Tumour therapy with proton beams has been used for several decades in many centres with very good results in terms of local control and overall survival. Typical pathologies treated with this technique are located in head and neck, eye, prostate and in general at big depths or close to critical organs. The Experimental Physics Department of the University of Turin and the local Section of INFN, in

collaboration with INFN Laboratori Nazionali del Sud Catania and Centre de Protontherapie de Orsay Paris, have developed detector systems that allow the measurement of beam position and fluence, obtained in real time during beam delivery. The centre in Catania (CATANA: Centro di AdroTerapia ed Applicazioni Nucleari Avanzate) has been treating patients with eye pathologies since spring 2002 using a superconducting cyclotron accelerating protons up to 62 MeV. This kind of treatments need high-resolution monitor systems and for this reason we have developed a 

256-strip segmented ionisation chamber, each strip being 400

μm wide, with a total sensitive area 13 × 13 cm2. The Centre de Protontherapie de Orsay (CPO) has been operational since 1991 and features a synchrocyclotron

used for eye and head and neck tumours with proton beams up to 200 MeV. The monitor system has to work on a large surface and for this purpose we have designed a 

pixel-segmented ionisation chamber, each pixel being 5×5 mm2, for a total active area of 16 × 16 cm2. The results obtained with two prototypes of the pixel and strip chambers demonstrate that the detectors allow the measurement of fluence and centre of gravity as requested by clinical specifications

Benati, C.

Boriano, A.

Bourhaleb, F.

Cirio, R.

Cirrone, G. A. P.

Cornelius, I.

Cuttone, G.

Donetti, M.

Garelli, E.

Giordanengo, S.

Guérin, L.

La Rosa, A.

Luparia, A.

Marchetto, F.

Martin, F.

Meyroneinc, S.

Peroni, C.

Pittà, G.

Raffaele, L.

Sabini, M. G.

