ESTRO 36 Abstract Book

S29 ESTRO 36 _______________________________________________________________________________________________

found to agree within one standard uncertainty or better, for all proton energies.

use of the 99% level of the proximal part (p99) of the normalized light curve, as the starting point of the SOBP seemed more appropriate to get accurate SOBP width measurement. The measured SOBP width corresponds then, to the distance between that identified point and the measured range.

Figure 1: Beam monitor chamber calibration curve in terms of DAP w (at z ref = 2 cm) per MU, as a function of nominal proton energy, obtained with the Bragg Peak chamber (direct) and Markus chamber (indirect). The uncertainty bars correspond to one standard uncertainty. Conclusion This work proves the feasibility of the reference dosimetry of proton pencil beams based on DAP w , as it agrees with the standard and well-established approach based on D w within one standard uncertainty. Its main advantage is that it is not affected by the uncertainty in beam position, which results in an uncertainty in δx and δy. Its main drawback is the slightly larger uncertainty of the ionization chamber calibration coefficient. This drawback, however, could potentially pay off in the dosimetry of small photon fields, where perturbation factors of small detectors might result in a larger source of uncertainty. OC-0061 Development of a 3D plastic Scintillator detector for a fast verification of ocular proton beam H. Ziri 1 , D. Robertson 1 , S. Beddar 1 1 MD Anderson Cancer Center, Department of radiation physics, Houston, USA Purpose or Objective Scintillator detectors have been recently used for beam verification in radiation dosimetry. However, when irradiated with charged particles, scintillators undergo an ionization quenching effect that causes a decrease of the scintillation light emission with increasing linear energy transfer (LET). The goal of this project is to develop a tool for ocular proton beam quality assurance (QA) using a solid plastic scintillator without the need for quenching correction. Material and Methods Figure: Landmarks identification for range and SOBP width measurements The measurements were done at the Proton Therapy Center-Houston (PTC-H). The detector system consists of a 5x5x5 cm 3 cube of a plastic scintillator, EJ-260, that converts the incident proton beam into visible light and a telecentric lens coupled to a CCD camera to image the emitted light distribution. Landmarks were determined directly on the quenched light-depth distribution to determine the range and spread-out Bragg peak (SOBP) width of the beam. A common behavior of having an inflexion point at the distal edge was noticed in the measured scintillation light profiles. The range was determined as the corresponding depth of that point. To identify the inflexion point, a linear function was fitted to the distal edge. Then, to determine the SOBP width, the assumption that quenching is negligible at the beginning of the SOBP was considered. A Gaussian fit was used to smooth the curve and get better estimation of the starting point of the SOBP. An empirical analysis showed that the

Figure: Landmarks identification for range and SOBP width measurements Results The validity and the accuracy of this method was evaluated by comparison to ionization chamber measurements. The measured ranges were in good agreement with the ionization chamber measurements. The mean difference was within 0.05 cm and the standard deviation was within 2%. The measured SOBP widths, for a nominal range of 3.5 cm, were compared to the ionization chamber measured SOBP widths. The mean difference was within 0.06 cm and the standard deviation was less than 4%. SOBP width measurements were also in a good agreement with ionization chamber measurements. Conclusion Even though quenching decreases the emitted light with the increase of LET with depth, landmarks can still be identified for fast beam verification without the need for quenching correction. It has been shown that the developed approach works for different beam energies with a sufficient accuracy and reproducibility for clinical use. The accuracy of this approach is within 0.06 cm for range and SOBP width measurement for the ocular proton beam. OC-0062 Correcting for linear energy transfer dependent quenching in 3D dosimetry of proton therapy E.M. Høye 1 , M. Sadel 2 , L.P. Muren 1 , J.B.B. Petersen 1 , P. Skyt 1 , L.P. Kaplan 2 , J. Swakon 3 , L. Malinowski 3 , G. Mierzwińska 3 , M. Rydygier 3 , P. Balling 2 1 Aarhus University Hospital, Medical Physics, Aarhus C, Denmark 2 Aarhus University, Department of Physics and Astronomy, Aarhus, Denmark 3 Polish Academy of Sciences, Institute of Nuclear Physics, Krakow, Poland Purpose or Objective Three dimensional (3D) dosimetry allows for detailed measurements of dose distributions in photon-based radiotherapy, and has potential to become a useful tool for verification of proton therapy (PT). However, linear energy transfer (LET) dependent quenching of the signal in the Bragg peak results in an under-response of the dosimeter. In this study we investigate whether the LET