ESTRO 36 Abstract Book

S178 ESTRO 36 _______________________________________________________________________________________________

1 Université Catholique de Louvain- Institute of Experimental & Clinical Research, Molecular Imaging- Radiotherapy & Oncology, Brussels, Belgium 2 Centre Antoine Lacassagne, Medical Physics, Nice, France 3 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria 4 National Physical Laboratory, Acoustics and Ionising Radiation Division, Teddington, United Kingdom 5 Cliniques Universitaire St-Luc, Radiotherapy and Oncology Dep., Brussels, Belgium 6 IBA Dosimetry GmbH, Schwarzenbruck, Germany Purpose or Objective The main application of calorimeters in standards laboratories is as primary standard of absorbed dose to water against which ionisation chambers (ICs) are calibrated. At present, no calorimeter is established as a primary standard instrument in proton beams. Based on the absorbed dose-formalism of IAEA TRS-398, this work describes a direct comparison between a water calorimeter (WCal) and plane-parallel ICs in clinical pulsed pencil beam scanning (PBS) proton beams, delivered by a synchrocyclotron. The temporal beam characteristics and the absence of a dosimetry protocol for such beams create significant challenges in absorbed dose determination. The aim of this work is to demonstrate the feasibility a water calorimetry in pulsed PBS beams. Material and Methods The method consisted in comparing the response of WCal and ICs (PPC40 and PPC05) in the same reference conditions. Measurements have been performed at a depth of 3.1 cm using two mono-layers maps of proton beams (10 x 10 cm²), with incident beam energies of 96.17 MeV (range in water = 6.8 g/cm²) and 226.08 MeV (range in water = 31.7 g/cm²), respectively. The response of the WCal is corrected for heat transfer (calculated using numerical simulations based on finite element method) and non-water material inside the WCal (using experimentally derived factors). Using hydrogen- saturated high-purity water in the WCal, the chemical heat defect is assumed to be zero. Classical correction factors are applied to the response of ICs: temperature and pressure, polarity and recombination (k s ). k s was studied in detail due to the very high beam dose rate used with the delivery method. Results Table 1 shows preliminary relative differences of D w measured with WCal and IC, during two independent experimental campaigns, for both energies. A small positioning uncertainty could explain that the ratios obtained during campaign B are higher for the low energy beam. For campaign A, however, ratios are higher for the high energy beam, which cannot be explained by a positioning uncertainty. A new campaign is planned to repeat the measurement of correction factors to improve the statistics of the results.

planned to confirm and consolidate correction factors and determine the overall uncertainty on absorbed dose-to- water obtained using each system. The next experimental step is to perform the same experimental comparison for a real clinical situation: a dose cube of 10 x 10 x 10 cm³, created by a superposition of mono-energetic layers. OC-0340 Validation of HU to mass density conversion curve: Proton range measurements in animal tissues J. Góra 1 , G. Kragl 1 , S. Vatnitsky 1 , T. Böhlen 1 , M. Teichmeister 1 , M. Stock 1 1 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria Purpose or Objective Proton dose calculation in the treatment planning system (TPS) is based on HU information taken from the CT scans and its relation to the relative stopping powers (RSP). However, tissue equivalent substitutes commonly used in the process of conversion curve definition may not reflect precisely the properties of real, human tissues. Therefore, various animal tissues were used for validation of the CT number to mass density (MD) conversion curves implemented in the TPS (RayStation v5.0.2). Material and Methods 10 animal tissue samples (pig) were used in this study (muscle, brain, bone, blood, liver, spleen, lung, fat, kidney and heart). Each sample was prepared and wrapped separately. 3-4 tissues were placed in dedicated phantoms (head and pelvis) at a time and CT scans were taken in the clinically accepted planning protocols. Specially designed PMMA phantoms where composed of two parts: a) an internal box, which could fit the animal tissues inside, b) the outer PMMA cover, designed to simulate pelvis (see fig.1c) and head during CT scan. The design of the phantoms not only helped to reduce imaging artefacts but also allowed to apply a slight pressure on the tissues in order to remove unwanted air. Subsequently, the tissue phantom was attached to the front of the water phantom, where with the use of 2 Bragg peak chambers, range measurements were performed. All measurements were performed within 24h after the animal was slaughtered with the use of one, central, 160.3 MeV pencil beam. For each sample, multiple irradiation positions were chosen in a very precise matter, as it was extremely important to choose the most homogeneous path through which the proton beam would pass. Acquired CT data was used to read out the HU, correlate them with the measured RSP and validate against implemented CT number to MD conversion curves. Results Figure 1, shows the comparison between measured RSP and HU for real tissue samples and implemented conversion curve in the TPS a), CT scan of the adult, abdomen protocol b), and measurement set-up c). The measured data for all soft tissues were found to be within 1% agreement with the calculated data. Only for lung tissue the deviations were up to 3.5%. For bone, both the difficulty in assessing the actual thickness of the part where the beam was passing through, as well as the inhomogeneous nature of this tissue, prevented us from the accurate RSP assessment. However, for 2 measurements out of 3, the measured RSP where within 3.5% uncertainty. Conclusion The experimental validation of the conversion curve resulted in good agreement between measured and calculated data, therefore we can use it in the clinical set- up with confidence. There is a number of uncertainty sources related to these measurements, starting from HU to RSP model, real tissue heterogeneities or uncertainties related to acquisition of the CT data due to beam hardening. The last one, we tried to minimize by using especially dedicated phantom.

Conclusion The preliminary results are very encouraging and demonstrate that water calorimetry is feasible in a clinical pulsed PBS proton beam. The absolute relative differences between D w derived from WCal and IC are inferior to 2%, which is within the tolerance of the IAEA TRS-398 protocol. Due to the depth-dose distribution, a depth inferior to 3.1 cm (e.g. 2 cm where the gradient is lower) would be more suitable to minimise the uncertainty in positioning. Further numerical and experimental investigations are