Heavy-ions shielding data for hadrontherapy application with Monte Carlo methods.

In hadrontherapy centers, ion beams are accelerated by a cyclotron or synchrotron through electric and magnetic fields and, once the treatment energy is reached, ions interact with the patient. The interaction of the beam with magnets, other elements of particle accelerator or with the patient generates secondary radiation and the calculation for the design of the shielding is required. A first assessment of the shielding, generally made by concrete, can be performed by using Monte Carlo simulations, in which neutron yields and the ambient dose equivalent, H*(10), spatial distribution are obtained. The aim of this work is to evaluate, with a Monte Carlo approach, ambient dose equivalent values at various concrete depths generated by the secondary radiation. Simulations are performed considering helium, lithium, carbon, oxygen and iron primary beams. For existing facilities that already accelerates carbon ion, an equivalent carbon ratio is introduced to estimate the ion currents to get, in a given point, the same ambient dose equivalent as a carbon ion. Numerical results have been obtained simulating an ion beam that impinges on a thick iron or international commission on radiation units and measurements (ICRU) tissue, respectively, representing magnets and patient. The secondary radiation is transported through a large concrete shielding where ambient dose equivalent values are calculated as function of concrete thickness.

Ultrasound-assisted carbon ion dosimetry and range measurement using injectable polymer-shelled phase-change nanodroplets: in vitro study.

Methods allowing for in situ dosimetry and range verification are essential in radiotherapy to reduce the safety margins required to account for uncertainties introduced in the entire treatment workflow. This study suggests a non-invasive dosimetry concept for carbon ion radiotherapy based on phase-change ultrasound contrast agents. Injectable nanodroplets made of a metastable perfluorobutane (PFB) liquid core, stabilized with a crosslinked poly(vinylalcohol) shell, are vaporized at physiological temperature when exposed to carbon ion radiation (C-ions), converting them into echogenic microbubbles. Nanodroplets, embedded in tissue-mimicking phantoms, are exposed at 37 °C to a 312 MeV/u clinical C-ions beam at different doses between 0.1 and 4 Gy. The evaluation of the contrast enhancement from ultrasound imaging of the phantoms, pre- and post-irradiation, reveals a significant radiation-triggered nanodroplets vaporization occurring at the C-ions Bragg peak with sub-millimeter shift reproducibility and dose dependency. The specific response of the nanodroplets to C-ions is further confirmed by varying the phantom position, the beam range, and by performing spread-out Bragg peak irradiation. The nanodroplets' response to C-ions is influenced by their concentration and is dose rate independent. These early findings show the ground-breaking potential of polymer-shelled PFB nanodroplets to enable in vivo carbon ion dosimetry and range verification.

A Novel Approach to Design and Evaluate BNCT Neutron Beams Combining Physical, Radiobiological, and Dosimetric Figures of Merit.

(1) Background:The quality of neutron beams for Boron Neutron Capture Therapy (BNCT) is currently defined by its physical characteristics in air. Recommendations exist to define whether a designed beam is useful for clinical treatment. This work presents a new way to evaluate neutron beams based on their clinical performance and on their safety, employing radiobiological quantities. (2) Methods: The case study is a neutron beam for deep-seated tumors from a 5 MeV proton beam coupled to a beryllium target. Physical Figures of Merit were used to design five beams; however, they did not allow a clear ranking of their quality in terms of therapeutic potential. The latter was then evaluated based on in-phantom dose distributions and on the calculation of the Uncomplicated Tumor Control Probability (UTCP). The safety of the beams was also evaluated calculating the in-patient out-of-beam dosimetry. (3) Results: All the beams ensured a UTCP comparable to the one of a clinical beam in phantom; the safety criterion allowed to choose the best candidate. When this was tested in the treatment planning of a real patient treated in Finland, the UTCP was still comparable to the one of the clinical beam. (4) Conclusions: Even when standard physical recommendations are not met, radiobiological and dosimetric criteria demonstrate to be a valid tool to select an effective and safe beam for patient treatment.

