Transmission calorimeter for measuring the dose of radiation

Abstract

A transmission calorimeter for measuring the dose of a radiation beam includes a core that receives the radiation and transmits the radiation along a radiation path, with the radiation path passing through the core, and at least one sensor for measuring a temperature change of the core, and the energy of the radiation absorbed by the calorimeter is less than or equal to the energy that would be absorbed by the radiation passing through 2 mm of water.

Description

Technical Field

[0001] The present invention relates to a transmission calorimeter for measuring the dose (or area dose or fluence) of a radiation beam, and more particularly to a transmission calorimeter for measuring radiation at a relatively high dose rate (e.g., a dose rate exceeding 40 Gy / second) delivered to a patient during radiotherapy.

Background Art

[0002] It is well known to use radiation for the treatment of patients (e.g., for the treatment of tumors), and the use of radiation can include the use of beams of protons, electrons or photons. In order to deliver an appropriate dose to the patient, it is important that the radiation beam (regardless of type) is properly quantified. Conventionally, this has been done by passing the radiation beam through a transmission ionization chamber in front of the patient and measuring the dose of the delivered radiation. In order to be able to use the ionization chamber in this way, the ionization chamber should first be calibrated, which is usually done by using an absorption calorimeter or a secondary standard ionization chamber (usually a calorimeter or a free air chamber) that is traceably calibrated against a primary standard and measuring the dose in a phantom as a substitute for the patient.

[0003] In a known arrangement (shown in FIG. 1), a radiation source (A) generates a radiation beam (B), the radiation beam (B) passes through a transmission ionization chamber (C), and the transmission ionization chamber (C) measures the beam with a minimum perturbation without blocking the beam. The beam then enters a conventional calorimeter (D) embedded in the phantom and raises the temperature of the calorimeter by an amount determined by the dose of the radiation beam. Thus, this calorimeter functions as a primary standard reference device and is used to calibrate the transmission ionization chamber, and the transmission ionization chamber functions as a beam monitor that quantifies the fluence proportional to the dose at the calorimeter measurement point that has passed through the transmission ionization chamber. After the transmission ionization chamber is calibrated, the transmission ionization chamber can be used to measure the dose of the radiation beam directed at the patient (E) for treating the tumor (F). In summary, the signal of the beam monitor ionization chamber is calibrated with respect to the dose in the phantom.

[0004] The problem with using ionization chambers to measure radiation dose is that ionization chambers are not suitable for higher dose rates (e.g., those used in FLASH radiotherapy). FLASH radiotherapy is a radiotherapy modality in which dose rates exceeding 40 Gy / second are used relative to conventional dose rates (usually 6 Gy / minute) due to their enhanced healthy tissue sparing effects. The popularity of FLASH radiotherapy regimen investigations is increasing, and preclinical and clinical trials are being actively studied. Conventional radiotherapy (5 - 10 minutes / day, approximately 30 days) can be condensed into a single high-intensity burst of radiation lasting fractions of a second.

[0005] However, it is difficult to perform dose measurements under these conditions due to the extremely high dose rates that result in unacceptably high levels of the "ion recombination" effect within conventionally designed and operating ionization chambers. This leads to a large loss of ion collection efficiency that requires numerous corrections, and such losses result in a large uncertainty in the measured dose, and thus a large uncertainty in the dose delivered to the patient. See, for example, "Correction for Ion Recombination in a Built-in Monitor Chamber of a Clinical Linear Accelerator at Ultra-High Dose Rates", Konradsson et al., Radiation Research 194(6), 580 - 586, (June 22, 2020), and "Beam Monitors for Tomorrow: The Challenges of Electron and Photon FLASH RT", Vignati et al., Front. Phys. 8:375.

[0006] Various solutions have been proposed to solve this problem, but they all have drawbacks. A transmission ionization chamber coupled with an electronic device operating at the FLASH dose rate has been developed, but this is a compromise solution and it has been recognized that it still has the drawback of resulting in higher uncertainties than desired in the quantification of dose delivery. Silicon detectors have been developed, but they are very expensive, have a limited lifespan (since silicon is damaged over time by ionizing radiation), and also have limitations at higher dose rates. Finally, detectors based on luminescence can be used, but these have a high level of uncertainty and also have saturation thresholds.

[0007] Historically, there were only two types of calorimeters used in external beam radiotherapy. · Absorbed dose calorimeter. A small sensitive volume is enclosed within a larger environmental "phantom". Radiation is delivered to this larger phantom volume (including the sensitive volume). A variation of this is to measure the surface dose. · Total absorption calorimeter. A sensitive volume designed to be larger than the lateral size of the radiation beam and longer than the range of the particles being measured. The total energy of the particles can be inferred from these devices. To provide additional information regarding the radiation beam, these detectors can be segmented as a function of depth.

[0008] U.S. Patent No. 4,620,800 (A) (Research Dynamics Inc.) discloses a direct dosimeter for measuring a gamma radiation beam having a dose rate exceeding 0.1 megarad per hour. This dosimeter includes a gamma heating material such as aluminum or lead disposed within an evacuated housing. A thermocouple measures the temperature of the material when it is subjected to a high level of gamma radiation. The absolute value of the gamma radiation beam may be determined from the time rate of change of the temperature measurement before the temperature of the material reaches a steady state value.

