Bundesmann, J.

Damerow, T.

Denker, A.

Kunert, C.

Weber, A.

HZB Repository

A MULTI-LEAF FARADAY CUP ESPECIALLY FOR PROTON THERAPY OF OCULAR TUMORS C. Kunert#, J. Bundesmann, T. Damerow, A. Denker, Helmholtz-Zentrum Berlin, Germany  A. Weber, Charité Universitätsmedizin Berlin, GermanyAbstract For tumor therapy with protons it is crucial to know the beam range with a high accuracy. The Multi-leaf Faraday Cup (MLFC) offers a quick and precise range or energy measurement. To adapt the principle of a MLFC to the eye tumor therapy requirements is a challenging task, due to the necessary accuracy in the sub mm regime. The first prototype has 42 channels; each consists of a 10 µm copper foil, connected to an ammeter, next to a 25 µm Kapton foil. An incoming proton beam creates a current of some pA in a certain channel with respect to its energy. MOTIVATION The eye tumor therapy benefits from the radiation therapy with protons in a lot of cases. Since 1998 the University Hospital Charité Berlin provides together with the Helmholtz-Zentrum Berlin (HZB) a treatment facility using the HZB isochronous cyclotron [1]. The main benefit of proton beams in tumor therapy is the determined range in tissue in contrast to the commonly used photon radiation. Additionally the protons create the highest dose just before stopping. This depth dose curve is called Bragg peak and an example is shown in fig. 1.  Figure 1: Typical Single Bragg Peak (SBP) of the HZB cyclotron With such proton beams are very well determined radiation fields achievable, the dose can be delivered mainly to the tumor tissue, and critical tissues, which are highly sensitive to radiation, can be spared. In our case this leads to a tumor control of 96% after 5 years and in most cases the eyesight can be conserved. The eye contains several critical structures in its small volume of 6-7 cm³, e.g. the optical nerve or the macula. Fig. 2 shows a typical planned dose distribution for a melanoma located near the optical nerve. The positioning of the radiation field is crucial for the successful therapy with low side effects. Due to the small structures in the eye compared to other organs the necessary precision is in the sub mm regime. Therefore the positioning of the patients has to be done very precisely and the range of the proton beam has to be known with a resolution of 0.1 mm or better.  Figure 2: Planned radiation field with marked critical structures and the tumor. Due to necessary quality checks, a range measurement has to be done in every therapy and after unexpected shut downs, e.g. an accidental shut-down due to instabilities in the power supply network. Thus, it would be a great advance to perform the necessary quality check in a short time to keep the therapy interruption as short as possible. THE MLFC  Figure 3: Principle of a MLFC and the comparison (top to bottom) of the fluence, differential fluence (range) and the Bragg peak as a function of depth [2], [3]. One idea for a quick and precise range measurement is a Multi-Leaf Faraday Cup. This is a stack of alternating conductor and insulator plates (fig. 3). Each conductor is connected to the ground potential via an ammeter. Incoming protons stop in a certain plate and this additional positive charge creates a current by pulling  ___________________________________________  #christoph.kunert@helmholtz-berlin.de electrons from the ground. Thus the differential fluence (range) of a proton beam can be measured relatively fast. By determining the foil size and number an MLFC can be set to the eye tumor therapy requirements. In our case copper was chosen for the conductor material with a thickness of 10 µm, which equals approx. 50µm H2O equivalent. As insulator we use Kapton foils of 25 µm thickness, which corresponds to approx. 32 µm H2O. We want to reach a native resolution better than 100µm water equivalent in the range measurement. Additionally, the MLFC is a tool to check the beam energy for radiation hardness tests (RHT). For RHT different energies are requested for the devices under test. In these cases the beam is degraded with a calculated amount of aluminum to achieve the requested energy. SIMULATIONS  To dump a 68 MeV beam 6.75 mm copper is necessary. So, over 600 foils would be needed to dump the whole beam in the MLFC. To confine the efforts for the current measurement electronics, the number of channels should be kept low. Therefore simulations were conducted with MCNPX 2.6 to investigate how many copper foils are necessary to cover at least the Gaussian range peak in one measurement and to estimate the expected ratio of incoming beam to measured current in one foil. The simulations included the end of the nozzle, a chamber with the 50 copper foils covered by Kapton of 25 µm, 50 µm or 75 µm thickness and in front of the chamber a preabsorber of 315 mm acrylic glass. The simulated proton beam had a Gaussian energy distribution with the center around 68 MeV comparable to our real beam. The charge deposition in each copper foil was simulated. As shown in fig. 4, 50 foils are enough and the signal in the center of the peak is 3.5-4% of the incident proton beam.  Figure 4: Simulated charge deposition in 10 µm copper foils for different Kapton foils and a 315 mm preabsorber.  EXPERIMENTAL SETUP To measure the range peak in the MLFC the beam has to be degraded with a preabsorber in front of the MLFC. For the first setup with three foils a variable preabsorber has to be used to shift the peak over foils. By changing the thickness of the preabsorber the range of the proton beam can be changed and the end of the proton beam could be driven over the foils step by step. At the treatment room this is done with the range shifter (see fig. 6), which is also used for the therapy. The range shifter has a fixed position in the beam line. Another range shifting tool is the double wedge made of acrylic glass. These are two wedges with their oblique sides in front of each other, but one is movable and a defined thickness can be determined. The double wedge can be located directly in front of the nozzle. Thus with the wedge the proton loss due to scattering is lower.  