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Dosimetric Evaluation of High Energy Electron Beams Applied in Radiotherapy

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Electron-beam therapy: is used to treat superficial tumors at a standard 100 cm source-to-surface distance (SSD). Characteristics of electron beams from an Elekta PreciseTM linear accelerator are presented at a nominal SSD of 100 cm. However, certain clinical situations require the use of an extended SSD. The effects of extended source-to-surface distance (SSD) on the electron beam dose profiles were evaluated for various electron beam energies 6, 8, 10,12 and 15 MeV and the accuracy of various output correction methods was analyzed on an Elekta PreciseTM linear accelerator using a radiation field analyzer (RFA). Effective SSDs was evaluated for field sizes ranging from 6×6, 10×10, 14×14 and 20×20 cm2 for various energies.
Aim of the work
1.Investigate the physical properties of electron beams
at different beam energies.
2.Evaluate the accuracy of dose calculated by
Treatment Planning System (TPS) and measured for
different field configurations.

Published in: Science

Dosimetric Evaluation of High Energy Electron Beams Applied in Radiotherapy

  1. 1. aymanstohy@yahoo.com
  2. 2. Dosimetric Evaluation of High Energy Electron Beams Applied in Radiotherapy aymanstohy@yahoo.com
  3. 3. Preparation by DR. AYMAN G. STOHY MEDICAL PHYSICIST Showing what has been done in the Master's Thesis, aymanstohy@yahoo.com
  4. 4. Supervisors  Prof. Dr. M. A. Abouzeid  Prof. Dr. M. A. Elleithy  Dr. T. A. Dawod aymanstohy@yahoo.com
  5. 5. Aim of the work 1. Investigate the physical properties of electron beams at different beam energy. 2. Evaluate the accuracy of dose calculated by Treatment Planning System (TPS) and measured for different field configurations. aymanstohy@yahoo.com
  6. 6. INTRODUCTION aymanstohy@yahoo.com
  7. 7. Electron beam therapy has become an important component in radiotherapy.  It used for treating superficial tumors (less than 5 cm depth). aymanstohy@yahoo.com
  8. 8. Such as the treatment of:  Skin and lip cancers.  Chest wall irradiation for breast cancer.  in the treatment of head and neck cancers. aymanstohy@yahoo.com
  9. 9. Electron Beam Therapy aymanstohy@yahoo.com
  10. 10. Percent depth dose  PDD is the dose along the beam central axis normalized to 100% at the depth of maximum dose (dmax).  Electron energy can be specified as the most probable electron energy Ep,0 at the surface of the water phantom. • Determined from this equation: 22.098.10025.0 2 0,  ppp RRE aymanstohy@yahoo.com
  11. 11. Beam profile The dose at any point in a plane perpendicular to the beam direction.  Beam flatness (F ) :obtained from equation:  Beam symmetry (S): Obtained from the profile in depth of maximum dose: rightleft rightleft areaarea areaarea S   100 minmax minmax 100 DD DD Flatness    aymanstohy@yahoo.com
  12. 12.  The penumbra is defined as the distance between the 80% and 20% dose points on a transverse beam profile measured 10 cm deep in the water phantom. aymanstohy@yahoo.com
  13. 13. Effective SSD  The distance between the beam virtual source and the surface of the water phantom, determined from equation: f = effective SSD, I0 = dose with zero gap, and Ig = dose with gap g between the standard SSD point and the phantom surface. 2 max max          df gdf I I g o aymanstohy@yahoo.com
  14. 14. Electron applicators and cutouts  Electron beam applicators are used to collimate the beam, the electron field is defined at distance as small as 5 cm from the patient.  Cutouts are used to shape the radiation field to the tumor, hence minimizing the dose to the surrounding tissues. aymanstohy@yahoo.com
