Radiotherapy with protons, due to the physical properties of these particles, offers several advantages for cancer therapy as compared to the traditional radiotherapy with photons. In the clinical use of proton beams, a pCT (proton

Computed Tomography) apparatus can contribute to improve the accuracy of the patient positioning and dose distribution calculation. In this paper a pCT apparatus

built by the PRIMA (PRoton IMAging) Italian Collaboration will be presented and the preliminary results will be discussed

Brianzi, M.

Bruzzi, M.

Bucciolini, M.

Cirrone, G. A. P.

Civinini, C.

Cuttone, G.

Lo Presti, D.

Randazzo, N.

Scaringella, M.

Sipala, Valeria

Stancampiano, C.

Talamonti, C.

Tesi, M.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10809-xColloquia: IFA 2010IL NUOVO CIMENTO Vol. 34 C, N. 1 Gennaio-Febbraio 2011Proton imaging apparatus for protontherapy applicationV. Sipala(1)(2)(∗), M. Brianzi(3), M. Bruzzi(3)(4), M. Bucciolini(3)(5),G. A. P. Cirrone(6), C. Civinini(3), G. Cuttone(6), D. Lo Presti(1)(2),N. Randazzo(2), M. Scaringella(3)(4), C. Stancampiano(2), C. Talamonti(3)(4)and M. Tesi(4)(1) Dipartimento di Fisica, Universita` di Catania - via S. Sofia 64, I-95123 Catania, Italy(2) INFN, Sezione di Catania - via S. Sofia 64, I-95123 Catania, Italy(3) INFN, Sezione di Firenze - via G. Sansone 1, I-50019 Sesto Fiorentino (FI), Italy(4) Dipartimento di Energetica, Universita` di Firenze - via S. Marta 3, I-50139 Firenze, Italy(5) Dipartimento di Fisiopatologia Clinica, Universita` di Firenze - v.le Morgagni 85I-50134 Firenze, Italy(6) INFN, Laboratori Nazionali del Sud - via S. Sofia 62, I-95123 Catania, Italy(ricevuto il 7 Luglio 2010; approvato il 31 Luglio 2010; pubblicato online il 15 Febbraio 2011)Summary. — Radiotherapy with protons, due to the physical properties of theseparticles, offers several advantages for cancer therapy as compared to the traditionalradiotherapy with photons. In the clinical use of proton beams, a pCT (protonComputed Tomography) apparatus can contribute to improve the accuracy of thepatient positioning and dose distribution calculation. In this paper a pCT apparatusbuilt by the PRIMA (PRoton IMAging) Italian Collaboration will be presented andthe preliminary results will be discussed.PACS 29.40.Vj – Calorimeters.PACS 87.57.Q- – Computed tomography.PACS 29.40.-n – Radiation detectors.PACS 07.05.Hd – Data acquisition: hardware and software.1. – IntroductionThe interest in the use of hadrons for the treatment of cancer results in a growingnumber of new projects in this area [1-3]. The main advantage in the use of theseparticles is a better conformation of dose to the target volume, linked to how theseparticles interact with matter. However, the accuracy of the proton therapy is currentlylimited by the uncertainty with which one knows the distribution of mass stopping powersof these particles in matter, which is determined, for each patient to be treated, fromthe distribution of the absorption coefficients of photons measured with CT [4]. Theuncertainty could be reduced by measuring directly the mass stopping power with beamof proton through the use of a computerized axial tomography system with protons(pCT) [5].(∗) E-mail: valeria.sipala@ct.infn.itc© Societa` Italiana di Fisica 1112 V. SIPALA, M. BRIANZI, M. BRUZZI, ETC.For this reason, the PRIMA (Proton IMAging) Collaboration [6] has designed andbuilt an apparatus able to acquire tomographic images and to measure the mass stoppingpower distribution, using proton beams. The use of the pCT apparatus is foreseen in thepre-treatment phase, i.e. it is useful for the planning of the treatment, besides for theverification of the patient positioning during the first session.The problem in using proton beams for imaging is that these particles do not havestraight trajectories in the matter because of multiple Coulomb scattering. The “singletracking technique” whose principle is to track each single proton and to measure itsresidual energy is a promising method to overcome this difficulty [7, 8]. The first steptowards developing a pCT apparatus is building a proton Computed Radiography (pCR)device, able to measure a single projection. Adding a rotating system in order to acquiremultiple projections the pCT apparatus construction will be possible.Supported by the MIUR and INFN, a pCR system has been designed and built [9-12],with field of view about 5× 5 cm2, consisting of a silicon tracker and a YAG:Ce crystalcalorimeter. The aim is to detect protons with kinetic energy high enough to cross ahuman torso (250–270MeV) with a rate of ∼ 1MHz and to collect data with times ofthe order of the second, which satisfies the clinical requirements. The parameters tobe measured are entrance and exit angles, positions and proton residual energy. Thesedata are necessary to determine the most likely path (MLP) for the creation of a map ofelectronic densities of the tissue [13]. The radiography reconstruction algorithm has beenalready tested with pre-existing data [14]. In the future plans