[0009] U.S. Patent No. 4,312,224 (A) (Domen) discloses a calorimeter for absorbed dose water that utilizes the low thermal diffusivity of water and the water impermeability of a polyethylene film.

[0010] U.S. Patent No. 4,614,635 (A) (GEC) discloses an apparatus and method for analyzing signals generated by a detector placed in a field of gamma rays and neutrons in cooperation with an electrical system.

[0011] U.S. Patent Application Publication No. 2018 / 250529 (A1) (Sun Nuclear Corporation) discloses a graphite probe calorimeter for clinical reference dosimetry that functions as a secondary standard and enables direct measurement of radiation therapy dose in an absolute manner.

[0012] CN106125123A (Northwest Institute Of Nuclear Technology) discloses a calorimeter, as well as a system and method for measuring absorbed dose. This calorimeter includes a central ball and a hollow ball arranged concentrically with the central ball. The hollow ball includes an inner case, an insulating layer, and an outer case in order from the inside to the outside. The interior of the central ball includes a first thermistor. A second thermistor is arranged between the insulating layer and the outer case. The lead wires of the first thermistor are connected to a conducting wire through holes.

[0013] SU989963A1 (Suchkov) is a calorimeter for measuring pulsed ionizing radiation, which is a calorimeter comprising a thermocouple and a radiation absorber in the form of a pair of parallel plates. For the purpose of increasing the measurement accuracy and simplifying the structure, a thermocouple in the form of absorber plates made of materials having the same absorption capacity and different absolute thermoelectric powers is made, which are connected in pairs at the center and insulated at the periphery.

[0014] SU593554A1 (Belarus) is a calorimeter for measuring the local power of the absorbed dose of electron radiation, a calorimeter comprising an absorber and a measuring unit, characterized in that, in order to expand the measurement range towards the low-energy side, increase the measurement accuracy, and simplify the structure, the absorber is made of a radiation-resistant and heat-resistant electrically conductive polyimide film and is electrically connected to the measuring unit.

Prior Art Documents

Patent Documents

[0015]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Non-Patent Documents

[0016]

Non-Patent Document 1

Non-Patent Document 2

[0017] According to a first aspect of the present invention, there is provided a transmission calorimeter as an alternative to a conventional transmission ionization chamber for measuring the dose of a radiation beam, the calorimeter comprising a core for receiving the radiation and transmitting the radiation along a radiation path, the radiation path passing through the core, and at least one sensor for measuring a temperature change of the core, wherein the energy of the radiation absorbed by the calorimeter is less than or equal to the energy that would be absorbed by the radiation passing through 2 mm of water.

[0018] Surprisingly, it has been found that the dose of a radiation beam can be measured using a calorimeter constructed according to the present invention, and then, due to the radiation transmission characteristics of the calorimeter, the radiation beam can be used to treat a patient. Furthermore, such a calorimeter can be used to measure relatively high doses and dose rates of radiation (e.g., protons, electrons or photons) without the problems associated with the use of ionization chambers. In fact, it has been found that the measurement uncertainty decreases as a function of the dose rate, which makes calorimetry ideal for use in the FLASH modality.

[0019] It is important to note that it is the entire calorimeter (i.e., the calorimeter core and any other materials on or within the radiation path) that must have the energy absorption characteristics defined above. This is different from prior art calorimeters that comprise a core embedded in a phantom environment that perturbs the radiation beam beyond an acceptable amount.

[0020] For example, in the case of prior art absorbed dose calorimeters, only some of the particle energy is absorbed by the core or sensor, but due to their structure with the core embedded in a phantom environment (which is an essential feature of the operation of these calorimeters), these calorimeters are considered to perturb the radiation beam inhomogeneously and thus completely disrupt the desired structure of dose delivery to the patient, and therefore cannot be used as in-beam monitors. These calorimeters are further designed as devices for measuring the dose within a phantom, which is a patient substitute, and thus these calorimeters are placed within the radiation beam in place of the patient and then removed before treating the patient. In contrast, the transmission calorimeter of the present invention is continuously placed within the beam path between the radiation source and the patient.

[0021] When measuring the output of a beam delivery system, two separate dose measurement tasks are required. The first task is the dose measurement of beam delivery, which is usually measured using an internal ionization chamber to monitor the amount of transmitted radiation with minimal perturbation. The second dose measurement task is related to "dose determination within the patient", or, in the context of industrial applications, "dose determination within the irradiated sample" instead. Within the scope of the field of radiation therapy, the transmission calorimeter of the present invention is designed with the former in mind, while all other calorimeters to date have been the latter. The calorimeter of the present invention can also perform the second task by calibrating it against an absorbed dose calorimeter. The calorimeter of the present invention further measures the dose directly, but it should be understood that the dose is not the dose within the patient or within the phantom medium, but rather the dose within its own core related to the particle fluence within the beam being monitored, as clarified in the section under the heading "Fluence Equation".

[0022] The prior art devices recognized above are mainly intended for use in standard test laboratories and not for use in a clinical environment. All of the calorimeters in question are placed within the beam path after the beam has passed through the monitor ionization chamber and are used to calibrate the ionization chamber, as described in the background art section.

