Transmission Calorimeter for Measuring Dose of Radiation

Abstract

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

Claims

1. A transmission calorimeter for measuring a dose of a beam of radiation comprising: a core for receiving and transmitting said radiation along a radiation path which passes through said core and at least one sensor for measuring a temperature change of the core, wherein the energy of said radiation absorbed by the transmission calorimeter is less than or equal to the energy that would be absorbed by transmitting said radiation through 2 mm of water.

2. The transmission calorimeter of claim 1, wherein the core is formed from aluminium, copper, titanium, silver, gold or from alloys thereof.

3. The transmission calorimeter of claim 1, wherein the core has a thickness in the direction of the radiation path of 1 mm or less.

4. The transmission calorimeter of claim 1, wherein the core has a thickness in the direction of the radiation path of 0.75 mm or less.

5. The transmission calorimeter of claim 1, wherein the core is formed from aluminium with a thickness in the direction of the radiation path of 0.94 mm or less, 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 of claim 1, wherein the core has a diameter in a plane perpendicular to the radiation path of 5 mm or greater.

7. The transmission calorimeter of claim 1, wherein the sensor is a thermistor.

8. The transmission calorimeter of claim 1 which has four sensors.

9. The transmission calorimeter of claim 8 wherein the sensors are distributed at equal angles about the centre of the core.

10. The transmission calorimeter of claim 1 wherein the core is substantially circular.

11. The transmission calorimeter of claim 1 further comprising a housing, wherein the core is contained in the housing and wherein the housing is substantially transparent to said radiation along said radiation path.

12. The transmission calorimeter of claim 1 further comprising: a body which is not located on or in said radiation path and which is thermally insulated from the core; and an at least one sensor for measuring a temperature change of the body, wherein in use said temperature change of said body caused by ambient temperature changes is measured and the result is used to compensate for changes in ambient temperature.

13. The transmission calorimeter of claim 12, wherein the body has the same surface area as the core.

14. The transmission calorimeter of claim 12, wherein the body is configured in the form of a torus, said torus being positioned around said core.

15. The transmission calorimeter of claim 12, wherein the body 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, each of which includes a core for receiving and transmitting said radiation along a radiation path which passes through said core and at least one sensor for measuring a temperature change of the core, wherein the energy of said radiation absorbed by the transmission calorimeter is less than or equal to the energy that would be absorbed by transmitting said radiation through 2 mm of water, wherein said plurality of transmission calorimeters are arranged in series along said radiation path and including a material between each calorimeter, and wherein the material has an absorption effect on the radiation.

17. An apparatus for treating a patient with radiation comprising: (a) an inlet for receiving radiation, an outlet for dispensing radiation, a radiation path from the inlet to the outlet, and a beam guide for guiding radiation through said apparatus from the inlet to the outlet along said radiation path; and (b) a transmission calorimeter comprising a core for receiving and transmitting said radiation along said radiation path which passes through said core and at least one sensor for measuring a temperature change of the core, wherein the energy of said radiation absorbed by the transmission calorimeter is less than or equal to the energy that would be absorbed by transmitting said radiation through 2 mm of water, wherein said transmission calorimeter is located between the inlet and the outlet along said radiation path or wherein the transmission calorimeter is located downstream of the outlet.

18. The apparatus of claim 17 further comprising a beam bender located upstream of the outlet for bending said radiation around a corner.

19. The apparatus of claim 18 further comprising a beam shaper for shaping the radiation before it is dispensed from the outlet.

20. The apparatus of claim 19 wherein the transmission calorimeter is located between the beam bender and the beam shaper.

21. A method of measuring a dose of a beam of radiation, comprising the steps of: (a)) providing radiation; (b) directing the radiation at a transmission calorimeter, wherein the transmission calorimeter comprises a core for receiving and transmitting said radiation; and at least one sensor for measuring a temperature change of the core, wherein the energy of said radiation absorbed by the transmission calorimeter is less than or equal to the energy that would be absorbed by transmitting said radiation through 2 mm of water, wherein the radiation passes through and exits the core of said transmission calorimeter and causes the temperature of the core to change; and (c) measuring said temperature change and using said change to calculate said dose of the radiation.

22. The method of claim 21 in which the radiation is provided in the form of said beam, and wherein the diameter of the beam is less than the diameter of the core.

