The present invention relates to a pixel detector system optimized for Pencil Beam Scanning Proton Therapy.

The PSI PROSCAN proton therapy facility started with Gantry 1 in 1995. It was worldwide the first proton therapy machine with

Pencil Beam Scanning (PBS) technology. The proton beam is applied to the patient spot by spot and can be deflected by a scanning magnet in one dimension. The first patient was treated with spot scanning technology in 1996. For more than 12 years Gantry 1 was the only machine using this technology. Today the technology is state of the art and supported by most of the new industrial facility installations. Gantry 2 is also an PSI development and implements advanced scanning technology. Next to Gantry 1 and Gantry 2 PSI operates a commercial gantry built by Varian Medical System and a horizontal beamline for treatment of ocular tumors.

The performance optimized Gantry 2 design allows fast energy changes and fast magnetic beam scanning in two dimensions. This allows using the machine not only for spot scanning but also for advanced scanning modes like the continuous delivery of lines or even contours. The basic principles of the different scanning modes are shown in Fig. 1.

The PSI beamline design and Gantry geometry with all its steering elements are the main components to deliver a thin proton pencil beam with an energy-dependent Gaussian beam of 2-5 mm sigma and a position accuracy of better than 1 mm. The properties of the PBS technology are the key to irradiate the tumor target with high precision. But to be sure that the machine works properly the beam precision has to be verified regularly with quality assurance measurements. Dedicated tests, using different dosimetry devices, verify important beam delivery parameters as dose, energy and position. A small and round proton beam will guarantee a precise treatment where important organs of risks next to the tumor are optimal spared.

It is therefore the objective of the present invention to provide a particle detector having a spatially modulated resolution that can be read-out efficiently even under the constraint of a limited number of read-out channels.

This objective is achieved according to the invention by two different designs of the particle detector.

The first design discloses a particle beam detector having

spatially modulated resolution, comprising:

a) a PCB bearing on both sides a plurality of individual particle detector pixels thereby offering a detector active area, wherein a first number of larger particle detector pixels are disposed on one side of the PCB regularly to cover substantially the complete detector active area and a second number of individual smaller particle detector pixels are disposed on the other side of the PCB regularly to cover substantially the complete detector active area wherein the smaller particle detector pixels are disposed in arrays of small detector pixels field and wherein the larger particle detector pixels each match with one small detector pixel field;

b) a data multiplexer being controlled to link the outputs of the individual particle detector pixels to a data evaluation instance; said data evaluation instance being enabled to control the data multiplexer in order to readout the outputs of the larger particle detector pixels and to readout the smaller particle detector pixels wherein the outputs of the small particle detector pixels being located at the same position in all small detector pixel fields are electrically connected to form one output channel thereby generating a number of output channels that equals the number of small particle detector pixel contained in one small detector pixel field; and

c) the data evaluation instance being further enabled to determine the position of the incoming particle beam by the analysis of the signals in the output channels for the larger particle detector pixels and of the signals thereby being enabled to determine the position of an incoming particle beam.

The second design discloses a particle beam detector having spatially modulated resolution, comprising:

a) a PCB bearing on both sides a plurality of individual particle detector pixels thereby offering a detector active area, wherein a first number of particle detector strips are disposed on one side of the PCB and a second number of particle detector strips are disposed on the other side of the PCB wherein the two groups particle detector strips are being oriented perpendicular to each other, wherein a third number of individual particle detector pixels are disposed on one side or both sides of the PCB between the particle detector strips, wherein the dimension of the

individual particle detector pixels is significantly smaller than the smaller dimension of the particle detector strips and wherein the particle detector pixels are disposed in arrays of small detector pixels fields ; and wherein each crossing point of the two groups of particle detector stripes is assigned to one of the small detectors pixel fields ;

b) a data multiplexer being controlled to link the output of the individual particle detector strips and/or particle detector pixels to a data evaluation instance; said data evaluation

instance being enabled to control the data multiplexer in order to readout the outputs of the particle detector strips and to read out the outputs of the particle detector pixels wherein the outputs of the particle detector pixels that are being located at the same position in all small detector pixel fields are

electrically connected to form one pixel group output channel thereby generating a number of pixel group output channels that equals the number of small particle detector pixel contained in one small detector pixel field; and

c) the data evaluation instance being further enabled to determine the position of the incoming particle beam by the analysis of the signals in the output channels of the particle detector strips and by the analysis of the signals of the pixel group output channels of the particle detector pixels thereby being enabled to determine the position of an incoming particle beam.

Both designs have in common that a structure with limited

resolution is used to identify roughly the position of the

incoming particle beam and a number of detector pixels having a higher resolution that has been assigned in advance to a subset of the structure with limited resolution where the particle beam came in is read-out subsequently. Using this approach, only a limited number of pixels have to be read-out in one detection cycle thereby shortening the read-out dead time of the detector

significantly.