Figure 6: Left: the range shifter, right: double wedge with the end of the nozzle. The beam comes from the left side. The first current measurements were conducted with three 617 Programmable Electrometers from Keithley. Thus the first setup was limited to three foils. Later, we used the “Rabbit Box” from iThemba Labs, South Africa, which can measure electrical currents in up to 48 channels simultaneously, but has a higher noise in the low pA regime than the electrometers from Keithley.  Figure 5: The open vacuum chamber with three copper foils with a diameter of approx. 4 cm (left), setup with 42 copper foils with a diameter of 10 cm (right). The actual setup is basically a board with a hole of 10cm in the middle and 50 spots in a circle of 12cm diameter, where the copper foils can be soldered on (fig. 5). Each spot has a 50 Ω impedance connection to a SMA connector. Despite careful soldering, in some channels dark currents of up to 1nA were observed and only 42 connections are usable. On top a 2.45 mm thick Aluminum plate is mounted for pressing on the foils and for shielding. For the connection to the Rabbit Box, we use special low noise cables. RESULTS & FIRST APPLICATIONS Fig. 7 shows the result of a measurement with the three foil setup. The chamber was evacuated but in the first two foils a high noise is present, as can be seen in the large y-error bars corresponding to the standard deviation. This is explained by the use of rather long BNC-cables (10 m) for the connection to the electrometers to keep them far away from the nozzle during irradiation.  Figure 7: Measured current for each foil for different positions of the range shifter for the chamber setup. Foil 1 saw the beam first. The main issue is the low current. By the conducted simulations (fig. 4) it was found that the expected signal in the foils is approx. 4% of the incoming proton current at the maximum height of the range curve. The therapy beam current is below 100 pA per cm² of beam field, the expected signal is around 4 pA per cm². In fact the ratio of approx. 4% was validated in the first setups. Therefore it is important to keep the dark current as low as possible. First checks with the Rabbit Box showed offset currents of about ±60 pA. A measurement with and without the beam was done to subtract the offset. Without any preabsorber the MLFC could measure proton energies from approx. 22 MeV – 29 MeV. This corresponds to the first and the last foil. Due to the Aluminum plate on top of the foils, lower energies are not reachable, but such low energies are not interesting either. By adding a particular preabsorber, higher proton energies can be measured.  Figure 8: First measurement of the 68 MeV beam at 700 pA with a 27.23 mm preabsorber of acrylic glass. The center is at foil 21,this corresponds to 67.6 MeV. Fig. 8 shows the measurement of the beam, which is extracted from the cyclotron with approx. 68 MeV. Then the beam passes through 50 µm scatter foil of Tantalum and the nozzle window of 50 µm Kapton. In front of the MLFC is an acrylic glass plate of 27.23 mm. The center of the Gaussian peak corresponds to 67.6 MeV. So, the beam energy of the cyclotron is verified. Also, RHT measurements are done at the HZB cyclotron and these measurements need different beam energies. Thus, measurements with the MLFC for checking the real beam energy against the calculation with SRIM [4] were recently conducted and tab. 1 shows the results. The calculation started with 68 MeV protons and calculated their energy after passing through the scatter foil (50 µm Ta), the nozzle (80 µm Kapton), the degrader stack of Aluminum in the particular thickness and air (24 cm minus the thickness of the absorber stack). The MLFC stood at the same position as the tested devices and the measured energy corresponds to the center of the each range peak for each measurement. Table 1: Results of energy check for radiation hardness tests (RHT). Requested energy Nominal energy* Measured energy 30 MeV 29.8 MeV 30.8 MeV 50 MeV 48.7 MeV 49.2 MeV 68 MeV 67.3 MeV 67.6 MeV *as calculated with SRIM [4] The MLFC measurement is in good agreement with the calculation. So, the MLFC offers a quick verification of the needed energies for RHT. OUTLOOK Further measurements for the MLFC characterization and tests of especially developed amplifiers are planned. Automated measurements require special preabsorbers and a software tool. Additionally an analytical model for Bragg peak calculations out of particular range values [5] will be tested concerning its accuracy. ACKNOWLEDGMENT Our sincere thanks go to iThemba Labs, South Africa, for the possibility to use their Rabbit Box and their support. REFERENCES [1] A. Denker et al., “Eye Tumor Therapy in Berlin,” Proceedings of IPAC’10, Kyoto, May 2010, MOPEA002, p. 64 (2010); http://www.JACoW.org [2] B. Gottschalk, “BGtalks: Multi-Layer Faraday Cup” (2007) [3] B. Gottschalk; “On the Characterization of Spread-Out Bragg Peaks and the Definition of ‘Depth’ and ‘Modulation” (2004) [4] “SRIM-the Stopping and Range of Ions in Matter”, software packages for calculating stopping power and range of ions in matter, http://www.srim.org [5] T. Bortfeld, “An analytical approximation of the Bragg curve for therapeutic proton beams,” Med.Phys. 24, 2024 (1997). 

