Multi-particle detection and imaging with quasi-monolithic organic detector arrays for treatment verification in particle therapy

Particle therapy is a modality for treating cancer using ionizing radiation from, e.g., protons or carbon ions. A growing number of patients worldwide receive particle therapy as a part of their cancer treatment due to its dosimetric advantages over the more conventional external beam radiotherapy using photons. The finite range of particles in tissue results in sparing of healthy tissue surrounding the tumour and thereby a reduced risk of adverse effects compared to conventional treatment. This opens possibilities for a more intensified treatment by dose escalation to the tumour and thereby an increased probability for controlling the disease. However, charged particles, when used for external beam radiotherapy, are much less forgiving than photons in case of treatment deviations. Treatment deviations may be caused by (1) sensitivity of charged particles to anatomical or density changes along their radiological path in the patient, e.g., due to organ motion or tumour shrinkage and (2) uncertainties associated with the estimation of the exact position of the Bragg-peak
(maximum dose deposition) in tissue, e.g., due to uncertainties in the estimation of stopping power ratios. Collectively, these are referred to as “range uncertainties”. The most important consequence of range uncertainties is that the tissue sparing potential of charged particles cannot be fully exploited. Therefore, there is an urgent need for reliable and robust treatment verification systems that can identify potential treatment deviations in real-time.

Despite recent advances in state-of-the-art prompt gamma-ray imaging, spectroscopy and timing systems as well as in-beam and offline PET imaging systems, the development of treatment verification systems still lags and as such, there is no system yet in wide routine clinical use. Common to all existing solutions is the fact that they rely on a single feature of a single particle species and will therefore suffer from limited counting statistics. This is an important limiting factor in applying state-of-the-art systems in a treatment verification system.
Recently, a collaboration of detector, nuclear and medical physicists, nuclear engineers and mathematicians initiated the NOVO (Neutron and gamma-ray imaging with quasi-monolithic organic detector arrays – a novel, holistic approach to real-time range assessment-based treatment verification in particle therapy) project. The aim of the NOVO project is to develop a sophisticated quasi-monolithic organic detector array (QuDA) that will combine detection and imaging of secondary fast neutrons and prompt gamma-rays produced in nuclear interactions of incident particles with tissue, the profiles of which show a strong correlation with the incident particle beam range. In addition to imaging spatial profiles of secondary fast neutrons and prompt gammarays,
the QuDA will be used to acquire information on additional features of each particle species, such as timing, energy and intensity, that can provide supplementary information on the range of particle beams in tissue, thus representing a major shift from single-feature, single-particle systems to a multi-feature, multiparticle system.
In this contribution we will report on first estimates of the imaging properties of a potential QuDA design, based on Monte Carlo radiation transport models with MCNP6.2, Geant4 and GATE, considering fast neutrons and prompt gamma-rays for various, realistic combinations of timing, energy and position resolution of the individual sensing elements. Furthermore, we will report on the expected detection efficiencies and the resulting range monitoring precision for proton beams with clinically relevant energies and intensities incident on homogeneous polymethyl methacrylate phantoms. The preliminary results demonstrate the potential to obtain a range monitoring precision of approximately 1 mm down to proton beam intensities of about 1 – 2 x
10⁷ [protons/pencil beam] when fast neutron and prompt gamma-ray data are combined.