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19 chap 14 electron beam therapy

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  • 1. Chapter 14Electron Beam Therapy 1
  • 2. 1. In early days, betatrons were used to produce electron beams, in modern times, linacs are used to produce electron beams.2. Clinically useful energies are between 6 and 20-MeV.3. Used for treating superficial tumors (skin, chestwall, boost to nodes, head/neck).4. Relatively uniform dose in the target, fast dose drop off beyond the electron range. 2
  • 3. 14.1 Electron InteractionsElectrons interact with atoms by different processes through the Coulombforce. These processes are (1) inelastic collisions with atomic electrons(ionization/excitation); (2) inelastic collisions with nuclei (bremsstrahlung);(3) elastic collisions with atomic electrons; (4) elastic collisions with nuclei(no energy loss, large angle deflection).In inelastic collisions, some of the kinetic energy is lost in producingionization or converted to other forms of energy.In elastic collisions, kinetic energy is not lost but it may be redistributedamong the emerging particles.In low-Z media (water, tissue), electrons lose energy through ionization andexcitation.In high-Z media (tungsten, lead), bremsstrahlung is important.In ionization, if the ejected electron is energetic enough to cause furtherionization, it is called secondary electron or δ-ray. (note: by definition, theenergy of the δ-ray is < ½ of the incident electron energy)Electrons continuously lose its energy traveling through the medium. 3
  • 4. 14.1 Electron Interactions – A. Rate of Energy Loss( S ρ ) col ∝ ρ e (electron density)ρ ewater = 3.34 × 10 26 e / kg 2 MeV/cmρ e = 2.38 × 10 26 e / kg lead ( S ρ ) col ( S ρ ) col → minimum ~ 1 - MeV ( S ) rad ∝ EZ 2 bremsstrahlung production more efficient for high - Z material, ( S ρ ) rad high - energy electrons ( S ρ ) tot = ( S ρ ) col + ( S ρ ) rad 4
  • 5. 14.1 Electron Interactions – A. Rate of Energy Loss(polarization or density effect) ( S ρ ) gas > ( S ρ ) dense medium Because of polarization of the condensed medium. Atoms close to the incident electron track screen those remote from the track. The ratio of (S/ρ)water to (S/ρ)air varies with energy, therefore, the conversion from dose-to-air(in chamber) to dose-to-water(phantom) varies with depth (because electron energy decreases with depth by ~ 2-MeV/cm in water). 5
  • 6. 14.1 Electron Interactions – A. Rate of Energy Loss (absorbed dose) (E>∆) energy carried away(unrestricted) Stopping power (S/ρ) by δ-rayrefers to the energy lost by acharged particle to the medium.Restricted stopping power(L/ρ)col,∆ (linear energy transfer Local energy deposition dueLET) refers to energy absorbed by to ionization and excitationthe medium. (collisions in whichenergy loss < ∆) E0 L D= ∫∆ Φ ( E )  ρ   col , ∆ dE L S   ρ <  ρ   col,∆   6
  • 7. 14.1 Electron Interactions – B. Electron ScatteringWhen an electron pencil beam passes through amedium, it suffers multiple scattering, resulting in lspread in both lateral position and direction. Thespread in approximately Gaussian. θ θ2 the mass angular scattering power : ρl where θ 2 is the mean square scattering angle. θ 2 ∝ Z 2 / E 2 High-Z materials are used for electron scattering foil to spread out the electron beam. (recall that photon beam is spread out by the production of bremsstrahlung itself.) 7
  • 8. 14.2 Energy Specification and Measurement phantom Scattering Accelerator z foil tube Electron beam At patient surface, energy degraded and spread due to collision At exit window, nearly At depth z, further energy with scattering foil, air monoenergetic degradation and spreadElectron fluence E(0) Emax(0) Ea Ep(z) Ep(0) Electron energy 8