23. A method of measuring a dose of a beam of radiation comprising the steps of: (a) providing radiation; (b) directing the radiation at a transmission calorimeter wherein said transmission calorimeter comprises a core for receiving and transmitting said radiation wherein the path of the radiation to and from said core defines a radiation path; an at least one sensor for measuring a temperature change of the core; a body which is not located on or in said radiation path and which body is thermally insulated from the core, and at least one sensor for measuring a temperature change of the body, wherein the energy of said radiation absorbed by the transmission calorimeter is less than or equal to the energy that would be absorbed by transmitting said radiation through 2 mm of water, wherein the radiation passes through and exits the core of said transmission calorimeter but does not pass through said body and causes the temperature of the core to change; (c) measuring a temperature change of the core; (d) measuring a temperature change of said body and using said change to calculate a change in the temperature of the core caused by ambient temperature changes; and (e) calculating the dose of the radiation based on the change in temperature of the core caused by said radiation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] A number of preferred embodiments of the invention will now be described, with reference to and as illustrated in the following drawings:

[0080] FIG. 1 is a schematic diagram showing a prior art arrangement of an ionisation chamber used to measure radiation dosage.

[0081] FIG. 2 is a photograph of a prototype transmission calorimeter in accordance with the invention.

[0082] FIG. 3 is a schematic diagram of an experimental set-up to test a prototype transmission calorimeter in accordance with the invention.

[0083] FIG. 4 is a graph of temperature against time to show a comparison of calorimeter response for 10 deliveries of radiation.

[0084] FIG. 5 is a graph of temperature against time to show a comparison of calorimeter response for two irradiation cycles.

[0085] FIG. 6 is a graph showing a temperature extrapolation of a prototype absorption calorimeter (SSCal).

[0086] FIG. 7 is a graph showing a temperature extrapolation of a prototype transmission calorimeter in accordance with the invention.

[0087] FIG. 8 is a graph of temperature response against dose of a prototype transmission calorimeter in accordance with the invention.

[0088] FIG. 9 is a graph of temperature response against time for a prototype transmission calorimeter and an absorption calorimeter in accordance with the invention when operated at a high doses.

[0089] FIG. 10 is a graph of the transmission calorimeter temperature response against absorption calorimeter dose from the data presented in FIG. 9.

[0090] FIG. 11 is a graph of temperature response of the transmission calorimeter in accordance with the invention against dose measured by the Secondary Standard calorimeter (SSCal) grouped according to prescribed proton beam current.

[0091] FIG. 12 is a graph showing the standard deviation of the relative response of the prototype transmission calorimeter in accordance with the invention and an absorption calorimeter against dose.

[0092] FIG. 13 is a graph showing the temperature response against time for a calorimeter in accordance with the invention and associated pre-drift and post-drift extrapolated curves fitted to the data.

[0093] FIG. 14 is a schematic diagram showing the geometric arrangement of the Compensated calorimeter simulation in accordance with the invention.

[0094] FIG. 15 is a schematic diagram of a Wheatstone bridge circuit for use in the Compensated calorimeter embodiment in accordance with the invention.

[0095] FIG. 16 is a graph of the simulated temperature response against time for a radiation induced temperature response of the Active Core in accordance with the invention.

[0096] FIG. 17 is a graph of the simulated temperature response against time for an ambient temperature change without radiation heating.

[0097] FIG. 18 is a graph of the simulated temperature response against time for a radiation induced temperature change with radiation heating during irradiation of the transmission calorimeter in accordance with the invention.

[0098] FIG. 19 is a simulated compensated temperature difference plot.

[0099] FIG. 20 is a plot of temperature against time, showing ten simulated irradiations and environmental fluctuations.

[0100] FIG. 21 is a plot of simulated temperature difference against time, showing ten simulated irradiations without environmental fluctuations by subtracting the temperature of the Active Core from the Compensator Core in accordance with the invention.

[0101] FIG. 22 is a plot of the simulated temperature change of the transmission calorimeter in accordance with the invention with and without the Compensated Core for simulated irradiations.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT(S)

Experimental

Example 1

[0102] A prototype transmission calorimeter was developed using domestic aluminium foil with thermistors embedded. The foil was a nominal 50 mm diameter, approximately 0.6 mm thick. The thermistors were networked together in series and parallel such that they had a resistance equal to one thermistor. Due to the low mass of the aluminium it was possible to suspend the aluminium core in air using the thermistor wires. The prototype calorimeter was suspended in a plastic case and was isolated from the external environment using transparent Mylar.

