ESTRO 38 Abstract book

S483 ESTRO 38

used as a conventional ionization chamber with a fixed electrode gap of 2 mm. Three Kapton® entrance windows with the thicknesses of 25 μm, 50 μm and 75 μm were employed. TLDs provided as powder pressed onto a piece of aluminium. TOPAS [2] in version 3.1.2 was used to model an IBA PBS Nozzle and score dose to water in voxels of 5x5x5 mm³ within a cubic phantom. Results Both figures show the relative dose, normalized to a reference depth of 3 cm for the various detectors as a function of depth for 150 MeV. The results in figure 1 show the proton build-up effect. The two ionization chambers, the TLDs and the combination of film and wedge have good agreement with each other, whereas TOPAS and the film stack can deviate from these by up to 1 %. The errors of the mean value are within 1-2 ‰, except for the TLDs, which show larger deviations of up to 2.3 %. From the extrapolation chamber and TLD measurements and TOPAS calculations respectively, the skin dose at 70 μm amounted approx. 87 % of the dose at reference depth of 3 cm, see figure 2 .

Conclusion Our experiments indicate that a fluence based beam quality correction function for EBT3 films may be limited in its applications for intensity modulated proton therapy. At high LET there was also a tendency of the RE-LET d relation to differ for different initial beam energies and fluences. A single parameter may not be sufficient to represent beam quality with respect to film response. Current work in progress is to develop a formalism for RE of EBT3 film response. PO-0908 Determination of surface dose in pencil beam scanning proton therapy A. Kern 1,2,3 , C. Bäumer 1,3,4 , K. Kröninger 2 , L. Mertens 2 , B. Timmermann 1,3,4,5 , J. Walbersloh 6 , J. Wulff 1,3 1 University Hospital Essen, West German Proton Therapy Centre Essen WPE, Essen, Germany ; 2 Technical University Dortmund, Experimental Physics IV, Dortmund, Germany ; 3 University Hospital Essen, West German Cancer Center WTZ, Essen, Germany ; 4 German Cancer Consortium DKTK, Radiation Oncology and Imaging, Essen, Germany ; 5 University Hospital Essen, Department of Particle Therapy, Essen, Germany ; 6 Materialprüfungsamt NRW, Strahlenschutz, Dortmund, Germany Purpose or Objective Quantification of surface dose within the first few hundred µm is challenging, albeit of large interest for proton therapy to study dose effects in the skin. Treatment planning systems are typically limited in their ability to calculate skin dose accurately. Experimental determination on the other hand is affected by the detectors’ volume and in case of ionization chambers by the entrance wall. Aim of this study is the estimation of the absorbed dose at and around depth of 70 µm according to ICRU [1] with different dosimetric approaches for surface dose in proton pencil beam scanning. Material and Methods Four different detectors were tested for determination of surface dose in water: EBT3 GAFCHROMIC®, LiF:Mg,Ti TLDs, IBA PPC05 plane-parallel and PTW 23391 extrapolation chamber. The irradiation setup consisted of a monoenergetic extended proton pencil beam with 100 MeV, 150 MeV and 226.7 MeV. Radiochromic films are used within a vertical stack and in wedge geometry and analyzed/calibrated with FilmQA Pro™ (triple channel dosimetry). The extrapolation chamber PTW 23391 was

Conclusion All measurement methods show a good agreement within the first 5 mm. With the extrapolation chamber, the TLDs and the EBT3 film, data in the micrometer range can be measured. The extrapolation chamber shows the smallest uncertainties and the largest dynamic range. The TLDs have the largest uncertainty. TOAPS MC reproduces the experimental results. With regard to the skin dose, it can be noted that at 70 µm it amounts to approx. 87 % of the total dose from the reference depth in 3 cm. Furthermore, a dose increase of 4 % can be observed in the first 200 µm. References [1] ICRU. 1985. “Determination of dose equivalents resulting from external radiation sources.” Report No. 39, International Commission on Radiation Units and Measurements [2] Perl J et al. 2012. “TOPAS: An innovative proton Monte Carlo platform for research,” Med. Phys. 39 6818-37 PO-0909 Development and experimental validation of a user code for time-resolved Monte Carlo simulations P. Sibolt 1 , C.E. Andersen 2 , C.F. Behrens 1 , R.O. Cronholm 3 , E. Heath 4 1 Herlev & Gentofte Hospital, Radiotherapy Research Unit- Department of Oncology, Herlev, Denmark ;