A filtering approach for PET and PG predictions in a proton treatment planning system

Positron emission tomography (PET) and prompt gamma (PG) detection are promising proton therapy monitoring modalities. Fast calculation of the expected distributions is desirable for comparison to measurements and to develop/train algorithms for automatic treatment error detection. A filtering formalism was used for positron-emitter predictions and adapted to allow for its use for the beamline of any proton therapy centre. A novel approach based on a filtering formalism was developed for the prediction of energy-resolved PG distributions for arbitrary tissues. The method estimates PG yields and their energy spectra in the entire treatment field. Both approaches were implemented in a research version of the RayStation treatment planning system. The method was validated against PET monitoring data and Monte Carlo simulations for four patients treated with scanned proton beams. Longitudinal shifts between profiles from analytical and Monte Carlo calculations were within -1.7 and 0.9 mm, with maximum standard deviation of 0.9 mm and 1.1 mm, for positron-emitters and PG shifts, respectively. Normalized mean absolute errors were within 1.2 and 5.3%. When comparing measured and predicted PET data, the same more complex case yielded an average shift of 3 mm, while all other cases were below absolute average shifts of 1.1 mm. Normalized mean absolute errors were below 7.2% for all cases. A novel solution to predict positron-emitter and PG distributions in a treatment planning system is proposed, enabling calculation times of only a few seconds to minutes for entire patient cases, which is suitable for integration in daily clinical routine.

Offline imaging of positron emitters induced by therapeutic helium, carbon and oxygen ion beams with a full-ring PET/CT scanner: experiments in reference targets

In vivo verification of light ion therapy based on positron-emission tomography (PET) imaging of irradiation induced patient activation relies on activity predictions from Monte-Carlo (MC) or analytical computational engines for comparison to the measurements. In order to achieve the necessary accuracy, experimental data are indispensable for the validation of the calculation models. For this we irradiated thick reference targets with mono-energetic helium, carbon and oxygen ion beams and measured the resulting material activation offline with a commercial full-ring PET/CT scanner located nearby the treatment room. Acquired PET data were analysed over time to separate the activity contribution of different radionuclides. Determined production yields were compared to published findings obtained from in-beam activation measurements with a limited-angle double-head PET camera. In addition, we investigated the time-dependence of the measured radionuclide-specific contributions and of the distal activity range, as well as the lateral spread of the activity signal as a function of beam penetration depth. We present radionuclide-specific depth-resolved activity distributions and production yields for the radionuclides [Formula: see text], [Formula: see text] and [Formula: see text], dominating irradiation-induced patient activation. We observe systematically lower production yields with a ratio between the dual-head and our full-ring PET measurements of, on average, 1.7 and 1.3 for the oxygen and carbon beam irradiations, and 1.7 (2.1) for the high (low) energy helium beam irradiations. Findings on the temporal development of the activity range confirm the expectation, with the oxygen beam induced signal being the most sensitive scenario. The experimental data reported in this work, acquired with a state-of-the-art full ring PET scanner, provide a comprehensive and consistent basis for the benchmarking of PET signal calculation engines. In particular, they can support a fine-tuning of the underlying physics models used by the respective implementation and therefore improve the accuracy of PET-based therapy verifications at current and future treatment facilities.

Prediction of positron emitter distributions for range monitoring in carbon ion therapy: an analytical approach

Range verification is one of the most relevant tasks in ion beam therapy. In the case of carbon ion therapy, positron emission tomography (PET) is the most widely used method for this purpose, which images the [Formula: see text]-activation following nuclear interactions of the ions with the tissue nuclei. Since the positron emitter activity profile is not directly proportional to the dose distribution, until today only its comparison to a prediction of the PET profile allows for treatment verification. Usually, this prediction is obtained from time-consuming Monte Carlo simulations of high computational effort, which impacts the clinical workflow. To solve this issue in proton therapy, a convolution approach was suggested to predict positron emitter activity profiles from depth dose distributions analytically. In this work, we introduce an approach to predict positron emitter distributions from depth dose profiles in carbon ion therapy. While the distal fall-off position of the positron emitter profile is predicted from a convolution approach similar to the one suggested for protons, additional analytical functions are introduced to describe the characteristics of the positron emitter distribution in tissue. The feasibility of this approach is demonstrated with monoenergetic depth dose profiles and spread out Bragg peaks in homogeneous and heterogeneous phantoms. In all cases, the positron emitter profile is predicted with high precision and the distal fall-off position is reproduced with millimeter accuracy.

