Particle Therapy (PT) is a non-invasive technique that exploits charged light ions for the irradiation of tumours that cannot be eﬀectively treated with surgery or conventional radiotherapy. While the largest dose fraction is released to the tumour volume by the primary beam, a non-negligible amount of additional dose is due to the beam fragmentation that occurs along the path towards the target volume. In particular, the produced neutrons are particularly dangerous as they can release their energy far away from the treated area, increasing the risk of developing a radiogenic secondary malignant neoplasm after undergoing a treatment. A precise measurement of the neutron ﬂux, energy spectrum and angular distributions is eagerly needed in order to improve the treatment planning system software, so as to predict the normal tissue toxicity in the target region and the risk of late complications in the whole body. The MONDO (MOnitor for Neutron Dose in hadrOntherapy) project is dedicated to the characterisation of the secondary ultrafast neutrons ([20–400]MeV energy range) produced in PT. The neutron tracking system exploits the reconstruction of the recoil protons produced in two consecutive (n,p) elastic scattering interactions to measure simultaneously the neutron incoming direction and energy. The tracker active media is a matrix of thin squared scintillating ﬁbers arranged in orthogonally oriented layers that are read out by a sensor (SBAM) based on SPAD (Single-Photon Avalanche Diode) detectors developed in collaboration with the Fondazione Bruno Kessler (FBK)

Battistoni, G.

De Simoni, M.

Dong, Y.

Fischetti, M.

Gioscio, E.