[0103] A preliminary investigation at the University of Birmingham's MC40 Cyclotron revealed a good correlation between induced temperature change in Aluminium from a FLASH beam and a Faraday cup. This validated the concept; however due to the limited amount of data, a decision was made to repeat the experiment with several changes.

[0104] The second-generation prototype transmission calorimeter was built using a 50 mm diameter 0.5 mm thick Aluminium disk; purchased from Goodfellow. The purity of the core is in excess of 99.9%. A more compact plastic case was used for the calorimeter body, lowering the volume of air inside; and aluminised Mylar was used to isolate the calorimeter from the external environment. Bench tests indicated that this was very effective at isolating the calorimeter core from infrared radiation.

[0105] The thermistors were placed 5 mm from the edge of the core and secured in place using thermally conductive glue. Circles of aluminium (0.05 mm thick and 6 mm in diameter) were glued to prevent the thermistors being directly exposed to the air. A photograph of the core with the upper portion of the plastic case removed can be seen in FIG. 2.

[0106] Although four thermistors were attached, providing mechanical support for the calorimeter, only one was electrically connected. The intention being to partially reduce the complexity, but more significantly, it was found that some of the thermistors were short circuiting themselves through the aluminium core. This is likely a consequence of the construction process of this demonstrator prototype and has not presented an issue in future iterations of the device.

[0107] The transmission calorimeter's thermistor 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 range of temperatures using an environmental chamber, referenced against thermocouples traceable to the NPL primary standard for temperature in accordance with the International Temperature Scale of 1990.

[0108] The transmission calorimeter was again taken to the MC40 Cyclotron at the University of Birmingham and irradiated with a 36 MeV proton beam, configured to deliver 1 second long irradiations upon pressing a button.

[0109] A PTW transmission ionisation chamber (type 34014, 786) was placed at the end of the nozzle to act as an in-beam monitor. This was connected to a USB PC Electrometer (from Sun Nuclear Corporation) to provide relative measurements for the beam. The transmission calorimeter was positioned immediately downstream of the PTW ionisation chamber and connected to the Wheatstone bridge, and Keithley DMM. The set-up is shown in FIG. 3.

[0110] Further downstream from the transmission calorimeter, a prototype Secondary Standard graphite calorimeter was placed. This device was also calibrated in an environmental chamber, similar to the transmission calorimeter. The Secondary Standard calorimeter was used to measure the dose delivered by the radiation beam and has a 16 mm diameter and a 2 mm thick core made of graphite, surrounded by a jacket made from 3D printed plastic. The Secondary Standard calorimeter was connected to a DC Wheatstone bridge with an excitation voltage of 5 V, the output of which was monitored by a Raspberry Pi using an analogue-to-digital convertor (ADC) hardware attachment.

[0111] Lastly, a Faraday Cup was positioned further downstream than the Secondary Standard calorimeter and connected to a Sun Nuclear PC Electrometer. It was predicted the Bragg peak of the 36 MeV proton beam would fall within the depth of the Secondary Standard calorimeter, and as such, little signal was expected to be recorded in the Faraday Cup.

[0112] Due to a failure of the Cyclotron on the day of the measurement, it was not possible to provide a uniform 50 mm diameter beam at high intensity as planned. Instead, the beam delivered was approximately 10 mm in diameter. This was not ideal, as it meant the transmission calorimeter core (50 mm diameter) was larger than necessary thus reducing the magnitude of the measured signal due to the absorbed radiation beam).

[0113] An attempt was made to use a transmission ionisation chamber for relative measurements.

Results

[0114] Due to the high beam current, it was impossible to use the transmission ionisation chamber for relative measurements because the instantaneous beam current was greater than the largest measurement range setting available on the USB electrometer.

[0115] A comparison of ten measurements between the Secondary Standard calorimeter (SSCal) and transmission calorimeter (TransCal) can be seen in FIG. 4 delivering a dose of approximately 250 Gy/s in the Secondary Standard calorimeter core. As there is a small amount of direct heating of the thermistors in the Secondary Standard calorimeter, a slight over response can be observed. This is due to the higher relative stopping power of the materials within the thermistors relative to the Secondary Standard calorimeter core they are embedded within, causing more energy to be absorbed locally.

[0116] A closer comparison of two irradiations can be seen in FIG. 5. As the radiation beam does not directly irradiate the thermistors in the transmission calorimeter, the induced temperature response from the radiation beam takes several seconds to peak; despite the beam delivery only lasting one second. Additionally due to this effect, the timing between the two calorimeter responses appears offset.

