Abstract Book

S1007

ESTRO 37

larger adults and whose life expectancy and sensitivity to radiation make reducing the risk of radiation induced late effects a high priority. One method of reducing imaging dose is to optimise bow-tie filtration of the x-ray field, using a filter with an optimal ‘child sized’ shape. In addition, the bow-tie filter may provide a means of lowering the lowest exposure setting by adding additional attenuation. This work aims to design such a filter, simulate CBCT images acquired with the new bow-tie filter and simulate the impact on image quality and registration accuracy for paediatric patients. Material and Methods 9 paediatric patient CBCT datasets with various tumour sites were simulated. A bow-tie filter profile was designed to add additional attenuation and achieve uniform signal across a 270mm diameter water cylinder. This corresponds to the maximum diameter fitting the small field of view collimator most commonly used for children, allowing children to be setup centrally as well as offset. Projection images were simulated by adding Gaussian noise of the correct magnitude according to the reduction in signal caused by the new bow-tie filter profile. Changes in patient scatter were estimated but ignored for now. CBCT scans were reconstructed using the modified projection images. The quality of the simulated CBCT scans was assessed visually and by testing registration accuracy, comparing the table correction to that for the original images. Results The central thickness of the simulated filter was 15.53 mm of Aluminium, chosen to reduce signal to 0.22 of the initial magnitude. This would decrease dose from 1.5mGy to 0.33mGy per scan, based on a commonly used preset for children in our hospital. The radius of curvature chosen was 41.55mm, which scales to a radius of 135 mm at the isocentre with a source-isocentre distance of 1000mm (Figure 1). Patient scans reconstructed with the simulated filter showed increased noise, however all bony anatomy remained visible with clear bone/soft tissue boundaries (Figure 2). Registration accuracy was not affected, with table correction vector discrepancies within 1mm (max 0.95mm) for all patients.

allows for considerable dose reduction to the patient without affecting visual image quality or registration accuracy. EP-1865 Cardiac Implantable Electronic Devices and Radiotherapy with 6 MV Photons: Are the Patients Safe? S. Blamek 1 , A. Konefał 2 , A. Wrońska 3 , D. Gabryś 1 , A. Orlef 4 1 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, Department of Radiotherapy, Gliwice, Poland 2 Department of Nuclear Physics and Its Applications- University of Silesia, Institute of Physics, Katowice, Poland 3 Faculty of Physics- Astronomy and Applied Computer Science- Jagiellonian University, Department of Nuclear Physics, Kraków, Poland 4 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, Department of Medical Physics, Gliwice, Poland Purpose or Objective According to the literature data and our previous studies, damage of cardiac implantable electronic devices (CIEDs) during irradiation with low energy photons is possible but much less probable than in case of photons of energy exceeding 10 MV. High-energy radiation is the source of secondary neutrons which are believed to cause damage to the integrated circuits of electronic devices. The exact cause of malfunctions in case of irradiation with lower energies is not clear because usually the dose delivered to the device comes only from the scattered radiation. The aim of the study was to analyze the environment during irradiation with 6 MV photons generated by various treatment machines in order to find evidence of production of secondary neutrons. Material and Methods A helium detector (InSpector 1000 with neutron probe) was used to test the presence of neutrons in the treatment rooms of a conventional linear accelerator (Clinac 23EX, Varian Medical Systems), CyberKnife and tomotherapy machine (Accuray). Additionally, samples of antimony, gold and manganese were placed in the treatment room in order to detect potential activation by thermal neutrons. The samples were then examined with HPGe (high purity germanium) detector. Results A relatively small number of neutrons was detected in the treatment rooms of all machines operating with 6 MV photons. The helium detector showed values in the order of 200-400 neutrons per second during beam-on, the background radiation was 0-2 neutrons/s, the background radiation measured during irradiation with 20 MV photons performed in the neighboring treatment room was 2-5 neutrons/s. The presence of the neutrons was confirmed by activation of Mn samples (Mn-56 was detected and identified by the presence of 846 keV and 1810.7 keV peaks registered by the HPGe detector). Detection of metallic samples activation confirmed the presence of neutrons and excluded potentially possible artifactual readouts from the helium detector. Conclusion The operation of CIEDs in patients irradiated with 6 MV photons should be closely monitored, especially in patients subject to long courses of irradiation because despite the number of neutrons is small relative to irradiation with high energies, the probability of malfunction may increase with the time of exposition.

Conclusion A new bow-tie filter has been designed to optimise CBCT imaging for children. Image quality and registration accuracy was assessed for CBCT scans that were simulated as if acquired with this filter. The filter design