ESTRO 36 Abstract Book

S481 ESTRO 36 _______________________________________________________________________________________________

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%).

permits computation of clinically meaningful differences in context, but differences in spinal cord dose mandating ART should be a rare event. PO-0880 Using accumulated delivered dose to predict rectal toxicity in prostate radiotherapy L.E.A. Shelley 1,2,3 , J.E. Scaife 1,4 , A.M. Bates 1,4 , J.R. Forman 1,5 , K. Harrison 1,6 , R. Jena 1,4 , D.J. Noble 1,4 , M.A. Parker 1,6 , M.R. Romanchikova 1,3 , M.P.F. Sutcliffe 1,2 , S.J. Thomas 1,3 , N.G. Burnet 1,4 1 Cambridge University Hospitals NHS Foundation Trust, Cancer Research UK VoxTox Research Group, Cambridge, United Kingdom 2 University of Cambridge, Department of Engineering, Cambridge, United Kingdom 3 Cambridge University Hospitals NHS Foundation Trust, Department of Medical Physics and Clinical Engineering, Cambridge, United Kingdom 4 Cambridge University Hospitals NHS Foundation Trust, Department of Oncology, Cambridge, United Kingdo 5 Cambridge University Hospitals NHS Foundation Trust, Cambridge Clinical Trials Unit, Cambridge, United Kingdom 6 University of Cambridge, Department of Physics- Cavendish Laboratory, Cambridge, United Kingdom Purpose or Objective Dose-volume tolerances for organs at risk (OARs) adopted during radiotherapy planning have been historically derived from normal tissue complication probability (NTCP) models linking toxicity with planned dose. On-treatment image guidance facilitates daily tumour localisation ensuring target coverage. However, the positional variation of neighbouring OARs is often disregarded. Anatomical deviations from the pre- treatment CT due to interfraction motion can introduce discrepancies between the planned and delivered dose. One objective of the VoxTox research programme is to test the hypothesis that delivered radiation dose can be a stronger predictor of toxicity than planned dose. Material and Methods For 109 prostate cancer patients treated with TomoTherapy® (74Gy/37#), daily megavoltage CT scans were acquired. An in-house autocontouring algorithm determines the rectal position, incorporating the effect of displacement and deformation, and an independent dose calculation is performed. Processing is fully automated within the VoxTox study. Dose surface maps (DSMs) of the rectal wall were generated following the virtual cutting and unfolding method of Buettner et al [ Phys. Med. Biol , 54, 21 (2009)], allowing conservation of spatial dose information (Figure 1). Daily delivered DSMs were summated to produce an accumulated DSM (Figure 1b) over the whole treatment. Planned and accumulated DSMs were parametrised by calculating 1) the equivalent uniform dose (EUD) and 2) the ‘DSM dose-width’, the lateral extent of an ellipse fitted to the largest isodose cluster, for 7 discrete dose levels between 30 and 75 Gy.

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