[0117] The impact of the lack of full environmental shielding can be observed in the transmission calorimeter between 130 and 150 seconds, where there is a change in voltage which is attributed to a small fluctuation in the ambient temperature.

[0118] For both the Secondary Standard calorimeter and transmission calorimeter, the radiation induced change in voltage/temperature was calculated by using a bespoke analysis script written in Python which extrapolates between the beam on/off positions (see FIG. 6). For the Secondary Standard calorimeter, due to the isolation from the environment this is relatively trivial to calculate the radiation induced temperature. The pre-beam temperature drift and post-beam temperature drift are extrapolated to the beam mid-point. As the thermistors in the Secondary Standard calorimeter are directly in the beam, there is a temporary overresponse in measured temperature as the thermistors are temporarily hotter than the graphite. This is due to the difference in specific heat capacity, density, and relative stopping power of the materials.

[0119] Extrapolating is significantly more challenging for the transmission calorimeter. As the beam was significantly smaller than the calorimeter core, the locally deposited thermal energy must transfer through the core before it can be measured. This results in a systematic delay not present in the Secondary Standard calorimeter. This can be seen in FIG. 7 where the post-beam fit for extrapolation requires some data to be excluded from the analysis.

Linearity Study

[0120] 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 one second, with a consistent energy of 36 MeV. Due to the prior issue with the cyclotron, the size, shape, and position of the beam were not able to be monitored throughout the measurement.

[0121] A comparison of all measurement data can be seen in FIG. 8. There is observed to be a good linearity between the two instruments, with the exception of the highest dose rate measurements.

[0122] Further investigation of the highest dose rate measurements (FIG. 9) reveals that the observed over response of the Secondary Standard calorimeter due to local thermistor heating appears to vary throughout the series of measurements. This implies that the beam changes shape and/or position.

[0123] As this was the first set of measurements after the cyclotron had recovered from the prior fault, it is therefore likely that it was still in the process of settling. Data omitting this run is presented in FIG. 10.

[0124] Alternatively, FIG. 11 presents data grouped by proton current. There is still a large variation in the readings of the (now) greatest beam current, attributed to the cyclotron still settling. However, this variation is linear with respect to the rest of the measurements-indicating that it is a variation of the beam current and not necessarily the beam shape/size/position.

Uncertainty Study

[0125] Explicitly studying the standard deviation of the ratio between the two instruments produces FIG. 12. Whilst it is not possible to draw any substantiated conclusions from this about the minimum dose rate the transmission calorimeter can quantify, it is important to note that this behaviour follows the expected response between dose rate and measured output signal of the transmission calorimeter. A higher dose rate measurement is observed to have a lower standard deviation than a lower dose rate measurement. It is anticipated that there exists a dose and dose/rate threshold at which variations in ambient temperature have minimal impact on the calculated dose and resulting value of measurement uncertainty.

Example 2

[0126] At the Oncoray research facility, Dresden, measurements were conducted using a revised version of the transmission calorimeter of the present invention (0.4 mm thickness, 78 mm diameter core) for evaluating its use as a monitoring device in conjunction with clinical quality assurance devices.

[0127] The series of instruments were irradiated with a 180 MeV proton beam, with a full width at half maximum (FWHM) 0.980.93 mm.sup.2. As the proton beam passes through all instruments (depositing energy as it does so), the relative response of the instruments can be correlated. At this energy, the water equivalent thickness of the transmission calorimeter core is approximately 1 mm. An example of a temperature response during a 165 Gy/s delivered in a 200 ms irradiation can be seen in FIG. 13.

[0128] Compared to the MC40 cyclotron measurement, there are several significant differences. For a comparable sized transmission calorimeter core, the beam at Oncoray is significantly smaller in size. This results in less total energy deposited in the core, and correspondingly a lower radiation induced temperature rise is observed. This highlights the importance of matching the size of the core to the size of the radiation beam, as an unnecessarily large core will have a poor signal to noise ratio (SNR).

[0129] Another key difference would be the larger variation in ambient temperature observed without any radiation. As the ambient temperature change should ideally be separated from the radiation induced temperature change for a measurement, this also results in a lower SNR.

[0130] Together these differences resulted in a standard deviation of the mean (SDOM) of approximately 10% for this series of measurements. This is significantly higher than prior measurements.

