A compact solution for ion beam therapy with laser

Authors: U Masood M Bussmann T E Cowan W Enghardt L Karsch F Kroll U Schramm J Pawelke

Publish Date: 2014/04/09

Volume: 117, Issue: 1, Pages: 41-52

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Abstract

The recent advancements in the field of laserdriven particle acceleration have made Laserdriven Ion Beam Therapy LIBT an attractive alternative to the conventional particle therapy facilities To bring this emerging technology to clinical application we introduce the broad energy assorted depth dose deposition model which makes efficient use of the large energy spread and high doseperpulse of Laser Accelerated Protons LAP and is capable of delivering homogeneous doses to tumors Furthermore as a key component of LIBT solution we present a compact isocentric gantry design with 360° rotation capability and an integrated shottoshot energy selection system for efficient transport of LAP with large energy spread to the patient We show that gantry size could be reduced by a factor of 2–3 compared to conventional gantry systems by utilizing pulsed aircore magnetsa Singlefielduniformdose scheme the depth dose profiles green are shown as a function of penetration depth in water displaying a pristine Bragg peak corresponding to a proton beam with energy spread of ∼1–3  A flattop SOBP red is achieved by nonlinear superposition of these energy and intensitymodulated Bragg peaks The SOBP has an acceptable ±3–5  dose uniformity within the tumor region bounded by the proximal near and distal far edges of tumor region depending upon the beam entrance More complex depth dose regimes than singlefielduniformdose are also commonly practiced with beams entering from two or more directions b shows two SOBPs dashed blue matched in the middle of the tumor region while c shows overlapped SOBPs dashed blue The first variant cover larger extent of tumor widths and can also be achieved by matching slanting SOBPs and the latter variant delivers a higher peak to entrance dose ratio In conIBT a combination of these schemes is used for patient treatment plans with higher order of complexity to optimize tumor conformity and normal tissue sparingIn IBT large conventional accelerators cyclotrons or synchrotrons are deployed to produce particle beams with high energies eg 70–250 MeV protons which are necessary to deliver doses at clinically relevant depths of up to 30 cm These beams are then transported via magnetic transfer lines to several treatment rooms and delivered to patients preferably via a 360° rotatable gantry system However at these high energies particle beams become highly rigid and beam transport by conventional ironcore magnets require big and heavy transfer lines and beam gantries Existing IBT isocentric gantries therefore are massive and large eg for protons above 100 tons ∼7–11 m diameter and ∼9–12 m in length and must be supported by enormous and massive architectural complexes and support structures which house and rotate the whole gantry systems around the patient table with high precision This all adds up to the complexity and cost of IBT facilities 7 with capital investments easily exceeding 100 million Euros This is the main reason for limiting IBT implementation to few large centers and hindering the wide spread of particle therapy around the worldIn order to reduce the size and cost of IBT systems several novel technologies are under investigation such as high field superconducting synchrocyclotron systems which even may be mounted onto a rotating gantry 8 combination of cyclotron and linear accelerators 9 nonscaling fixedfield alternative gradient accelerator concepts 10 11 dielectricwall accelerators 12 and laser particle acceleration mechanisms 13 14 15 16 17 However recent huge advancements in the field of laserdriven particle acceleration have made Laserdriven Ion Beam Therapy LIBT a very promising and attractive alternative to conventional IBT conIBT facilities 15 16 17 18 By replacing conventional accelerators with table top high power laser systems may considerably reduce the size and cost of IBT facilities Moreover laser pulses can be guided to target assemblies inside several treatment rooms by compact optical lines with mirrors making heavy magnetic transfer lines obsolete Nonetheless apart from actual accelerator and transferlines the size of the gantry is still a limiting factor and the size and cost reduction of IBT facilities through laserdriven accelerators can only be capitalized on if the size and weight of the associated gantry systems can be reducedThe properties of laserdriven beams eg ultraintense particle bunches with large energy spread and divergence are different from conventional beams Therefore