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Diagnostics for Medical Accelerators

Anna Parravicini – Fondazione CNAO (Pavia – Italy). Diagnostics for Medical Accelerators. Medical Accelerators versus Experimental Accelerators /1. Medical machines are similar to LINAC, Cyclotron , Synchrotron used for low energy physics experiments , but…

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Diagnostics for Medical Accelerators

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  1. Anna Parravicini – Fondazione CNAO (Pavia – Italy) DiagnosticsforMedicalAccelerators

  2. MedicalAcceleratorsversusExperimentalAccelerators /1 Medicalmachines are similarto LINAC, Cyclotron, Synchrotron usedfor low energyphysicsexperiments, but… • PatientSafety-> strictcontroltoavoid over-dose topatient • Treatment Quality-> dose distributionovercancer volume hastorespect treatment planning requirementsrigidly

  3. MedicalAcceleratorsversusExperimentalAccelerators /2 • Dose&Beam Delivery -> the medicalmachinerequires a “brain” (calledNozzle) receiving treatment planning detailsfrom the Medical World and controllingMachine Side, depending on the delivered dose • Diagnostics -> beamdiagnosticsformedicalmachineisbasically the sameusedforexperimentalmachine, butitsgoals and control can besignificantlydifferent

  4. MedicalAcceleratorsOverview /1 • GeneralRequirements: compact size, costkept down, easy touse and tomaintain, reliable, efficient • @ MedicalCentres: • LINAC: compact size/ easy touse/ dedicatedtoconventionalradiotherapy/ notsuitableto accelerate hadrons up to hadrontherapy energies (prototypesof LINAC for hadrontherapy exist, butstillnot commercial)

  5. MedicalAcceleratorsOverview /2 • Cyclotron: compact size/ easy touse/ continuousbeam/ high intensity, smallintensityripples/ fixedenergy (passive modulatorsmustbechangedtomodifyenergy), notnegligibleenergy spread/ maximumenergynotenoughforC-iontreatments • Synchrotron: largesize/ elaborate operationbutflexiblemachine/ wide intensityrangesuitablefortreatments (1Gy/min in 1 litre), despiteriskofsignificantripples/ wide energyrangewithsmallenergy spread/ suitableforC-iontreatments

  6. Synchrotron forHadrontherapy /1 • Hadronsreleaseenergy in the mattermakingBragg’s peak, differentlyfrom the otherparticles • Hadrontherapy key-pointis the abilityofkillingdiseasedcellsselectively, minimizingdamages in the allaroundregion • In this scenario, beam position accuracy at patientlevelbecomesmandatorynottosave ‘bad’ cells -> DoseUniformity [±2.5%] Bragg’s peak

  7. Synchrotron forHadrontherapy /2 • Wide Energy Range (@ CNAO: source 8keV/u, after LINAC 7MeV/u, extraction 60-250MeV forprotons, 120-400MeV/u for C6+ ions, energyresolution 1mm stepequivalent) • Wide IntensityRange (@ CNAO, onenominalspillcontains 1E+10 protons or 4E+08 C6+ ionsreducibleof a factor 100, without emittance increasing) • Cycle-basedMachine: beamparameterschangequickly (1-2 sec), depending on treatment requirements ->Different detector principlesmustbeused

  8. 2 DC sources (protons and C6+ ions) • synchrotron circumference 76.84m • 3 horiz + 1 vert treatment lines • 1 experimentalline (for future) • protonsextracted at 60-250MeV • C-ionsextracted at 120-400MeV/u • bar-of-chargefornarrowbeam in the horizontalplane • active scanning • first patient in fewmonths CNAO Layout

  9. MonitorsRequirements /1(peculiarforMedicalMachine) • Fast Setting Up and Debugging[patientsshouldnotwait, for comfort & cost] -> don’t forgetBeamInstrumentation at design time • Monitorsshouldnotperturbbeam-> non (or low) interceptivemonitorsonly can beusedduringtreatments [ifweadd/ removetargets on beampath, wechangeBragg’s peakdepth] • Dose Delivery System -> systemdedicatedtomeasure the dose deliveredto the patient

  10. MonitorsRequirements /2(peculiarforMedicalMachine) In particular, toguarantee treatment quality (@ CNAO): *** A long listhere: details in the nextpages *** • MinimizeIntensityRipples: [magnets] debuncher, betatron – [operation] slow extraction – [diagnostic] Chopper Faraday Cup, Current Transformer, BeamQualificationMonitors (intensity), Servo-Spill • QualifyBeamSize and Intensity: BeamQualificationMonitorsintensity&position (beforesendingbeamtopatient), NOZZLE (controlof scanning magnets)

  11. DiagnosticsControl /1 • CNAO synchrotron workswith a cycleabout 2sec long • Itmeansthatevery 2sec a messageis sent by the Timing System toall the subsystems (magnets, diagnostics, …) providingbeamparameters (particlestype, energy, intensity,...) neededfor the nextmachinecycle

  12. DiagnosticsControl /2 • Nextcycle can start onlyifall the subsystemshave sent back their right acknowledge • Monitorssetting (amplifiergain, slits position, expectedparameterstoqualify the beam…) changesautomaticallydepending on the newbeamparameters: in few ms machineconfiguration can changedrastically (using the Timing message) • The listofcyclestobeexecutedisfixedby the treatment planning *ofcourse*