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DOI 10.1393/ncc/i2018-18206-5Colloquia: SIRR 2018IL NUOVO CIMENTO 41 C (2018) 206Characterisation of the secondary-neutron production in particletherapy treatments with the MONDO tracking detectorM. Fischetti(1)(5), G. Battistoni(6), M. De Simoni(4)(5), Y. Dong(7)(6),E. Gioscio(2), I. Mattei(6), R. Mirabelli(4)(2)(5)(∗), V. Patera(1)(2)(5),A. Sarti(1)(2)(3), A. Schiavi(1)(5), A. Sciubba(1)(2)(5), G. Traini(4)(2)(5),S. M. Valle(7)(6) and M. Marafini(2)(5)(1) Università di Roma “La Sapienza”, Dipartimento di Scienze di Base e Applicate perl’Ingegneria Rome, Italy(2) Museo Storico della Fisica e Centro Studi e Ricerche “E. Fermi” - Rome, Italy(3) Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati - Frascati, Italy(4) Università di Roma “La Sapienza”, Dipartimento di Fisica - Rome, Italy(5) Istituto Nazionale di Fisica Nucleare, Sezione di Roma - Rome, Italy(6) Istituto Nazionale di Fisica Nucleare, Sezione di Milano - Milan, Italy(7) Università degli Studi di Milano, Dipartimento di Fisica - Milan, Italyreceived 4 December 2018Summary. — Particle Therapy (PT) is a non-invasive technique that exploitscharged light ions for the irradiation of tumours that cannot be effectively treatedwith surgery or conventional radiotherapy. While the largest dose fraction is releasedto the tumour volume by the primary beam, a non-negligible amount of additionaldose is due to the beam fragmentation that occurs along the path towards the targetvolume. In particular, the produced neutrons are particularly dangerous as they canrelease their energy far away from the treated area, increasing the risk of developinga radiogenic secondary malignant neoplasm after undergoing a treatment. A pre-cise measurement of the neutron flux, energy spectrum and angular distributionsis eagerly needed in order to improve the treatment planning system software, soas to predict the normal tissue toxicity in the target region and the risk of latecomplications in the whole body. The MONDO (MOnitor for Neutron Dose inhadrOntherapy) project is dedicated to the characterisation of the secondary ultra-fast neutrons ([20–400] MeV energy range) produced in PT. The neutron trackingsystem exploits the reconstruction of the recoil protons produced in two consecutive(n, p) elastic scattering interactions to measure simultaneously the neutron incomingdirection and energy. The tracker active media is a matrix of thin squared scintil-lating fibers arranged in orthogonally oriented layers that are read out by a sensor(SBAM) based on SPAD (Single-Photon Avalanche Diode) detectors developed incollaboration with the Fondazione Bruno Kessler (FBK).(∗) Corresponding author. E-mail: riccardo.mirabelli@roma1.infn.itCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 M. FISCHETTI et al.1. – IntroductionParticle Therapy (PT) is a non-invasive technique mainly devoted to the treatmentof tumours that are radioresistant or localised closely to organs at risk. Even if a largefraction of the dose is released by the charged particles directly in the tumour volume, anon-negligible dose amount, carried mainly by neutrons, can reach other body regions faraway from the target volume increasing the risk of developing a radiogenic second malig-nant neoplasm, after undergoing a treatment. A precise measurement of the secondaryneutrons flux, production energy and angular distributions is not yet available [1], while itwould greatly help understanding how to reduce such unwanted additional dose release.Measurements are eagerly needed to improve the Treatment Planning System (TPS)software, so as to predict the normal tissue toxicity in the target region and the risk oflate complications in the whole body. The request becomes particularly relevant in thecase of paediatric treatments where life expectancy is a fundamental parameter [1]. TheMONDO (MOnitor for Neutron Dose in hadrOntherapy) project addresses the require-ments of a high efficiency and good backtracking precision for a complete characterisationof secondary ultra-fast neutrons in the energy range [20–400] MeV. In this contributionthe detector is described in sect. 2 together with the description of the simulation workperformed for the layout optimisation while sect. 3 presents the experimental results.2. – The MONDO projectThe MONDO project principal aim is the development of a compact, high-resolutiontracking device optimised for the detection of secondary ultra-fast neutrons producedin PT treatments. The technique used for the secondary neutrons tracking aims at thesimultaneous reconstruction of the neutron energy and direction exploiting the detectionof two consecutive Elastic Scattering (DES) interactions in the tracker active material(fig. 1).In a (n, p) elastic interaction, the proton and the neutron momenta pn and pp arecorrelated by the relation(1) pn =ppcos2 θp,where θp is the angle between the recoil proton and the primary neutron tracks. Ifthe direction and the energy of the incoming neutron are unknown, DES events can beused to get simultaneous access to both physical quantities reconstructing the two recoilprotons [2].Fig. 1. – Scheme of the DES reaction [3].CHARACTERISATION OF THE SECONDARY-NEUTRON PRODUCTION ETC. 3The detector is composed by a matrix of thin squared scintillating fibers (250μm side)arranged in orthogonal layers and it has the total size of 10 × 10 × 20 cm3. The readoutof the fibers is performed through a dedicated sensor, SBAM (SPAD Based Array for theMONDO experiment) that implements several SPAD (Single-Photon Avalanche Diode)arrays. The sensor, implemented using the CMOS technology, has been developed incollaboration with the Fondazione Bruno Kessler (FBK) [4]. SBAM is composed by a tileof several small sensors (each 2×2mm2 area) equipped with 16×8 pixels (125×250μm2is the area of each pixel). A two-level trigger logic is implemented: the readout of theevent is performed only when the number of fired pixels and the number of fired sensorsbecomes greater than a user programmable threshold.2.1. Monte Carlo simulation. – A FLUKA Monte Carlo (MC) simulation [5] hasbeen used to optimise the detector layout and evaluate its expected performance. Thesimulation was used to evaluate the proton energy spectrum expected in clinical-likescenarios, the predicted neutron energy resolution and the background contaminationfrom anelastic neutron collisions. Figure 2 shows the energy spectrum of the protonsgenerated by a monochromatic neutron beam of 100 MeV impinging on the detector,both for Single Elastic Scattering (SES, black) and for DES (green and orange) events.When studying DES events, both protons are requested to be fully contained in thedetector.As expected, for single elastic scattering interactions, the recoil proton energies aredistributed uniformly from ∼12 MeV (at least 3 x-y planes of fibres traversed) up to theprimary energy of the neutrons (the maximum energy available). For DES interactions,where the presence of a second elastic scattering interaction is required, the maximumenergy of the first recoil proton is forced to be lower than the maximum available energy(80 MeV) [3].The neutrons kinetic energy was studied using DES events. The relative energy reso-lution is shown in fig. 3 as a function