DOSIMETRY WITH THE ABILITY TO DISTINGUISH PULSED AND NON-PULSED DOSE CONTRIBUTIONS.

The concept of an active dosimetry system for pulsed radiation dose rate measurements is presented. Real-time distinction of pulsed and non-pulsed radiation contributions is based on the time structure of a single interaction. A fast tissue equivalent plastic scintillator is exploited to minimize the pile-up effect influence on absorbed energy measurements. Being connected to a fully digital signal processing board, the detector creates an active dosimetry system with adjustable parameters. With this system, absorbed dose rate measurements were carried out in a photon field with a time structure mimicking a radiotherapeutic beam, but also in the presence of a constant radiation field. Measurements show a linear dependence of a pulsed radiation contribution on the accelerator current in the investigated range of the total dose rate up to 8 µGy h-1. While increasing the accelerator current by 1 µA, the pulsed radiation dose rate grows by (26.2 ± 0.9) nGy h-1 when considering pile-up events.

In-beam PET at clinical proton beams with pile-up rejection.

Positron emission tomography (PET) is a means of imaging the ß+-activity produced by the radiation field in ion beam therapy and therefore for treatment verification. Prompt γ-rays that are emitted during beam application challenge the detectors and electronics of PET systems, since those are designed for low and medium count rates. Typical PET detectors operated according to a modified Anger principle suffer from multiple events at high rates. Therefore, in-beam PET systems using such detectors rely on a synchronization of beam status and measurement to reject deteriorated data. In this work, a method for pile-up rejection is applied to conventional Anger logic block detectors. It allows for an in-beam data acquisition without further synchronization. Though cyclotrons produce a continuous wave beam, the radiation field shaping technique introduces breaks in the application. Time regimes mimicking synchrotrons as well as cyclotron based ones using double-scattering or pencil beam scanning field shaping at dose rates of 0.5, 1.0 and 2.0Gy/min were investigated. Two types of inhomogeneous phantoms were imaged. The first one simulates cavity structures, the other one mimics a static lung irradiation. It could be shown that, depending on the dose rate and the beam time structure, in-beam measurement including a few seconds decay time only, yield images which revealed all inhomogeneities in the phantoms. This technique can be the basis for the development of an in-beam PET system with traditional detectors and off-the-shelf electronics.

Compton Camera and Prompt Gamma Ray Timing: Two Methods for In Vivo Range Assessment in Proton Therapy.

Proton beams are promising means for treating tumors. Such charged particles stop at a defined depth, where the ionization density is maximum. As the dose deposit beyond this distal edge is very low, proton therapy minimizes the damage to normal tissue compared to photon therapy. Nevertheless, inherent range uncertainties cast doubts on the irradiation of tumors close to organs at risk and lead to the application of conservative safety margins. This constrains significantly the potential benefits of protons over photons. In this context, several research groups are developing experimental tools for range verification based on the detection of prompt gammas, a nuclear by-product of the proton irradiation. At OncoRay and Helmholtz-Zentrum Dresden-Rossendorf, detector components have been characterized in realistic radiation environments as a step toward a clinical Compton camera. On the one hand, corresponding experimental methods and results obtained during the ENTERVISION training network are reviewed. On the other hand, a novel method based on timing spectroscopy has been proposed as an alternative to collimated imaging systems. The first tests of the timing method at a clinical proton accelerator are summarized, its applicability in a clinical environment for challenging the current safety margins is assessed, and the factors limiting its precision are discussed.

Range assessment in particle therapy based on prompt γ-ray timing measurements.

Proton and ion beams open up new vistas for the curative treatment of tumors, but adequate technologies for monitoring the compliance of dose delivery with treatment plans in real time are still missing. Range assessment, meaning the monitoring of therapy-particle ranges in tissue during dose delivery (treatment), is a continuous challenge considered a key for tapping the full potential of particle therapies. In this context the paper introduces an unconventional concept of range assessment by prompt-gamma timing (PGT), which is based on an elementary physical effect not considered so far: therapy particles penetrating tissue move very fast, but still need a finite transit time--about 1-2 ns in case of protons with a 5-20 cm range--from entering the patient's body until stopping in the target volume. The transit time increases with the particle range. This causes measurable effects in PGT spectra, usable for range verification. The concept was verified by proton irradiation experiments at the AGOR cyclotron, KVI-CART, University of Groningen. Based on the presented kinematical relations, we describe model calculations that very precisely reproduce the experimental results. As the clinical treatment conditions entail measurement constraints (e.g. limited treatment time), we propose a setup, based on clinical irradiation conditions, capable of determining proton range deviations within a few seconds of irradiation, thus allowing for a fast safety survey. Range variations of 2 mm are expected to be clearly detectable.