Device for energy determination and determination of a depth dose distribution of particle radiation

Abstract

The invention relates to a device for energy determination and determination of a depth dose distribution of particle radiation, at least comprising a radiation detector (3) having an objective lens for the spatially resolved detection of photons, and a phantom (1) which is in the form of a wedge and has a first and a second face (1F, 2F) arranged at a 90° angle relative to one another, a third face 3F opposite the angle formed by the first and second face (1F, 2F), as well as an interaction layer (2) at least made of a luminescent material. According to the invention, the interaction layer (2) is arranged on the phantom (1) on the third face (3F), and the second face (2F) is parallel to a plane of the radiation detector (3).

Description

[0001]Designation Device for determining energy and a depth dose distribution of particle radiation Technical field The invention relates to a device for determining energy and a depth dose distribution of particle radiation, such as is used, for example, in proton therapy for tumors. State of the art Devices for determining energy and a depth dose distribution of particle radiation are known from the prior art. The depth dose distribution of particle radiation describes the absorbed radiation energy of a particle beam as a function of the so-called travel distance, i.e. the distance traveled by the particles in the particle beam in the irradiated material. To determine the depth dose distribution in so-called soft tissue (all tissue except bone), the depth dose distribution is usually measured in a so-called water phantom.Phantoms are structures that replicate physical properties, such as the absorption behavior of biological tissue. The behavior of particle beams in soft tissue is very similar to that in water, so the insights gained from the water phantom about the properties of the particle beam can be transferred to soft tissue. Instead of water phantoms, a plastic or similar material can be used that exhibits a permeability to ionizing radiation from a source comparable to that of living tissue. Here, too, the water equivalence is crucial, which is equivalent to that of a water phantom. Common water-equivalent plastics and other water-equivalent materials are those with a density close to 1 g / cm. 3and are predominantly composed of the light elements hydrogen, oxygen and carbon, such as those from the open-ended group of polymethyl methacrylate (PMMA, also known as “Plexiglas®”), polyethylene, polystyrene, epoxy resins, urethane and petrolatum. The energy determination of a particle beam, in particular a predominantly mono-energetic beam, can be determined via the penetration depth in a material and thus also in a phantom. For this purpose, the devices are usually calibrated accordingly. In addition to a phantom made of a corresponding material with an absorption behavior adapted to soft tissue, the depth dose distribution of a particle beam and also the energy determination in materials whose absorption coefficient is not adapted to soft tissue can be determined if corresponding conversion factors are known through calibration.For the purposes of the invention, the term phantom therefore also includes a material made of a material that has a different absorption coefficient than that of soft tissue. The absorbed dose is the energy transferred by the particle radiation to the matter in a volume element, i.e. the energy absorbed by the matter per mass of the volume element. The unit is Gray (symbol: Gy, unit: J / kg). The absorbed dose rate is the quotient of the absorbed energy dose within a specific time (Gy / h). After entering matter, the particles of a particle beam are gradually absorbed on their path of travel and come to a complete stop after a path characteristic of their energy, i.e. they are completely absorbed after this distance.The change in the degree of absorption over the travel distance, which determines the depth dose distribution, depends on the type of particles in the particle beam and is characteristic of these. For the purposes of the invention, particle beams are all those that consist of charged particles, in particular positively charged particles such as protons and carbon ions or other cations. Predominantly monoenergetic proton or cation beams, i.e. those with predominantly uniform energy, display a so-called Bragg curve in the depth dose distribution with a steep increase in the energy dose, i.e. the absorption, in the form of a peak (also referred to as a Bragg peak) at the end of the travel distance. The use of proton or cation radiation, i.e. the formation of a Bragg peak in the depth dose distribution in tissue, is based on this property of the absorption of proton or cation radiation, i.e. the formation of a Bragg peak in the depth dose distribution in the tissue, for example.for the treatment of tumors, in which a volume of deeper tissue can be specifically exposed to a high dose depending