of the neutrons kinetic energy. For monochromaticneutrons, the achievable accuracy improves as a function of the secondary-protons energyincrease, as expected in the case of straighter and longer tracks. The average preliminaryrelative energy resolution for neutrons has been evaluated to be in the 4–6% range.The occurrence of elastic scattering interactions events must be compared with theexpectations of the main intrinsic backgrounds represented by events in which inelas-Fig. 2. – Energy spectrum of protons emitted in a single elastic scattering interaction (black)compared with the energy spectrum of the first (green) and second (orange) protons emitted ina DES interaction for a neutron incoming beam of 100 MeV [3].4 M. FISCHETTI et al.Fig. 3. – Left: the relative energy resolution (in %) for DES is shown as a function of the neutronkinetic energy. Only statistical uncertainties are included. Right: example of a DES interactionevent display (in a projected X-Y view): the first and the second recoil protons are clearlytracked by the detector.tic scattering interactions occur (fig. 4). In this analysis monochromatic neutrons withenergies in the range [20–300] MeV have been studied considering only protons thatcross at least 3 layers of fibers in both directions. In fig. 4 the single elastic scatter-ing interactions (black full circles) are dominant below 100 MeV, while the inelastic scat-tering interactions (red empty squares) are not negligible for higher energies. As a result,DES interactions (green full circles) are dominant for low-energy neutrons.The probability of mixed interactions elastic-inelastic/inelastic-elastic or double in-elastic interactions (magenta and grey triangles) could have a considerable impact on thedetector performance below 120 MeV. The double inelastic scattering interactions (blueempty squares) are dominant in the upper energy range. These contributions could havea non-negligible impact on the detector performances and have to be taken into accountFig. 4. – Number of neutrons interacting in the detector as a function of the kinetic energy ofthe neutrons for: single elastic scattering (black full circles), double elastic scattering (green fullcircles), single inelastic scattering (red empty squares), double inelastic scattering (blue emptysquares), elastic scattering followed by inelastic scattering (magenta full triangles), inelasticscattering followed by elastic scattering (grey empty triangles). The error bars (not visible) areonly accounting for the statistical contribution [3].CHARACTERISATION OF THE SECONDARY-NEUTRON PRODUCTION ETC. 50.60.811.21.4Pixel X0 2 4 6 8 10 12 14 160123456780.50.60.70.80.911.11.2Pixel X0 2 4 6 8 10 12 14 16Pixel Y0123456780.30.40.50.60.70.80.9Pixel X0 2 4 6 8 10 12 14 16Pixel Y01234567880 MeVCharge [# SPAD_On]910111213Pixel0 2 4 6 8 10 12 14 168Beam Energy80 MeV90 MeV110 MeV90 MeV 110 MeVFig. 5. – Energy released in spad-net sensor from different energies protons. (a) Top left: Chargerelease in number of detected photon per pixel column. At Ep = 80 MeV there is an increasein the energy release thanks to the Bragg Peak presence; (b)–(d) Top right, bottom left andbottom right: Average energy sensor map. Each square box represent a pixel. The averagenumber of photoelectrons detected are shown with different brightnesswhen calculating the intrinsic background. The reduction of the background contamina-tion and the rejection of inelastic events is currently being studied: the track multiplicityat the interaction vertex will be one of the main ingredients of the algorithm, but otherstrategies are being pursued as well.3. – Experimental resultsIn May 2017 a first MONDO prototype has been tested at the TIFPA experimentalline of the Trento Proton Therapy Center. The prototype consists in a small tracker(PENELOPE, 4×4×4.8 cm3) read out by a SPAD-based sensor prototype, spad-net [6].The total area of the sensor is 1 × 0.5 cm2 and only a small area of the tracker could beread out: the sensor position was chosen to allow the observation of the less energeticprotons, that stopped inside PENELOPE. The aim of the test was the characterisationof the dE/dx release reconstruction capability of the sensor. Three proton beams of80, 90 and 110 MeV kinetic energies have been considered. In fig. 5 (top right, bottomleft and bottom right, respectively, for the three different energies) the energy sensormap, averaged over 75k events, is shown. Each bin in the x-y plane represents one pixel(side 600μm) while the content of each bin in the z-axis represents the average numberof detected photons in that pixel: the lighter band represents the track of the protonstraversing the detector. The total number of photo-electrons as a function of the pixelcolumn is shown in fig. 5 (top left). When looking at the 80 MeV distribution, the Bragg6 M. FISCHETTI et al.peak appears in the last two pixels along the X view. When going to higher energies, theBP slowly exits from the detector and the energy release does not show a peak anymore.The deposited energy (shown in terms of the total measured charge) in the pixels isgreater for protons of 80 MeV kinetic energy, when compared to the 110 MeV one, asthe Bragg peak is fully contained in the detector portion read by spad-net in the formercase.4. – ConclusionsMONDO is a project dedicated to the complete reconstruction of neutrons that havea kinetic energy in the range [20–400] MeV. The MONDO detector layout has been op-timised for the characterisation of secondary neutrons emitted during Particle Therapytreatments, exploiting the detection of the double elastic scattering events in the trackerscintillating fibers. A tracker prototype, with dimensions that are smaller than thoseof the final full size detector, has been tested with protons of different energies and areadout prototype. A new sensor, capable of reading out the full size final detector iscurrently under development in collaboration with FBK. The new design will be opti-mized considering the final tracker geometry and will implement a two-levels trigger logiccapable of reducing the combinatorial background events to a negligible level.∗ ∗ ∗The authors thank the APSS Proton Therapy Centre staff for beam-time and supportduring the data taking. The authors wish to thank Marco Magi for his help in therealisation of the Penelope prototype and mechanical structure. The project MONDOis supported by INFN Gruppo V with a Young Researchers Grant (2015–2016), by theCentro Fermi and by MIUR-SIR (Scientific Independence of young Researchers) (2015–2018).REFERENCES[1] Newhauser W. and Durante M., Nat. Rev. Cancer, 11 (2011) 438.[2] Marafini M. et al., Phys. Med. Biol., 62 (2017) 3299.[3] Giacometti V. et al., Radiat. Meas., 119 (2018) 144.[4] Mirabelli R. et al., Trans. Nucl. Sci., 65 (2018) 744.[5] Battistoni G. et al., Ann. Nucl. Energy, 82 (2015) 10.[6] Braga L. H. C. et al., IEEE J. Solid-State Circuits, 49 (2014) 301.

Characterisation of the secondary-neutron production in particle therapy treatments with the MONDO tracking detector

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Characterisation of the secondary-neutron production in particle therapy treatments with the MONDO tracking detector

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