofthis group.Discussion: Although the time to radiotherapy has improved since2000, the number of patients who progress while waiting fortreatment remains at 20%. In spite of increased radiotherapyresources, waiting times for certain categories of patients remaincritical. The proportion of NSCLC patients suitable for radicaltreatment is low, but it is this select group with a chance of longterm survival whose outcome is most at risk on a waiting list.Cancer waiting time targets ignore the differing urgency in treatingdifferent tumour types and prioritise all equally. We contend thatthis approach can only have an adverse effect on the survival ofthose patients with rapidly growing tumours. Solutions to thiscontinuing problem require a new approach to the prioritisation ofradiotherapy waiting lists.

P69 Use of Rectal DSHs to Compare Forward and Inverse IMRTTreatment Planning for Prostate Cancer

O. F. Naismith*, D. P. Dearnaleyy, A. M. Bidmead**Department of Physics, Royal Marsden NHS Foundation Trust,Fulham Road, London, UK; yDepartment of AcademicRadiotherapy, Institute of Cancer Research and Royal MarsdenNHS Foundation Trust, Sutton, Surrey, UK

Introduction: The doseesurface histogram (DSH) provides analternative to the doseevolume histogram (DVH) for estimatingdose to the rectal wall. This study compares rectal surface dosedistributions for forward and inverse IMRT plans for treatingprostate cancer in the CHHIP trial, and contrasts the results witha DVH evaluation. A model linking the DSH and DVH results isinvestigated.

Method: Forward and inverse IMRT plans were created for targetsof both prostate with seminal vesicles (SVs) and prostate-only for10 patients. DSH analysis of the rectum was performed to comparethe two planning methods. The trends identified were comparedfor the prostate-only and prostate+SV plans.Results: The inverse plans achieved superior dose conformality tothe PTVs, reducing dose to the rectum. Rectal DSH curves forinverse plans were significantly lower than for forward plans fordoses R 60 Gy.The DSH results followed similar trends to the DVH analysis.However, the mean differences between forward and inverseplans, evaluated using relative surface areas and relative volumes,differed. At 100% prescribed dose (74 Gy) the difference betweenforward and inverse plans in terms of rectal exposure was greaterusing DSHs than DVHs: the mean relative rectal surface areaexposed to 74 Gy for the 10 forward-planned prostate+SV cases was2.2%, whereas the mean relative volume exposed was 0.6%. Bothwere 0% for inverse plans. At doses from 60 to 70 Gy the differencesbetween forward and inverse plans using relative rectal surfaceareas were lower than the relative volume differences.A theoretical model of DSHs and DVHs, treating the rectum asa cylinder, predicted lower rectal volumes than measured due tothe irregularity of the rectum and the curvature of the isodoseswithin the rectum.Conclusion: Analysis of rectal dose distributions using DSHs andDVHs confirms that, for treatment of the prostate, inverse planningmay reduce the risk of rectal complications for the same degree oflocal tumour control, in particular when treating prostate+SVs.

P70 Redefining Rectal Volume d with Particular Reference toProstate Cancer Radiotherapy

H. E. O’Donnell, P. N. PlowmanDepartment of Radiotherapy, St Bartholomew’s Hospital,West Smithfield, London, UK

With the development of conformal radiotherapy, scrutiny of thedosimetry of normal structures abutting a target volume hasintensified, particularly when new technology allows higherradiation doses to be delivered to tumours with improved cureresults, for example prostate cancer. During prostate radiotherapyplanning, the rectum is defined by its mural perimeter (axial plane)between the dentate line (caudally) and peritoneal reflection(rostrally). Such a definition in ‘avoidance’ planning presupposesthat rectal tolerance depends on ‘whole organ’ radiation tolerance(as does, for example, lung or kidney tolerance). However, rectalmorbidity with modern prostate radiotherapy is determined byanterior rectal wall tolerance d occurring between the rostral andcaudal limits of the PTV that, we would argue, is not dependent onwhole organ tolerance. Particularly in the IMRT era, a redefinitionof the relevant rectal volume is recommended. Dose volumehistograms presented demonstrate the improved usefulness of thenew method appraising the anterior rectal wall dose.

P71 The Simulator d Has it Had its Day? A Study of RadiotherapyVerification

H. E. O’Donnell, M. S. Griffiths, V. A. Harris, M. E. B. PowellDepartment of Radiotherapy, St Bartholomew’s Hospital,West Smithfield, London, UK

Aim: Accuracy and reproducibility are essential in radiotherapy. Inour centre the verification process starts in the simulator andcontinues with electronic portal imaging (EPI) during the course oftreatment. Alteration to the set up during treatment using EPI isprotocol-defined and dependent on accepted tolerances fortreatment site and method of immobilisation and not based ona single image. At initial simulation, however, change to the set upmay be made on this single visit without using such protocols.This prospective study looked at all stages of verification for CTplanned patients to see whether any set up alterations made atsimulation were validated at subsequent verification.

S41CLINICAL ONCOLOGY