Further, the objective of the present invention can be achieved by a 3D particle beam detector having spatially modulated resolution, comprising a number of stacked particle beam detectors according to claim 1 or claim 2. In the stack, the particle beam detectors are oriented parallel to each other allowing a particle beam to penetrate into the stack substantially perpendicular to the orientation of the flat 2D particle beam detectors.

Preferred embodiments of the present invention are hereinafter described in more detail with reference to the attached drawings which depict in:

Figure 1 schematically possible pencil beam scanning modes in

particle beam therapy; A. spot scanning, B. Line scanning, and C. Contour scanning;

Figure 2 schematically a design for a pixel detector having a

small and big pixel PCB design;

Figure 3 schematically a cross-sectional view on a pixel detector ionization chamber in PCB stack design;

Figure 4 schematically a pixel detector having a strip design for raw position determination; Figure 5 schematically the pixel detector and readout electronics system;

Figure 6 schematically beam size measurements at different

energies with the detector design according to Figure 2;

Figure 7 schematically a spot scanning as integrated data

measurement results and its corresponding GUI visualization;

Figure schematically a 3D and 2D reconstructed 12 cm scan line applied with a beam scanning speed of 2 mm/ms;

Figure 9 schematically: (A) a reconstruction of a 2D line 90°

rotated (length 12 cm / speed 2 mm/ms; and (B) same 2D scanline rotated by 45°; and

Figure 10 schematically the concept of a 3d detector comprising a number of stacked 2D detectors.

To get real 2-D profile measurements of the spot scanning beam PSI developed a Pixel Detector prototype using standard, inexpensive printed circuit board (PCB) technology. Quality and precision of the measurement depend on the resolution of pixels. With a

decreasing pixel size, the overall number of required pixels will increase and the PCB layout becomes more complex to fan-out all signals. A limiting factor in the design approach is the number of readout channels. In a preferred but not limiting example of the present invention, the goal is using not more than 256 readout channels. With this constraint developing a detector with

thousands of pixels requires an intelligent PCB layout. A special channel recycling technique was used to connect several pixels to one sensor channel.

Prototype I - Pixel detector with small and big pixel structure This channel recycling method was verified with a first prototype design and an active area of 12 x 12 cm2. On the detector, there are two active signal areas on the top and bottom side of the PCB . The bottom side has a big pixel structure, each with a size of 30 x 30 mm2. In total 16 big pixel fields cover the whole active detector area. The PCB top side is covered by 16 fields of small copper pixels. Each field has a 15 x 15 pixel structure (225 small pixels/field) . Size of one pixel is 1.85 x 1.85 mm2 and the spacing between pixels is 150 ym. This results in a pixel pitch of 2 mm. In total the whole detector has 3600 pixels.

To fill the gap between 3600 pixels and only 256 readout channels the small pixels are routed with a multiplexed structure on the PCB. By connecting the same pixel of each field to one PCB copper trace the information of 16 different pixels is mapped to only one readout channel. Each big pixel on the bottom side of the PCB has the same size as one small pixel field on the top side. The small and big pixel arrangement of the PCB is depicted in Fig. 2.

As the size of a big pixel is chosen such that it covers the whole area irradiated by even the largest beam spots, the detector can be read out using the following two steps: a) With the signal information of the 16 big pixels, the raw beam position is calculated; and b) Depending on the information of the big pixels the detailed beam position and real 2-D profile can be calculated from the signals measured on the 225 small pixels array.

For the final ionization detector design, two high voltage (HV) planes are required - one covers the top and the other the bottom side of the PCB. To keep the design as simple as possible the HV distribution is also realized with PCB technology. The HV power supply can easily be connected to the pixel detector PCB. The HV is distributed to two carrier boards on both sides of the detector over copper lines and PCB spacers. Both carrier boards have a hole in the middle with the size of the active detector area. The hole is covered with double metalized 20 ym thick Mylar foil. It is electrically connected to the HV and generates the HV field of the ionization chamber. Fig. 3 shows the final ionization chamber detector stack where the HV and pixel detector PCBs are screwed together with spacers. The result is a very compact and simple detector design including a 5 mm air gap between HV and pixel PCBs on both sides.