Compensating Core

[0131] An example of a transmission calorimeter having a compensating core is described below.

1. Modelling Parameters

a. Simulation Software

[0132] Simulations of the heat flow within the Compensated transmission calorimeter were performed using COMSOL Multiphysics 6.0 (Build: 318), using the Heat Transfer package. This software performs Finite-Element Modelling (FEM) of temperature flow for many industrial and scientific applications (including calorimetry).

b. Geometry

[0133] In its simplest iteration, the Compensated transmission calorimeter was modelled with an inner Active Core with radius 2.50 cm. The Compensator Core was a ring with an outer radius of 3.92 cm and an inner radius of 3.02 cm. Realised in this manner, both cores had the same surface area, and should both be subject to the same changes in the environment. Both cores had the same thickness of 0.4 mm. FIG. 14 presents the geometric arrangement of the Compensated transmission calorimeter.

c. Material Assignment

[0134] Both cores were assigned to be made of Aluminium (density 2.70 g/cm3, specific heat capacity 900 J/kgK), using the default library in COMSOL for all other parameters.

d. Radiation Heating

[0135] A beam of radiation was simulated by heating the Active Core with a Gaussian heat source. This was positioned in the centre of the Active Core, with a FWHM of 1 cm. This localised heating was configured to last for 1 second. The intensity of the heating was tuned such that the average temperature in the Active Core would increase by 3.5 mK, representative of experimental results. The effect of this heating can be seen in FIG. 16.

e. Environmental Influence

[0136] To simulate the effect of a changing ambient environment, a global heat source was added to all components. This composed of two sinusoidal waves (with periods of 10.0 and 6.5 seconds) to represent small thermal fluctuations, with a negative heat source added to represent thermal drift. This change in ambient temperature can be seen in FIG. 17.

[0137] As the radiation induced heating is localised to the Active Core, the variation in temperature response of the two cores can be seen. This is shown in FIG. 18.

f. Temperature Compensation

[0138] By subtracting the temperature response of the Compensating Core from the Active Core, the influence of environmental effects can be removed. This may be performed electronically using opposite arms of a DC Wheatstone Bridge circuit such as is shown in FIG. 15. For the COMSOL model however, this is simulated data so it is performed analytically, as shown in FIG. 19.

g. Simulation Repeats

[0139] The model was configured to repeat ten simulated irradiations every 60 seconds, using the output of the proceeding model as the input to the next. This is shown in FIG. 20.

[0140] For completeness, the temperature difference between the Active Core and the Compensator can be shown in FIG. 21. A plot of temperature change of the simulated transmission calorimeter with and without the Compensated Core for simulated irradiations is shown in FIG. 22.

[0141] Due to the sinusoidal ambient conditions (discussed previously), each irradiation would have slightly different instantaneous heat flow. The ten readings for temperature change could be compared for statistical analysis.

2. Analysis

a. Analysis Software

[0142] Analysis was performed using the National Physical Laboratory (NPL) software Python calorimetry Software Analysis v 0.3. This software package is used to analyse radiation induced temperature changes of the transmission calorimeter and NPL Secondary Standard calorimeter (10.1259/bjr.20220638). This software is used for calorimetry analysis both internally to NPL and by scientific collaborators.

b. Analysis Results

[0143] The temperature data from the Active Core and the Compensated Core designs were independently analysed by another member of NPL staff who was not provided details about the beam parameters (duration, cycle).

[0144] For the 10 simulations, the Active Core observed an average temperature rise of 3.502 mK with a standard deviation of 0.93%. For the device with the Compensated Core, the average temperature rise was found to be 3.496 mK with a standard deviation of 0.17%. The two methods of deriving the temperature rise in the transmission calorimeter Core agree, however the introduction of the Compensated Core significantly increases accuracy and reduces the uncertainty in the measurement.

SUMMARY

[0145] In this short exercise it has been demonstrated how the introduction of an additional temperature sensitive core to the transmission calorimeter, that is sensitive to only the ambient temperature and not the radiation treatment beam, could be used to improve the accuracy whilst reducing the uncertainty of the measurement. In a medical setting, increased accuracy and reduction of uncertainty in measurement is desirable, with the expectation of an improvement in a patient's treatment outcome.

[0146] Although not investigated here, it is anticipated that this reduction in the uncertainty would reduce the minimum dose rate capable of being measured by the transmission calorimeter.

[0147] All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

[0148] The disclosures in UK patent application number 2207878.6, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.