on the energy of the protons or cations. When planning therapeutic radiation, it is therefore essential to determine the depth dose distribution and energy of a particle beam to be used in advance and to adapt and control it to the therapeutic requirements. In Article 1 by C. Kunert et al. (A Multi-leaf Faraday Cup Especially for the Therapy of Ocular Tumors with Protons, in 5th Proceedings of the International Particle Accelerator Conference 2014 (IPAC'14), Dresden, Germany, June 2014, pp. 2149-2152), a device for determining the energy and depth dose distribution of particle radiation, in particular proton beams, is disclosed, which device comprises a multi-leaf Faraday cup (MLFC).The MLFC consists of a stack of alternating electrically insulating and electrically conductive plates. Each conductive plate is connected to an ammeter. The number and layer thickness of the plates used (here 50 channels) allows the absorption behavior of particle radiation in a specific distance of water to be simulated. However, the manufacturing costs for an MLFC are high. The energy of the particle radiation to be determined is also limited by the thickness of the pre-absorber, which brings the Bragg peak into the measurement range of the MLFC. The measurement resolution is 0.1 mm, and the measurement time is in the range of 10 s to 20 s. Furthermore, only the Bragg peak is recorded, which limits the information. DE.TEC.TOR. Devices and Technologies Torino Srl offers a medical device ("QEye") for energy determination and depth dose distribution of particle radiation, particularly proton beams, based on MFC technology with 512 channels.The resolution is 0.12 mm. Ionization chambers in conjunction with different phantoms are also used in a variety of ways to detect particle beams, as described, for example, in Article 2 by J. Medin et al. (Ionization chamber dosimetry of proton beams using cylindrical and plane parallel chambers. Nw versus NK ion chamber calibrations, Physics in Medicine & Biology, Vol. 40, 1995, pp. 1161 - 1176). Measurements of this type have the disadvantage of long measurement times. Instead of ionization chambers, other detection systems such as diodes or camera systems such as so-called CCD (Charge Coupled Device) cameras can be used, as described, for example, in Article 3 by S. Beddar et al. (Exploration of the potential of liquid scintillators for real-time 3D dosimetry of intensity modulated proton beams, Medical Physics, Vol.36, No.5, 2009, pp.1737 -1743) in combination with a scintillator.Such measurements also suffer from the disadvantage of long measurement times. In article 4 by Y. Fukushima et al. (Development of an easy-to-handle range measurement tool using a plastic scintillator for proton beam therapy, Physics in Medicine and Biology, Vol. 51, 2006, pp. 5927–5936), the depth dose profile of a proton beam in a scintillator is measured using a CCD camera. The setup described here is considerably large and therefore unwieldy. The Bragg peak is only recorded indirectly by calculating various ratios. The resolution is 0.24 mm / pixel and can therefore be classified as rather coarse.Task The task of the present invention is to provide a device for determining the energy and depth dose distribution of particle radiation, which delivers results on the energy and depth dose distribution of particle radiation in a time period that is shorter than the prior art, in an energy range that is wider than the prior art, and which can also be manufactured at low cost. This task is solved by claim 1. Advantageous embodiments are the subject of the dependent claims. The device according to the invention for determining the energy and depth dose distribution of particle radiation comprises at least the components listed below.The device initially comprises a phantom which, in particular, as also corresponds to one embodiment, is formed from a water-equivalent material and which has at least a first and a second surface which are arranged at a 90° angle to one another and have the shape of a wedge. The first and the second surface are advantageously square. The first surface is intended for the perpendicular incidence of a particle beam, and the second surface is intended to be arranged parallel to a planar, spatially resolving radiation detector. The wedge further comprises a third surface which is arranged opposite the 90° angle between the first and second surfaces and, together with the first and the second surface, completes an irregular triangular prism which, in the case of the wedge, is also closed off by two lateral surfaces (base surfaces of the prism).The volume of the phantom is designed to encompass the entire path of the particle beam irradiated by the particle beam, i.e. the entire beam from the first surface until complete absorption or, in the case of electrons, at least sufficient absorption. The volume must therefore be adapted, if necessary, to the energy and diameter of the particle beam. According to the invention, the material is in particular, as is also