ESTRO 36 Abstract Book

S476 ESTRO 36 2017 _______________________________________________________________________________________________

transformation parameters, we created ‘voxel histories’ for the spinal cord relative to the planning CT, and calculated delivered dose. Maximum planned and delivered spinal cord dose (D 2% ) were then compared. Results A summary of auto-contouring algorithm performance is shown in Table 1. Auto-contouring performance appeared comparable to manual segmentation, and we proceeded to calculate delivered dose. These results are shown in Figure 1 (A-C). Fig. 1A shows a waterfall plot of planned D 2% minus delivered D 2% for each patient. Mean spinal cord D 2% was 35.96Gy (planned) and 36.01Gy (delivered), and the mean absolute difference between planned and delivered dose was 1.1Gy (3% of mean planned D 2% ). Differences between planned and delivered dose were plotted as a histogram, which appears to be normally distributed around the mean difference (Fig 1B). The mean difference (µ, -0.05) and standard deviation (σ, 1.448) were used to approximate a normal distribution to this data – as shown in Fig.1C. Using this model, a z statistic can be calculated for a chosen difference (e.g. Prob. of delivered D 2% being 4Gy higher than planned is 2.5%).

Conclusion The use of plan adaptations to correct patient setup errors was experimentally validated on the MRL for the first time. The reference dose distribution was reproduced at the shifted location for rectum cancer patients. In addition, high gamma pass rates were measured. Acknowledgements: The autors like to thank Robert Spaninks (Elekta). PO-0879 Differences between planned and delivered maximum spinal cord dose in Head &Neck cancer patients D. Noble 1,2 , P. Yeap 3 , K. Harrison 3 , S. Thomas 1,4 , M. Parker 3 , N. Burnet 1,2 1 VoxTox Research Group - University of Cambridge., Oncology, Cambridge, United Kingdom 2 Addenbrooke's Hospital - Oncology Centre University of Cambridge, Oncology, Cambridge, United Kingdom 3 VoxTox Research Group - University of Cambridge., Cavendish Laboratory- Department of High Energy Physics, Cambridge, United Kingdo 4 Addenbrooke's Hospital - Oncology Centre University of Cambridge, Medical Physics, Cambridge, United Kingdom Purpose or Objective Adaptive radiotherapy (ART) for head and neck cancer remains resource intensive, and there is little consensus on which patients will benefit most from having it done. Concerns regarding maximum spinal cord dose sometimes trigger re-planning at our centre, and we sought to compute and model differences between planned and delivered maximum dose to the spinal cord in patients undergoing IMRT with daily image-guidance (IG) on the We drew planning kVCT, IG MVCT and planned dose datasets from archive for 33 patients who were treated for head & neck cancer (HNC) on TomoTherapy units at our centre. All patients underwent daily IG, with matching to high dose PTV (close to the spinal cord), or cervical spine vertebrae. To automatically contour the spinal cord, we developed an intensity based deformable image registration (DIR) algorithm using the open source Elastix toolkit to propagate manual contours from the planning CT. Using ‘gold standard’ contours of an expert observer, the algorithm was optimised on 30 MVCT datasets (567 slices) and validated on a further 90 (2203). Conformity was measured with Jaccard conformity index (JCI) and distance between centres (DBC), and compared with results from intra- and inter-observer studies. Using in-house dose recalculation software (CheckTomo), TomoTherapy sinograms, MVCT datasets and algorithm TomoTherapy system. Material and Methods

Conclusion Differences between planned and delivered D 2% to the spinal cord in patients receiving daily IG are small in HNC patients treated with daily IG on TomoTherapy. Our model