Design of a BNCT irradiation room based on proton accelerator and beryllium target.

Preliminary studies for the design of an accelerator-based BNCT clinical facility are presented. The Beam Shaping Assembly neutron activation was evaluated experimentally and with Monte Carlo simulations. The activations of patient, air and walls in the room, the absorbed doses by the patient and the in-air dose distributions were evaluated. Based on these calculations, different walls compositions were tested to optimize the environmental conditions. Borated concrete, advantageously reducing the thermal flux in the room, was proven the best choice.

[Exposure risk for the workers of a hadrontherapy center and collective and individual protection measures]. / Rischio espositivo per i lavoratori di un centro di adroterapia e misure di protezione collettiva e individuale.

SUMMARY: In the last few years a wide dissemination of hadrontherapy facilities is taking place. In these facilities, proton or heavy ion (mainly carbon) accelerators are used to treat cancers in peculiar positions (i.e. close to critical organs), or with peculiar biological features that make them not eligible for conventional radiation therapy with photons. During the design, the commissioning and the use of these facilities many radiation safety issues are to be addressed, that are different from the ones that the professionals in the field are used to facing. Many problems need to be solved, among which the characterization of the radiations fields produced by the accelerators, the shielding design, the design of the interlock systems, and the management of the activated materials (PE11). Both the personal and environmental dosimetry systems need to be set up and implemented, taking into consideration the peculiarities of the involved radiation fields, that are often made of many different high energy particles. So, the approach to this kind of problems is usually much more complex than the one that is required for lower energy machines, and the adopted techniques are much more similar to the ones used for the high energy research accelerators. Due to the complexity of the physics involved in the radiation/matter interaction at these energies, the radiation safety calculations are often based on Monte Carlo simulations (that take into account all the physical processes for all the particles involved), and the data should be cross-checked with the experimental data available in literature (e.g. Na06). Moreover, all the radiation measurements must be carried out with instruments conceived for this kind of radiation fields, or anyway with instruments whose behavior can be foreseen also when measuring in high energy mixed fields (Na04). The shielding design and the activation evaluations obviously depend on the different accelerator technologies (e.g. if synchrotrons, or cyclotrons, are used) and on the energy and nature of the accelerated beam. On the other hand, while the technologies used for the interlock safety systems are well known, a big research and development effort is still ongoing about the technologies adopted for personal or environmental dosimetry. Anyway, while the state-of-the-art of instrumentation is still far from being completely satisfactory, many detectors are available, that can be a good option to solve some of the measurement problems found in such environments.

Impact of the newly proposed ICRU/ICRP quantities on neutron calibration fields and extended range neutron rem-counters.

In July 2017, the International Commission on Radiation Units and Measurements (ICRU) and the International Commission on Radiological Protection (ICRP) proposed the introduction of new operational quantities for external radiation exposure, with the aim of improving coherence between protection quantities and operational quantities within the system of radiological protection. A change in operational quantities will impact both instrumentation and reference radiation fields used for their calibration. This paper evaluates the potential impact of the new quantity ambient dose, H*, meant to replace ambient dose equivalent, H*(10), on two neutron reference fields, the Am-Be source and the CERF high-energy workplace field, and on the response of two models of extended-range neutron rem counters (LINUS and LUPIN). The conclusions are that calibration procedures should in general not be affected and that changes should only be expected in calibration coefficients. Considering the acceptable measurement uncertainties for operational radiation protection, for the extended-range rem counters changes in their design would not be required for measurements outside particle accelerators shielding and for aircrew dosimetry. One can expect that this type of instrument can still be calibrated with Am-Be source neutrons and employed in neutron fields with energy distributions spanning several decades. For uses in radiation fields with very peculiar neutron energy distribution, a specific workplace field calibration may instead be required.

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

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 12 C and 4 He 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 4 He2+ is about three times the 12 C6+ 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.