ESTRO 36 Abstract Book

S959 ESTRO 36 _______________________________________________________________________________________________

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 CBCT parameters, patient dose and image quality for both 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. bone and soft tissue. Material and Methods

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 2 Karolinska Institutet, Department of Oncology and Pathology, Stockholm, Sweden Purpose or Objective For several decades unidirectional photon-grid therapy has been a useful tool in radiation oncology. Its main advantage is to limit the normal tissue toxicity when irradiating the patients with bulky tumors. In this work we use proton grid therapy (PGT). PGT delivered with a crossfiring technique has been used instead of a unidirectional approach. The physical properties of proton beams allow for the protection of risk organs posterior to the target while the crossfiring technique enables a larger separation between the beams, thus better preserving the normal tissue. Here we evaluate the possibility to use PGT as a therapeutic option in certain clinical situations. For example, due to the ability of interlaced proton-beam grids to significantly spare normal tissue, this technique may be useful in re-irradiation cases not otherwise eligible for radiotherapy treatment because of too high doses to organs at risk. Material and Methods CT data from patients previously treated with conventional photon therapy at Karolinska Hospital, Stockholm, were reused in order to create PGT treatment plans with the TPS Eclipse (Varian Medical Systems). Patients that could benefit re-irradiations or palliative care were selected. The aim was to deliver a high and nearly homogeneous target dose, while keeping the grid pattern of the dose distribution, made of peak and valley doses, as close to the target as possible. A low grid dose, with low peak and valley doses, was also preferable to better protect the normal tissue. The dosimetric characteristics of those plans were then evaluated, with a focus on the overall homogeneity of the target dose, as well as dose profiles outside of the target (i.e. evaluation of the grid dose distribution through peak and valley doses analysis). Results All the studied cases presented dose distributions for which the grid pattern was preserved until the direct neighborhood of the targets. When normalizing the minimum target dose to 100%, the valley doses reached around 5%, while the peak doses were approximately 60- 70%, depending on the grid geometry used. Inside the targets, a good dose homogeneity could be achieved (σ= ±10 %). The volumes of organs at risk irradiated with high doses remained small and limited spatially to the dose peaks of the grids. Conclusion PGT produces a combination of nearly homogeneous and high target dose. The grid pattern can be preserved in the normal tissue, from the skin to the direct vicinity of the target, preventing extensive damage to the organs at risk. The PGT approach could present a therapeutic possibility in difficult clinical situations where conventional radiotherapy would fail to provide any suitable option for the patients. EP-1744 Failure modes and effects analysis of Total Skin Electron Irradiation (TSEI) technique B. Ibanez-Rosello 1 , J.A. Bautista-Ballesteros 1 , J. Bonaque 1 , J. Perez-Calatayud 1,2 , A. Gonzalez-Sanchis 3 , J. Lopez-Torrecilla 3 , L. Brualla-Gonzalez 4 , M.T. Garcia- Hernandez 4 , A. Vicedo-Gonzalez 4 , D. Granero 4 , A. Serrano 4 , B. Borderia 4 , J. Rosello 4,5

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 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.