  • 9. 14.2 Energy Specification and MeasurementRp : practical range 100 Percent depth doseMost probable energy Ep: Ep(0)=C1+C2Rp+C3Rp2 50 for water C1 = 0.22 MeV C2 = 1.98 MeV/cm C3 = 0.0025 MeV/cm2 depth R50 RpMean energy at surface E(0):   Energy at depth: E p ( z ) = E p (0)1 − z  E(0)=C4×R50  Rp    for water, C4 ~ 2.33 MeV/cm  z  E ( z ) = E (0)1 −   Rp    9
  • 10. 14.3 Determination of Absorbed Dose Absolute dose can be measured with: ionization chamber calorimetry Fricke dosimetry Relative dose can be measured with: film: energy independence for electron beam TLD diode: often used for electron beam measurement. 10
  • 11. 14.3 Determination of Absorbed Dose – output calibration For photon beams, the output varies smoothly with field size. For electron beams, the output does NOT vary smoothly with field size. This is because each applicator has its own collimator setting. For example, the output of a 10x10 applicator with a 10x10 insert may be different from that of a 15x15 applicator with a 10x10 insert. Thus, for electron beams, it is important to measure the output of every applicator and every insert in clinical use. Do not assume that the output very smoothly with field size, especially when different applicators are involved. For elongated or irregularly-shaped cutouts, the output should be individually measured. 11
  • 12. 14.3 Determination of Absorbed Dose – depth dose distributionIf ion chamber is used tomeasure electron beam depthdoses, the conversion fromdepth-ionization to depth-doseinvolves the water-to-airstopping power ratio, which isdepth-dependent. In addition,if the chamber is cylindrical,the measured depth-dosecurve needs to be shifted toaccount for the effective pointof measurement.If diode is used, the dioderesponse is taken as the Med Phys 14, 1060 (1987)depth-dose, no correction isneeded. 12
  • 13. 14.3 Determination of Absorbed Dose – film dosimetryEnergy independence for electron relative dose measurement. The opticaldensity can be taken as proportional to dose without correction. Med Phys 14, 1060 (1987) Med Phys 16, 911 (1989) 13
  • 14. 14.3 Determination of Absorbed Dose – film dosimetry Med Phys 16, 911 (1989) 14
  • 15. 14.3 Determination of Absorbed Dose – film dosimetry Things to avoid with film dosimetry Air gaps adjacent to film Film sticking out Film recess inside the phantom the phantom 15
  • 16. 14.3 Determination of Absorbed Dose – phantomWater is the standard phantom.Water-equivalent Plastic phantom (polystyrene, electron solid-water): sameelectron density (# electrons/cc), same effective-Z → same linear stoppingpower S, same linear angular stopping power.Depth-dose measured in plastic phantom converted to depth-dose inwater: Dw (d w ) = Dmed (d med )( S ρ ) med Φ med water water  R50 water  d w = d med × ρ eff = d med  med   R   50  Polystyrene Polystyrene Electron water Acrylic (clear) (white) solid water ρ 1.000 1.045 1.055 1.18 1.04ρeff 1.000 0.975 0.99 1.15 1.00 16
  • 17. 14.4 Characteristics of Clinical Electron Beams Central axis depth dose curves Relatively dmax increases with energy for low- uniform dose energy electrons, ~ 2.5cm for high-energy electrons (12-20 18 MeV)Modest skin sparing R90(cm) ~ E(MeV)/3.2 increases↑energy ↑skin dose R80(cm) ~ E(MeV)/2.8 6 with energy Rapid dose drop-off Bremsstrahlung for low energy x-ray electron beams, but contamination disappears for high- energy electron beamsThe choice of beam energy is much more critical for electrons than for 17photons.