A Multi Leaf Faraday Cup especially for Proton Therapy of Ocular Tumors

A MULTI-LEAF FARADAY CUP ESPECIALLY FOR THE THERAPY OF OCULAR TUMORS WITH PROTONS C. Kunert#, J. Bundesmann, T. Damerow, A. Denker,  Helmholtz-Zentrum Berlin, Germany  A. Weber, Charité Universitätsmedizin Berlin, Germany Abstract For tumor therapy with protons it is crucial to know the beam range with high accuracy. The Multi-leaf Faraday Cup (MLFC) offers a quick and precise range and energy measurement. Our MLFC is adapted to the eye tumor therapy requirements and has 47 channels; each consists of a 10 μm copper foil (connected to an ammeter) next to a 25 μm Kapton™ foil. A system of absorbers has been constructed to achieve an energy range of 30 MeV to 70 MeV for the measurements. First results and characteristics are presented in this work. MOTIVATION The Helmholtz-Zentrum Berlin (HZB) and the University Hospital Charité Berlin provide together a facility for treating eye tumors with proton radiation using the HZB isochronous cyclotron facility [1]. The main benefit of proton beams in tumor therapy is the determined range in tissue in contrast to the commonly used photon radiation. Furthermore protons create the highest dose just before stopping. This depth dose curve is called Bragg curve (SBP) and the HZB SBP is shown in Fig. 1.  Figure 1: Typical Single Bragg Peak (SBP) of the HZB cyclotron used for the therapy with a distal dose fall off below 1 mm H2O (one of the sharpest worldwide) and an energy of approx. 62 MeV at the isocenter. Such proton beams achieve very well determined radiation fields. Thus, critical tissues, which are highly sensitive to radiation, can be spared. In our case this leads to a tumor control of 96 % after 5 years and in most cases the eyesight can be conserved. The eye is a rather small organ of 6-7 cm³ and contains several critical structures, e.g. the optical nerve or the macula. Fig. 2 shows a typical dose distribution for a melanoma located near the optical nerve. To achieve a successful treatment with the side effects as low as possible, the positioning of the radiation field is crucial. Therefore the necessary precision of the positioning of the patient, the beam and the location of the tumor is in the sub mm regime.   Figure 2: Planned radiation field with marked tumor (1) and the critical structures lens (2) and optic nerve (3). So, especially the quality checks measurements done before and during the therapy have to fulfill high standards. Additionally, the quality checks consume a lot of time and therefore it makes always sense to improve the tools used for measurements concerning the beam properties. MULTI-LEAF FARADAY CUP (MLFC)  Figure 3: Principle of a MLFC and the comparison (top to bottom) of the fluence, differential fluence (range) and the Bragg curve as a function of depth for protons of the same energy, but a different energy spread [2], [3].  A MLFC is a stack of alternating conductor and insulator sheets (Fig. 3). Each conductor is connected to  ___________________________________________  #christoph.kunert@helmholtz-berlin.de Proceedings of IPAC2014, Dresden, Germany WEPRO08208 Applications of AcceleratorsU01 Medical ApplicationsISBN 978-3-95450-132-82149 Copyright©2014CC-BY-3.0andbytherespectiveauthorsan ammeter. Incoming protons stop in a certain sheet and due to the additional positive charge electrons are pulled from the ground potential to the sheet. By measuring the resulting currents the range (differential fluence) of the proton beam in matter can be measured relatively fast. By defining the foil size and number an MLFC can be set to the eye tumor therapy requirements. To achieve a native resolution in water of approx. 100 μm our MLFC consist of 10 μm thick copper foils, which equals approx. 50 μm H2O. As insulator we use Kapton™ foils of 25 μm thickness, which corresponds to approx. 32 μm H2O.  The MLFC is a tool to check the beam energy in general and we want to use it for radiation hardness tests (RHT), too. For RHT different energies are requested for the devices under test. In these cases the beam is degraded with a calculated amount of aluminum to achieve the requested energy. Typical maximal energies used for eye tumor therapy are between 60 MeV and 70 MeV. For the RHT it is common to use energies down to 30 MeV. This sets the required energy limits for our MLFC to a range from 30 MeV to 70 MeV. SETUP  The protons are extracted from the cyclotron at 68 MeV and are decelerated until they reach the eye of the patient in the treatment room to approx. 