[0023] Using those calorimeters, it is not possible to perform the same functions as the calorimeter of the present invention. This is due to the following reasons. · The temperature-sensitive area is located within a phantom environment (i.e., the entire calorimeter), and as a result, the beam perturbation becomes larger than 2 mm of water equivalent thickness. Thus, although not all, a significant proportion of the beam is attenuated / absorbed, which, in combination with some material layers that cause significant inhomogeneous scattering of the beam, makes those calorimeters unsuitable for use as in-beam monitors. · Those calorimeters rely on the beam directly irradiating a specific part of the temperature-sensitive area and require the beam to be made parallel before the measurement point, thus limiting the area of the beam that passes through the calorimeter and hits the patient / measurement device located downstream. For these reasons, those calorimeters have to be removed from the beam, and instead, the patient / measurement device has to be placed. · Those calorimeters measure the radiation dose at one point within the beam, rather than the integrated measurement value (often called the area dose) over the entire radiation beam required for an in-beam monitor.

[0024] The calorimeter of the present invention is formed to absorb a minimal amount of energy from the radiation beam in such a manner that radiation having sufficient radiation energy to treat a patient passes through the calorimeter and exits the calorimeter (similar to existing technologies that use a transmission ionization chamber). This is more generally described as minimizing beam perturbation. As described above, the definition used in the present application is that the energy of the radiation absorbed by the calorimeter is less than the energy that would be absorbed by 2 mm of water (equivalent "water depth"). The concept of "absorbed dose to water" is a well-known measure of radiation energy loss in the field of dosimetry, as described, for example, in Luoni F, Weber U, Boscolo D, Durante M, Reidel C-A, Schuy C, Zink K, and Horst F (2020) Beam Monitor Calibration for Radiobiological Experiments With Scanned High Energy Heavy Ion Beams at FAIR. Front. Phys. 8:568145. doi:10.3389 / fphy.2020.568145.

[0025] There are several factors that affect the beam perturbation effect of the calorimeter, and they are mainly the identification of the materials of the core and the jacket surrounding the core (whose stopping power is approximately proportional to the atomic number of the pure elemental material), and the thickness of the calorimeter components through which the beam passes. The perturbation of the radiation beam appears in the form of energy loss of the incident beam due to absorption in the medium and an increase in angular divergence due to scattering in the medium. These two factors are interdependent, so that if it is thin enough, a material with a greater stopping power (i.e., a greater perturbation effect) can be used, while a material with a minimal stopping power can be made thicker. To form a transmission calorimeter that operates according to this definition, the user can easily adjust these factors.

[0026] Using reference tables (such as the NIST PSTAR database at https: / / physics.nist.gov / PhysRefData / Star / Text / PSTAR.html), the energy lost by protons passing through a thin material can be estimated using Equation 1. ∂E~ρS(E)∂x (1)

[0027] In the above equation, ρ is the density of the material and S(E) is the mass electronic stopping power (which is a function of energy). Rearranging this gives Equation 2, which can be used to estimate the amount of material required to produce an equivalent energy loss for a given amount of water.

[0028]

Number

[0029] Using a typical water attenuation value of 2 mm in the monitor chamber, it is possible to determine the amount of material required to produce the same attenuation. For proton therapy, energies of 70 MeV and 225 MeV were selected as indicative clinical minimum and maximum values, as shown below.

[0030]

Table 1

[0031] Although it is certainly possible to make the core of the calorimeter of the present invention thinner, this introduces manufacturing difficulties and the potential for increased sensitivity to thermal noise. Using this analysis, it was confirmed that aluminum is an ideal material for minimizing perturbations due to its low density and relatively low mass stopping power.

[0032] A similar analysis for X-ray radiation can be performed using the conventional water attenuation equation (3) (https: / / physics.nist.gov / PhysRefData / XrayMassCoef / chap2.html).

[0033]

Number

[0034] In the above equation, μ is the mass attenuation coefficient, and I is the measured value of the radiation intensity. Assuming that the intensity losses for two different media are equal, this equation can be rearranged as shown in Equation 4.

[0035]

Number

[0036] Using the publicly available data (https: / / physics.nist.gov / PhysRefData / XrayMassCoef / ComTab / water.html), (https: / / physics.nist.gov / PhysRefData / XrayMassCoef / tab3.html) for low-energy (100 kV) and high-energy (6 MV) beams, the following table can be created.

[0037]

Table 2

[0038] Using this analysis, it can be determined that a transmission calorimeter having an aluminum core with a thickness of 0.74 mm would be suitable for monitoring low and high energy X-rays as well as low and high energy proton beams.

[0039] Accordingly, in a preferred embodiment, the core is formed from aluminum, copper, titanium, silver, gold, or an alloy thereof. The thickness of the core in the direction of the radiation path may be 1 mm or less, preferably 0.75 mm or less, and most preferably 0.4 - 0.7 mm.

[0040] The sensor is preferably a thermistor. Most preferably, the calorimeter has a plurality of sensors (more preferably four sensors), and these plurality of sensors may be arranged at equal angles around the center of the core (preferably circular).

[0041] The diameter of the core in a plane perpendicular to the radiation path is preferably 5 mm or more. Therefore, in order to measure the integrated value of the entire radiation field rather than a single point, during use, the diameter of the radiation beam is preferably smaller than the diameter of the calorimeter core.

