Abstract
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.
Highlights
In the first decades of the 20th century, during the rise of particle accelerators, physicists studied the interaction of fast charged particles with matter
Rather than an in-depth review of the literature, this manuscript provides a summary of two separate topics developed within the collaborative framework of ENTERVISION [52]: the characterization of detector components for the absorber plane of a clinical Compton camera and the first test of the Prompt Gamma Ray Timing (PGT) method with heterogeneous phantoms at a clinical proton center, corresponding to the publications [53] and [29], respectively
For the acquisition of PGT spectra, the detection time of the gamma rays with respect to the arrival of the protons to the target has to be measured
Summary
In the first decades of the 20th century, during the rise of particle accelerators, physicists studied the interaction of fast charged particles with matter. The absence of tools in clinical routine for measuring in vivo and in real time, the actual distal fall-off edge, together with the high sensitivity of the proton range to tissue composition, force medical physicists to add safety margins and apply field patching techniques in order to obtain a robust treatment plan [17]. The spatial emission distribution correlates to the dose deposition map of the incident protons [21, 22] and provides an indirect measurement of the particle range Such correlation is dependent on prompt gamma ray energy and tissue composition [23,24,25,26] and stems from the maximum of the nuclear cross section at low (~10-MeV) proton energies [27]. Rather than an in-depth review of the literature, this manuscript provides a summary of two separate topics developed within the collaborative framework of ENTERVISION [52]: the characterization of detector components for the absorber plane of a clinical Compton camera and the first test of the PGT method with heterogeneous phantoms at a clinical proton center, corresponding to the publications [53] and [29], respectively
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