  15. 15. Output factors The radiation output is a function of field size  Output is measured at standard SSD with a small volume ionization chamber at dmax on the central axis of the field.  Output factors are defined as the ratio of the dose for any field to the dose for 10x10 cm2 field at dmax.  The output factors for cutouts depend on the electron energy, applicator and the area of the cutout. aymanstohy@yahoo.com
  16. 16. IAEA (International Atomic Energy Agency) protocol  Determination of water absorbed dose Dw,Q in quality beam radiation Q by used ionization chamber from eq. ; where MQ is the reading of the dosimeter ND,W,Q0 is the calibration factor kQ Q0 is the beam quality correction factor 00,,, QQQwDQQw kNMD  aymanstohy@yahoo.com
  17. 17. Surface dose (Dose build-up):  The build-up effect with electron increases when beam penetrates the surface until a build-up reach maximum dose. Mean energy gradually decreases with depth. aymanstohy@yahoo.com
  18. 18. Treatment planning system  A computer consists of hardware and software components of the system.  Description of dose calculation models used in the planning system and generate beam shapes to maximize tumor control and minimize normal tissue complications.  The pencil-beam algorithm used for Fermi–Eyges pencil- beam theory to calculate dose in patients.  Enter patient data and machine parameters into the system and comparing treatment plans calculated for standard phantom. aymanstohy@yahoo.com
  19. 19. MATERIALS AND METHODS aymanstohy@yahoo.com
  20. 20. The radiation facilities used in this study is aymanstohy@yahoo.com
  21. 21. Linear Accelerator, Elekta Precise  It is used as a source of electron energies aymanstohy@yahoo.com
  22. 22.  Open applicator field sizes of 6x6, 10x10, 14x14, and 20x20 cm2 aymanstohy@yahoo.com
  23. 23. PTW-UNIDOS Electrometer  It used to measure dose ( the absolute and relative dose ) aymanstohy@yahoo.com
  24. 24. Ionization Chambers  Markus Ionization Chamber  0.125 cc ion chamber with build up cap aymanstohy@yahoo.com
  25. 25. Therapy Beam Analyzer  A computer system used for measuring dose distribution and radiation analysis in radiotherapy. aymanstohy@yahoo.com
  26. 26. Water phantom  It is made of prespex in form of cubic tank. aymanstohy@yahoo.com
  27. 27. The treatment planning system FOCUS Plan 2D radiation treatment planning system Precise Plan 3D treatment planning system aymanstohy@yahoo.com
  28. 28. Low Temperature Melting Alloy Blocks  Cerrobend shielding block.  It used to make of cutout field. aymanstohy@yahoo.com
  29. 29. Methods aymanstohy@yahoo.com
  30. 30.  Used a water phantom system with the dual 0.125 cc ionization chamber and reference detector.  The water surface is set to the standard treatment distance 100 cm, giving a 5 cm stand off between the end of applicator and the water surface, the gantry set at 0° position.  Used applicators 6x6 to 20x20 cm2 at 6 to15 MeV at SSD = 100 cm. aymanstohy@yahoo.com
  31. 31. This work consists of 7 steps 1. Physical parameters of the electron beams. 2. Percent Depth Dose and Beam Profile. 3. Beam Output Correction Factors. 4. Effective SSD. 5. Relative output factors. 6. Surface dose (Dose build-up). 7. Effect of gantry angle on output dose rate. aymanstohy@yahoo.com
  32. 32. RESULTS AND DISCUSSION aymanstohy@yahoo.com
  33. 33. Physical parameters of the electron beams aymanstohy@yahoo.com
