Principles and Practice of Radiation Therapy Chapter 26 Electron Beams in Radiation Therapy Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Electron (e-) Negative charge Mass is 2000 times smaller than proton Proton No charge No mass Electron beams Interact differently with matter than photon beams do because of their mass and negative charge Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Mass stopping power (S/p) Rate of energy loss per unit length (S) divided by density of the medium (p) Total mass stopping power = Sum of all energy losses (S/p)tot = (s/p)col + (S/p)rad Includes losses caused by collisions of e- with atomic e- Also includes radiation losses or bremsstrahlung production Bremsstrahlung (Gr. braking radiation) is caused by e- decelerations when passing through the field of atomic nuclei Contribution of each process is affected by energy of e- beam and atomic (Z) number of irradiated material Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Restricted mass collisional stopping power Better describes absorbed dose than total mass stopping power because it accounts for energy transferred by delta rays Delta rays – e- scattered with enough energy to cause further ionization and excitation in other atoms Task Group 51 Protocol Uses stopping power ratios for “realistic electron beams” More precise representation of dose Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Collisional losses Predominant interaction is that of incident e- interacting with e- of an atom Low Z number materials have greater electron density than high Z number materials Electron density Number of e- per unit mass Probability of occurrence of energy loss resulting from radiation process increases with increasing energy or increasing Z number of absorbing material Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Energy dependence of e- interactions Radiation losses range from 0.01 to 0.4 MeV/cm in the 1- to 20-MeV range In radiation therapy in the 1- to 20-MeV range, e- beam energy losses result from collisional interactions in tissues because of low Z number of tissue Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Electron beam energy spectrum dependence on depth Beam moves through linear accelerator, accelerator window, scattering foils, ionization monitor chambers, and air to patient When beam passes through patient, it undergoes a decrease in energy and broadening of energy spectrum Energy spectrum of beam in patient depends on depth of patient and energy spectrum of surface of patient Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Production of clinically useful electron beams Most commonly produced by linear accelerators Modifications must be made Remove “target” and flattening filter Decrease “electron gun” current to lower dose rate A “pencil beam” is produced Beam can be widened for clinical use by scattering foil or by scanning electron beam Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Scattering foil Thin sheet of material with high Z number placed in path of pencil beam “Dual scattering foil” arrangements First widens beam Second improves beam’s flatness Most common method of producing a wide beam for clinical use Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Scanning beams Pencil beam is scanned by magnetic fields across treatment area The constantly moving pencil beam distributes dose evenly Especially useful with energies greater than 25 MeV Thickness of required scattering foil would result in difficulties due to their size and would cause problems with e- contamination Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Physics of Electron Beams Scattering foils vs. scanning beams Scattering foils cause bremsstrahlung contamination + Scattering foils are simple and reliable + Scanning beams have no bremsstrahlung contamination Scanning beam requires maintenance of complex electronic system Failure of electronic system could allow entire dose to be delivered in one small area of patient with disastrous results Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Characteristics of Therapeutic Electron Beams Dosage gradients of clinically useful electron beams Electron beam therapy allows superficial treatment of lesions with almost no dose to underlying deep tissues Gradient – rate of change of a value (dose) with a change in position High-energy beams approximate depth dose characteristics of low-energy photon beams with no “true” skin-sparing effect Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Characteristics of Therapeutic Electron Beams Shape of plot of percent depth dose vs. depth Surface dose is approximately 85% of maximum Builds up to 100% in the first few centimeters below the surface Beyond the 80% to 90% depth dose, there is rapid fall-off The curve does not reach 0 but “flattens out” at a value of a few percent Result of bremsstrahlung-produced x-ray contamination See Figure 26-3 on page 554 of the textbook Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Characteristics of Therapeutic Electron Beams Shape of electron beam isodose curves Lateral bulge or ballooning of isodose curves Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Treatment Planning of Electron Beam Therapy Electron beam rules of thumb