Prototype II - Pixel detector with small pixels and strip

structure

Fig. 7 shows that the spot size of a proton beam with an energy of 70 MeV almost covers the area of one big pixel field. In case the beam shape is greater than one field the channel recycling

approach won't work anymore. Next to this the active scanning area of Gantry 2 is 12 x 20 cm ² and can't be covered by the first prototype detector which in this example has the limitation of 256 read-out channels. The second prototype of pixel detector tackles these limiting factors of the field size and active detector area. One field has still the same number of 225 small pixels but the size of each pixel was change to 2.2 x 2.2 mm ² . With a 300 ym pixel to pixel spacing, the field size was increased to 37.5 x 37.5 mm ² . The detector has now 7 x 5 fields with total 7875 pixels and covers an area of 26.25 x 18.75 cm ² . In this specific example, a limiting element is the number of sensor channels. With 225 small pixels and 35 big pixel fields, the total number of sensor

channels would exceed 256. Instead of big pixels, a structure of strip grids in horizontal and vertical orientation was designed on the top and bottom side of the PCB. The strip grid is between the small pixels array. The copper grid around the pixels has a size of 100 ym. The strip structure on the top and bottom side of the PCB is depicted in Fig. 4. To increase the sensor signal each pixel has an active area on the top and bottom side of the PCB to measure a current from both ionization chamber sides of the detector. It has been assured that this design delivers a better signal to noise ratio for the pixel signal.

Frontend Readout Electronic System

An important part of the detector system is the frontend readout electronics. The current signals from the pixels are typically in the range of 10 ¹⁵ to 10 ⁹ ampere. Another important parameter defining the electronic requirements is the readout cycle time. Gantry 2 with its optimized design for line and contour scanning, operates with a maximum scanning speed of 1 cm/ms. Satisfying the performance of PBS, especially Gantry 2, a detector readout cycle time in the range of 100 - 200 ys is required. With a 200 ys cycle time it is possible to measure a 2 mm beam path of a line with the highest scanning speed at Gantry 2. This cycle time is enough to be sure that not more than one small pixel field will be

irradiated during this time.

Multi Channel Current Sensor frontend electronics board

A commercial readout chip, fulfilling these requirements is the ADAS1128 from ANALOG Devices. It is a highly integrated current- to-digital converter with 128 input channels. A digital

configuration interface gives the flexibility to adapt the

measurement range, readout cycle time and calibrate the analog electronics part of the chip from remote. Based on this chip, PSI developed a Multi Channel Current Sensor board (MCCS) with an analog interface to the detector and a digital interface to an FPGA processing system. Besides the readout chip the MCCS board handles the complete power supply distribution and supervision, as well as chip and PCB ambient temperature measurements.

For the pixel detector, only two readout boards are required to connect all signals. They are directly connected with cables to the FPGA processing platform. FPGA data processing board

The core of the processing platform is an eNCLUSTRA module with a XILINX KINTEX-7 FPGA. The module itself is a complete FPGA system with DDR SRAM, FLASH, complex FPGA power distribution and a flexible user 10 interface. Integrating this FPGA module into the system reduced the time and risk of the hardware development significantly. Only a simple and easy to design carrier board with two digital interfaces to the MCCS boards and optical high-speed communication lines to the proton therapy control system had to be developed .

Over the optical high-speed communication line, the detector system is fully integrated into the proton therapy control system. The deep integration into the control system, shown in Fig. 5, allows triggering a detector readout cycle synchronously to the beam delivery.

FPGA application

The FPGA application running on the eNCLUSTRA board is written in VHDL . There are two interfaces in the application.

Control system interface

The proton therapy control system has a high-speed optical

communication link to the FPGA board. This link operates typically with 2 GBit/s. Data access within the FPGA is handled over an internal set of registers or dual port memory. The register access is typically used for system configuration data or simple

measurement data like temperatures and power supply supervision. Dual port memory is used for the pixel detector sensor signals. In case of single sample logging of each sensor channel, the memory size will become quite big.

MCCS board interface The interface to the MCCS board is not only one interface with a protocol but several interfaces. The ADAS1128 chip will be

configured over a 4-wire serial peripheral interface (SPI) . The configuration of measurement range and sampling time is possible as well as starting of an automatic gain and offset calibration of all 128 measurement channels. For measurement data readout the ADAS1128 chip has a serial 125 MHz LVDS interface. From

configuration side, it is possible to define a flexible cycle time in the range of 50 to 900 ys . The system monitor on the MCCS board with power and temperature measurements is accessible over an I2C interface. Data from the system monitor is accessible over the internal FPGA registers from the proton therapy control system.

Depending on the information of the big pixels the detailed beam position and real 2-D profile can be calculated from the signals measured on the 225 small pixels array. The detector readout cycle is initiated by a start command from the proton therapy control system. From the start to the end of a single spot, line or contour the FPGA application reads all channels in a 200 ys loop and integrates the detector signals until the delivery stop command is received. Additionally all 256 signal channels are continuously logged. The logging limit for one channel is 512 samples. With a readout cycle time of 200 ys, it is possible to have a full data logging for the duration of 102.4 ms.