  34. 34. The physical parameters of the electron beam for each applicator at different energy:
  35. 35. The central axis depth–dose curve of electron beams depends on many factors such as: the beam energy, field size, SSD, collimation, depth of penetration, and angle of beam incidence. aymanstohy@yahoo.com
  36. 36. Percent Depth Dose (PDD) aymanstohy@yahoo.com
  37. 37. PDD curves of energy 10 MeV at SSD=100 cm for different applicators. Central axis PDD curves for all energies from linear accelerator. All curves are normalized to 100% at dmax 0 20 40 60 80 100 0 20 40 60 80 100 Depth (mm) Dose(%) 6 MeV 8 MeV 10 MeV 12 MeV 15 MeV 10 MeV 0 20 40 60 80 100 0 10 20 30 40 50 60 70 Depth (mm) Dose(%) 6x6 10x10 14x14 20x20 aymanstohy@yahoo.com
  38. 38. (a) Measured and Calculated open field depth-doses for all applicators , at SSD=100cm, 10 MeV. (b) The differences between measured and calculated doses. (a) 0 20 40 60 80 100 0 20 40 60 80 Depth (mm) Dose(%) calc.6x6 meas.6x6 calc.10x10 meas.10x10 calc.14x14 meas.14x14 calc. 20x20 meas.20x20 (b) -5 -3 -1 1 3 5 0 20 40 60 Depth (mm) Difference(%) aymanstohy@yahoo.com
  39. 39. The mean difference and SD between measured and calculated depth- doses for all energies and for all field sizes . Appl. Energy (MeV) Precise Plan Focus Plan Mean ±SD Mean ±SD 6x6 6 0.48 1.43 -0.52 1.16 8 0.55 1.30 0.42 0.84 10 0.92 1.58 0.35 1.02 12 0.73 1.17 0.15 0.65 15 0.97 1.33 0.11 0.53 10x10 6 0.36 1.61 0.26 1.03 8 0.22 1.09 0.27 0.71 10 0.23 1.24 0.20 0.65 12 0.42 0.99 0.04 0.50 15 0.73 1.04 0.21 0.60 14x14 6 0.28 0.80 0.23 0.96 8 0.16 0.82 0.39 1.03 10 0.02 0.76 0.34 0.79 12 0.14 0.65 0.20 0.69 15 0.28 0.57 0.17 0.64 20x20 6 0.50 1.39 0.20 1.03 8 0.58 1.36 0.31 0.77 10 0.71 1.42 0.35 0.77 12 0.73 1.15 0.29 0.65 aymanstohy@yahoo.com
  40. 40. The deviation between measured and calculated values for smaller applicator of size 6x6 cm2, is larger than the large applicators. The differences between measured and calculated central axis percent depth doses for all field sizes and at all energies are found to be within 2%. aymanstohy@yahoo.com
  41. 41. Dose profile aymanstohy@yahoo.com
  42. 42. Calculated and measured beam profiles for 10-MeV for Precise Plan Calculated and measured beam profiles for 10-MeV for Focus Plan 0 20 40 60 80 100 -120 -90 -60 -30 0 30 60 90 120 Off axis distance (mm) Relativedose 0 20 40 60 80 100 -150 -100 -50 0 50 100 150 Off axis distance (mm) Relativedose aymanstohy@yahoo.com
  43. 43. The differences between the calculated beam profile and measured data found to be less than ±2%. aymanstohy@yahoo.com
  44. 44. The beam profile scans for applicator 10x10 cm2 at different depths by using energy 10 MeV. The beam profile scans for applicator 20x20 cm2 at different depths by using energy 10 MeV. The beam profile scans for applicator 10x10 cm2 at different depths by using energy 15 MeV. The beam profile scans for applicator 20x20 cm2 at different depths by using energy 15 MeV. aymanstohy@yahoo.com
  45. 45. The flatness, symmetry and penumbra (left and right) for energy 10 MeV, applicators 10x10 and 20x20 cm2. Energy Appl. (cm2) Depth (cm) Pen. Left Pen. Right Flatness (%) Symmetry (%)(mm) (mm) 10 MeV 10x10 0 7.3 7.8 1.97 101.17 0.5 11.25 11.03 2.72 101.31 1 15.03 14.61 3.94 101.26 1.5 19.33 19.05 5.02 101.29 2 23.61 23.13 6.53 101.34 2.5 28.15 27.74 8.42 100.97 20x20 0 5.71 6.11 1.13 100.2 0.5 9.35 9.5 1.41 100.34 1 13.22 12.9 1.97 100.3 1.5 16.72 16.77 2.4 100.45 2 21 20.75 3.24 100.72 2.5 24.74 24.79 3.92 100.66