  • 18. 14.4 Characteristics of Clinical Electron Beams Central axis depth dose curves – buildup region Lower energy electrons scatter Higher energy electrons scatter more and through larger angles, less and through smaller angles, causing more rapid buildup, thus, causing less rapid buildup (in the the difference between the extreme case, if there is no surface dose and maximum dose scatter, there will be no buildup). is larger. 18
  • 19. 14.4 Characteristics of Clinical Electron Beams Isodose curves Different machines → different collimation systems (scattering foil, monitor chamber, jaws, cones, air-gap to surface) → dose distribution For low energy electron beams, isodose curves bulging out for all dose levels 19
  • 20. 14.4 Characteristics of Clinical Electron Beams Isodose curves For high energy electron beams, isodose curves constrict for high dose levels But bulge out for low dose levels 20
  • 21. 14.4 Characteristics of Clinical Electron Beams Field flatness and symmetry uniform = A 90% index A geometric edge or uniform = A 90% ( > 0.7 ) index A 50% -p +p ● + ● on a reference plane at a reference depth (e.g. d max ) Dmax < 103% Dcentral axis D(+ p ) − D(− p) symmetry = < 2% [ D (+ p ) + D(− p )] 2 21
  • 22. 14.4 Characteristics of Clinical Electron Beams Beam collimation Variation of output with collimator jaws opening Dual scattering foil systemCollimator jaws open toa fixed predeterminedsize for a given electronenergy and applicatorsize (do NOT change it !) applicator 22
  • 23. 14.4 Characteristics of Clinical Electron Beams Field size dependence Output increases smoothly with field size (as defined by the insert with collimator jaws size fixed) due to increased phantom scatter and in-air scatter. 23
  • 24. 14.4 Characteristics of Clinical Electron Beams Field size dependence large field size, PDD nearly constant The PDD increases with field size until it exceeds the lateral range of the electrons, then the PDD is almost constant with field size. The depth of maximum Small field dose, dmax, also size, PDD increases field size until increases the lateral range is significantly reached. with field sizeThe output and PDD for small field electron beams need to be individuallymeasured. 24
  • 25. 14.4 Characteristics of Clinical Electron Beams Field equivalence and square root method For large fields (>10x10), the PDDs are nearly the same, thus, they are all equivalent. For small circular fields, the equivalent field radius, Requiv, to a 2ax2a square field is: Requiv ~ 1.116a. For small rectangular fields XxY, as a result of Gaussian pencil beam distribution, the PDD is related to that of square fields by: D X ,Y = D X , X • D Y ,Y 25
  • 26. 14.4 Characteristics of Clinical Electron Beams Electron source (virtual source) 2 VirtualI0  f + dm + g  I0 g source =  f +d   or = +1Ig  m  Ig f + dm 1 I0 I g −1f = − dm slope = slope g f < 100 cm g dmax I 26 Q
  • 27. 14.4 Characteristics of Clinical Electron Beams X-ray contaminationDose due to x-ray contaminationgenerated in the collimatingsystem and in phantom 6-12 MeV 0.5-1% 12-15 MeV 1-2% 15-20 MeV 2-5%Dose due to x-ray contamination generally is not a concern, except for totalskin electron therapy (TSET) in which the entire body is irradiated (sixdirections, thus, the x-ray contamination dose is increased 6-times). 27
  • 28. 14.5 Treatment Planning Choice of energy and field size Choice of beam energy is dictated by the depth of the prescribed level, typically, 80-90%, (thus ~ E(MeV)/3 in cm). Similarly, the choice of field size depends on the constriction of the isodose curve of the dose level. (the margin between 90% and geometric field edge typically > 0.5 cm) 28
  • 29. 14.5 Treatment Planning Correction for air gaps and beam obliquity dmax shifts toward the surface with increasing incident angle Decreased penetration Dose affected by air- with gap (inverse square) increasing and obliquity incident angle 29
  • 30. Effect of oblique incidence dIncreased dose at shallow depth dueto greater side scatter fromneighboring pencil beams traversingthrough a larger depth (d’ > d) d’Decreased dose at larger depth dueto lack of side scatter since it isbeyond the range of neighboringpencil beams 30