the case in one embodiment, a plastic or similar material which has a permeability comparable to that of living tissue for ionizing radiation from a source used. In radiation medicine, such a phantom is also referred to as a water phantom if it is predominantly made of water. Common water-equivalent plastics are, for example,those from the open-ended group of polymethyl methacrylate (PMMA, also known as “Plexiglas®”), polyethylene, polystyrene, epoxy resins, urethane and petroleum jelly. The material of the phantom is advantageously radiation-hard, i.e. durable against particle radiation but also against electromagnetic radiation. An interaction layer comprising at least one phosphor is applied to the third surface of the wedge. A phosphor within the meaning of the invention is a material which, when charged particles pass through it, is converted into an excited state through interaction with the particles in the form of collision processes, which spontaneously relaxes (fluorescence) while emitting electromagnetic radiation, usually in the spectrum of visible light. The interaction layer can be in the form of a film arranged on the third surface of the wedge.The interaction layer can also be deposited on the third surface of the wedge. The material of the interaction layer is advantageously radiation-hard, meaning it is resistant to both particle radiation and electromagnetic radiation. Materials suitable for a phosphor are, in particular, those that emit spontaneously, such as those from the open group consisting of: thallium-doped cesium iodide (Th:CsI), cesium iodide (CsI), terbium-doped gadolinium oxysulfide (Tb:Gd2O2S), and bismuth germanium oxide (Bi4Ge3O). 12 ), barium fluoride (BaF), yttrium aluminum garnet (Y3Al5O 12), calcium tungstate (CaWO4), europium-doped calcium fluoride (Eu:CaF2). The phosphor can also be produced from an organic scintillator in combination with another fluorescent material. For each phosphor, the signals obtained may need to be corrected for undesired effects, in particular so-called “quenching,” as described, for example, in Article 5 by L. Kelleter and S. Jolly (A mathematical expression for depth-light curves of therapeutic proton beams in a quenching scintillator, Medical Physics, Vol. 47 (5), 2020, pp. 2300 – 2308) and otherwise known to the person skilled in the art. The interaction layer is always arranged on the third surface of the wedge so that it extends to the common edge of the third and first surfaces of the wedge. This is essential for the application of the device for determining the energy and the depth dose distribution, since the first point of contact (or-line) of the particle beam with the interaction layer must correspond to a zero point (starting point) of the travel path in the phantom, since otherwise the travel path is not completely recorded and thus the corresponding analyses for determining the energy and the depth dose distribution are incorrect. In addition, the interaction layer advantageously covers the entire third surface of the wedge. Calculations for the dimensioning of the wedge and the interaction layer, so that a particle beam lies within the volume of the phantom over its entire travel path and can be detected by the interaction layer, do not have to be carried out separately between the interaction layer and the wedge. The material of the phantom must be largely (> 95%) transparent to the electromagnetic radiation emitted by the interaction layer during irradiation (usually in the spectrum of visible light).Furthermore, a radiation detector for the spatially resolved detection of photons (the electromagnetic radiation from the interaction layer) is included as a component of the device, such as a so-called CCD camera or, advantageously, an active pixel detector using CMOS (complementary metal-oxide-semiconductor) technology. Other detector systems such as so-called imaging plates, X-ray films or similar are possible, but may require adaptation in the evaluation. The detector must be sensitive at least in an expected intensity range and energy range of an electromagnetic radiation (photons, mostly in the spectrum of visible light) incident on it and emitted by the interaction layer. The spatially resolved detection must also be designed in such a way that a depth dose distribution occurring depending on the particle radiation to be characterized, e.g.in the form of a Bragg curve, a particle beam can be represented with sufficient resolution, usually in a range of 0.01 mm. 2 / pixel - 1 mm 2 / pixel. The surface of the spatially resolving radiation detector is arranged parallel to the second surface of the phantom. The size of the surface of the spatially resolving radiation detector must be adapted to the penetration depth (travel path) of a particle beam in the phantom, taking into account a lens possibly arranged between the phantom and the detector, as corresponds to one embodiment (see below), and perpendicular to this, to its diameter. These values ​​are usually in the range of a few