  46. 46. The flatness, symmetry and penumbra (left and right) for energy 15 MeV, applicators 10x10 and 20x20 cm2. Energy Appl. (cm2) Depth (cm) Pen. Left Pen. Right Flatness (%) Symmetry (%)(mm) (mm) 15 MeV 10x10 0 6.64 6.83 1.59 101.31 0.5 8.8 8.02 2.31 101.52 1 10.99 10.98 2.88 101.42 1.5 14.22 13.98 3.33 101.44 2 17.58 17.01 4.05 101.29 2.5 20.46 19.89 5.13 101.06 20x20 0 4.96 5.17 1.22 100.59 0.5 7.19 7.18 1.2 10.54 1 9.32 9.92 1.45 100.73 1.5 12.28 12.43 1.86 100.68 2 15.16 14.97 2.03 100.94 2.5 17.72 18.01 2.40 100.67
  47. 47.  The results showed that for applicators 10x10 and 20x20cm2 at energy 10 MeV the flatness increased from 1.97 at water surface to 8.43%.  At energy 15 MeV the flatness increased from 1.59 at water surface to 5.13%. aymanstohy@yahoo.com
  48. 48. The penumbra linearly with the depths for energy 10 &15 MeV 10 MeV 15 MeV aymanstohy@yahoo.com
  49. 49. The flatness, symmetry and penumbra (left and right) for 6 MeV and applicator 10x10 cm2 at extended SSD. SSD (cm) Profile Felid (50%) (cm) Pen. Left (mm) Pen. Right (mm) Flatness (%) Symmetry (%) 100 X 10.51 9.69 9.40 3.35 106.35 Y 10.16 9.81 9.21 1.77 101.33 105 X 11.19 14.70 13.90 3.65 104.33 Y 11.05 14.31 14.42 3.23 102.75 110 X 11.77 19.10 18.72 4.94 103.62 Y 11.76 19.42 19.23 5.09 101.28 115 X 12.46 22.60 21.25 7.45 101.88 Y 12.32 23.50 23.27 7.30 101.75 120 X 13.02 27.73 25.54 9.70 103.31 Y 13.05 28.63 27.85 9.79 103.29 aymanstohy@yahoo.com
  50. 50. Profiles for 6 MeV electron beam and 10 × 10 cm2 field size at extended SSD. The effect of extended SSD on transverse beam profiles found that loss of flatness and increase in the penumbra. aymanstohy@yahoo.com
  51. 51. Beam Output Correction Factor 1. At fixed SSD 2. At extended SSD aymanstohy@yahoo.com
  52. 52. 1. The output factor at fixed SSD, dmax, in the electron beam
  53. 53. 2. The output factor at extended SSD, dmax, in the electron beam
  54. 54. Output factor should be checked annually for the accuracy, and a commissioning process would also verify these factors. aymanstohy@yahoo.com
  55. 55. Effective SSD aymanstohy@yahoo.com
  56. 56. Variation of extended SSD for different energies of applicator sizes (cm2) 6x6 (a), 10x10(b),14x14 (c) and 20x20(d) aymanstohy@yahoo.com
  57. 57. From these figures, found SSDeff from the slope of straight line by Khan equation: max 1 d slope f  aymanstohy@yahoo.com
  58. 58. SSDeff values in cm from Elekta Precise with the variation of applicator and energy by using Markus chamber. Energy Appl. (cm2) Electron Beam Energy (MeV) 6 8 10 12 15 6x6 65.4 72.4 74.4 75.4 77 10x10 87.3 91 94.1 96.4 96.7 14x14 93.1 93.6 96.9 96.9 97 20x20 97.7 99.3 102 102.7 102.9 aymanstohy@yahoo.com
  59. 59.  The effective SSD increased from 65.4 cm for energy 6 MeV to maximum of 102.9 cm for energy 15 MeV.  SSDeff increases by increasing applicator size and energy. aymanstohy@yahoo.com
  60. 60. Correction of output  The dose distribution values of the electron beam can be calculated from the following equation: at extended SSD and dmax. 2 max max 100             dgSSD dSSD DD eff eff SSDcalc aymanstohy@yahoo.com
  61. 61. Comparison between measured and calculated values:
  62. 62.  Found that small deviation for the mean value of relative ranged from 0.34 to 0.83, and relative SD range of 1.5 to 3.7%,the deviation of SSDeff was ≤4 % for combination of energy and field size. aymanstohy@yahoo.com
  63. 63. The relative output factor 1. For fixed SSD using cutouts 2. For extended SSD aymanstohy@yahoo.com
  64. 64. 1. The ROFcut for 10 x 10 cm2 applicator at SSD = 100 cm (a) Measured ROFcut for all energies electron beam. (b) Calculated ROFcut for all energies electron beam. (c) Difference between measured and Calculated ROFcut. aymanstohy@yahoo.com
