ESTRO 36 Abstract Book

S804 ESTRO 36 2017 _______________________________________________________________________________________________

in the process, to assess the impact of each sub-step and to rank the most relevant errors by setting a numerical value- Risk Priority Number (RPN) obtained with the scores attributed to the occurrence, severity and detectability by questionnaires submitted to staff (doctors, physicists and therapists). Results For breast the unanimous responses between professional classes were initial placement, lateral displacement in the location of the isocenter and image acquisition. The causes of positioning errors were during treatment for physicians losses marks on the skin is the most important factor, to the physicists, error in the use of accessories results in major failures and for therapists, changes in the weight of the patient may cause major errors. For H&N cases there was not unanimous response. In simulation-CT scan, doctors point out patients lack of cooperation as the leading cause of errors, physicists an improperly made mask generates the greater number of failures and therapists did not have unanimous answers. In the initial position sub-step, the most important point proved to be the inclusion or exclusion of tracheostomy/nasal probe for therapists and physicists. Physicists also considered non- coincidence of location marks a factor of great importance. In location of the treatment isocenter sub- step, physicists and therapists pointed to the poor positioning of the mask as a cause of failure, but with different impact in the treatment. For physicians, the wrong initial displacement is the main cause of errors. In acquisition of portal sub-step, the most frequent cause of errors was inaccurate comparison of images and mistaken correction, for all. For therapists and physicists, the use of DRR associated with other phases was the root cause of failures in this step. Positioning errors causes during treatment received different answers: for doctors, the main causes of failure are problems with the mask accessories and change in patient weight. For physicists the patient's weight change was the most important failure. Conclusion The FMEA introduces a subjective analysis, since it is dependent on personal judgment criteria relevant points were highlighted in the analysis of positioning routine. To the answers with relevant frequency or high RPN, solutions could be suggested in order to prevent failures and minimizing human erros. Further studies are in progress to other anatomical sites. EP-1518 Various activation foils for photo neutron measurements in medical linac A.H. Kummali 1 , S. Cyriac 2 , S. Deepa 3 , A. BAKSHI 3 1 Nanavati Hospital, Medical Physics, Mumbai, India 2 Apollo Hospitals Navi Mumbai, Medical Physics, Navi Mumbai, India 3 BARC, RPAD, Mumbai, India Purpose or Objective Photo neutrons produced from medical linear accelerators while operating above 10 MV is a concern for radiation protection and safety for patients and radiation workers [1] . Different methods are used to quantify the neutron production in clinical situation. In our study we used various activation foils for the photo neutron measurements in medical LINAC. This study discusses the measurement techniques of neutron absorbed dose for various treatment parameters of clinical importance. Material and Methods Absolute measurements of photo-neutrons using the Indium activation foil [2] having both thermal and fast neutron cross-sections through the nuclear reactions 115 In (n, γ) 116m In and 115 In (n, n’) 115m In, the thermal neutrons using 197 Au(n,γ) 198 Au, 63 Cu (n,γ) 64 Cu were evaluated in the present study. Photo-neutron measurements for various field size opening using MLC, and for various wedge angles

for 15 MV photon beam from a Medical LINAC model Elekta Precise have been carried out in the present study. Results Photo neutrons were measured using 3 foils mentioned above for various field sizes [3] such as 10 x 10 cm 2 to 20 x 20 cm 2 and for 40 x 40 cm 2 . Irradiation time for each field size took approximately 10 min and the total MU delivered is 5000 at a dose rate of 590 MU/min. Dose calculated at Dmax is 50Gy and 10 cm back up of PMMA phantom is ensured for the scattering and to mimic the TPS treatment planning. The result shows that, the total neutron dose increases as the field size increases from 10 x 10 cm 2 to 20 x 20 cm 2 and for 40 x 40 cm 2 . The photo neutron measurements using activation foils for Omni wedged fields in Elekta LINAC is uniquely studied. The irradiation time of about 20 min were taken to deliver 50 Gy at Dmax with the dose rate of 640 Mu/min. Wedged fields were defined for a field size of 30 x 30 cm 2 and the wedge used for each set of measurements are 15°, 30° and 60°. The fast neutron dose decreases and thermal neutron dose increases with wedge angles from 15°, 30° and 60°. Open beam gives the highest fast neutron dose and the lowest thermal neutron dose. Conclusion Insensitivity nature of activation foils for gamma/photons and the possibility of absolute measurements using the primary quantity of nuclear reaction cross-section makes activation foil best suited for photon induced neutron measurement. The present results indicate that the total neutron dose represents a small contribution to the therapeutic photon dose, meaning that it is much smaller than 1% of the photon dose delivered to the patient. However, the amount of this extra dose in the vicinity of the patient position cannot be neglected in view of radiological protection assessment related to the patients. EP-1519 Implementation of a hybrid superfast Monte Carlo-Pencil beam dose optimizer for proton therapy A.M. Barragán Montero 1 , K. Souris 1 , D. Sánchez- Parcerisa 2 , A. Carabe-Fernández 3 , J.A. Lee 1 , E. Sterpin 1,4 1 Université Catholique de Louvain- Institute of Experimental & Clinical Research, Molecular Imaging- Radiotherapy and Oncology MIRO, Brussels, Belgium 2 Universidad Complutense de Madrid, Departamento de Física Atómica- Molecular y Nuclear, Madrid, Spain 3 Hospital of the University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA 4 KU Leuven - University of Leuven, Department of Oncology, Leuven, Belgium Purpose or Objective Monte Carlo (MC) dose calculation plays an important role in treatment planning for proton therapy due to the limited accuracy of analytical algorithms, especially in very heterogeneous tumor sites. The new dedicated MC engines for pencil beam scanning (PBS) achieve reduced computation times for a single dose calculation. However, computing spot-per-spot doses is still very time- consuming, since typically 10000 to 20000 spots are needed. The presented strategy combines the speed of analytical algorithms and the accuracy of MC to get the best outcome for PBS treatment planning in a reasonable amount of time for clinical practice. Material and Methods An in-house treatment planning system was used to create the plans. The optimizer combines the analytical pencil beam (PB) algorithm in FoCa (Sánchez-Parcerisa et al. Phys Med Biol 2014) and the super-fast Monte Carlo engine Electronic Poster: Physics track: Treatment plan optimatisation: algorithms