ESTRO 36 Abstract Book

S814 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective Modern dose calculation algorithms in radiotherapy treatment take into account the scattered dose and lateral electrons transport, such as point kernel model. The impact of scattered radiation dose from radiotherapy treatment is more significant for children. In this study, secondary cancer risk (SCR) resulting from scattered dose and the contribution of electrons transport were compared. Material and Methods Clinical examples of treatment plans for pediatric medulloblastoma were used to estimate the SCR for lungs. For each case, two treatment plans with conformal radiotherapy were generated. The same dose prescriptions for posterior fossa and craniospinal irradiation were used for both plans. The dose in first plan was calculated with algorithm taking account only scattered dose. The dose in second plan was calculated taking account scattered dose and lateral electron transport, as point kernel algorithms. The organ equivalent dose (OED) concept with a linear, linear- exponential and plateau dose response curves was applied to dose distributions, dose volume histograms, for lungs to estimate SCR. The excess absolute risk ratio (EAR) was also evaluated as EAR = OED from scattered dose divided to OED from scattered with lateral electrons transport doses. Results The calculated DVH with algorithm modeling lateral electron transport were significantly increased predicting more average dose for lungs by a factor of 1 to 1.1. The SCR was also increased (8%-16%) depending on model prediction. The EAR ratio were 1.08, 1.2 and 1.13, respectively, using linear, linear-exponential and plateau models. Conclusion The considerable impact of dose calculation methods in radiotherapy, integrated in TPS, can significantly influence the secondary cancer risk prediction and plan optimization, since OED is calculated from DVH for a specific treatment. The modern algorithms such as AAA, Acuros XB or Monte Carlo showed a better prediction of dose distribution. On the other hand, they provided more “trust” DVH metrics, as input in the SCR models, avoiding the uncertainties of dose distribution as well as significantly contribute to better estimations. EP-1517 Analysis of radiotherapy risk profile applied to the patient positioning G. Menegussi 1 , M.M. Vasques 1 , G.R.D. Santos 1 , L. Furnari 1 , L.N. Rodrigues 1 1 Hospital das Clinicas -FMUSP, Radiotherapy, Sao Paulo, Brazil Purpose or Objective The purpose of this work is to recognize and understand the risks of the processes of Radiotherapy positioning. Material and Methods Risk analysis methods were applied Failure Mode Effect Analysis (FMEA) to key steps in each sub-step of the positioning process (simulation, initial positioning, displacement, images acquisition and treatment) of patients in the treatment of breast and head&neck (H&N) tumors. This tool enabled us to identify the risks involved 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