62 MeV, due to adaptions of the beam to each patient.  To dump the whole 68 MeV beam in the MLFC 6.75 mm of copper are necessary, or more than 600 foils in our case. But the first 500 or more would only decelerate the beam and the interesting peak of the differential fluence (see Fig. 3) would be located in the last foils.  Simulations were conducted [4] to investigate how many copper foils are necessary to cover at least the Gaussian shaped differential fluence peak (DFP or range peak) in one measurement. As a result less than 50 channels (one channel is the combination of a Kapton™ foil in front of a copper foil behind) are enough to cover the whole peak, if there is a degrader in front of the foil stack to decelerate the beam to an energy that the beams stops in the foil stack, which means below 19 MeV.  Figure 4: Setup with the absorber system (stair B1, moveable table B2), the MLFC stack (A) and the Rabbitbox (C).  Because the MLFC should be able to measure different energies, a particular absorber system had to be developed and constructed (see Fig. 4). This system has to fulfill that beam energies can be measured without knowing the energy in advance. The absorber system consists of two main parts, a stair and a double wedge, both made out of Aluminum with a purity of 99.5 %. Both are mounted on moveable tables in front of the MLFC stack. The stair has three steps (S1-S3), each 4 mm thick (S1=12.06±0.01 mm, S2=7.98±0.01 mm, S3=4.05±0.005 mm). The stair can be moved entirely out of the beam (S4=0 mm). The double wedge has an adjustable thickness range from 3.14±0.09 mm to 6.30±0.13 mm. The actual MLFC-Stack, shown in Fig. 5, consists of 48 foils sets slightly pressed between a 7 mm thick copperplate in the back, which works as a beam dump, and a 0.2 mm thick Aluminum sheet in the front. Each copper foil is connected to the Rabbitbox, an ammeter with 48 channels for simultaneous measurements from iTHEMBA labs, South Africa. Each copper foil is soldered onto an especially designed board, which connects (50 ȍ impedance) the foils via SMA couplings and low-noise cables to the Rabbitbox. The copper foil of the last channel lies directly on top of the beam dump so a direct readout of the beam dump signal is achieved. The copper foils have a diameter of 10 cm and the Kapton™ foils have a diameter of 12 cm. The active area of the MLFC is a circular area with a diameter of 10 cm. For controlling and analysis a LabVIEW™ program was written. This program includes an automatic search for the correct position of the absorber system to have the range peak in the MLFC stack and a measurement procedure with an analysis of the measured data.  During this procedure the currents in every channel are measured several times and a previous measured background value for each channel is subtracted. The mean value of each channel is saved and a Gaussian fit, weighted with the standard deviation of the mean values, is done. The fit can be done in units of channels, energy or water range. The outputs are the fit data as well as typical values like the center, standard deviation and amplitude of the Gaussian curve and their errors.  Figure 5: A: Back plate of the MLFC with beam dump, B:  Board on top of the back plate, C: Copper foils soldered onto the board with Kapton™ foils in between. WEPRO082 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-82150Copyright©2014CC-BY-3.0andbytherespectiveauthors08 Applications of AcceleratorsU01 Medical ApplicationsRESULTS Fig. 6 shows a typical result of a measurement. As expected it shows a Gaussian shaped curve which do not go to zero in the first foils, due to protons that stops earlier via nuclear interactions. In the last channel (48) is the dump signal. The error bars are different, because every channel has a different noise. Additionally, the first one has the biggest noise due to the worst shielding (only the 0.3 mm Al plate), every following channel is shielded by the channels in front of it. The second channel seems to be broken.  Figure 6: Example of a measurement (274 pA beam with 67.59 MeV) in units of channels and the Gaussian fit done by LabVIEW™. The error bars represent the standard deviation of the mean values in each channel. The beam extracted from the cyclotron goes through a 50 μm Tantalum foil for scattering and an 80 μm Kapton™ nozzle window. In the treatment room the beam passes also an air path and a few devices for measurements and beam shaping. With one device it is possible to vary the range of the protons in precise steps of 10 μm H2O. This was used to investigate the relative resolution for the MLFC shown in Fig. 7.  