  • 31. f θ d g D0(f,d) d D(f+g,d )f = effective SSD(surface-to-virtualsource distance) Obliquity factor 2  f +d D ( f + g , d ) = D0 ( f , d ) •   f + g + d  • OF (θ , d )    31
  • 32. Obliquity factor 32
  • 33. 14.5 Treatment Planning Irregular surface Low dose due to loss of scatter High dose due to extra scatter High dose due to extra scatter Low dose due to loss of scatter 33
  • 34. 14.5 Treatment Planning Tissue inhomogeneity An approximation: fd eff = (d − z ) + z • ρ e A = d − z • (1 − ρ e ) ρe zρ e = CET d(coefficient of equivalent thickness) D 2  f + d eff D = PDD( f , d eff , A) •   f +d     34
  • 35. Lunginhomogeneity Without lung inhomogeneity correction Homogeneous water phantom With lung inhomogeneity correction 35
  • 36. Small inhomogeneityMaterial M’ has greater scatteringpower than material M M’ M Cold spot hot spot 36
  • 37. 14.5 Treatment Planning Use of bolus and absorbersBolus is used to (a) flatten out an irregular surface, (b) reduce penetration inparts of the field, and (c) increase surface dose 37
  • 38. 14.5 Treatment PlanningProblems of adjacent fields Big gap, cold spot Small gap, hot spot 38
  • 39. 14.5 Treatment Planning Problems of adjacent fields (due to setup 9-MeV e- 6-MV x-ray clearance) 9-MeV e- 6-MV x-raySSD = 100 SSD = 100 SSD = 120 SSD = 100 Increased SSD leads to wider e- beam penumbra, resulting in larger areas of hot/cold spots 39
  • 40. 14.6 Field Shaping External shielding Electron beam can be shaped by cutouts made of cerrobend or lead, placed at the applicator (cone) or directly on the skin. Shielding too thin (e.g. eye shield), causing dose buildup at depth immediately under shield Shielding thickness to achieve transmission < 5% 40
  • 41. 14.6 Field Shaping Measurement of transmission curves Shielding thickness vs electron energy Lead thickness (in mm) required to stop primary electrons (transmitted dose due to bremsstrahlung photons generated in the shield) ~ MeV/2. For cerrobend, increase the thickness by 20%. 41
  • 42. 14.6 Field Shaping Effect of blocking on dose rate Blocked field < electron lateral range (~ Rp/2) Output ratio = 1 / output-factorWhen in doubt, individual dosimetry (output, depth-dose, isodose 42distribution) should be made for irregularly-shaped cutouts.
  • 43. 14.6 Field Shaping Internal shielding (e.g. protection of eye in the treatment of eyelid) Lower energy, more scatter ~30-60% enhancement D D’ Energy (MeV)Dose enhancement at the interface dueto extra backscatter from the lead shield= D’/D 43
  • 44. 14.6 Field Shaping Internal shielding Range of backscattered electrons 1-2 cm in water No lead shield 44
  • 45. 14.6 Field Shaping Internal shielding Incident primary electrons A thin layer of low-Z material (e.g.wax) can be placed in front of the lead shield to reduce backscatter Depth in polystyrene upstream from the interface 45
  • 46. 14.6 Field Shaping Internal shielding - example aluminum 9-MeV cheek Oral electrons structure 2 cm Pb shield(a) Energy immediately beyond cheek ~ 9-MeV – 2-MeV/cm x 2cm = 5 MeV To protect oral structure, lead shield thickness ~ 5 MeV/2 = 2.5 mm(b) Backscatter from 5-MeV electrons on lead ~ 56%(c) Depth upstream from the interface to reach 10% dose is ~ 10 mm inpolystyrene, or about 4mm of aluminum 46
  • 47. 14.7 Electron Arc Therapy Calibration of arc therapy beams Suitable for treating superficial tumors along curved surfaces.  n D0 • ∆θDarc ( P ) = 2πn ∑ D ( P) • Inv(i) i =1 iwhereD = dose rate (MU/min) 0n = number of rotations / minD i (P) = dose to P (from isodose chart) at the i th angleInv(i) = inverse sqaure correction at the i th angle. 47
  • 48. 14.7 Electron Arc Therapy Treatment planning Beam energy ‘velocity effect’ (?) The effect depends on the fieldDose increased at larger depth width and arc-size, i.e., the rangeDose decreased at shallow depth of angles a given point is irradiated (exposed), it has nothing to do with 48 the rotation speed.