there is the developmentof adequate tomographic reconstruction algorithms and the validation of the developedalgorithms on real data obtained with the pCR system rotating the phantom.2. – pCR apparatus architectureThe pCR apparatus consists of a silicon tracker to sample the particle trajectory and acalorimeter to measure its residual energy. As shown in fig. 1, the tracker is composed offour x-y modules to measure the proton coordinates in the plane orthogonal to the beamline and a YAG:Ce scintillating crystal employed as calorimeter. Each tracker modulecommunicates with the control terminal (PC) via an Ethernet (IEEE 802.3) connection.The calorimeter outputs are used in order to generate the trigger signal, necessary to thetracker as acquisition start, and the global event number (GEN), attached to all the data(calorimeter and the tracker) in order to allow an easy and unambiguous event building.Each x-y tracker plane is composed of two identical tracker modules rotated by 90◦one with respect to the other. The module includes a 256 channels single-side siliconmicrostrip detector, 8 VLSI (Very Large Scale Integrated) front-end chips and a FPGA-based digital electronics for data acquisition and transmission. The front-end chip in-cludes 32 channels with a charge sensitive amplifier, a shaper and a comparator withexternal threshold. When a particle crosses the detector the electronic front-end gives adigital output. Moreover, by the Time Over Threshold (TOT) technique, the energy lossin the silicon microstrip detector can be reconstructed by digital output. The data arememorized in a RAM and sent off-line to the PC. The RAM chosen is able to acquireabout 500000 events necessary for a clinical image.The calorimeter is made of four YAG:Ce optically separated crystals, with size3× 3× 10 cm3, arranged as a 2 × 2 matrix, produced by Crytur [15]. This materialhas been selected due to its short decay time (70 ns) and because its scintillation lightis in the photodiode sensitivity range. The photodiode used is S3204 made by Hama-matsu [16]. A requirement of the system is the acquisition data rate which must reachPROTON IMAGING APPARATUS FOR PROTONTHERAPY APPLICATION 13Fig. 1. – Schema of the pCR apparatus. It consists of 4 x-y tracker planes, a calorimeter,a trigger generator board, a commercial acquisition board. Tracker data are transferred byEthernet connection to the PC.1MHz. For this reason, the calorimeter has been sectioned into four parts, so each crys-tal must be able to acquire at 250 kHz. The photodiode outputs are processed by acharge sensitive amplifier with “cold discharge” method [17] and a commercial hybridshaper. Four signals are sampled in parallel mode by UF2-4032 acquisition commercialboard [18].3. – pCR apparatus statusThe pCR apparatus has been completed. Each x-y plane has been tested separatelywith β-source. About the calorimeter, different tests were performed at LaboratoriNazionali del Sud (LNS) in the CATANA facility (proton energy up to 60MeV) [19]and Loma Linda University Medical Center (LLUMC) (proton energy up to 200MeV)in order to measure the resolution and the homogeneity of the crystal, the acquisitionperformance at high rate. An x-y plane has been coupled to the calorimeter and testedwith 60MeV proton beam at LNS.Finally, all planes have been assembled with the calorimeter in the final configurationand in fig. 2 a picture of the apparatus placed along the beam line is shown. The testhas been performed in the 1–10 kHz acquisition rate range.Data taken during this recent test is currently under analysis. Several radiographiesof an experimental phantom have been taken to measure the system resolution.4. – Preliminary experimental resultsSeveral tests have been carried out to understand the electrical performances ofthe front-end (by injecting a test pulse inside ASIC test inputs) and particle detectioncapabilities (with β-source and whit 60MeV proton beam). Different output DC levelsfor channel outputs have been observed, so a fine threshold regulation on chip-by-chipbasis through potentiometers has been implemented. In this configuration all the chipswork in optimal conditions even with higher proton beam energy. In fig. 3, pulse dura-tion, measured at 60MeV proton beam and with 200mV threshold voltage value over14 V. SIPALA, M. BRIANZI, M. BRUZZI, ETC.Fig. 2. – The picture of the pCR apparatus realized, positioned along the CATANA beam line.the DC level, is plotted versus particles kinetic energy. Both the tracker modules exhibitthe same slope but different intercept. This plot confirms the different chip DC voltage.The tracker detection efficiency as a function of the threshold voltage has been mea-sured using the 60MeV proton beam: the efficiency remains at its maximum value(∼ 98%) for about 150mV above the noise limit. Using β-source, the threshold voltagevalue interval whit high efficiency is about 10mV. However, according to Monte Carlosimulation, the charge released in the detector by β-source is equal to 1.5 · 104 electronswhile the 250MeV proton beam is about 3·104 electrons. So, these results ensure that oursystem is able to detect protons from a radiotherapy beam with good efficiency, becauseit will always have better performances than those we can measure with β-source.Figure 4 shows the histogram of the charge released in the silicon strip from 60MeVproton beam obtained using the TOT technique and the calibration curve of the singleFig. 