ESTRO 2022

Session Item

Sunday

May 08

16:55 - 17:55

Room D5

Application of functional & quantitative imaging

Co-Chair: Adam Szmul, United Kingdom;

Chair: , Denmark

Session Code: 2550

Session Type: Proffered Papers

Track: Physics

17:45 - 17:55

Proton therapy treatment verification with prompt gamma rays and fast neutrons – a feasibility study

Helge Egil Seime Pettersen, Norway

Presentation Number: OC-0628

Abstract

Abstract Title:

Proton therapy treatment verification with prompt gamma rays and fast neutrons – a feasibility study

Authors:

Helge Egil Seime Pettersen¹, Enver Alagoz^2,3, Liv Bolstad Hysing¹, Toni Kögler⁴, Danny Lathouwers⁵, William Lionheart⁶, Jasmina Obhodas⁷, Guntram Pausch⁸, Hunter M. Ratliff³, Marta Rovituso⁹, Sonja Schellhammer⁴, Lena M. Setterdahl³, Kyrre Skjerdal³, Davorin Sudac¹⁰, Joseph A. Turko⁴, Kristian Smeland Ytre-Hauge¹¹, Ilker Meric³

¹Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway; ²University of Bergen, Department of Earth Science, Bergen, Norway; ³Western Norway University of Applied Sciences, Department of Computer Science, Electrical Engineering and Mathematical Sciences, Bergen, Norway; ⁴OncoRay — National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden, Dresden, Germany; ⁵Delft University of Technology, Section of Nuclear Energy and Radiation Applications, Department of Radiation, Science and Technology, Delft, The Netherlands; ⁶University of Manchester, Department of Mathematics, Manchester, United Kingdom; ⁷Ruđer Bošković Institute, Division of Experimental Physics, Zagreb, Croatia; ⁸Target Systemelektronik GmbH & Co. KG, -, Wuppertal, Germany; ⁹Holland Proton Therapy Centre - HollandPTC, -, Delft, The Netherlands; ¹⁰Ruđer Bošković Institute, Division of Experimental Physics, Zagreb, Croatia; ¹¹University of Bergen, Department of Physics and Technology, Bergen, Norway

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Purpose or Objective

Robust and sufficiently quick treatment verification for proton therapy is a well-known challenge. While detection systems for prompt gamma rays (PG) are maturing, the concept of measuring fast neutrons (FN) produced by inelastic proton interactions in the patient is relatively new.

In this study, the feasibility of a novel treatment verification concept measuring both FN and PG was evaluated through MC simulations. The hypothesis was that the combination of FN and PG would increase the treatment validation sensitivity. To this end, a hybrid, beamspot-specific Range Landmark (RL) was defined and its value was correlated to deviations in the proton range. Simulations of a patient geometry revealed the upper limits of the sensitivity of the method.

Material and Methods

We propose the use of a quasi-monolithic detector array (QuDA, figure 1), consisting of densely stacked and optically segmented organic bar-shaped scintillator elements with 1-2 ns decay times and pulse shape discrimination (PSD) for FN / PG identification. Fast photodetectors placed on both ends of a scintillator element measure the relative interaction position.

MC simulations of a 30x20x20 cm³ QuDA and a lung cancer patient CT from the Cancer Imaging Archive were performed with GATE / Geant4. Fifteen range-shifted versions of the patient dataset were made by removing or adding surface tissue, ranging between -5 and +5 mm. An 85 MeV posterior-anterior proton beam was stopped centrally in the lung tumor 3 cm in diameter.

Consecutive PG or FN interactions in the QuDA allows for kinematic reconstruction of their respective production vertices. In the present study, to estimate the upper limit of the setup sensitivity, the MC truth production vertices of PGs and FNs interacting in the QuDA were used as a basis for the RL: calculated as the mean value plus one standard deviation of the distribution of production vertices along the beam. RL distributions were created by subsampling bootstrapping, allowing for variable proton intensity. The minimum detectable range shift was found by comparing the RL distributions of several artificial range shifts using AUROC with a detection threshold of 0.9.

Results

While the PG production was isotropic (allowing for several possible QuDA positions), the FNs were produced predominantly in the forward direction. The optimal QuDA position in terms of particle detection was anterior to the patient, along the beam axis.

The proton intensities for the minimum detectable range shifts are shown in figure 2. Combining FN and PG in a single detector considerably increased the sensitivity, and only 20 M protons were necessary to detect a spot-wise range shift of 1 mm.

Conclusion

Combining the FN and PG measurements effectively halved the required proton intensity.

The presented results reflect an ideal detector and reconstruction setup: the next project milestone is to extend the work with reconstructed vertices including resolution effects.