P76 Evaluation of Margining Algorithms in Commercial Treatment Planning Systems

CLINICAL ONCOLOGY Introduction: When used in a consistent manner, in-vivo dosimetry enhances existing QA programs and helps identify set-up and planning errors. We have recently commissioned a semiconductor diode system for use in routine photon beam in-vivo dosimetry. Method: The first phase of commissioning included sensitivity calibration and checks on intrinsic precision, leakage, dose linearity and field perturbation at depth. The second phase included the determination of correction factors to be applied in order to allow for variations in source-to-skin distance (SSD), field size and the angle of incidence of the beam with respect to the diode. Results: Sensitivity calibration was carried out under reference conditions (10  10 cm2 field size at 100 cm SSD, with the diode dosimetry reference point at dmax). The diodes were found to exhibit negligible leakage and repeatability of measurement within a standard deviation of 0.1%. The diode reading was linear with dose. Field perturbation at 5 cm depth was measured to be 7.3% and 6.0% at 6 MV and 10 MV, respectively. Significant correction factors arise from the angle of incidence, particularly in the axial direction, measuring e6.2% at 6 MV and e4.9% at 10 MV for an angle of 70(. Temperature correction factors are also significant and are based on the manufacturer’s measurements of 0.4%/(C. Less significant are the SSD and field size correction factors, being within 2% and 1.5%, respectively. The diodes are used clinically for measuring photon entrance doses. All in-vivo dosimetry results obtained so far have been within 8% of expected entrance doses, without correction factors. With correction factors, the noted percentage difference is significantly smaller, in some cases achieving 0.5% agreement. Conclusion: The diodes can be used in two ways: (1) without correction, allowing the identification of gross errors (typically O10%) early in treatment; (2) with extensive correction, allowing ‘near-absolute’ in-vivo dosimetry (typically !2%) for longer-term audit purposes. Ongoing clinical use will allow assessment of the stability of the system and determination of recommendations relating to calibration frequency and overall tolerance margins. P76 Evaluation of Margining Algorithms in Commercial Treatment Planning Systems A. M. Pooler*, H. M. Mayles*, O. F. Naismithy, D. P. Dearnaley on behalf of CHHIP collaboratorsz *Physics Department, Clatterbridge Centre for Oncology NHS Foundation Trust, Bebington, Wirral, UK; yDepartment of Physics, Royal Marsden NHS Foundation Trust, Fulham Road, London, UK; zDepartment of Academic Radiotherapy, Institute of Cancer Research and Royal Marsden NHS Foundation Trust, Sutton, Surrey, UK Introduction: During commissioning, the margining algorithm of the Philips Pinnacle TPS was investigated and was found to produce larger volumes than Plato for identical starting volumes. Subsequent comparison of the results of a QA outlining exercise for the CHHIP (Conventional or Hypofractionated High Dose IMRT for Prostate Cancer) trial confirmed that margining algorithms for different planning systems in the UK produced significantly different results. We investigate the clinical impact of the differing resulting PTVs and suggest a test to establish self-consistency. A recommendation is made to reduce inconsistencies. Methods: Margining algorithms were tested for consistency by drawing an octahedron on 5 mm CT slices. It was margined once by 50 mm and five times by 10 mm. The two GTVs in the CHHIP QA data sets were margined using five TPSs to produce PTV3 (prostate + 5 mm except 0 mm post), PTV2 (PTV3 + 5 mm) and PTV1 (prostate + SVs + 10 mm). GTVs and PTVs were imported into one system and the volumes measured. Plans created using the smallest PTVs were recalculated on the largest PTV data set and vice versa. Results: Consistency was best with Nucletron Plato (0.4% difference) and worst for Eclipse (e15%) and Pinnacle (+15%). These inconsistencies were reduced to 5% by adjusting the supero-infero margins.

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The SD of the imported GTV volumes was !0.5%. However, PTV1 and PTV3 had a SD of 6% and PTV2 (margined twice) of 11%. The Varian Eclipse PTV2 was 25% smaller than Pinnacle’s. When the Eclipse plan was recalculated on the Pinnacle structures the minimum PTV2 dose dropped by 5%. When the reverse was done there was a 6% increase in rectal volume treated to 50 Gy. Conclusion: In the context of a national trial requiring consistent plans, differences in margining algorithms become significant. A simple consistency check can be applied and margins adjusted to ensure volumes are equivalent.

P77 Time for a Change: Is it Time to Consider Job Redesign for Therapy Radiographers? H. Probst*, S. Griffithsy *Sheffield Hallam University, Sheffield, UK; yLeeds Teaching Hospitals NHS Trust, Leeds, UK Background: High therapist vacancy rates and an unsatisfied workforce reduces the opportunity to meet waiting time targets or maintain high standards of care. Current vacancy rates reported by the Department of Health for therapy radiographers are double those for nurses, midwives and other allied health professions. The purpose of this presentation is to propose the case for job redesign to enhance therapists’ job satisfaction and commitment to the organisation with the ultimate goal of enhancing performance and service quality. Method: A prospective case study of job satisfaction utilising an interpretive research design was conducted across three radiotherapy departments in England. Individual interviews were held with a sample of therapy radiographers across a range of grades, including specialist and generalists posts, and a range of time in post (n ¼ 18). Grounded theory was used to analyse the transcribed interviews and sampling continued until no new themes emerged from the interviews. Results: Job design, leadership and burnout were the main themes identified as significant to job satisfaction. Focussing on the theme of 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 and evidence from the wider literature on job satisfaction to propose the case for some consideration of job redesign. Using wellvalidated models from the literature and the results of this phase I study, it will be argued that in terms of job design it is necessary to ensure skill variety through regular job rotations in some roles with attention to the range of activities therapy radiographers undertake. Allowing sufficient time for education and training opportunities should enhance the scope for mental challenge, limiting the chances of stagnation with subsequent effects on job satisfaction and retention. P78 Digitally Reconstructed Radiographs: A New Phantom for Verifying Accuracy and Optimising Quality A. J. Reilly*, A. S. MacLeod*, D. I. Thwaitesy *Oncology Physics, Edinburgh Cancer Centre (ECC), Western General Hospital, Edinburgh, UK; yRadiotherapy Physics, Cookridge Hospital, Leeds, UK Introduction: A new phantom has been developed for verifying the geometrical accuracy of digitally reconstructed radiographers (DRRs) and performing a fully objective and quantitative assessment of their image quality. Methods: The phantom consists of tungsten spheres embedded in a tissue-equivalent resin. For any beam geometry the expected locations of the spheres in DRRs are calculated and compared against those in actual DRRs. Any number of scenarios can therefore be investigated from a single CT scan. For image quality