3. – Most probable value of pulse duration T vs. proton kinetic energy E at fixed threshold(ΔVth = 200mV). Duration is expressed in terms of clock cycles (clock period is 10 ns).PROTON IMAGING APPARATUS FOR PROTONTHERAPY APPLICATION 15Fig. 4. – Histogram of the released charge from 60MeV proton beam in a silicon strip.channel. In the plot measured data (dots) is compared to the Landau fit (circles andline). The released charge mean value is equal to 8.7 · 104 electrons while the MonteCarlo prediction is 1.05 · 105 electrons. The discrepancy of 20% can be ascribed to thetolerance of integrated ASIC input capacitance values used in the calibration phase.The YAG:Ce calorimeter was tested with protons at LNS, and at Loma Linda Uni-versity Medical Center (LLUMC). In fig. 5 typical charge spectra at different energiesare shown. The lower energies are obtained using PMMA slices to degrade the 60MeVprimary beam. The FWHM resolution at 60MeV is 3.7%. The crystal response is linear(about 1% error) in the 30–60MeV proton energy range. Tests performed at LLUMCshow a good resolution also whit proton energy up to 200MeV (1–2%). Moreover, usinga tracker, we have verify that the crystal is homogeneous in the whole area.Fig. 5. – Charge spectrum obtained with different proton energy beam and YAG:Ce calorimeter.16 V. SIPALA, M. BRIANZI, M. BRUZZI, ETC.Fig. 6. – Counting map of the event triggered by the calorimeter with 60MeV proton beam.The x-y coordinates correspond to the last x-y silicon plane near the calorimeter.The test results of the complete apparatus, with the tracker and the calorimeter, arein advanced status of analysis. Figure 6 shows the map of the events triggered by thecalorimeter in the alignment phase of the system. In order to plot this map, where thebeam spot is evident, the x-y coordinates of the last silicon plane have been used. In theradiographic reconstruction it is necessary to apply to the data an algorithm, currently inreview phase, that employs the calculus of the particle most likely path in the phantom.5. – ConclusionsA system for pCR has been developed and the first test with proton beam has beenperformed. The apparatus includes a tracker based on Si microstrip detectors and asegmented YAG:Ce calorimeter. Previously, all the components had been manufacturedand tested separately with β-source and with protons of various energies. Results of beamtests, performed at INFN-LNS with 1–50 kHz proton rate range, have been discussed inthis paper. Finally, tests of the completed apparatus have been performed and havedemonstrated the good performances of the data acquisition system. The data fromvarious tracker modules and from the calorimeter have been correlated offline by usingGEN tag, confirming that an unambiguous event building can be carried out. Theimage of the beam spot has been appreciated as shown in the previous section, the firstphantom image has been acquired and the data analysis is in progress. The reconstructionalgorithm is in the review phase and will be applied to the existing data. In order todemonstrate the functionality as radiography system, new tests need to use higher protonbeam energies. For this reason, a beam test with 200MeV protons at Essen WPC isscheduled by the end of 2011.PROTON IMAGING APPARATUS FOR PROTONTHERAPY APPLICATION 17REFERENCES[1] Miller D. W., Med. Phys., 22 (1995) 1943.[2] Amaldi U. et al., Rep. Prog. Phys., 68 (2005) 1861.[3] Dosanjh M. et al., Nucl. Instrum. Methods A, 571 (2007) 191.[4] Schneider U. et al., Phys. Med. Biol., 41 (1996) 111.[5] Zygmanski P. et al., Phys. Med. Biol., 45 (2000) 511.[6] Cirrone G. A. P. et al., Nucl. Instrum. Methods A, 576 (2007) 194.[7] Schneider U. et al., Med. Phys., 21 (1994) 1657.[8] Hanson K. M. et al., Phys. Med. Biol., 26 (1981) 965.[9] Sipala V. et al., Nucl. Phys. B (Proc. Suppl.), 197 (2009) 39.[10] Sipala V. et al., Nucl. Instrum. Methods A, 612 (2010) 566.[11] Menichelli D. et al., IEEE TNS, Vol. 57, No. 1, February 2010.[12] Civinini C. et al., Nucl. Instrum. Methods A, 623 (2010) 588.[13] Williams D. C., Phys. Med. Biol., 49 (2004) 2899.[14] Talamonti C. et al., Nucl. Instrum. Methods A, 612 (2010) 571.[15] www.crytur.com.[16] http://sales.hamamatsu.com/assets/pdf/parts S/s3204 etc kpin1051e08.pdf.[17] Pullia A. et al., IEEE TNS, Vol. 48, No. 3 (2001).[18] www.strategic-test.com.[19] http://www.policlinico.unict.it/Adroterapia/def.htm.

Proton imaging apparatus for protontherapy application

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Proton imaging apparatus for protontherapy application

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