Valastro, L.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2005-10008-6IL NUOVO CIMENTO Vol. 27 C, N. 5 Settembre-Ottobre 2004Online measurement of fluence and positionfor protontherapy beamsC. Benati(1)(2), A. Boriano(1)(2), F. Bourhaleb(3), R. Cirio(2)(∗)G. A. P. Cirrone(4), I. Cornelius(2), G. Cuttone(4), M. Donetti(2)(5)E. Garelli(2)(6), S. Giordanengo(2)(∗∗), L. Gue´rin(7), A. La Rosa(1)(2)A. Luparia(2)(8), F. Marchetto(2), F. Martin(7), S. Meyroneinc(7)C. Peroni(1)(2), G. Pitta`(3), L. Raffaele(4)(9), M. G. Sabini(4)(10) andL. Valastro(4)(11)(1) Dipartimento di Fisica Sperimentale, Universita` di TorinoVia Giuria 1, 10125 Torino, Italy(2) INFN, Sezione di Torino - Via Giuria 1, 10125 Torino, Italy(3) TERA Foundation - Via Puccini 1, 28100 Novara, Italy(4) INFN, Laboratori Nazionali del Sud - Via S. Sofia 62, 95100 Catania, Italy(5) CNAO Foundation - Via Caminadella 16, 20123 Milano, Italy(6) ASP, Viale S.Severo 65, 10133 Torino, Italy(7) Institut Curie - Centre de Protontherapie (CPO), Orsay, France(8) COREP - C.so Duca Degli Abruzzi 24, 10129 Torino, Italy(9) Dipartimento di Radiologia e Radioterapia, Ospedale UniversitarioVia S. Sofia 78, 95100 Catania, Italy(10) A.O. Cannizzaro - Via Messina 829, 95126 Catania, Italy(11) Dipartimento di Fisica, Universita` di Torino - Via. S Sofia 64, 95100 Catania, Italy(ricevuto il 20 Gennaio 2005)Summary.— Tumour therapy with proton beams has been used for several decadesin many centres with very good results in terms of local control and overall survival.Typical pathologies treated with this technique are located in head and neck, eye,prostate and in general at big depths or close to critical organs. The ExperimentalPhysics Department of the University of Turin and the local Section of INFN, incollaboration with INFN Laboratori Nazionali del Sud Catania and Centre de Pro-tontherapie de Orsay Paris, have developed detector systems that allow the mea-surement of beam position and fluence, obtained in real time during beam delivery.The centre in Catania (CATANA: Centro di AdroTerapia ed Applicazioni NucleariAvanzate) has been treating patients with eye pathologies since spring 2002 using asuperconducting cyclotron accelerating protons up to 62 MeV.(∗) E-mail: cirio@to.infn.it(∗∗) Partially supported by Ion Beam Application (IBA), Chemin du Cyclotron 3, B-1348Louvain-la-Neuve, Belgium.c© Societa` Italiana di Fisica 529530 C. BENATI, A. BORIANO, F. BOURHALEB, R. CIRIO G. A. P. CIRRONE, ETC.This kind of treatments need high-resolution monitor systems and for this reasonwe have developed a 256-strip segmented ionisation chamber, each strip being 400µm wide, with a total sensitive area 13 × 13 cm2. The Centre de Protontherapiede Orsay (CPO) has been operational since 1991 and features a synchrocyclotronused for eye and head and neck tumours with proton beams up to 200 MeV. Themonitor system has to work on a large surface and for this purpose we have designeda pixel-segmented ionisation chamber, each pixel being 5×5 mm2, for a total activearea of 16 × 16 cm2. The results obtained with two prototypes of the pixel andstrip chambers demonstrate that the detectors allow the measurement of fluenceand centre of gravity as requested by clinical specifications.PACS 87.53.Qc – Proton, neutron, and heavier particle dosimetry: measurements.PACS 87.56.Da – Ancillary equipment.1. – IntroductionProton therapy allows a very precise energy deposition in the tumour while sparinghealthy tissue taking advantage of the Bragg peak in the depth dose distribution. Adrawback of this technique is the need for complicated apparatuses needed to insure theconstant delivery of the therapeutical beam. In particular it is of great importance to con-tinuously measure the beam shape and intensity. Normally the beam shape is measuredwith 4-sector ionisation chambers, where the information fed back to the control systemis up-down and left-right; the fluence measurement is performed with full-area ionisa-tion chambers. To improve such measurements we have developed electrode-segmentedionisation chambers. The two kinds of segmentation (strip or pixel) involve a very dif-ferent number of electronics channels, as it is intuitive. The choice of one of the two isthus driven by the need for having a faster or slower readout, being this linear with thenumber of channels.2. – Description of the detectors and the readout electronicsThe two detectors are based on the architecture of a parallel-plate ionisation cham-ber with one of the two electrodes segmented in strips or pixels [1]. Figure 1 shows anexploded view of the chamber used at CATANA. Two anodes, segmented in horizon-tal and vertical strips, respectively, are mounted facing each other on the two externalframes, as schematically shown. Each anode consists of 256 strips each 400 µm wide,separated by a 100 µm electrically insulated interspace. The total covered active areais (12.8× 12.8) cm2, exceeding the typical dimensions of the proton beam. The anodesare made of a 35 µm thick Kapton foil, covered by a 15 µm thick aluminium layer. Theconducting strips, placed on the aluminised side of the foil, have been obtained by usingthe standard printed circuit board (PCB) technique. Signals from the strips are broughtto four 68 pin connectors located on the edge of the anode frame. The single cathode issandwiched between the two anodes. It