centimeters. Advantageously, the particle beam to be characterized is a homogeneous beam. The beam width and travel path of the beam must also correlate with the dimensions of the phantom and the interaction layer. If necessary, the diameter of the particle beam must be adapted to the phantom size, e.g., by cropping it with an aperture.Furthermore, an objective lens is advantageously arranged between the phantom and the spatially resolved radiation detector, which focuses the electromagnetic radiation from the interaction layer and images it onto the radiation detector, thus achieving a favorable ratio of image to image on the radiation detector, as also corresponds to one embodiment. The distance between the objective lens and the phantom and / or the radiation detector must therefore also be adapted to the path of the particle beam in the phantom. The sizes of the first and second surfaces of the phantom and the size of the interaction layer, as well as the angles formed between them and the surface normals of the first and second surfaces, must be adapted to a diameter and a penetration depth in the phantom of a particle beam to be characterized with the device with regard to a depth dose distribution.The interaction layer is therefore arranged at an angle to the first surface that ensures complete traversal of the interaction layer by a particle beam incident perpendicularly on the first surface up to its maximum or at least achievable (in the case of electrons) penetration depth, forming the irradiated volume in the phantom, in the direction of the particle beam. The device is therefore adapted to an energy range that determines the penetration depth in the phantom and the diameter of the particle beam to be characterized by the device. The orders of magnitude and ranges of these parameters of a particle beam are known to the person skilled in the art, at least to a sufficient approximation, by other means, or can be determined separately experimentally.The irradiated volume that can be generated by a particle beam of a given energy and diameter in a material from which the phantom is formed should advantageously be completely encompassed by the phantom of the device. In a further embodiment of the invention, the interaction layer is arranged between two identical wedges. This design offers improved stability and handling of the device.In a method in which the device according to the invention is used for determining energy and a depth dose distribution of particle radiation to characterize a particle beam, the first surface of the phantom of the device is to be aligned perpendicular to a provided, incident particle beam in such a way that the particle beam covers the interaction layer and the edge of the first and third surfaces of the wedge is always included, so that the zero point of the path of the particle beam in the phantom is always included by the interaction layer. Advantageously, the diameter of the particle beam is smaller than the dimensions of the first surface and can be completely covered by the interaction layer. The dimensioning of the particle beam can be influenced and adjusted using apertures.The particle beam then forms an irradiated volume in the wedge of the phantom, which is delimited by the interaction layer along the entire path, depending on the distance already traveled. The particle beam causes the emission of electromagnetic radiation (usually in the visible light spectrum) in the interaction layer, corresponding to the depth dose distribution, i.e., the absorbed radiation energy at each location along the path of the beam's penetration up to its maximum penetration depth.The electromagnetic radiation emitted by the interaction layer after irradiation is then detected by the spatially resolving radiation detector, which is arranged parallel to the second surface of the phantom, in the projection of the irradiated interaction layer (the one located in the irradiated volume of the phantom) onto the plane of the radiation detector, and its intensity distribution is imaged. Thus, an image of the depth dose distribution is generated. In a third embodiment, the device according to the invention is housed in a housing for shielding the influence of electromagnetic radiation (light), which housing has an opening as a beam inlet opposite the first surface of the phantom. All components of the device are arranged within the shield.The spatially resolving radiation detector is usually equipped with readout electronics and is connected to a device for data processing and graphic display for data processing. The advantage of the device according to the invention lies in particular in the measurement time of < 1 second, which is achieved by choosing the phantom with interaction layer, the type of interaction layer and the spatially resolving radiation detector. In addition, the device is cost-effective and simple in construction and easy to adapt to given conditions. Exemplary embodiment The invention will be explained in more detail using an exemplary embodiment and