[0042] The calorimeter core is preferably uniformly flat so that perturbations to the beam are uniformly applied, and preferably there is no protruding structure due to additional materials that cause changes in the cross-section of the radiation beam.

[0043] The transmission calorimeter may further include an object that is not located on the radiation path and is thermally insulated from the calorimeter core, and at least one sensor for measuring the temperature change of the object. By this at least one sensor, during use, the temperature change of the object due to the change in the ambient temperature can be measured, and the result can be used to compensate for the change in the ambient temperature.

[0044] This object preferably has the same surface area as the core, preferably has the same thickness as the core, and is preferably formed of the same material as the core. This object may be shaped in the form of an annular body, and the annular body may be arranged around the core.

[0045] This object or "compensation core" (hereinafter referred to as such in this specification) is exposed to the same environmental conditions as the "effective core". By comparing the temperature of the effective core with the compensation core, the change in the ambient temperature can be isolated, and a more accurate measurement of the radiation-induced temperature rise can be enabled.

[0046] The transmission calorimeter of the present invention can be further defined in terms of the measurement of the "relative fluence" of the incident beam, in addition to the area dose (DAP) or the dose. A series of equations relating the temperature rise to the fluence of the radiation beam are shown below.

[0047] I. Fluence Equation For charged particles such as electrons, protons, or carbon ions, the dose (D med ) deposited in the medium is related by Equation 5 to the particle fluence (Φ med ), the density (ρ) of the medium, and the unrestricted mass collision stopping power ((S col / ρ) med ) of the medium at the energy of the particles. D med = Φ med (S col / ρ) med (5)

[0048] The dose deposited in the medium induces a temperature change (ΔT) related to the specific heat capacity (c p (T)) of the medium, as related by Equation 6. D med = c p ΔT (6)

[0049] Combining Equation 5 and Equation 6 yields the relationship between the radiation-induced temperature rise and the particle fluence in the medium, as described by Equation 7. c p ΔT = Φ med (S col / ρ) med (7)

[0050] When functioning as a monitor device for radiation therapy, the core of the transmission calorimeter is consistent, and as a result, the values of c p and (S col / ρ) med become constant. This results in the reduced relationship described by Equation 8. α represents all other constants. Φ med = αΔT (8)

[0051] Using Equation 8, by measuring the radiation-induced temperature rise induced by the radiation beam, the transmission calorimeter can function as a radiation monitor and demonstrate the linearity of the fluence through the transmission calorimeter core.

[0052] As described in Equation 9, there is a similar relationship with the therapeutic (or other) photon fluence interaction with the transmission calorimeter core of thickness (x) using the mass attenuation coefficient (μ / ρ).

[0053]

Number

[0054] The energy deposited in the transmission calorimeter core, i.e., the dose, is Φ med and

[0055]

Number

[0056]

Number

[0057] For a sufficiently thin material, Equation 10 further reduces as described in Equation 11. D med ∝ Φ med (11)

[0058] Relating Equation 8 and Equation 11, it can be shown again that the radiation-induced temperature rise of the thin transmission core is directly proportional to the photon fluence.

[0059] By measuring the radiation-induced temperature rise of the transmission calorimeter of the present invention, the particle-invariant relationship with respect to fluence can be observed. Therefore, this calorimeter can be used as a monitoring device.

[0060] II. Multiple Scattering Equation An analytical approximation for the angular distribution resulting from passing protons through a material was developed by Rossi in 1941. This uses the value of the radiation length (L R ) that can be pre-calculated or determined by the particle momentum "p", velocity "v", material thickness "L", and chemical composition. Using this, the induced angular divergence (θ0) can be determined as shown in Equation 12.

[0061]

Equation

[0062] In 1975, Highland developed a modified version of this equation, as shown in Equation 13, which introduced the dependence on the atomic number "Z" of the medium.

[0063]

Equation

[0064] A significantly reduced version of this equation is shown in Equation 14.

[0065]

Equation

[0066] This transmission calorimeter is preferably designed to be uniformly flat and thin so that the induced angular divergence of the incident beam is minimized and is equally distributed with respect to the incident beam. The presence of inhomogeneous zones of different materials within the beam monitoring equipment will lead to complex and non-trivial scattering that is difficult to account for within the beam delivery system.

[0067] III. Energy Loss Equation For charged particles, the Bethe-Bloch formula determines the average energy loss per unit distance

[0068] [Eqn.] as a function of the electron mass (m e ), the atomic number (Z) of the incoming particle, the speed of light (c), the electron density (n), the fractional speed of light (β), the permittivity of free space (ε0), and the mean excitation energy (I) of the medium, Equation 15.

[0069] [Eqn.]

[0070] The complexity of Equation 15 is beyond the scope of this application. Using the relationship described in Equation 16 that relates the electron density (n) of a material to Avogadro's number (N A ), the atomic number (Z) of the material, the relative atomic mass (A) of the material, and the molar mass constant (M u ), Equation 15 can be further reduced to the equation described in Equation 17. Equation 17 shows that the energy loss is proportional to the thickness and atomic number (Z) of the absorbing material and the density of the material.

