ESTRO 37 Abstract book

S971

ESTRO 37

Purpose or Objective In scanned proton therapy multiple narrow pencil beams (PBs) are superposed to yield a 3D dose distribution. Due to the high number of PBs small dose calculation uncertainties of the individual PBs can result in considerable dose calculation errors. The aim of this study is to validate the electromagnetic and nuclear physics models employed by Geant4 on transverse dose In total, 3 sets of measurements were evaluated. Lateral dose profile of the high dose region in the center of the PB, the so called core (Gottschalk et al. 2015), were measured with a MicroDiamond (PTW, Freiburg, Germany) detector for 5 energies (62, 97, 148, 198 and 252 MeV). Measurements in the core region aim to evaluate the accuracy of the multiple Coulomb scattering model. The dose halo, i.e. the low dose region mostly deposited by charged secondaries, was measured for 3 energies (62, 148 and 252 MeV) with 24 PinPoint (PP, TN31015, PTW) ionization chambers (ICs) to evaluate the nuclear effects on a single PB. Since the PPs were cross calibrated, MC simulations could be evaluated in terms of absolute dose, as recommended in Gottschalk et al. 2015. The width of the beam was quantified as the full width at a ratio x of the maximum (FWxM). In this notation, the FW50%M is equivalent to the FWHM. In order to quantify the nuclear effects on the total dose delivered by numerous PBs, field size factors (FSF, normalized to a 10x10cm 2 field) were measured for field sizes ranging from 2 to 20 cm for 3 energies (62, 148 and 252 MeV) using a SemiFlex IC (TN31010, PTW). Gate v8.0/Geant4.10.3 using the QBBC_EMZ physics list was used to carry out the Monte Carlo (MC) simulations. Results The FWHM of the MC simulated PBs deviated by less than 8% (or 1.2 mm) from the MicroDiamond measurements, showing slight overestimation of the broadening of the FWHM with depth. This could result from overestimating the multiple Coulomb scattering in the WentzelVI model (see Figure 1). MC generated FW0.05%M reveal the midrange bump of the beam halo (see Figure 1 b) and agree within 6% to measurements. The overestimation of FW1%M up to 7% at high depths for the highest energy reveals the moderate overestimation of the transverse dose profile in the intermediate dose level at deep layers (see figure 2). Overall, all FWxMs agree within 10% to measurements (excluding 1 sample, due to too low resolution). FSFs derived from MC simulations agreed within 1% with measurements for field sizes larger than 4x4cm 2 for energies 64 and 148 MeV. However, deviations up to 6% were observed for the 2x2cm 2 field for 64 MeV. Deviations up to 3% were found for the 252 MeV at deep positions, which is consistent with the observations in the lateral profile. distributions in water. Material and Methods

measured doses (see also Table I). - Fig1f: For any dose threshold, the average length of the strip with a measured dose above this threshold was larger for 3D than for breast IMRT. It was also larger for H&N IMRT than for breast IMRT.

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Table I shows that average values for H&N higher than for breast. Measured SBRT dose values were low. The max recorded dose for SBRT was 12.71 Gy in 5 fractions, corresponding to EQD2=14.4 Gy 2 .

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Conclusion - The TPS does not calculate accurately doses at 1mm depth but differences with respect to measured doses were not significant. The skin structure can be used as a conservative and safe warning. - 3D irradiation of breast is related to larger skin doses and larger areas of high dose than IMRT. H&N IMRT delivers larger doses to larger areas of skin than breast IMRT. - Breast and H&N IMRT are expected to show skin toxicity but for SBRT treatments it should not be a concern. EP-1805 Dose calculation accuracy of Gate/Geant4 on transverse dose profiles of proton pencil beams in water A. Resch 1 , A. Carlino 2 , H. Fuchs 1 , A. Elia 2 , M. Stock 2 , D. Georg 1 , L. Grevillot 2 1 Medizinische Universität Wien, Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology and Department of Radiotherapy, Vienna, Austria 2 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria