Mixed particle beam for simultaneous treatment and online range verification in carbon ion therapy: Proof‐of‐concept study

Abstract

Purpose Radiation therapy with ion beams provides a better conformation and effectiveness of the dose delivered to the tumor with respect to photon beams. This implies that a small uncertainty or variation in the crossed tissue shape and density may lead to a more important underdosage of the tumor and/or an overdosage of the surrounding healthy tissue. Although the online control of beam fluence and transverse position is well managed by an appropriate beam delivery system, the online measurement of the longitudinal position of the Bragg peak inside the patient is still an open issue. In this paper we propose a proof‐of‐concept study of a technique that would allow the online verification of the patient thickness along the beam direction, which could permit detecting a subset of possible range error causes, such as morphological variations. Methods The nuclei 12C and 4He have the same magnetic rigidity: the two species could be accelerated together in an accelerator and a mixed particle beam delivered to the patient. In the same medium and with the same energy per nucleon, the range of 4He2+ is about three times the 12C6+ one. It is, thus, conceivable to achieve a dual goal with a single mixed beam: carbon, stopping into the tumor, is appointed to cure, while helium, emerging from the patient, to control: by detecting and measuring the residual range and position of He, it would be possible to determine the integrated relative stopping power of the patient and prove that it is the expected one. For the detection of helium particles, a plastic scintillator and an optical sensor are proposed. Being helium ions not available at CNAO, the detection system has been characterized using a proton beam. Nevertheless, since the light emitted by a proton is less than the one produced by a helium ion, the helium signal is expected to be more pronounced than the proton one (for the same number of particles). To predict the magnitude of the light signal measured by the sensor, two Monte Carlo models have been setup and validated by measuring the photons per pixel impinging on the sensor. To deal with the many optical issues and to reliably describe the physical process, some corrections have been included into the models. Results The predictions of both the models are in good agreement with the measurements (within the 20% in terms of absolute photons per pixel). The proposed detection system is able to measure the range of a proton beam with sub‐millimetrical precision also in the presence of the background produced by carbon ion fragments and discrepancies in the expected range were detected with a resolution better than 1 mm. Conclusions Although many technical issues have still to be addressed for a real implementation in a clinical environment, the preliminary results of this study suggest that a surrogate of real‐time verification of the beam range inside the patient during a treatment with carbon ions is possible by adding a small fraction of helium ions to the primary beam.

Development and Validation of Single Field Multi-Ion Particle Therapy Treatments

“…Mixed particle beams have been theorized for simultaneous treatment and online range verification in carbon ion therapy. 46 Similarly, CICR C-p treatments could provide a more ideal signal ([2.5x) for prompt gamma imaging techniques than carbon ions alone (Dal Bello et al, unpublished data, 2019), considering the increased physical dose from the distal proton branch (Fig. 2b).…”

Radioactive Beams in Particle Therapy: Past, Present, and Future

“…The method has been tested also for high-energy C-ions in phantoms [24,25]. Several other methods have been proposed, such as ionoacoustic measurements [26] or mixed beams [27]. For C-ions, it is also possible to measure secondary charged particles, such as protons emitted at large angles [28,29].…”

A Single-Particle Trigger for Time-of-Flight Measurements in Prompt-Gamma Imaging

“…Despite the absence of fiber cladding, we were able to decompose the cross-talk events between neighboring fibers by evaluating the energy deposited in the alternating fibers. This evaluation may be further used in beam diagnostics, e.g., in mixed beams [49,50], where the carbon beams are used for treatment and the helium beams are used for imaging. Such prototype of a trigger system may also be used for cross-section measurements of carbon ions hitting a thin target.…”