[0071] [Eqn.] dE ∝ ρZ dx (17)

[0072] According to a second aspect of the present invention, there is provided a method of measuring the dose of a radiation beam, the method comprising (a) providing a radiation beam; and (b) guiding the radiation along a radiation path towards a transmission calorimeter as defined above, the radiation passing through the core of the calorimeter, exiting the core of the transmission calorimeter, and changing the temperature of the core. (c) Measuring the temperature change and calculating the dose of this radiation using the change including.

[0073] When a calorimeter with a compensating core is used, a method for measuring the dose of a radiation beam is (a) providing radiation; (b) guiding this radiation along a radiation path towards the transmission calorimeter, the radiation passing through the core of the calorimeter and emerging from the core of the transmission calorimeter but not passing through an object and changing the temperature of the core; (c) measuring the temperature change of the core; (d) measuring the change in the temperature of the object and using the information to calculate the change in the temperature of the core due to the change in the ambient temperature; (e) calculating the dose of radiation based on the change in the temperature of the core due to the radiation including.

[0074] In a third aspect of the invention, an apparatus for measuring the intensity of radiation as a function of depth is provided, the apparatus comprising a plurality of transmission calorimeters as defined above, the plurality of transmission calorimeters being arranged in series along the radiation path, the plurality of transmission calorimeters each including a material (e.g., a transparent plastic material) between the respective calorimeters, the material having an absorption effect on the radiation.

[0075] In a fourth aspect of the invention, an apparatus for treating a patient with radiation is provided, the apparatus comprising (a) an entrance for receiving radiation, an exit for administering radiation, and a beam guide for guiding the radiation through the apparatus from the entrance to the exit along a radiation path, and (b) a transmission calorimeter as defined above, located along the radiation path between the entrance and the exit or downstream of the exit including.

[0076] The device may further include a beam bender for bending the radiation along a corner, located upstream of the outlet.

[0077] The device may further include a beam shaper for shaping the radiation before the radiation is administered from the outlet.

[0078] The transmission calorimeter is preferably located between the beam bender and the beam shaper.

[0079] Next, several preferred embodiments of the present invention will be described as shown in the following drawings with reference to the following drawings.

Brief Description of the Drawings

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Embodiments for Carrying Out the Invention

[0081] Experiment Example 1 Using domestic aluminum foil, a prototype transmission calorimeter with a thermistor embedded was developed. The diameter of the foil was nominally 50 mm and the thickness was about 0.6 mm. Those thermistors were networked together in series and in parallel so that they had the same resistance as one thermistor. Due to the low mass of aluminum, it was possible to suspend the aluminum core in the air using thermistor wires. The prototype calorimeter was suspended in a plastic case and isolated from the external environment using transparent Mylar.

[0082] In a preliminary investigation at the MC40 cyclotron of the University of Birmingham, a good correlation was revealed between the induced temperature change of aluminum by the FLASH beam and the Faraday cup. This confirmed the validity of the concept, but due to the limited amount of data, some changes were made and the experiment was repeated.

[0083] Using a 50-mm-diameter and 0.5-mm-thick aluminum disk purchased from Goodfellow, a second-generation prototype transmission calorimeter was constructed. The purity of the core is over 99.9%. A more compact plastic case was used for the calorimeter body to reduce the volume of the internal air, and aluminum-treated Mylar was used to isolate the calorimeter from the external environment. Bench tests showed that doing so was very effective in isolating the calorimeter core from infrared radiation.

[0084] The thermistor was placed 5 mm from the edge of the core and fixed in place using a thermally conductive adhesive. A circular piece of aluminum (0.05 mm thick, 6 mm in diameter) was adhered to prevent the thermistor from being directly exposed to air. A photograph of the core with the top of the plastic case removed is shown in Figure 2.

[0085] Four thermistors were attached to provide mechanical support to the calorimeter, but only one was electrically connected. The intention was to partially reduce complexity, but more importantly, it was found that some of the thermistors were shorted to each other through the aluminum core. This was probably the result of the construction process of this proof-of-concept prototype and this would not be a problem in future iterations of this device.

[0086] This thermistor of the transmission calorimeter was wired to a single unshielded coaxial cable connected to a DC Wheatstone bridge. The voltage output of the Wheatstone bridge was measured using a Keithley DMM6500 multimeter. The transmission calorimeter was calibrated over a temperature range using an environmental chamber referenced to a thermocouple traceable to the NPL primary standard for temperature according to the 1990 International Temperature Scale.

[0087] This transmission calorimeter was transported back to the MC40 cyclotron at the University of Birmingham and irradiated with a 36 MeV proton beam configured to deliver an irradiation of 1 second in length when a button was pressed.

[0088] A PTW transmission ionization chamber (type 34014, 786) was placed at the end of the nozzle to function as an in-beam monitor. This was connected to a USB PC potentiometer (Sun Nuclear Corporation) to provide relative measurements with respect to the beam. The transmission calorimeter was placed immediately downstream of the PTW ionization chamber and connected to the Wheatstone bridge and the Keithley DMM. This configuration is shown in Figure 3.

[0089] Further downstream of the calorimeter, a prototype secondary standard graphite calorimeter was placed. This device was also calibrated in the environmental chamber in the same manner as the transmission calorimeter. This secondary standard calorimeter was used for the purpose of measuring the dose delivered by the radiation beam and had a graphite core with a diameter of 16 mm and a thickness of 2 mm surrounded by a jacket made of 3D printed plastic. This secondary standard calorimeter was connected to a DC Wheatstone bridge with an excitation voltage of 5 V, and its output was monitored by a Raspberry Pi using an analog - digital converter (ADC) hardware attachment.

