ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 518 (2004) 411–414 Position-sensitive silicon detectors for real-time dosimetry in medical applications W. Kucewicza,*, M. Alemib, M. Amatib, L. Badanoc, V. Bartschd, D. Berste, C. Bianchif, W. de Boerd, H. Bold, A. Bulgheronib, M. Cacciab, C. Cappellinib, F. Cannilloe, G. Clause, C. Colledanie, L. Contef, A. Czermakg, G. Deptuche, A. Dierlammd, K. Domanskih, W. Dulinskie, B. Dulnyg, O. Ferrandoc, P. Grabiech, E. Grigorievi, B. Jaroszewiczh, L. Jungermannd, K. Kucharskih, S. Kutaa, G. Leoi, R. Lorussof, J. Marczewskih, G. Mondryj, W. Machowskia, H. Niemieca, R. Novariof, M. Pezzettac, Y. Poposkii, M. Prestb, J-L. Riestere, C. Sampietrof, M. Sapora, H. Schweickertk, D. Tomaszewskih, A. Zalewskag a ! 30-059, Poland AGH University of Science and Technology, al.Mickiewicza 30, Krakow " Universita degli Studi dell’Insubria, Dipartimento di Scienze Chimiche Fisiche e Matematiche,via Vallegio 11, Como 20100, Italy c Fondazione per Adroterapia Oncologica, via Puccini 11, Novara 28100, Italy d Universitat . Karlsruhe, Kaiserstrasse 12, Karlsruhe 76128, Germany e Laboratoire d’Electronique et de Physique des Systemes Instrumentaux, rue du Loess 23, Strasbourg 67037, Centre National de la Recherche Scientifique, rue Michel-Ange 3, Paris 75794, Universit!e Luis Pasteur/IN2P3, rue Blaise Pascal 4, Strasbourg 67030, France f Universita" degli Studi dell’Insubria,Dipartimento di Scienze Cliniche e Biologiche, viale Borri 57, Varese 21100, Italy g ! 31-352, Poland H.Niewodniczanski Institut of Nuclear Physics, u. Radzikowskiego 152, Krakow h ! 32/46, Warszawa 02-668, Poland Instytut Technologii Elektronowej (Institute of Electron Technology), al.Lotnikow i Universit!e de Geneve, rue General-Dufour 24, Gen"eve 4121, Switzerland j Europtope Entwicklungsgesellschaft fur . Isotopentechnologien, Robert-Roessie Strasse 10, Berlin 12125, Germany k ZAG-Zyklotron AG, Hermann-von-Helmholtz Platz 1, Karlsruhe 76344, Germany b Abstract Real-time dosimetry is a critical issue in most radiotherapy applications. Silicon Ultra fast Cameras for electron and gamma sources In Medical Applications (SUCIMA) is a project addressing the development of an imaging system of extended radioactive sources based on monolithic and hybrid position-sensitive silicon sensors, where ‘‘imaging’’ has to be intended as the record of a dose map. The detector characteristics are constrained by the main applications, namely brachytherapy and real-time monitoring of a hadron beam for oncology. The key issues in the sensor and DAQ development are described together with the most relevant medical applications. SUCIMA1 is a project approved by the EC within the V Framework Program. r 2003 Elsevier B.V. All rights reserved. Keywords: Silicon detector; Active pixel detector; SOI technology; Dosimetry; Brachytherapy *Corresponding author. Tel.: +48-12-6173045; fax: +48-12-6332398. E-mail addresses: kucewicz@agh.edu.pl (W. Kucewicz). 1 E.C. Contract No. G1RD-CT-2001-00561. 0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.039 ARTICLE IN PRESS 412 W. Kucewicz et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 411–414 1. Introduction Coronary artery diseases are some of the most widespread pathologies in the western world and they are quite often linked to a reduction (stenosis) of the artery cross-section affecting the blood flow. There are two ways to recover a normal situation: angioplasty [1–3] and coronary bypassing, by far more invasive than the first option. Angioplasty reestablishes the artery lumen by inflating a balloon introduced by a catheter. Unfortunately, even supporting the artery by implanting a stainless-steel tube (stent) during the angioplasty, re-occurrence of the stenosis (restenosis) affects B25% of the patients. Local radiotherapy (brachytherapy) delivering a high dose (8–30 Gy) at high rate (B0.2 Gy/minute) has been proven to be quite effective, reducing the restenosis rate to the 10% level. Intravascular brachytherapy is currently used in the US and Europe; nevertheless, an optimization of the treatment plan remains an open question, as it requires a full characterization of the radiation interaction with vascular tissues, a record and analysis of the vessel morphology and a source profile customization. The Silicon Ultra fast Cameras for electron and gamma sources in Medical Applications (SUCIMA) project addresses the development of the tools required for the definition of a treatment plan, from the software for the vessel ultrasonic image analysis to the simulation of the energy deposition and real-time dosimetry, based on position-sensitive silicon sensors, which are the main subject of this paper. It is worth mentioning other applications foreseen; in particular, the use of the sensors under the development for real-time monitoring of a hadron beam for oncology is a key activity in the project plan. detector with analog readout information to provide an activity and dose map, with a granularity matching the resolution of ultrasonic vessel images (o100 mm). Moreover, the detector active area should cover full size of the source train injected during the treatment (up to 60 mm long) and be optimized to stand the high dose rate. The major goal of the SUCIMA project is the development of monolithic devices in CMOS and silicon on insulator (SOI) technologies, the latter being based on the ‘‘wafer bonding’’ process. To provide an initial evaluation of the perspectives opened up by the use of position sensitive silicon sensors, a dedicated strip detector has been engineered. The main characteristics of the CMOS prototypes are summarized elsewhere [4]; test structures in AMIS 0.35 mm and AMS 0.35 mm technology are being processed to evaluate the radiation tolerance and validate the new architecture. 