ESTRO 36 Abstract Book

S480 ESTRO 36 _______________________________________________________________________________________________

Material and Methods The planning CT and delineated structures of four rectum cancer patients were selected. For each patient, a MRL treatment plan was generated with Monaco using a 7- beam IMRT technique (25 x 2.0 Gy) including all MRL- specific properties (7MV, 1.5 T magnetic field, collimator 90°, FFF, SAD: 143.5 cm). Patient setup errors of 1.0 cm and 2.0 cm in the CC and LR directions were simulated by shifting the planning CT with respect to the isocenter position. For each setup error, the initial plan was adapted by first adjusting the leaves of each segment to approximate the shift and second re-optimize the weight of each segment. Also, a reference plan was generated by adapting the initial plan with a 0.0 cm shift, as the second phase of plan adaptation was observed to introduce dose changes even for a 0.0 cm shift. All plans were rescaled (PTV V 95% = 99%). The reference and adapted plans were irradiated on the MRL on a slab phantom with a 2D detector array (PTW Octavius 1500 MR ) inserted parallel to the couch at the center position of the PTV. For each plan, the phantom position was changed according to the introduced shift. Patient setup errors in the AP direction cannot be evaluated using this measurement setup. The measured 2D dose distribution of the reference plan was rigidly registered to the measured 2D dose distribution of the adapted plans in order to assess the positional accuracy of the simple dose shift. After alignment, the similarity between the 2D dose distributions of the reference plan and the adapted plans was evaluated using a 3%/3mm γ analysis (local dose, 20% low dose threshold). Results For all adapted plans, the measured positional accuracy was within 0.1 cm. The γ analysis between the dose distributions of the reference plan and the adapted plans resulted in an average pass rate (γ≤1) of 96.2% (range: 83.3% – 99.9%). Smaller values of γ mean were observed for dose shifts in the CC direction compared to the LR direction as well as for 1.0 cm dose shifts compared to 2.0 cm dose shifts (Table 1). Figure 1 shows an example of a 2D γ distribution. High γ values are measured in the low dose area mainly. A simple dose shift to correct for a 1.0 cm setup error in the CC direction resulted in limited dose differences. Various γ hotspots were observed for the 2.0 cm setup error in the LR direction.

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