[0090] Finally, a Faraday cup was placed further downstream of the secondary standard calorimeter and connected to a Sun Nuclear PC potentiometer. The Bragg peak of the 36 MeV proton beam was predicted to fall within the depth range of the secondary standard calorimeter, and therefore, little signal was expected to be recorded by the Faraday cup.

[0091] Due to a cyclotron failure on the measurement day, it was impossible to provide a uniform beam with a diameter of 50 mm at high intensity as planned. Instead, the diameter of the delivered beam was approximately 10 mm. This was not ideal because the transmission calorimeter core (50 mm in diameter) was larger than necessary, thus reducing the magnitude of the signal being measured due to the absorption of the radiation beam.

[0092] An attempt was made to use the transmission ionization chamber for relative measurements.

[0093] Results Due to the high beam current, it was impossible to use the transmission ionization chamber for relative measurements. This was because the instantaneous beam current was larger than the largest measurement range setting available on the USB potentiometer.

[0094] The comparison of ten measurements between the secondary standard calorimeter (SSCal) and the transmission calorimeter (TransCal) is shown in Figure 4 where a dose of approximately 250 Gy / s was delivered to the secondary standard calorimeter core. Since there is a small amount of direct heating of the thermistor in the secondary standard calorimeter, a slight overresponse can be observed. This is due to the higher relative stopping power of the material in the thermistor compared to the secondary standard calorimeter core in which the thermistor is embedded, whereby more energy is locally absorbed.

[0095] A more detailed comparison of the two irradiations is shown in Figure 5. Since the thermistor in the transmission calorimeter is not directly irradiated by the radiation beam, it takes several seconds for the induced temperature response from the radiation beam to reach its peak even though the beam delivery lasts only for 1 second. Further due to this effect, the timing between the two calorimeter responses appears shifted.

[0096] In the transmission calorimeter, the effect of not being completely shielded from the environment can be observed between 130 and 150 seconds. During this period, there is a change in voltage, which is thought to be due to small fluctuations in the ambient temperature.

[0097] For both the secondary standard calorimeter and the transmission calorimeter, the radiation-induced changes in voltage / temperature were calculated (see Figure 6) by using a custom analysis script written in Python that extrapolates between the on / off positions of the beam. For the secondary standard calorimeter, since it is isolated from the environment, calculating the radiation-induced temperature is relatively straightforward. The pre-beam temperature drift and the post-beam temperature drift are extrapolated to the midpoint of the beam. Since the thermistor of the secondary standard calorimeter is directly in the beam, there is a temporary overresponse in the measured temperature. This is because the thermistor becomes temporarily hotter than the graphite. This is due to the differences in the specific heat capacity, density, and relative stopping power of these materials.

[0098] For the calorimeter, extrapolation is much more difficult. Since the beam was significantly smaller than the calorimeter core, the locally deposited thermal energy had to move through the core before it could be measured. This results in a systematic delay that does not exist in secondary standard calorimeters. This is shown in Figure 7. In Figure 7, the beam backfitting for extrapolation requires excluding some data from this analysis.

[0099] Investigation of linearity To investigate the linearity of the transmission calorimeter, the beam current of the cyclotron was varied. The time for each beam delivery was kept at 1 second, and the energy was consistently 36 MeV. Due to the previous problems with the cyclotron, the beam size, shape, and position could not be monitored throughout the measurements.

[0100] Figure 8 shows a comparison of all the measurement data. Good linearity is observed between the two instruments, except for the highest dose rate measurement.

[0101] Additional investigation of the highest dose rate measurement (Figure 9) reveals that the observed overreaction of the secondary standard calorimeter due to local thermistor heating appears to vary throughout the series of measurements. This implies that the beam is changing in shape and / or position.

[0102] This measurement was the first after the cyclotron recovered from the previous failure, so the cyclotron may still have been in the stabilization process. Figure 10 shows the data excluding this run.

[0103] Instead, Figure 11 shows the data grouped by proton current. There are still large fluctuations in the measured values of the (currently) largest beam current, which is thought to be due to the cyclotron still being in the process of stabilizing. However, this fluctuation is linear with respect to the remaining measurements, indicating that this fluctuation is due to the variation of the beam current and not necessarily due to the variation of the beam shape / size / position.

[0104] Consideration of Uncertainty Explicitly consider the standard deviation of the ratio between two devices and create Figure 12. Although it is not possible to draw a proven conclusion from this figure about the minimum dose rate that the transmission calorimeter can quantify, it is important to note that this behavior follows the expected response between the dose rate and the measured output signal of the transmission calorimeter. It is observed that measurements at higher dose rates have a lower standard deviation than measurements at lower dose rates. It is expected that there are dose and dose / rate thresholds where fluctuations in ambient temperature have a minimal impact on the calculated dose and the resulting measurement uncertainty values.

[0105] Example 2 At the Oncoray Institute in Dresden, measurements were carried out using a modified version of the transmission calorimeter of the present invention (core with a thickness of 0.4 mm and a diameter of 78 mm) to evaluate its use as a monitoring device together with a clinical quality assurance device.

