Lecture 7.pdf

Engineering of electron Beam Therapy Lecture : 7 Dr. Samar Tarish  INTRODUCTION The most clinically useful energy range for electrons is 6 to 20 MeV. At these energies, the electron (less than 5 cm deep) with a characteristically sharp drop-off in...

Engineering of electron Beam Therapy Lecture : 7 Dr. Samar Tarish  INTRODUCTION The most clinically useful energy range for electrons is 6 to 20 MeV. At these energies, the electron (less than 5 cm deep) with a characteristically sharp drop-off in dose beyond the tumor beams can be used for treating superficial tumors. The principal applications are – (a) the treatment of skin and nodes – (b) the treatment of head and neck lip cancers. – (c) chest wall irradiation for breast cancer – (d) administering boost dose to cancers.  Electron Interactions Coulomb force interactions 1- Inelastic collisions with atomic electrons (ionization and excitation) 2- Inelastic collisions with nuclei (bremsstrahlung) 3- Elastic collisions with atomic electrons (electron- electron scattering) 4- Elastic collisions with nuclei (nuclear scattering) Low Z  ionization Ionization High Z  bremsstrahlung Bremsstrahlung Excitation  Collisional Losses Depending on the electron density of the medium ZMass stopping power (S/, MeV-cm2/g) – Z  electrons/g  – Z  tightly bound electrons  Fig 14.1 1 MeV, the energy loss rate in water  2 MeV/cm The rate of energy loss depends on the electron density of the medium. (b) The rate of energy loss per gram per centimeter squared, which is called the mass stopping power, is greater for low-atomic- number (Z) materials than for high-Z materials (compare the water curve to the lead curve in Fig. 14.1). There are two reasons for this: First, high-Z materials have fewer electrons per gram than low-Z materials have and, second, high-Z materials have more tightly bound electrons, which are not as available for this type of interaction.  Rate of energy loss in MeV per g/cm2 as a function of electron energy for water and lead (From Johns HE, Cunningham JR. The Physics of Radiology. 3rd ed. Springfield, IL: Charles C Thomas; 1969, with permission.) As seen in Figure 14.1, the energy loss rate first decreases and then increases with increase in electron energy with a minimum occurring at about 1 MeV. Above 1 MeV, the variation with energy is very gradual. (d) The rate of energy loss of electrons of energy 1 MeV and above in water is roughly 2 MeV/cm.  Radiation Losses (Bremsstrahlung) Energy loss/cm  electron energy & Z2 The probability of radiation loss relative to the collisional loss  electron energy & Z That means that x-ray production is more efficient for higher-energy electrons and higher-atomic-number absorbers.  Most Probable Energy The Nordic Association of Clinical Physics ,recommends the specification of most probable energy, (Ep)0 (defined by the position of the spectral peak in Fig. 14.2) at the phantom surface and the use of the following relationship: (Ep)0 = C1 + C2Rp + C3Rp2 (Ep)0 the most probable energy at the phantom surface (defined by the position of the spectral peak) Rp the practical range in centimeters For water, C1=0.22 MeV, C2=1.98 MeV cm-1, C3=0.0025 MeV cm-2 Rp is the depth of the point where the tangent to the descending linear portion of the curve intersects the extrapolated background.  In clinical practice, an electron beam is usually characterized by the energy at the body surface Figure 14.2. Distribution of electron fluence in energy, øE, as the beam passes through the collimation system of the accelerator and the phantom. Report No. 21. Washington, DC: International Commission on Radiation Units and Measurements; 1972, with permission.  Energy Specification Depth-dose curve of electron beam Although the range measurements are usually made using the depth ionization curve, the result is only slightly different from what would be obtained using depth dose curves The practical range, Rp, is the depth of the point where the tangent to the descending linear portion of the curve (at the point of inflection) intersects the extrapolated background, as shown in Figure 14.3  Mean Energy The mean energy of the electron beam, Ē0, at the phantom surface is related to R50 (the depth at which the dose is 50% of the maximum dose) by the following relationship: E 0  C4 R50 for water, C4= 2.33 MeV Energy at Depth(z) The most probable energy and the mean energy of the spectrum decreases linearly with depth.as relationship z Harder’s equation E z  E 0 (1  ) Rp This equation is important in dosimetry to know the mean electron energy at the location of the chamber.  7 We will complete in the next lecture