  13. Monitors in details • Let’s go intodetailsfor some monitorsusedtoimprovebeamquality, in position, dimension and intensityuniformity • Wereferto CNAO experience. Some monitors are available on the market, others are home-made and are aimingtosatisfypeculiarneedsof CNAO machine

  14. Chopper Faraday Cup /1 • Source beamis DC. Beforeentering LINAC, beamhastobebunched • Chopper is amagnetdeflectingparticlestowardsvacuumchamber,tomakebuncheda continuousbeam(@ CNAO injectionline: beamagainstvacuumchamberfor morethan 95% time)

  15. Chopper Faraday Cup /2 • The Chopper Faraday Cup (CFC) aimstodetect source currentstability and rippleswith 10kHz bandwidth. 1% resolutionfor the minimum beamcurrent (C-ions, 0.15mA) requires 1.5uA absoluteresolution • CFC High Voltageswitched ON/ OFF when Chopper OFF/ ON, nottoperturbbeamentering LINAC ->Beamuniformityis a key-pointformedicalmachine, in particularfor hadrontherapy

  16. Degrader • The machineispreparedfor ‘nominal’ intensities, that can bereducedaccordingto treatment planning requirements • At CNAO weuse a Degradermadeof 3 pepper-pot metal plates, reducingintensities down to 10% ofnominal value, without affecting beam emittance

  17. CurrentTransformers • Charge flow measurement: beamcurrentchangesduringacceleration / beamcharge *hopefully* remainsconstant • Non-interceptiveintensitymeasurement, forcontinuous (DC) and pulsed (AC) beams

  18. AC-Beam Transformer • Forbunchedbeam (up tofew ms long batches) • Beamactsas a primary winding • Beamcurrentisreadfrom the secondar winding • tDroop [sec]= WindingInductance [H] / Read-OutResistance [Ω] After 1tDroop, signalisexponentiallyreducedof 1/e

  19. DC-Beam Transformer /1 • Forcontinuousbeams (eitherDC-componentofbunchedbeams) • Useoftwo ‘identical’ cores • Beamcurrentmeasuredthrough the magneticfieldinducedby the beam in the core • Blindto fast currentvariations

  20. DC-Beam Transformer /2

  21. Scintillatingplate 90 fibersY (2-by-2 binned) Y Z BEAM X 34 fibersX BeamQualification /1 • Beamsize, position (QPM) and intensity (QIM) qualitycheckedbeforesendingbeamtopatient or during treatment suspensions: • Avoid treatment interruption • Improve treatment quality • QPM+QIMplaced in frontofextractionlinedump. During first 100ms ofeachspill, beamisdumped and qualified. Ifqualificationresultis positive, beamis sent to treatment room

  22. IN/OUTmotion BeamQualification /2 • Qualificationmeasurementscannot take more than 100ms: numberoftreatments per daymustbemaximized • Automaticbeam qualification: • Cycle-Code dependent • Algorithmtobe commissioned

  23. Servo-SpillLoop • Dose Delivery system monitors ‘online’ (1MHz – ifstatisticssignificant) the numberofparticles sent topatientBUT…ifintensityripples are “very high” itisnotenoughtoprotect the patientfrom over-dose • Machinedetectorsshalllimitcurrentintensityripplestoavoidvoxel over-dose [fromspecifications, voxel dose peak < 4% voxel dose] • Weforesee open/ closedloop (10kHz bandwidth) between a largebandwidthintensity monitor in the extractionline (or the Nozzle) and the air-corequadrupole in the synchrotron, for fast correctiontointensityripples • Intensitymeasurementperturbance on beamforpatientmustbeminimized

  24. Watch-Dog /1 • At extractionlines end, the Nozzlecontrols scanning magnetspowersupplies so todirect the beamto the ‘right’ target • Watch-Dog monitor aimstomeasurebarycenter position at scanning magnetsentrance, during treatment, so tocheckbarycenter offset and itsdrift, duringtreatments (at 100Hz-300Hz) • Extractionlinecorrectorssettingischangedconsequently, in such a way tomovebeambarycenterto the center, ifitisnot • Tosuppressbeam position offset and driftismandatorytoavoid ‘dose holes’ at patients (-> Dose Uniformity)

  25. Watch-Dog /2

  26. Nozzle • Nozzleis the usualnameforBeam Delivery (beamsteeringtoreach the right target position) and Dose Delivery (measuring the flow ofparticlesgoingto the patient) systems • Itactsas interface betweenaccelerator and medicalworlds • In case ofactivescanning (like CNAO) itcontrolsscanningmagnets

  27. CNAO Nozzle /1 • CNAO Nozzleismadeof BOX1 and BOX2, eachonemadeof 2 ionizationchambers (->directmeasurementof the chargedeliveredtopatient) • Nozzleisglobally < 1mm water-equivalentthick • Each BOX measuresintensity (@1MHz / 1 cnt=200fC ~8000 protonsat 60MeV) and position (@ 10kHz / 50um accuracy)

  28. CNAO Nozzle /2 • Toconclude… • …anexampleof scanning magnetscontrolby the Nozzle Single Spot AllSpots Strip Chambers (X white – Y red) Pixel Chamber

  29. - Thanksforyourattention -

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