Depth of 50% dose in cm (R50) is multiplied by a constant (C4). Product is mean energy of the electron (Eo) beam stated in MeV at phantom surface Eo = C4R50 The value of C4 has varied 2.33 MeV in American Association of Physicists in Medicine (AAPM) Task Group 21 2.4 MeV in AAPM Task Group 25 Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Treatment Planning of Electron Beam Therapy Electron beam rules of thumb Reduction of energy of an electron beam as it moves through matter Mean energy at surface/2 Practical range (Er) in cm of an electron beam in tissue Er = MeV/2 Depth of the 80% isodose line in cm in tissue 80% isodose = MeV/3 Depth of the 90% isodose line in cm in tissue 90% isodose = MeV/4 Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Treatment Planning of Electron Beam Therapy Electron beam characteristics at surface Factors affecting surface dose Scattering system Atomic number of absorber Beam energy Field size Beam collimation Beam energy, field size, and beam collimation may be manipulated for treatment planning Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Treatment Planning of Electron Beam Therapy Electron beam characteristics in the buildup region The buildup region is an underdose risk in many clinical situations Most true for electron beams less than 12 MeV Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Treatment Planning of Electron Beam Therapy Use of bolus Partial bolus should never be used “Edge effect” Areas under the bolus have decreased dose Unbolused areas have increased dose Tissue compensator for irregular surfaces or air cavities Shape isodose distributions Better conform to the treatment volume Decrease the dose to critical structures at depth Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Treatment Planning of Electron Beam Therapy Energy dependence of the width of the 80% isodose curve With increasing electron beam energy, the ballooning of the isodose lines decreases To cover an area at depth with the 80% isodose line, a larger area must be treated on the skin surface Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Treatment Planning of Electron Beam Therapy Distance correction factors for electron beam treatments The use of extended source-skin distances (SSDs) has minimal effect on the central-axis depth dose and the off-axis ratios Recommended that standard depth dose curves be used Output and beam penumbra change dramatically with a change in treatment distance Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Treatment Planning of Electron Beam Therapy Effective point source method D’max = Dmax (SSDeff + dmax)2/(SSDeff + g + dmax)2 D’max = The dose to Dmax at extended distance Dmax = The dose to Dmax at normal distance SSD = Normal SSD SSD’ = Extended SSD SSDeff = Effective SSD dmax = Depth of maximum dose on central axis g = Difference between the extended SSD and the nominal SSD (SSD’ – SSD) Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Treatment Planning of Electron Beam Therapy Virtual source method D’max = (Dmax)[(SSDvir + dmax)2/(fair)(SSDvir + g + dmax)2] SSDvir = Virtual SSD for calibration fair = Air gap correction factor All other parameters are identical to those of the effective point source method Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Irregular Fields and Electron Beams Shielding dependence on electron beam energy Various methods have been used Lead strips, cutouts, or masks placed directly on the patient’s skin or at the end of the treatment cone or collimator Approximate shielding thickness rule of thumb MeV/2 = Shield thickness in millimeters of lead The thickness of Lipowitz alloy required may be obtained by multiplying the thickness of lead indicated by 1.2 Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Irregular Fields and Electron Beams Internal shielding Tissues directly in front of the shield may receive 30% to 70% higher dose Results from electron backscatter from the shield May be minimized by placing a low Z number material between the shield and the tissue Dental acrylic Wax Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Irregular Fields and Electron Beams Effects of irregularly shaped electron fields on dose Variables Include field size, thickness of shielding material, amount of blocking, and treatment distance Almost all the absorbed dose results from electrons that have been scattered Lateral equilibrium Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Irregular Fields and Electron Beams Tissue heterogeneities and their effects on electron beams Small heterogeneities Changes in dose range from a 20% underdose behind bone to a 15% to 35% overdose behind air cavities Large heterogeneities The major factor responsible for changes in the dose distribution of large tissue heterogeneities of uniform density is absorption Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

Irregular Fields and Electron Beams Gaps in electron beam therapy Because of the lateral bulge of the isodose curves, electron fields with adjacent treatment areas that abut each other on the surface will result in an overlap at depth. Placing a “gap” on the patient’s surface results in underdosing on the surface. Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.