ESTRO 36 Abstract Book

S803 ESTRO 36 2017 _______________________________________________________________________________________________

can be used to attach and remove the cutouts to the applicator in an easy and fast manner. It is important to note, that the mounted frame must not interfere with the radiation field. Hence, the frame has an identical size as the used applicator. VELCRO® strips were deployed as an attachment modality between applicator, frame and cutout. Those were glued to a PMMA frame with the same size of a standard applicator which attach to VELCRO® strips elongating around the applicator. The skin facing side of the frame is also covered with VELCRO® strips. In addition, cutouts were covered on the one side with VELCRO® strips and on the other side with a plastic foil, respectively. Straight-lined cutouts can be adjusted within seconds to shape TVs.

should be considered to the prescribed dose using the presented frame-based attachment system. EP-1516 Prediction of secondary cancer risk from lateral electrons transport from pediatric radiotherapy A. Chaikh 1 , J. Balosso 2 1 CHU de Grenoble - A.Michallon, Radiothérapie et physique médicale, Grenoble, France 2 University Hospital of Grenoble- University Grenoble- Alpes, Department of Radiation Oncology and Medical physics, Grenoble, France 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

Results All used straight-lined shields from our database could be exactly reproduced contouring 100% of the TV using the developed attachment system. Nevertheless, the system increases the distance of the applicator to the patient skin by non-negligible 8.8mm. This has to be taken into account by an output factor (OF). A comparison of the measured and calculated (by the ISL) OFs shows a maximum deviation of ±3.6% (12keV) and ±0.8% (>12keV) for our set-up. Conclusion Shielding healthy tissue in kilovoltage x-ray beam therapy using the reusable frame-based system promises a fast exchange of cutouts. It also ensures a high reproducible accuracy to irradiate multiple TVs in a row with the same applicator but with different frames and cutouts. Moreover, it provides a complete coverage of healthy tissue of straight-lined TVs. Furthermore, a measured OF