[0106] A series of devices were irradiated with a 180 MeV proton beam having a full width at half maximum (FWHM) of 0.98×0.93 mm 2 . Since this proton beam passes through all the devices (depositing energy when passing through), the relative responses of those devices can be correlated. At this energy, the water equivalent thickness of the transmission calorimeter core is approximately 1 mm. An example of the temperature response in 165 Gy / s delivered in a 200 millisecond irradiation is shown in Figure 13.

[0107] When compared to MC40 cyclotron measurements, there are several significant differences. Looking at transmission calorimeter cores of comparable size, the beam at Oncoray is significantly smaller in size. As a result, the total energy deposited in the core is smaller, and correspondingly, a lower radiation-induced temperature rise is observed. Since an unnecessarily large core has an inferior signal-to-noise ratio (SNR), this emphasizes the importance of matching the core size to the size of the radiation beam.

[0108] Another key difference would be the greater variation in ambient temperature observed without radiation. Ideally, the change in ambient temperature should be separated from the radiation-induced temperature change for measurement, which also results in a lower SNR.

[0109] As a result of these combined differences, the standard deviation of the mean (SDOM) for this series of measurements was approximately 10%. This is significantly higher than previous measurements.

[0110] Compensation core An example of a transmission calorimeter with a compensation core will be described below.

[0111] 1. Modeling parameters a. Simulation software The simulation of heat flow within a compensated transmission calorimeter was performed using COMSOL Multiphysics 6.0 (Build: 318) with the "Heat Transfer" package. This software performs finite element modeling (FEM) of temperature flow for many industrial and scientific applications, including calorimetry.

[0112] b. Shape dimensions In its simplest iteration, a compensated transmission calorimeter with an inner "effective core" of radius 2.50 cm was modeled. The "compensator core" was a ring with an outer radius of 3.92 cm and an inner radius of 3.02 cm. Realized in this way, the surface areas of both cores are the same, and both cores should be subject to the same environmental changes. Both cores had the same thickness of 0.4 mm. Figure 14 shows the geometric arrangement of the compensated transmission calorimeter.

[0113] c. Material specification Both cores were specified as made of aluminum (density 2.70 g / cm 3 , specific heat capacity 900 J / kgK), and the default library in COMSOL was used for all other parameters.

[0114] d. Radiation heating The simulation of the radiation beam was performed by heating the "effective core" using a Gaussian heat source. The heat source was placed at the center of the "effective core" and the FWHM was set to 1 cm. This localized heating was configured to last for 1 second. The intensity of this heating was adjusted so that the average temperature within the "effective core" increased by 3.5 mK, which represents the experimental results. The effect of this heating is shown in Figure 16.

[0115] e. Environmental impact To simulate the influence of the changing ambient environment, a global heat source was added to all components. This consisted of two sine waves (with periods of 10.0 seconds and 6.5 seconds) to represent small thermal fluctuations, and a negative heat source was added to represent thermal drift. This change in the ambient temperature is shown in Figure 17.

[0116] Since this radiation-induced heating is confined to the "effective core", the variation in the temperature responses of the two cores can be observed. This is shown in Figure 18.

[0117] f. Temperature compensation By subtracting the temperature response of the "compensation core" from that of the "effective core", the influence of the environmental effect can be excluded. This can be performed electronically using the opposite arms of a DC Wheatstone bridge circuit, such as those shown in Figure 15. However, for this COMSOL model, since this is simulated data, it is instead performed analytically as shown in Figure 19.

[0118] g. Simulation iteration This model was configured to repeat the simulated irradiation 10 times every 60 seconds, using the output of the previous model as the input to the next model. This is shown in Figure 20.

[0119] To complete, the temperature difference between the "effective core" and the "compensator" is shown in Figure 21. Plots of the temperature changes of the simulated transmission calorimeter for simulated irradiation with and without the "compensated core" are shown in Figure 22.

[0120] For the sinusoidal ambient conditions discussed previously, each irradiation will have a slightly different instantaneous heat flux. Ten measurements of the temperature change can be compared for statistical analysis.

[0121] 2. Analysis a. Analysis Software The analysis was performed using the National Physical Laboratory (NPL) software "Python Calorimetry Software Analysis v0.3". This software package is used to analyze the radiation-induced temperature changes of this transmission calorimeter and the NPL secondary standard calorimeter (10.1259 / bjr.20220638). This software is used for both calorimetry analysis within NPL and calorimetry analysis by co-researchers.

[0122] b. Analysis Results The temperature data from the "effective core" design and the "compensated core" design were independently analyzed by another member of the NPL staff who was not informed of the details about the beam parameters (duration, cycle).

[0123] For 10 simulations, the "effective core" showed an average temperature rise of 3.502 mK, and the standard deviation was 0.93%. For the device with a "compensated core", it was found that the average temperature rise was 3.496 mK and the standard deviation was 0.17%. These two methods of deriving the temperature rise of the transmission calorimeter core are consistent, but the introduction of the "compensated core" significantly increases the accuracy and reduces the measurement uncertainty.

[0124] Summary This short experiment demonstrated how the introduction of an additional temperature-sensitive core into a transmission calorimeter that is only sensitive to ambient temperature and not to the radiotherapy beam can be used for the purpose of improving accuracy while reducing measurement uncertainty. In a medical environment, an increase in measurement accuracy and a reduction in uncertainty are desirable, thereby expecting an improvement in the treatment outcome of patients.