Figure 7: Result of measurements for the relative energy resolution. Each fit curve is based on data measured with a bit less water equivalent material in the beam path.  In Fig. 7 the dotted (green) and dash-dotted (blue) curve can be clearly separated from the reference. In contrast the dashed (red) one is too close to the reference. Differences in the proton range below 50 μm cannot be clearly separated because they are in the same order as the fitting error (see table. 1). Table 1: Results of the Relative Resolution Test Range difference Center of the fit Error of the fit 0 (Reference) 35.0 0.26 -10 μm H2O 35.1  0.25 -50 μm H2O 35.5 0.28 To check the energies used for RHT two programs, based on stopping power tables, are used to calculate the expected energy at the target room out of the extraction energy of 68.63±0.1 MeV (measured by varying the current in a dipole magnet) and degraded by the scattering foil, the nozzle window, Aluminum degraders (0, 8 mm, 14 mm) to achieve the requested energy and the air path to the MLFC. One program is “SRIM” [5], a standard program in physics to calculate ion ranges and the other program is “lookup” [6] a tool to plan beam lines of proton therapy facilities. The results are shown in table 2. Table 2: Results of Energy Check for Radiation Hardness Tests (RHT) for the Extraction Energy of 68.63 MeV. Requested energy  Nominal energy 1* Nominal energy 2** Measured energy 30 MeV 30.66 MeV 30.87 MeV 31.02 MeV 50 MeV 49.28 MeV 49.34 MeV 49.29 MeV 68 MeV 67.71 MeV 67.68 MeV 67.59 MeV *calculated with SRIM [5], **calculated with lookup [6] OUTLOOK The energies in table 2 are in good agreement, but there are still discrepancies in absolute range measurements of the order of 0.1 mm H2O compared to measurements with a water phantom. One reason could be small variations in the foil thicknesses, which will be tested by experiments. A LabVIEW™ program was written, which uses an analytical model for Bragg curve calculation [7] to calculate a Bragg curve out of the values for the center and standard deviation in units of water range measured by the MLFC. But some improvements have to be done for a quick comparison with a Bragg curve measured in a water phantom. Furthermore it is planned to investigate whether it is possible to measure even Spread-Out Bragg Peaks, which would make a further developed MLFC a more powerful tool for the therapy.  ACKNOWLEDGMENT Our sincere thanks go to iThemba Labs, South Africa, for the possibility to use their Rabbitbox and for their support and to the German Bundesministerium für Bildung und Forschung and Land Berlin for funding and supporting this work. Special thanks go to P.-E. Winter for manufacturing the absorber system and his kind support. Proceedings of IPAC2014, Dresden, Germany WEPRO08208 Applications of AcceleratorsU01 Medical ApplicationsISBN 978-3-95450-132-82151 Copyright©2014CC-BY-3.0andbytherespectiveauthorsREFERENCES [1] A. Denker et al., “Eye Tumor Therapy in Berlin,” Proceedings of IPAC’10, Kyoto, May 2010, MOPEA002, p. 64 (2010); http://www.JACoW.org [2] B. Gottschalk, “BGtalks: Multi-Layer Faraday Cup” (2007) [3] B. Gottschalk, “On the Characterization of Spread-Out Bragg Peaks and the Definition of ‘Depth’ and ‘Modulation” (2004) [4] C. Kunert et al., “A Multi-Leaf Faraday Cup especially for proton therapy of ocular tumors,” Proceedings of CYCLOTRONS’13, Vancouver, Sep. 2013, TH2PB04, http://www.JACoW.org, http://cyc13.triumf.ca/index.html [5] “SRIM-the Stopping and Range of Ions in Matter”, software packages for calculating stopping power and range of ions in matter, http://www.srim.org  [6] B. Gottschalk, “lookup” part of “BGware”, http://users.physics.harvard.edu/~gottschalk/ [7] T. Bortfeld, “An analytical approximation of the Bragg curve for therapeutic proton beams,” Med.Phys. 24, 2024 (1997) WEPRO082 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-82152Copyright©2014CC-BY-3.0andbytherespectiveauthors08 Applications of AcceleratorsU01 Medical Applications

A Multileaf Faraday Cup especially for the therapy of occular tumors with protons

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A Multileaf Faraday Cup especially for the therapy of occular tumors with protons

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