  • 49. 14.7 Electron Arc Therapy Treatment planningScanning field width:Smaller field width →Lower dose rate (greater MU) →Greater x-ray contamination dose(at the isocenter)Smaller field width →~Normal incidence at all angles(less surface curvature/obliquityeffect)Typically, field width 4 – 8 cm at isocenter. 49
  • 50. 14.7 Electron Arc Therapy Treatment planningLocation of isocenter:Approximately equi-distance fromthe contour surface from allangles, andThe depth of isocenter > electronrange, so that dose from primaryelectrons is not accumulated (butdose from contaminatedbremsstrahlung x-rays cannot beavoided). 50
  • 51. 14.7 Electron Arc Therapy Treatment planning Field shaping Isodose distribution calculated by computer treatment planning system Use surface shield to better Gradual dose falloff at define the dose distribution both ends of the arc 51
  • 52. 14.8 Total Skin Irradiation 2-9 MeV electron beams are useful for treating superficial lesions covering a large areas of the body (e.g. mycosis fungoides) A. Translational technique B. Large field technique Stanford technique 52
  • 53. 14.8 Total Skin Irradiation Field flatness Arc field vs stationary field 3 weighted fields 53
  • 54. 14.8 Total Skin Irradiation X-ray contamination X-ray contamination along the beam central-axisReduce x-ray contamination by angling the central axis away from the patient 54
  • 55. 14.8 Total Skin Irradiation Field arrangement 15 ° 15 ° 55
  • 56. 14.8 Total Skin Irradiation Dose distribution The depth-dose curve and dmax shift toward the surface, due to oblique incident angles. With the 6-field technique, dose uniformity of ±10% can be achieved in general, except in areas with large surface irregularities (e.g. inner thigh) where supplementary irradiation may be needed. Bremsstrahlung dose in patient midline is approximately doubled due to the opposed beam arrangement. 56
  • 57. 14.8 Total Skin Irradiation Modified Stanford technique (dual field angle) films ~ 400 cm 10-15° 10-15° Plastic screen 57
  • 58. 14.8 Total Skin Irradiation Modified Stanford technique (calibration) Single Dual-angle polystyrene field P Parallel plate chamber poly =1 for parallel- L ( DP ) poly = M • CT , P • N gas •   ρ • Pion • Prepl plate chamber   air water =1 P at surface S ( DP ) water = ( DP ) poly •   ρ • Φ water poly   poly 58
  • 59. 14.8 Total Skin Irradiation Modified Stanford technique (treatment skin dose) All 6 fieldsDual-anglefieldTotal skin dose from all 6 dual-angle fields: (D ) S poly = ( DP ) poly • B 2.5 ~ 3.0 (D ) S water = ( DP ) water • B 59
  • 60. 14.8 Total Skin Irradiation Modified Stanford technique (in-vivo dosimetry) Although an overall surface dose uniformity of ±10% can be achieved, there are localized regions of extreme non-uniformity of dose on the patient’s skin. (e.g. sharp body projections, curved surfaces,…) TLDs are most often used for in-vivo dosimetry. 60
  • 61. 14.9 Treatment Planning Algorithms Pencil beam based on multiple scattering theory −r 2 σ r ( z) 2d p (r , z ) = d p (0, z )eσ r2 = 2σ x = 2σ y 2 2 ∞ ∞∫ ∫ −r 2 σ r ( z ) 2 d p (r , z )2πrdr = d p (0, z ) e 2πrdr r =0 r =0D∞ ( z ) = d p (0, z )πσ r2 ( z ) y r⇒ d p (0, z ) = D∞ ( z ) / πσ r2 ( z ) x −r 2 σ r ( z ) 2 ed p (r , z ) = D∞ ( z ) πσ r2 ( z ) 61 Dose due to an infinite field size beam
  • 62. 14.9 Treatment Planning Algorithms Pencil beam based on multiple scattering theory x2 + y2 − 2 2σ x , y ( z ) ed p ( x, y, z ) = D∞ ( z ) 2 2πσ x , y ( z )D ( x, y , z ) = ∫∫ d p (x − x , y − y , z )dx dy for rectangular field size 2a × 2b : D∞ ( z )   a + x   a − x    b + y   b − y D ( x, y , z ) = erf   σ ( z )  + erf  σ ( z )  erf  σ ( z )  + erf  σ ( z )         4   r   r    r    r where 2 x ∫e −t 2erf ( x) = dt π 0 62
  • 63. 14.9 Treatment Planning AlgorithmsPencil beam based on multiple scattering theory (lateral spread parameter, σ) Mass angular Eyges equation : scattering power 1 z θ 2  ∫ 2 σ x ( z) =  ( z )ρ ( z )( z − z ) 2 dz 2 z = 0  ρl    Modified Eyges equation 63
  • 64. 14.9 Treatment Planning AlgorithmsPencil beam based on multiple scattering theory (implementation) For more accurate electron-beam dose calculation, Monte Carlo methods are available on modern- day commercial treatment planning systems. 64