Experimental validation of the FLUKA Monte Carlo code for dose and [Formula: see text]-emitter predictions of radioactive ion beams

In the context of hadrontherapy, whilst ions are capable of effectively destroying radio resistant, deep seated tumors, their treatment localization must be well assessed to ensure the sparing of surrounding healthy tissue and treatment effectiveness. Thus, range verification techniques, such as online positron-emission-tomography (PET) imaging, hold great potential in clinical practice, providing information on the in vivo beam range and consequent tumor targeting. Furthermore, [Formula: see text] emitting radioactive ions can be an asset in online PET imaging, depending on their half-life, compared to their stable counterparts. It is expected that using these radioactive ions the signal obtained by a PET apparatus during beam delivery will be greatly increased, and exhibit a better correlation to the Bragg Peak. To this end, FLUKA Monte Carlo particle transport and interaction code was used to evaluate, in terms of annihilation events at rest and dose, the figure of merit in using [Formula: see text] emitter, radioactive ion beams (RI [Formula: see text]). For this purpose, the simulation results were compared with experimental data obtained with an openPET prototype in various online PET acquisitions at the Heavy Ion Medical Accelerator in Chiba (HIMAC), in collaboration with colleagues from the National Institute of Radiological Sciences' (NIRS) Imaging Physics Team. The dosimetry performance evaluation with FLUKA benefits from its recent developments in fragmentation production models. The present work estimated that irradiations with RI [Formula: see text], produced via projectile fragmentation and their signal acquisition with state-of-the-art PET scanner, lead to nearly a factor of two more accurate definition of the signals' peak position. In addition to its more advantageous distribution shape, it was observed at least an order magnitude higher signal acquired from ¹¹C and ¹⁵O irradiations, with respect to their stable counterparts.

An overview of recent developments in FLUKA PET tools

The new developments of the FLUKA Positron-Emission-Tomography (PET) tools are detailed. FLUKA is a fully integrated Monte Carlo (MC) particle transport code, used for an extended range of applications, including Medical Physics. Recently, it provided the medical community with dedicated simulation tools for clinical applications, including the PET simulation package. PET is a well-established imaging technique in nuclear medicine, and a promising method for clinical in vivo treatment verification in hadrontherapy. The application of clinically established PET scanners to new irradiation environments such as hadrontherapy requires further experimental and theoretical research to which MC simulations could be applied. The FLUKA PET tools, besides featuring PET scanner models in its library, allow the configuration of new PET prototypes via the FLUKA Graphical User Interface (GUI) Flair. Both the beam time structure and scan time can be specified by the user, reproducing PET acquisitions in time, in a particle therapy scenario. Furthermore, different scoring routines allow the analysis of single and coincident events, and identification of parent isotopes generating annihilation events. Two reconstruction codes are currently supported: the Filtered Back-Projection (FBP) and Maximum-Likelihood Expectation Maximization (MLEM), the latter embedded in the tools. Compatibility with other reconstruction frameworks is also possible. The FLUKA PET tools package has been successfully tested for different detectors and scenarios, including conventional functional PET applications and in beam PET, either using radioactive sources, or simulating hadron beam irradiations. The results obtained so far confirm the FLUKA PET tools suitability to perform PET simulations in R&D environment.

A novel approach to electron data background treatment in an online wide-angle spectrometer for laser-accelerated ion and electron bunches

Laser-based ion acceleration is driven by electrical fields emerging when target electrons absorb laser energy and consecutively leave the target material. A direct correlation between these electrons and the accelerated ions is thus to be expected and predicted by theoretical models. We report on a modified wide-angle spectrometer, allowing the simultaneous characterization of angularly resolved energy distributions of both ions and electrons. Equipped with online pixel detectors, the RadEye1 detectors, the investigation of this correlation gets attainable on a single shot basis. In addition to first insights, we present a novel approach for reliably extracting the primary electron energy distribution from the interfering secondary radiation background. This proves vitally important for quantitative extraction of average electron energies (temperatures) and emitted total charge.