  65. 65. 2.The relative output for 10 MeV electron beam for different field sizes was plotted against extended SSD. The relative output at extended SSD decreased more rapidly. aymanstohy@yahoo.com
  66. 66. Surface dose (Dose build-up) aymanstohy@yahoo.com
  67. 67. Central axis depth dose curves for electron beams of 6 to 15 MeV measured in prespex sheet by using applicator 10x10 cm2. aymanstohy@yahoo.com
  68. 68. The build-up effect of electrons varies with electron energy. The electron scattering is strongly energy independent and decrease when the energy increase. aymanstohy@yahoo.com
  69. 69. Effect of gantry angle on output dose rate:  Rotate gantry angle by 5 degree for left and right side until 25º using linac, comparison between the measured and calculated on (TPS) we found that the relative output for rotate gantry angle, by using Precise Plan and Focus Plan ,the % Difference between calculated and measured less than 2%. aymanstohy@yahoo.com
  70. 70. Comparison between measured and calculated relative output for rotate gantry angle by steps 5º, applicator 10x10 cm2 for all energies at SSD=100cm for Precise plan.
  71. 71. The % difference between measured and calculated from precise plan for different energy with rotate gantry angle by steps 5º.
  72. 72. Comparison between calculated from precise plan and measured relative output for rotate gantry angle by steps 5º, use applicator 10x10 cm2 for energies 6,8,10,12 and 15 MeV. 6 MeV 0.93 0.95 0.97 0.99 1.01 -30 -20 -10 0 10 20 30 Gantry angle (o ) RelativeOutput Meas. Calc. 8 MeV 0.95 0.97 0.99 1.01 -30 -20 -10 0 10 20 30 Gantry Angle (o ) RelativeOutput Meas . Calc. 10 MeV 0.96 0.98 1.00 -30 -20 -10 0 10 20 30 Gantry Angle (o ) RelativeOutput Meas. Calc. 12 MeV 0.95 0.97 0.99 1.01 -30 -20 -10 0 10 20 30 Gantry Angle (o ) RelativeOutput Meas. Calc. aymanstohy@yahoo.com
  73. 73. 15 MeV 0.95 0.97 0.99 1.01 -30 -20 -10 0 10 20 30 Gantry Angle (o ) RelativeOutput Meas. Calc. aymanstohy@yahoo.com
  74. 74. Conclusion aymanstohy@yahoo.com
  75. 75. From the previous measurements we can conclude that: aymanstohy@yahoo.com
  76. 76. 1. The central axis depth–dose curve of electron beams depends on many factors such as: the beam energy, field size, SSD, collimation, depth of penetration, and angle of beam incidence. 2. In central axis percent depth dose the deviation between measured and calculated values for smaller applicator 6x6 cm2, is larger than the large applicators, the differences between measured and calculated for all sizes and at all energies are found to be within 2%. aymanstohy@yahoo.com
  77. 77. 3. The differences between the calculated beam profiles and measured data found to be less than ±2%. 4. The flatness, symmetry, and penumbra depends on energy, field size, and depth. 5. The OUF should checked annually for accuracy, and a commissioning process would also verify these factors. 6. The SSDeff, depends on energy and field size. It is necessary to measure SSDeff for each field insert and energy. aymanstohy@yahoo.com
  78. 78. 7. It is recommended to enter SSDeff values in the treatment planning system. 8. The relative output at extended SSD decreased more rapidly. 9. The build-up effect of electrons varies with electron energy. The electron scattering is strongly energy independent and decreases when the energy increases. aymanstohy@yahoo.com
  79. 79. THANK YOU Ayman Gomaa Stohy aymanstohy@yahoo.com

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