[0125] Although not investigated here, it is expected that this reduction in uncertainty will result in a decrease in the minimum dose rate that can be measured by the transmission calorimeter.

[0126] All optional features and modifications of the described embodiments and dependent claims, as well as all preferred features and modifications, are usable in all aspects of the invention taught herein. Further, the individual features of the dependent claims, as well as all optional features and modifications of the described embodiments and all preferred features and modifications, are combinable with one another and interchangeable with one another.

[0127] The disclosure of UK Patent Application No. 2207878.6, for which this application claims priority, and the abstract accompanying this application are incorporated herein by reference.

Claims

1. A transmission calorimeter for measuring the dose of a beam of radiation, wherein the calorimeter comprises a core that receives the radiation and transmits the radiation along a radiation path, the radiation path passing through the core, and at least one sensor for measuring a change in the temperature of the core, wherein the energy of the radiation absorbed by the calorimeter is less than or equal to the energy of the radiation that would be absorbed by the radiation passing through 2 mm of water.

2. The permeable calorimeter according to claim 1, wherein the core is formed from aluminum, copper, titanium, silver, gold, or an alloy thereof.

3. The transmission calorimeter according to claim 1 or 2, wherein the thickness of the core in the direction of the radiation path is 1 mm or less.

4. The transmission calorimeter according to claim 1 or 2, wherein the thickness of the core in the direction of the radiation path is 0.75 mm or less.

5. The transmission calorimeter according to claim 1, wherein the core is formed of aluminum with a thickness of 0.94 mm or less in the direction of the radiation path, copper with a thickness of 0.33 mm or less, titanium with a thickness of 0.62 mm or less, silver with a thickness of 0.31 mm or less, or gold with a thickness of 0.20 mm or less.

6. The transmission calorimeter according to claim 1, 2, or 5, wherein the diameter of the core in a plane perpendicular to the radiation path is 5 mm or more.

7. The heat transfer meter according to claim 1, 2, or 5, wherein the sensor is a thermistor.

8. A transmission calorimeter according to claim 1, 2, or 5, having four sensors.

9. The transmission calorimeter according to claim 8, wherein the sensors are arranged at equal angles around the center of the core.

10. The heat transfer meter according to claim 1, 2, or 5, wherein the core is substantially circular.

11. A transmission calorimeter according to claim 1, 2, or 5, further comprising a housing, wherein the core is placed within the housing, and the housing is substantially transparent to the radiation along the radiation path.

12. The transmission calorimeter according to claim 1, further comprising an object that is not located on or within the radiation path and is insulated from the calorimeter core, and at least one sensor for measuring a temperature change of the object, thereby enabling the measurement of a temperature change of the object due to a change in ambient temperature during use, and using the results to compensate for the change in ambient temperature.

13. The heat transfer meter according to claim 12, wherein the object has the same surface area as the core.

14. The permeable calorimeter according to claim 12, wherein the object is shaped into a ring, and the ring is arranged around the core.

15. The heat transfer meter according to claim 12, wherein the object is formed of the same material as the core.

16. An apparatus for measuring the intensity of radiation as a function of depth, comprising a plurality of transmission calorimeters as described in claim 1, wherein the plurality of transmission calorimeters are arranged in series along the radiation path, and the plurality of transmission calorimeters include a material between each calorimeter, the material having an absorption effect on the radiation.

17. A device for treating patients with radiation, (a) an inlet for receiving radiation, an outlet for administering radiation, and a beam guide for guiding radiation through the apparatus along a radiation path from the inlet to the outlet, (b) A calorimeter according to claim 1, wherein the calorimeter is located between the inlet and the outlet along the radiation path, or located downstream of the outlet. A device including a device.

18. The apparatus according to claim 17, further comprising a beam bender located upstream of the outlet for bending the radiation along a corner.

19. The apparatus according to claim 18, further comprising a beam shaper for shaping the radiation before it is administered from the outlet.

20. The apparatus according to claim 19, wherein the calorimeter is located between the beam bender and the beam shaper.

21. A method for measuring the dose of a radiation beam, (a) The step of providing radiation, (b) A step of directing the radiation along a radiation path toward the transmission calorimeter described in claim 1, wherein the radiation passes through the core of the calorimeter, exits the core of the transmission calorimeter, and changes the temperature of the core. (c) The steps of measuring the temperature change and using the change to calculate the dose of the radiation Methods that include...

22. A method for measuring the dose of a radiation beam, (a) The step of providing radiation, (b) A step of directing the radiation along a radiation path toward the transmission calorimeter described in claim 12, wherein the radiation passes through the core of the calorimeter and exits the core of the transmission calorimeter but does not pass through the object and changes the temperature of the core, (c) A step of measuring the temperature change of the core, (d) The steps of measuring the temperature change of the object and using that information to calculate the temperature change of the core caused by the change in ambient temperature, (e) A step of calculating the dose of radiation based on the change in the temperature of the core caused by the radiation; Methods that include...

23. The method according to claim 21 or 22, wherein the radiation is provided in the form of a beam, and the diameter of the beam is smaller than the diameter of the calorimeter core.

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