is made by a 25 µm thick Mylar foil, aluminisedon both sides. The thickness of each gas gap is 6 mm, filled with air for the present test.A special effort was taken to minimize the total thickness crossed by the beam; the totalwater equivalent thickness has been found to be about 100 µm by measuring the shift inposition of the Bragg peak.ONLINE MEASUREMENT OF FLUENCE AND POSITION FOR PROTONTHERAPY BEAMS 531Fig. 1. – Schematic drawing of the strip ionisation chamber tested on the CATANA beam.In figs. 2 and 3 are shown, respectively, the schematic drawing and a picture of thechamber tested at CPO. The construction details are equal to the strips chambers usedat CATANA, the major difference being that the pattern drawn on the anode has pixelsinstead of strips. There is a total of 1024 pixels, arranged on a 32 × 32 square 16 cmof side; each pixel has a 5 mm side and the interspace between two neighbouring pixelsis 0.1 mm. In this case it was not possible to have a single-layer printed circuit boardfor the anode, as it would have been impossible to bring out the electric signals of thecentral pixels; thus the anode is made of 50 µm thick Kapton with 25 µm thick copper.The signal of each pixel is brought to the input of the readout electronics through ametallized hole and a copper track that runs on the opposite face of the Kapton.Fig. 2. – Schematic drawing of the pixel ionisation chamber tested on the CPO beam.532 C. BENATI, A. BORIANO, F. BOURHALEB, R. CIRIO G. A. P. CIRRONE, ETC.Fig. 3. – Picture of the pixel ionisation chamber on the CPO beam line. The chamber is locatedat the isocenter, inside the treatment room. One can notice the final collimator on the rightside of the picture, reflecting on the chamber’s aluminised Mylar cathode.The electric field is generated by polarizing the cathode Mylar foil. Typically thehigh-voltage value is set to −500 V. The strip voltage is set to around 2 V, the samevoltage as the front-end input circuit.The design of the front end is based on the recycling integrator architecture [2]. Briefly,the input current from each pixel is integrated and a number of pulses proportional tothe charge collected by an individual channel is sent to a 16-bit counter. An ensemble of64 channels has been integrated in a single Very Large Scale Integration (VLSI) chip [3].At any given time all the counters can be latched and the results are stored in aregister. This is like taking a snapshot of the collected charges. The relevant features ofthe front-end circuit are summarized below:– the charge corresponding to a pulse (charge quantum) can be adjusted between100 fC and 800 fC via an externally regulated voltage;– maximum pulse frequency is 5 MHz which results in a limit for the input currentof 4 µA for a charge quantum set at 800 fC;– linearity from 100 pA up to the maximum current is within 1%;– dark current counting rate is at the level of ∼1 Hz;ONLINE MEASUREMENT OF FLUENCE AND POSITION FOR PROTONTHERAPY BEAMS 533Fig. 4. – Projections of the CATANA beam on the vertical (X) and horizontal (Y) axis.– charge quantum spread is less than 1%;– the change of charge quantum as a function of temperature in the range 20–30 ◦Cis less than 1 fC/◦C for a charge quantum set at 600 fC;– no dead time is introduced due to readout, because the latch located at the outputof the counters does not affect the counting itself.Each of the two CATANA chambers has a total of 256 channels and is read out withone custom printed circuit board that houses four VLSI chips and all the electronicsneeded to receive and send the digital signals to the computer that can be located ata maximum distance of 200 meters from the chamber; the CPO chamber needs 4 suchboards.Data acquisition was performed using a commercial input/output register read outby a Personal Computer running National Instrument LabView software. The readoutrate can be as high as 1kHz; in these preliminary tests the readout rate of the wholedetector was set at 5 Hz at CATANA and 10 Hz at CPO.3. – Experimental set-upThe two detector prototypes have been tested on the CATANA and CPO beam lines.In the former, the 62 MeV proton beam from the superconducting cyclotron operating atthe Laboratori Nazionali del Sud of INFN (Catania) exits in air through a 50 µm Kaptonwindow placed at about 3 meters from the isocenter. A first 15 µm tantalum scattering534 C. BENATI, A. BORIANO, F. BOURHALEB, R. CIRIO G. A. P. CIRRONE, ETC.Fig. 5. – Pedestals measured with the pixel ionisation chamber in the CPO treatment room. Onthe “Pixel” axis the 1024 pixels are shown, whereas the “Readout” axis runs from 0 to 300, thenumber of acquisitions taken every 100 ms in a 30 s run.foil is placed upstream of the window in vacuum, followed by a second 25 µm tantalumfoil provided with a central brass stopper of 4 mm in diameter, 7 mm thick and placedin air. The double-foil scattering system is optimised to obtain a good homogeneityin terms of lateral dose distribution, minimizing the energy loss. The range shifter andrange modulator are mounted in two different boxes downstream of the scattering system.Two transmission monitor chambers, used for on-line control of the dose delivered to thepatients during treatment, are placed further down along the beam line followed by thestrip ionisation chambers. The nozzle, a 37 cm long brass cylinder with a 45 mm hole,terminates with the final collimator (25 mm in diameter for the reference measures). TheFig. 