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Hillbrand, Martin; Landry, Guillaume; Ebert, Sandy; Dedes, George; Pappas, Eleftherios; Kalaitzakis, Giorgo; Kurz, Christopher; Wuerl, Matthias; Englbrecht, Franz; Dietrich, Olaf; Makris, Dimitris; Pappas, Evangelos and Parodi, Katia (2019): Gel dosimetry for three dimensional proton range measurements in anthropomorphic geometries. In: Zeitschrift für Medizinische Physik, Vol. 29, No. 2: pp. 162-172

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Proton beams used for radiotherapy have potential for superior sparing of normal tissue, although range uncertainties are among the main limiting factors in the accuracy of dose delivery. The aim of this study was to benchmark an N-vinylpyrrolidone based polymer gel to perform three-dimensional measurement of geometric proton beam characteristics and especially to test its suitability as a range probe in combination with an anthropomorphic phantom. For single proton pencil beams as well as for 3 x 3 cm(2) mono-energy layers depth dose profiles, lateral dose distribution at different depths and proton range were evaluated in simple cubic gel phantoms at different energies from 75 to 115 MeV and different dose levels. In addition, a 90 MeV mono-energetic beam was delivered to an anthropomorphic 3D printed head phantom, which was filled with gel. Subsequently, all phantoms underwent magnetic resonance imaging using an axial pixel size of 0.68-0.98 mm and with slice thicknesses of 2 or 3 mm to derive a 3-dimensional distribution of the T-2 relaxation time, which correlates with radiation dose. Indices describing lateral dose distribution and proton range were compared against predictions from a treatment planning system (TPS, for cubic and head phantoms) and Monte Carlo simulations (MC, for the head phantom) after manual rigid co-registration with the T-2 relaxation time datasets. For all pencil beams, the FWHM agreement with TPS was better than 1 mm or 7%. For the mono-energetic layer, the agreement with TPS in this respect was even better than 0.3 mm in each case. With respect to range, results from gel measurements differed no more than 0.9 mm (1.6%) from values predicted by TPS. In case of the anthropomorphic phantom, deviations with respect to a nominal range of about 61 mm as well as in FWHM were slightly higher;namely within 1.0 mm and 1.1 mm respectively. Average deviations between gel and TPS/MC were similar (-0.3 mm +/- 0.4 mm/-0.2 +/- 0.5 mm). In conclusion, polymer gel dosimetry was found to be a valuable tool to determine geometric proton beam properties three-dimensionally and with high spatial resolution in simple cubic as well as in a more complex anthropomorphic phantom. Post registration range errors of the order of 1 mm could be achieved. The additional registration uncertainty (95%) was 1 mm.

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