Transcription of Physics of Electron Beam Radiation Therapy
1 1 Physics of Electron beam Radiation TherapyPhysics of Electron beam Radiation TherapyGeorge Starkschall, of Radiation Anderson Cancer Center George Starkschall, Starkschall, of Radiation PhysicsDepartment of Radiation Anderson Cancer Center Anderson Cancer Center Why use electrons?Why use electrons? Electron beam characteristics: Rapid rise to 100% Region of uniform dose Rapid dose fall-off Electron beam Electron beam characteristics:characteristics: Rapid rise to 100%Rapid rise to 100% Region of uniform Region of uniform dosedose Rapid dose fallRapid dose fall--offoffAAPM TG-21 Med Phys 10(6), 741-771 (1983)Why use electrons?Why use electrons? Tumors that can be treated with electrons Superficial tumors Lymph node boosts Chest walls CNS TSI Tumors that can Tumors that can be treated with be treated with electronselectrons Superficial tumorsSuperficial tumors Lymph node Lymph node boostsboosts Chest wallsChest walls CNSCNS TSITSIAAPM TG-21 Med Phys 10(6), 741-771 (1983)2 Review Electron interactionsReview Electron interactions Long-range interactions with orbital electrons Excitation and ionization Long-range interactions with atomic nuclei Bremsstrahlung (x-rays)
2 LongLong--range interactions with orbital range interactions with orbital electronselectrons Excitation and ionizationExcitation and ionization LongLong--range interactions with atomic range interactions with atomic nucleinuclei Bremsstrahlung (xBremsstrahlung (x--rays)rays)Review Electron interactionsReview Electron interactions Characteristics of energy deposition Continuous loss of energy approx 2 MeV/cm (in water) Multiple Coulomb scatter spreading out of dose distribution at depth Electron interactions result in reductions in beam energy Characteristics of energy depositionCharacteristics of energy deposition Continuous loss of energy Continuous loss of energy approx 2 MeV/cm approx 2 MeV/cm (in water)(in water) Multiple Coulomb scatter Multiple Coulomb scatter spreading out of spreading out of dose distribution at depthdose distribution at depth Electron interactions result in Electron interactions result in reductions reductions in beam energyin beam energyFrom.
3 KhanSpecification of Electron energySpecification of Electron energy (Ep)0(most probable energy at phantom surface) E0(mean energy at phantom surface) Ez(mean energy at depth z) (E(Epp))00(most probable (most probable energy at phantom energy at phantom surface)surface) EE00(mean energy at (mean energy at phantom surface)phantom surface) EEzz(mean energy at (mean energy at depth z)depth z)3 Specification of Electron energySpecification of Electron energy (Ep)0 most probable energy at phantom surface (E(Epp))00 most probable energy at phantom most probable energy at phantom surfacesurface() )(pppRRMeVE++= Rp practical range (expressed in cm) RRpp practical range (expressed in cm)practical range (expressed in cm)Specification of Electron energySpecification of Electron energy E0 mean energy at phantom surface EE00 mean energy at phantom surfacemean energy at phantom )(RMeVE= R50 depth at which dose is 50% of maximum dose (expressed in cm) RR5050 depth at which dose is 50% of depth at which dose is 50% of maximum dose (expressed in cm)maximum dose (expressed in cm)Specification of Electron energySpecification of Electron energy Ez mean energy at depth z EEzz mean energy at depth zmean energy at depth z =pzRzEE10 Note that energy decreases linearly with depth Note that energy decreases linearly with depthNote that energy decreases linearly with depth4 Electron energy measurementElectron energy measurement Average Energy (E0): Most Probable Energy (Ep0).