Measurement data visualization

Afterward the beam delivery of a spot scanning application, the proton therapy control system reads out the integrated signal of all channels, calculates the position and beam width from the 2-D profiles of each pencil beam spot. The deep integration of the detector into the proton therapy control system gives the user the possibility to investigate each spot directly after the delivery on a graphical user interface. The readout and storage of the continuous logged data into a file are synchronized to the beam delivery. Analysis of the logging data is handled outside of the GUI with Matlab. The Matlab

application supports the properties of the second pixel detector optimized for Gantry 2.

Measurement Results

Measurements with beam have been done with both prototypes of the pixel detector. The design of pixel detector prototype I is used to verify the proof of principle of the PCB channel recycling layout. Prototype II is used to extend the application for

advanced scanning modes (lines and contours) .

Prototype I - Measurement results

First measurements were performed with proton beam and spot scanning technology on Gantry 2. The whole detector area was irradiated spot by spot including the variation of the energy from 70 MeV to 230 MeV. With decreasing energy, the size of the spot will increase. As shown in Fig. 6, the detector resolution is good enough to reconstruct the beam size and shape for different beam energy settings.

Even at 70 MeV, the shape of the pencil beam almost perfectly fits into the small pixels field. But there is less margin if the beam shape is destructed by unexpected material in the beam path.

Considering all results, the channel multiplexing of the detector PCB perfectly fits the properties of pencil beam scanning.

Prototype II - Measurement Results

Prototype II detector is fully supporting spot scanning

applications. Reconstruction of a spot uses the integrated

measurement data of the detector. Based on the strip information the raw position will be calculated and used to define the offset for the small pixel array within the 7875 pixel grid. Fig. 7 shows the measurement data of a spot applied to the detector center. The strip measurements have a significant signal on two channels, while the small pixel channels have different signal data

measured, representing the ionization chamber charge. With these measurement data, the GUI reconstructs the position and shape of the proton beam and shows it as 2-D visualization.

Reconstruction of lines uses the single sample data logging from the FPGA. The Matlab algorithm for line reconstruction treats each sample as one spot. Strip measurement data define the raw position of the spot. With the information of the small pixel measurements the dose distribution of the spot is defined. The basic steps of the Matlab algorithm to reconstruct a line are:

In a first step the small pixel measurement data will be

rearranged in such a way that the spot is centered within the 225 pixel array. With the strip data the position of the small pixel array within the detector grid is defined. In a last step the single sample spot is added to the measurement data from the samples before. With the integration of all spots, a line can be reconstructed.

Fig. 8 shows the line reconstruction result of a 12 cm line applied in 60 ms. Depicted in Fig. 9 is the same scan line from Fig. 8 but applied to different orientations of the detector position .

Extension for 3D measurements in order to measure spatial dose development

In order to measure not only the dose distribution in one plain but also the development of the dose in different depth, several detectors could be combined in a stack of detectors. In this stack, detector layers are interleaved with gaps of air, which acts as counting gas and active volume for the detector layers. Each layer is designed in such a way, that one face acts as a plain electrode carrying the high voltage for the detector plane facing it via the air gap. The other face carries the detector pattern of small pixels and strips in form of a grid-pattern around the individual small pixels. In order to resolve the coarse two-dimensional position, where the detector layers are hit by the particle beam, two planes of detector layers act together in the following way. The layers are oriented such that the grid- patterned strip structures of one plane are perpendicular to the ones of the corresponding plane. Therefore, with one pair of detector planes, the coarse position can be resolved and so for each plane the dose distribution can be reconstructed. As the particle beam transverses the layers, the energy of the particles is reduced. Therefore, with several consecutive layers, the particle beam can be scanned at different descending energies, giving the full 3D distribution of the applied dose, if the detector has enough layers to completely stop the particle beam.

As the particle beam broadens while travelling through the

detector planes, it is important to choose an adequate size for the small pixels, which is usually bigger than for the single plain detector. The spacing of the measurements in depth can be modulated by proper selection of the detector material. It is also possible to insert additional absorber material in the air gap between the detector layers as long as the electrical properties of the detector are not changed. This could be achieved by using conducting absorber materials or non-conducting materials coated with a conducting layer.

The benefit of such a detector is the full reconstruction of the dose in 3D measured online in only one application of the dose distribution. The Analysis and possible comparison of the measured dose with the expected distribution can then be done offline in any of the measured depth and has not to be chosen before the application of the dose distribution, as it is the case with a single detector layer and an absorber in front. Thus, it is more versatile and faster in comparison to single layer systems.

This 3D concept using a number stacked 2D detectors is

schematically shown in Fig. 10.