two figures. The figures show: Fig. 1: Schematic representation of a cross-section through the device according to the invention with a) a higher-energy incident particle beam and b) a particle beam with lower energy than in a). Fig. 2: Depth dose distribution of a particle beam (count rate vs.Extension along the travel path); evaluation from a measurement with the device according to the invention a) according to Fig. 1a) and b) according to Fig. 1b). In the exemplary embodiment of a device according to the invention for energy determination and depth dose distribution of particle radiation, as such is also shown schematically in Fig. 1 (a) and b)), the phantom 1 is made of polymethyl methacrylate and has the dimensions of width (b) 5 cm, height (h) 3 cm and depth (perpendicular to the plane of the sheet) 3 cm and is formed from two wedges. The interaction layer 2 is provided by a film which comprises terbium-doped gadolinium oxysulfide (Tb:Gd2O2S) as the phosphor. The interaction layer 2 is arranged at an angle ^ of 15° to the second surface 2F of the phantom 1 on the third surface 3F and extends from one edge to the diagonally opposite edge (figure not to scale).The wedge, which is formed from the first surface 1F, the second surface 2F, and the third surface 3F, is visible. For easier handling, a second wedge is arranged so that the interaction layer 2 lies between the wedges. The spatially resolving radiation detector 3 is provided by a commercially available digital camera based on CMOS technology with an integrated lens, which has 1.3 MP at a resolution of 1280x1024 and a frame rate of 85 fps, with a minimum illumination duration of 11 µs. The device is arranged within a metal shield 4, which has an opening 5 for the inlet of a particle beam and otherwise completely encloses the device and protects it from light. The opening 5 is arranged opposite the first surface 1F of the phantom 1, so that a particle beam can be irradiated perpendicularly onto the first surface 1F of the phantom 1. In Fig.1, incident particle beams (solid lines) are also shown for illustration purposes. The area of ​​a potential further beam path of the particle beam behind the interaction layer 2 (dashed line) and the aperture of the objective of the radiation detector 3 (dash-dotted line) are also indicated. In Fig. 1a), a particle beam is shown, which in the exemplary embodiment is a proton beam, with an energy of 68 MeV, which is sufficient for the beam path to end after 3.5 cm. The diameter of the beam is 10 mm (image is not to scale). In Fig. 1b), the proton beam has a lower energy of 50 MeV, with the same diameter. The path ends here after 2.1 cm. Therefore, only a part of the beam reaches the evaluation. If the portion of the particle beam that is detected by the interaction layer 2 is too small orIf the path of the beam extends beyond the last intersection point with the interaction layer 2, the dimensions of the phantom 1 and the arrangement of the interaction layer 2 must be adjusted. Fig. 2 shows the evaluation of the depth dose profile of the particle beams, which in the exemplary embodiment are proton beams, for a) Fig. 1a) and b) Fig. 1b). The plot corresponds to the count rate ([au]) over the distance in the detector image in mm. On the one hand, the depth dose profile (---) along the path of travel is shown, and on the other hand, the cross-section ( ^ ^ ^) of the image of the particle beam on the detector 3. The extension of the depth dose profile begins after the first peak, which results from a reflection within the wedge, and ends at the top of the Bragg peak. The cross-section is obtained by a section perpendicular to the path of travel of the particle beam, and the depth dose profile is obtained by summing over the cross-section in the image in the detector.The underlying images with the radiation detector for analysis are each acquired at a time of 1 second. As shown, the invention advantageously combines cost-effective, simple, and time-saving acquisition of the energy and depth dose profile of a particle beam.

Claims

Claims 1. Device for determining the energy and depth dose distribution of particle radiation, comprising at least: - a radiation detector (3) with a lens for spatially resolved detection of photons, - a wedge-shaped phantom (1) with a first and a second surface (1F, 2F) arranged at a 90° angle to each other and a third surface (3F) opposite the angle of the first and second surfaces (1F, 2F), - an interaction layer (2), at least formed of a phosphor, wherein the interaction layer (2) is arranged on the phantom (1) on the third surface (3F) and wherein the second surface (2F) is arranged parallel to a plane of the radiation detector (3).

2. Device according to claim 1, characterized in that a second wedge is arranged such that the interaction layer (2) lies between the wedges. 3.Device for determining the energy and depth dose distribution of particle radiation according to claim 1, characterized in that the device is housed in a shielding casing (4).

Images