4 Energy (Ez) at depth z Average Energy (Average Energy (EE00):): Most Probable Energy (Most Probable Energy (EEp0p0):): Energy (Energy (EEzz) at depth ) at depth zz500332) R. ( =Enominal(MeV)(Ep)0(MeV)Eo(MeV) 6 9 + +=Varian Clinac 2100 CAAPM TG-25 Med Phys 18(1), 73-109 (1991))Rz- (E Epz10=Properties of depth dose distributionsProperties of depth dose distributions Characterized by uniform irradiation to specified depth, followed by sharp fall-off Characterized by Characterized by uniform uniform irradiation to irradiation to specified depth, specified depth, followed by followed by sharp fallsharp fall--offoffDepth Doses for Varian 21EX25 x 25 coneDepth (cm)024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeVClinical consequenceClinical consequence Deliver uniform dose to superficial tumor ( , nodes, chest wall)
5 , with sparing of distal tissue Deliver uniform Deliver uniform dose to dose to superficial superficial tumor ( , tumor ( , nodes, chest nodes, chest wall), with wall), with sparing of distal sparing of distal tissuetissueDepth Doses for Varian 21EX25 x 25 coneDepth (cm)024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeV5 Regions of Electron depth dose distributionsRegions of Electron depth dose distributions1. Shallow depths --build-up region caused by side-scattered electrons Surface dose increases with increase in depths Shallow depths ----buildbuild--up region up region caused by sidecaused by side--scattered scattered electronselectrons Surface dose Surface dose increases with increases with increase in increase in energyenergyDepth Doses for Varian 21EX25 x 25 coneDepth (cm)
6 024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeVIncrease surface dose with increased energyIncrease surface dose with increased energyIncrease surface dose with increased energyIncrease surface dose with increased energy Lower energies Electrons scattered through larger angles Dose builds up more rapidly over shorter distance Ratio of surface dose to maximum dose less Lower energiesLower energies Electrons scattered Electrons scattered through larger anglesthrough larger angles Dose builds up more Dose builds up more rapidly over shorter rapidly over shorter distancedistance Ratio of surface dose Ratio of surface dose to maximum dose lessto maximum dose less6 Increase surface dose with increased energyIncrease surface dose with increased energy Higher energies Electrons scattered through smaller angles Dose builds up more slowly over longer distance Ratio of surface dose to maximum dose greater Higher energiesHigher energies Electrons scattered Electrons scattered through smaller anglesthrough smaller angles Dose builds up more Dose builds up more slowly over longer slowly over longer distancedistance Ratio of surface dose to Ratio of surface dose to maximum dose greatermaximum dose greaterRegions of Electron depth dose distributionsRegions of Electron depth dose distributions2.
7 Region of sharp dose falloff begins at approximately 90% of sharp egion of sharp dose falloff dose falloff begins at begins at approximately approximately 90% dose90% doseDepth Doses for Varian 21EX25 x 25 coneDepth (cm)024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeVRegion of sharp dose falloffRegion of sharp dose falloff Select Electron energy so that 90% or 80% isodose encloses target volume Select Electron Select Electron energy so that energy so that 90% or 80% 90% or 80% isodose isodose encloses target encloses target volumevolumeDepth Doses for Varian 21EX25 x 25 coneDepth (cm)024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeV7 Region of sharp dose falloffRegion of sharp dose falloff Depth of 80% central-axis dose easy to estimate Depth of 80% Depth of 80% centralcentral--axis axis dose easy to dose easy to estimateestimateDepth Doses for Varian 21EX25 x 25 coneDepth (cm)024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeV()()MeVcm3180Ed.
8 Region of sharp dose falloffRegion of sharp dose falloff Depth of the 80% Dose: Equal to approximately 1/3 nominal energy: Depth of 90% is approximately 1/4 nominal energy Depth of the 80% Dose:Depth of the 80% Dose: Equal to approximately 1/3 nominal energy:Equal to approximately 1/3 nominal energy: Depth of 90% is approximately 1/4 nominal energyDepth of 90% is approximately 1/4 nominal energyEnominalEnom / 3 Actual 2100 CRegion of sharp dose falloffRegion of sharp dose falloff Slope of falloff becomes more gradual with increasing energy Slope of falloff Slope of falloff becomes more becomes more gradual with gradual with increasing increasing energyenergyDepth Doses for Varian 21EX25 x 25 coneDepth (cm)024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeV8 Regions of Electron depth dose distributionsRegions of Electron depth dose distributions3.
9 At end of falloff region, beyond range of electrons, tail is due to end of falloff At end of falloff region, beyond region, beyond range of range of electrons, tail is electrons, tail is due to due to bremsstrahlungbremsstrahlungDepth Doses for Varian 21EX25 x 25 coneDepth (cm)024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeVRegion of bremsstrahlung tailRegion of bremsstrahlung tail Magnitude of tail increases with increasing energy Typically around 5% of maximum dose Magnitude of tail Magnitude of tail increases with increases with increasing energyincreasing energy Typically around Typically around 5% of maximum 5% of maximum dosedoseDepth Doses for Varian 21EX25 x 25 coneDepth (cm)024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeVRegion of bremsstrahlung tailRegion of bremsstrahlung tail X-Ray Contamination: Increases with energy: Varies with accelerator design XX--Ray Contamination:Ray Contamination: Increases with energy:Increases with energy.
10 Varies with accelerator designVaries with accelerator designEnom X-ray % 6 9 12 16 20 Varian 2100C9 Region of bremsstrahlung tailRegion of bremsstrahlung tail Practical range of electrons can be estimated using approximate relation Practical range of Practical range of electrons can be electrons can be estimated using estimated using approximate approximate relationrelationDepth Doses for Varian 21EX25 x 25 coneDepth (cm)024681012% Depth Dose0204060801006 MeV9 MeV12 MeV16 MeV20 MeV()()MeVcm21 ERp.