new methods and techniques for beam transport irradiation field formation and treatment planning 19 20 21 along with beammonitoring dosimetry and dosecontrolled irradiation 22 23 24 25 26 are required Moreover determination of radiobiological effects induced by ultrashort intense particle bunches 26 27 28 29 30 31 32 is necessary In addition to laser particle accelerator development a parallel oncologyfocused research and development is essential to bring this highly promising technology to the clinicsIn this paper we present a depth dose deposition model optimized for LIBT and an energyselective compact 360° isocentric gantry design with efficient capturing of divergent bunches and integrated energy filtering system The pulsed nature of the laser accelerated ion beam generation has allowed us to utilize aircore high field pulsed magnet systems over ironcore magnets for our gantry design Pulsed magnets can achieve higher magnetic field strengths at a comparatively smaller size but a beamline system consisting of pulsed magnets has never been deployed before Our proposed design for laserdriven beams results in a substantial reduction in size by a factor of 2–3 and hence weight compared to the most compact conIBT gantry systems for coasting beamsIn laserdriven ion acceleration a highly focused ultraintense laser pulse with peak light intensity of 1019 W cm−2 or higher interacts with thin ∼μm solid density targets The most commonly used and best understood mechanism to accelerate ion beams by lasers is Target Normal Sheath Acceleration TNSA 13 33 In this robust acceleration mechanism first the light field generates a plasma plume in the laser focal region The electrons in the plasma are accelerated by the laser field pass through the target and exit it at the rear side These electrons form a negatively charged sheath that extends up to the Debye length μm to nm scale depending on laser and target parameters which generates a quasistatic acceleration field for positively charged ions on the target rear side which is of the order of TV/m The accelerated ion bunches are pulsed and have an exponential energy spectrum and large energydependent divergence angles with an upper limit for repetition frequency coming from the high power laser systems which extends from 10 Hz for ultrashort pulse durations 50 fs to few pulses per minute for long pulse ∼700 fs laser systems Although several ion species can be accelerated through lasermatter interactions we focus our work on Laser Accelerated Protons LAP since protons are much more often used than heavier ions in conIBT 34 The maximum proton energies currently published could reach up to ∼70 MeV 35 by a long pulsed laser system of few 100 Terawatt power and are not yet sufficient for most radiation therapy purposes However scaling models show higher energies are reachable with increased laser power 36 37 38 and/or new target geometries 39 Also several laser particle acceleration mechanisms are under investigation which could be more efficient yet experimentally much more demanding than TNSA such as laserpiston regime 40 radiation pressure acceleration 41 42 and breakout afterburner regime 43 44 which could provide higherenergy ion beams with potentially better beam quality ie lowerenergy spread with better collimation Nevertheless with the development of next generation Petawatt 1000 Terawatt laser systems protons with much higher energies are expected to be reached in the near future However in this paper we have used a scaled TNSA spectrum for input parameters as worst case scenario to design the beamlineLaserdriven ion beam therapy will be different from conIBT in several ways 45 For instance due to the pulsed nature of high power laser systems with low repetition rates of a few laser pulses per second the accelerated proton bunches are also pulsed with bunch durations of nsec range and with up to ∼1012 protons per bunch depending upon laser parameters 28 46 Such intense bunches can attain pulsed peak dose rate values up to 1010 Gy/s which exceeds conIBT mean values of 15–30 Gy/s through quasicontinuous beam by many orders of magnitude Therefore LIBT poses a whole new set of challenges on both physical and biological levels Laserdriven irradiation technology with all the necessary main components such as high power laser system and laser target to produce the particle beam and also beam transport and monitoring as well as dose delivery technique has already been developed to perform invitro cell 26 27 28 29 30 31 32 and small animal 24 26 irradiation with low energy LAP within radiobiological experiments These recent promising results encourage a goahead with further LIBT solutions

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