6. – Standard deviation of the pedestals measured on each of the 1024 pixels in 5 subsequentruns at CPO.ONLINE MEASUREMENT OF FLUENCE AND POSITION FOR PROTONTHERAPY BEAMS 535Table I. – The linearity of response of the pixel ionisation chamber versus dose has been mea-sured at CPO with 4 different doses (Monitor Units, MU). The experimental column shows theaverage on 3 subsequent runs of the response of 16 central pixels, σ being the standard devia-tion. In the last two columns the result of the linear fit and the difference between fitted andexperimental are listed.MU Experimental σ (%) Fit Difference (%)1 4631.9 2.45 4615.7 0.35110 44421.3 0.12 44427.3 0.01450 221630.1 0.23 221367.6 0.119100 442241.7 0.21 442543.0 0.068final collimator is located 8 cm upstream of the isocenter, where a diode beam scanningsystem is placed for off-line beam profile measurements.The beam line used at CPO is indeed very similar to the one found at CATANA,although it is designed for a higher-energy beam and for the treatment of larger sizetumors. The major difference in the two test set-ups was that at CPO the pixel ionisationchamber was placed at the isocenter, inside the treatment room; this allowed an easieraccess with respect to when the chamber would have been placed along the beam line.Moreover, the pixel chamber tested at CPO was slightly different from the final one,described above; it had 7.5 mm side pixels for a total active area of 24× 24 cm2.4. – Experimental resultsIn both tests the goal was to verify that the detectors were able to measure the beamshape and fluence with minimum noise and good stability.At CATANA data were taken in 15 runs of 30 s, reading out the strip ionisationchamber at a rate of 5 Hz and the CATANA ionisation monitor chambers. The typicalbeam current was 2.5 nA. The detectors results for the integrated beam fluence agreewithin 0.4%. In fig. 4 the horizontal and vertical projections of the beam profile areshown; indeed the large number of strips permits to reach a high spatial resolutionallowing the measurement of very small modifications in the beam shape. A quantitativemeasurement of this sensitivity will be performed in the following months. So far theskewness, defined as γ = µ3/σ3 with µ3 =∑i(xi−µ)3ci∑iciand σ =√∑i(xi−µ)2ci∑ici, has beenmeasured for extreme values of the setting of tuning magnets placed along the beam line.The relative variation ∆γ/γ is around 0.5%.In the CPO tests the attention was driven by noise, linearity versus dose and stabilityof response. The count rate of the chamber with no beam (pedestals) was measured in5 different runs of 30 seconds each. As showed in fig. 5, for each reading of the chamberthe single pixel does not read more than 2 counts, while the counting frequency (meancount per second) has a mean of about 4-5 count/s, a max value of 16 count/s and astandard deviation equal to 2.4 count/s. The standard deviation calculated on the 5 runsis shown in fig. 6; it features a mean value of about 0.1 counts/s, which demonstrates thegood stability of the pixel chamber in the pedestal measure. The linearity of response asa function of the total dose has been measured irradiating the chamber with 4 different536 C. BENATI, A. BORIANO, F. BOURHALEB, R. CIRIO G. A. P. CIRRONE, ETC.Fig. 7. – Shape of the CPO beam used for the reproducibility tests as reconstructed by the pixelionisation chamber.dose values. The shape of the beam, kept constant for the 4 irradiations, was a circle15 cm in diameter. The total dose was measured in two ways: summing the countsof the 4 × 4 central pixels and summing the counts of the whole chamber. In bothcases the pedestals have been subtracted. Both methods resulted equivalent inside theexperimental uncertainty of 0.2%. The results of the linear fit are shown in table I, wherethe difference between fit and experimental value is below 0.35%. The very high error(2.45%) obtained for the 1 Monitor Unit measurement has been caused by the uncertaintyof the accelerator system in delivering such small dose. The stability of response has beenmeasured irradiating the chamber with 100 MU of a circular shaped beam (a typical runis shown in fig. 7) and comparing the relative response of each pixel along 10 runs. Thestandard deviation of the relative gain variation was below 0.5%.5. – ConclusionsTwo detector systems have been built with the goal of measuring in real time thefluence and the shape of proton beams used for radiotherapy. They have been testedin operating conditions on the beam lines of CATANA (at LNS-INFN) and CPO. Theresults show that noise, stability of response, linearity versus dose and sensitivity to beamshape variation are adequate. At present both detectors are being installed in permanentpositions along the beam lines to be routinely used during patient treatment.REFERENCES[1] Bonin R. et al., Nucl. Instrum. Methods Phys. Res. A, 519 (2004) 674.[2] Gottschalk B., Nucl. Instrum. Methods Phys. Res. A, 207 (1983) 417.[3] Bonazzola G. C. et al., Nucl. Instrum. Methods Phys. Res. A, 405 (1998) 111.

Online measurement of fluence and position for protontherapy beams

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Online measurement of fluence and position for protontherapy beams

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