Speaker
Description
Particle therapy is a promising and rapidly developing methodology of modern tumor treatment. In order to reach its full potential, however, it requires detailed verification that the clinical target volume receives the planned dose while sparing surrounding healthy tissues.
Although the applicability of in-beam positron emission tomography and prompt gamma rays has already been demonstrated in patients, range verification is not yet part of the clinical routine in particle therapy. This is due both to the methodological limitations of previous systems, but also to commercial, clinical and physical boundary conditions.
In pencil beam scanning, the most advanced particle therapy treatment method, the number of secondary particles available per spot ($\Delta t =$ 10 to 100 ms) is limited. In order to develop a clinically usable treatment verification system, as much information as possible must be extracted from the secondary radiation field. On the other hand, the instantaneous fluence rate of secondary particles is high ($5\cdot10^6$ to $1\cdot10^8$ cm$^{-1}$ s$^{-1}$), which challenges modern digital data acquisition systems connected to monolithic inorganic scintillators with typical sizes used in prompt gamma ray verification systems. In order to reduce the load on the detectors, and also with regard to the ever increasing current intensities of next generation medical accelerators, future systems have to be granular. Multi-Feature Treatment Verification (MFTV) combines and extends established methods (prompt gamma-ray imaging, spectroscopy, timing, etc.) in order to achieve higher reliability and performance.
This idea was taken up by the NOVO project and expanded by a multi-particle approach. The NOVO (A novel and holistic approach to real-time dose verification enabling a new era of radiotherapy for cancer treatment) consortium is a collaboration of medical, nuclear and detector physicists, nuclear engineers, and mathematicians, and aims at developing a holistic real-time treatment verification system in particle therapy. In addition to the development of the multi-particle detector, the NOVO consortium focuses on the development of crucial elements of a clinical system including image reconstruction methods, deterministic predictors, dose estimators, decision making schemes and the determination of neutron and photon production cross-sections of therapy-relevant isotopes in the proton energy range of 70 to 230 MeV.
Elements of a potential multi-channel MFTV system were characterized in a double time-of-flight experiment at the pulsed photo-neutron source nELBE. The essential properties (time resolution, light yield, detection efficiency and pulse shape discrimination) of EJ-276 plastic scintillators and novel organic glass scintillators from Sandia National Laboratories were determined. Organic glasses are reported to have higher light yield, improved pulse shape discrimination (PSD) properties and faster fall times compared to commercially available plastic scintillator materials such as the EJ-276. Therefore, organic glasses represent an interesting alternative to materials currently available in the market.
The first experimental results show that the time resolution of a $10\times10\times200$ mm$^3$ organic glass scintillator read-out at both ends will reach the high demands of such a proposed range verification system. Preliminary results show a time resolution of $\approx$390 ps, an energy resolution of $\approx$ 22 % / 17 % at 340 keV / 1061 keV and a pulse-shape discrimination figure-of-merit greater than 1.03 for separating neutrons and $\gamma$-rays over the entire usable kinetic energy range of nELBE. These properties make organic glasses a promising candidate for an MFTV system.