ESTRO 36 Abstract Book

S815 ESTRO 36 _______________________________________________________________________________________________

statistical accuracy) and D PB * w . Updates of C can be triggered as often as necessary by running the MC engine with the last corrected values of w as input. The performance of the method is illustrated on two extreme cases: prostate (relatively easy case) and lung (considered to be complex due to the high heterogeneity). For simplicity, we created PTV-based plans but the findings can be equally applied to robust optimized plans. Results For the prostate case, the recomputed MC dose after initial optimization ( C =0, before correction) revealed a decreased target coverage (D95=90% of the prescribed dose, D p ) that improved significantly after just one correction (D95 corrected =97%D p ). For the lung case, the difference between MC and PB doses before correction was very large: D95=63%D p and D5=137%D p . But still the hybrid strategy was able to partially improve target coverage (D95 corrected = 84%D p ) as well as reducing overdose (D5 corrected = 111%D p ), after two updates of C. In both cases, further corrections did not lead to better results. The results proved that the hybrid method allows us to improve dose accuracy even for very complicated cases as lung tumors. However, the success of the correction is limited by the order of magnitude of the term C, i.e, very large difference between MC and PB doses are only partially corrected. = P PB

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 MCsquare (Souris et al. Med Phys 2016) able to compute a final dose in less than 1 minute. The hybrid optimization strategy calculates the optimal spot weights ( w ) using the analytical beamlets matrix ( P PB ) and a correction term C . After a first optimization where C = 0, the method alternates optimization of w using P PB with updates of C = D MC – D PB , where D MC results from a regular MC computation (using 10 8 protons to ensure good Electronic Poster: Physics track: Treatment plan optimatisation: algorithms