ESTRO 36 Abstract Book

S945 ESTRO 36 2017 _______________________________________________________________________________________________

CBCT parameters, patient dose and image quality for both bone and soft tissue. Material and Methods In our institution we currently image this cohort with daily kV imaging and weekly CBCT. A daily ‘shift to zero’ is applied for set-up variations of less than 3mm with residual error evaluated on a weekly basis via a standard Full-Fan, partial arc CBCT template (Table 1-Template 1) The CBCT image quality was evaluated by comparing three progressive dose-reduction trial imaging templates +/- length reduction (Table1-Templates 2, 3, 4) with the standard CBCT template. Initial tests were carried on Catphan ® and Rando ® phantoms to assess image quality and scan artefact limitations. Three consecutive skull-base meningioma patients were then imaged with the four templates on sequential weekly imaging sessions during treatment. Each CBCT was reviewed by an expert group of a clinician, two radiographers and two physicists to evaluate, by consensus, the discernibility of the optic nerves, ventricles, cranial bones, temporalis muscle, and the external contour. In addition, the fidelity of the co- registration to the skull-base anatomy (ROI) was assessed. A standard threshold was applied throughout the investigation.

Results The

basic couch-bending, reproducibility and stability were within 0.10mm within the treatment volume (see Fig.2 a, b). The relative spatial deviation was smaller than 0.40mm for rotations ranging up to 200° (see Fig.2 c). For different vertical positions, the couch drifted less than 0.25mm for 2 different loads and rotations. The differences between the prescribed and measured absolute position were evaluated in terms of histograms showing the overall 3D deviation. In 95% of all measurement points the 3D accuracy was better than 0.63 mm (see Fig.2 d). Regarding the weight-induced couch-bending no correlation between the accuracy and payload could be found. characteristics of

Results A total of 15 CBCT images were acquired and reviewed. All structures were visible for each template except for the ventricles which were assessed as indistinct with templates 3 and 4 (Figure 1). The fidelity of registration was satisfactory for each template.

Conclusion The results show that the important spatial properties of the patient positioning system are well within the acceptable clinical tolerances. The very high reproducibility of the PPS allows further optimization of the absolute position. The measured datasets serve as new input for a high accuracy calibration. EP-1742 Optimisation and implementation of brain CBCT templates; an institutional pilot study. S. Petkar 1 , N. Lalli 2 , F. Solda' 1 , C. Gillies 2 , S. Moinuddin 1 , N. Fersht 1 1 UCLH NHS Foundation Trust, Radiotherapy, London, United Kingdom 2 UCLH NHS Foundation Trust, Radiotherapy Physics, London, United Kingdom Purpose or Objective Volumetric imaging (CBCT) in brain has facilitated the use of volumetric delivery techniques and reduction of uncertainty margins by quantifying bone set-up variation, and, by inference, OAR position. The aim of this work is to optimise the current imaging practice on a Truebeam Linac STx(2.5) for adult brain tumour patients by investigating the relationship between

Conclusion A length-reduction template can be utilised in brain tumours providing the skull-base is included in the CBCT ROI. In cases where the PTV is distant to the skull base, length reduction should be used with care. A concomitant dose reduction is also feasible unless the discernibility of the ventricles is essential, such as when changes in ventricular volume/position are of concern. In our institution we will adopt a new imaging strategy for brain tumour patients employing template 4 for all benign skull based tumours and retaining template 1 (standard) for all other lesions. Further similar work will be carried out to optimise the CBCT protocols in other cohorts of patients (e.g. paediatrics). EP-1743 Evaluation of proton grid therapy in challenging clinical cases T. Henry 1 , A. Valdman 2 , A. Siegbahn 1 1 Stockholm University, Department of Medical Physics, Stockholm, Sweden