2.1. The SOI sensor In SOI technology, two oxidized silicon wafers (identified as the ‘‘handle’’ and the ‘‘device’’ wafer) are bonded together. The device wafer, horsing the electronics circuit, before being processed is thinned down to a thickness of 1.5 mm range, actually separated by an oxide layer B1 mm thick from the handle wafer, acting as a mechanical support. For our specific application, a highresistivity handle wafer is used fully depleted and exploited as a sensitive volume [5]—See Fig. 1. To characterize the new technology a special test vehicle was designed and produced. Besides different transistors and capacitors it also includes fundamental circuits used in classical CMOS 2. Detector specification The aim of the proposed detector is the reconstruction of the energy deposited by a continuous flow of particles springing off an extended radioactive source. The detector has to be sensitive to b radiation as such sources are typically used in brachytherapy. It has to be a pixel Fig. 1. Schematic cross-section of a SOI ready device. ARTICLE IN PRESS W. Kucewicz et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 411–414 413 Fig. 2. Data acquisition and analysis performed with rolling shutter CDS. The charge coversion coefficient: 4.2 mV/fC. sensor design. The technological parameters have been extracted and grouped as a technological file. As a part of the validation process of the proposed read-out architecture, a front-end ASIC prototype in a standard 0.8 mm AMS bulk technology has been designed and successfully manufactured. Detailed measurements of DC and AC characteristics on the VLSI chip have been performed validating the architecture. The architecture includes a novel readout sequence based on the rolling technique [6] but including two samples of each channel during the readout cycles; immediately after the reset of the diode and the after the integration time. This method not only guarantees very short detector dead time (limited to the reset time of the integrating element) and well-defined integration time, but also enables external ‘‘correlated double sampling’’ (CDS) processing for noise suppression. Results of implementation of such methods [7], are presented in Fig. 2. 2.2. The hybrid silicon strip detector Since the feedback by the end users is crucial for the detector design finalization, a preliminary evolutionary hybrid solution with silicon strip detectors has been engineered, based on the 9.5  9.5 cm2 sensors designed for the AGILE experiment [8] and produced by HAMAMATSU Fig. 3. Attenuation of the dose in water equivalent tissue for 90 Sr measured with a strip sensor and GaFChromic films. Curves are normalized (according to the AAPM prescription) to the dose deposited at 2 mm depth in water [10]. Photonics.2 The 121 mm pitch strips are readout by the current integrating VASCM2 chips of the VIKING family,3 via a dedicated data acquisition system. Tests with radioactive sources used in brachytherapy have been performed, for the quality control of the dose profile and the dosedepth curve in water as prescribed by the American Association of Physicists in Medicine (AAPM) [9]. The main result is shown in Fig. 3, where the strip sensor response at different water depths is overlaid to the results in dose obtained with a calibrated GaFChromic film.4 Observed agreement within both curves is well promising and provide the starting point for passing from relative dosimetry to absolute one. To perform the absolute dosimetry it will be essential to have a well-defined and limited active silicon volume (i.e. pixel detector) together with an output signal energy calibration. 2 Hamamatsu, Hamamatsu City, Japan. Produced by IDEas, Veritasveien 9, Box 315, N-1323 Hvik, Norway. 4 ISP Performance Enhancing Products, Via Dei Gracchi 30 20146 Milano Italy, www.ispcorp.com. 3 ARTICLE IN PRESS 414 W. Kucewicz et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 411–414 4. Conclusions Three parallel research options of real-time dosimeters are being developed: CMOS sensors, SOI sensors and hybrid silicon strip detectors. Test results of the latter validate the principle of realtime dosimetry with position sensitive silicon sensors. The parameters of the novel SOI technology for sensor production were defined and prototype sensors are on the way. A PC compatible DAQ system with fast USB interface was designed and produced. References Fig. 4. Passing b particles measurement using the MIMOSA 5 chip with DAQ board. The projections on the axes are also shown. [1] [2] [3] [4] 3. Data acquisition system A dedicated data acquisition system has been developed [11], based on a XILINX VIRTEX-II FPGA and featuring four 12-bit ADC channels for fast analogue data digitization at 50 MHz. The data transmission to the equipment computer is based on the high-speed USB 2.0 port for communication and guarantees a throughput of up to 32 MB/s. The VIRTEX-II FPGA features 1 million general purpose programmable system gates allowing not only controlling the board, but also effective digital signal processing. 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