ESTRO 36 Abstract Book

S781 ESTRO 36 _______________________________________________________________________________________________

1 Pavia University, Physics, Pavia, Italy 2 Istituto Nazionale di Fisica Nucleare INFN, Physics Departiment, Pavia, Italy 3 Ludwig Maxmilians University, Medical Physics, Munich, Germany Purpose or Objective MONET (Model of ioN dosE for Therapy) is a code for the computation of the 3D dose distribution for protons in water. MONET accounts for all the physical interactions and is based on well known theories. Material and Methods The first part is the evaluation of the lateral profile. Our model is based on the Molière theory of multiple Coulomb scattering. To take into account also the nuclear interactions, we add to Molière distribution the Cauchy- Lorentz function, where two free parameters are obtained by a fit to a FLUKA simulation (Bellinzona et al., PMB 2016). The next step is the passage from the projected lateral distribution to a 2D distribution. The projected distributions are uncorrelated but not independent and we have to use the Papoulis theorem that allows, in case of cylindrical symmetry, to rebuild the radial distribution starting from projected one. We have implemented the Papoulis algorithm in the code. The second part is the study of the energy deposition in the longitudinal profile. We have implemented a new calculation of the average energy loss that is in agreement with simulations and other formulas published in the literature. The inclusion of the straggling is based on the convolution of energy loss with a Gaussian function. In order to complete the longitudinal dose profile, also the nuclear contributions are included in the calculation using a linear parametrization with only two free parameters for energy. The total dose profile is calculated in a 3D mesh by evaluating at each depth the 2D lateral distributions and by scaling them at the value of the energy deposition. Results We have compared MONET results with the FLUKA simulations and we have obtained a good agreement for different energy of protons in water. We have reproduced a lateral scan as a sum of many pencil beams in order to estimate the accuracy of the model focusing on the tails of the distribution that give rise to the low-dose envelope. Also in this case, the agreement between MONET and FLUKA is good. We have also estimated the calculation time: for each depth, it is about 2 seconds for the single beam and 4 seconds for the beam scan. Conclusion The advantages of MONET are the physical foundation, the fast calculation time and the accuracy. A possible development of this study is the creation of a dose database of clinical interest and an online fast dose evaluation tool. In the next future, we would like to extend MONET to the case of Helium beam and other ions. Preliminary results for helium ion will be shown. EP-1464 Investigation on beam width tolerances for proton pencil beam scanning B. Ackermann 1 , S. Brons 1 , M. Ellerbrock 1 , O. Jäkel 1,2 1 Heidelberg Ion Beam Therapy Center HIT, Medical Physics, Heidelberg, Germany 2 German Cancer Research Center, Medical Physics in Radiation Oncology, Heidelberg, Germany Purpose or Objective Beside beam spot position and proton range, the beam spot width is one of the central parameters for the correct application of a proton therapy plan utilizing pencil beam

CT scan used for radiotherapy treatment planning can contain temporary gas pockets inside the target volume for patients with tumours in the pelvis that later disappear during the course of radiotherapy. In this study we are interested to explore the dosimetric impact of applying the type C dose calculation algorithm for patients treated in the pelvic area using VMAT, where gas pockets appear and disappear in the rectum.

Material and Methods Ten clinical cervix cancer patients were selected for this study. The patients had different sizes of gas pockets on the planning CT. The treatment plans were optimized and calculated using one type B and one type C dose calculation algorithm (Eclipse, AAA and Acuros XB(Dose to Medium), respectively). Gas pockets of these patients were delineated on the planning CT and the contours and CT data was duplicated. On the first series the original CT the HU were kept as is, and on the second CT set the HU on the gas pockets were overridden with HU zero. Thus, we simulate a worst-case scenario where the gas pocket is present on the planning CT but disappear during the course of treatment. The original treatment plan optimized using the type C algorithm was recalculated on both CT data sets. The volume and maximum diameter of these gas pockets were measured and the difference in average and maximum dose in these pockets were calculated. Results The average volume of the pockets was 28.6 cm 3 [range: 1.7-77.7 cm 3 ] and the average maximum extent of the gas pocket was 4.2 cm [range: 2.8–5.3 cm]. Volumes up to 20 cm 3 show a decrease about 1 % in the average and maximum dose to the delineated air pocket for type C and an increase of less than 1% for type B algorithm. For volumes larger than 20 cm 3 the average and max dose to the delineated air pocket increases with more than 5% and up to 30%, respectively for the type C algorithm. The type B algorithm shows a decrease up to 2% in average dose and a small increase in maximum dose.

Conclusion Gas pockets above a volume of 20 cm 3 in the initial CT scan can induce dose hotspots during the actual treatment, when using the type C dose calculation algorithm for treatment planning and when deriving dose to medium for VMAT plans. This could increase risk for radiation toxicity in the worst case scenario. This effect is smaller, opposite and probably clinically negligible for the type B algorithm. EP-1463 MONET: an accurate model for the evaluation of the ion dose in water A. Embriaco 1,2 , E.V. Bellinzona 1,2,3 , A. Fontana 2 , R. Alberto 1,2