Khan Chapter 4.5 Summary

4.3, 4.7, & 11.3: Linear Accelerator, Machines Using Radionuclides, & Parameters of Isodose Curves

• The linear accelerator uses high-frequency electromagnetic waves to accelerate electrons to high enough energies for x-ray production by target bombardment or used outright as an electron beam to treat superficial tumors.
  • Linear accelerators are built such that the gantry rotates about a horizontal axis and that the collimator rotates about a vertical axis. The intersection of these axi is known as the isocenter.
• The magnetron is a high-power oscillator that produces microwaves.
  • It has a cylindrical construct with a central cathode surrounded by an outer anode.
  • Inside the construct resides resonant cavities by which a static magnetic field is applied perpendicular to their cross-section. A pulsed DC field is applied between the electrodes to accelerate electrons towards the anode.
    • The two fields in tandem will move the electrons in complex spirals toward the resonant cavities and radiate microwave energy.
• The klystron does not generate microwaves, but rather amplifies microwaves. The processes of a klystron will be explained using an elementary two-cavity klystron.
  • The first cavity takes in accelerated electrons from a cathode and low-power microwaves from the magnetron. The microwaves will set up an alternating electric field across the cavity and cause the electrons to bunch up by velocity modulation as they travel towards the second cavity via the field-free drift tube.
  • The second cavity is induced with charges on the ends of the cavity that will create a retarding electron field. When the velocity modulated electron beam enters the second cavity, the electrons will decelerate (lose kinetic energy), and by the principle of conservation of energy, this energy loss is converted into high-power microwaves to supply the accelerator waveguide.
    • An electron gun will inject electrons into the accelerator waveguide, and the electrons will become energized to high energies by the high-power microwaves obtained via the klystron.
    • As the electrons exit the accelerator waveguide, a 270° bending magnet will focus the disorganized electrons into a narrow pencil beam before target bombardment.
• The newly generated photon beam is immediately met with the primary collimator that is a conical opening.
  • It will define the largest available circular field size and also shield leakage radiation.
• The x-ray beam will then encounter the flattening filter to obtain flat isodose curves at a depth of 10 CM.
  • When the electron beam bombards the target, bremsstrahlung x-rays will be produced primarily in the forward direction.
  • This beam is characterized as “forward peaked” as the central axis of the beam has greater fluence relative to its peripheries.
    • Thus, the flattening filter is designed with the thickest region in the center.
  • The consequence of achieving a flat beam is the occurrence of “horns” or areas of high dose near the phantom surface by the peripheries of the field.
    • The design of the flattening filter will harden the central axis of the beam, resulting in those photons having a higher average energy compared to that of the peripheries.
    • As a result, dose profiles at depths beyond 10 CM will demonstrate the opposite of horns, that is, the peripheries having lower dose compared to the central axis.
• For an electron beam to be used for treatment, the target and flattening filter are removed, and a scattering foil is equipped in place of the flattening filter.
  • The scattering foil is a thin, high-Z metallic foil used to spread the beam and achieve a uniform electron fluence.
  • The thickness of the scattering foil must be thin for to minimize scatter by bremsstrahlung.
• The flattened x-ray beam or an electron beam pass through ion chambers that will monitor the dose rate, integrated dose, and field symmetry.
• The beam will then pass through two pairs of secondary collimators that will define the field size for treatment. The collimators are made or positioned such that the collimator edges match the divergence of the beam to reduce the effect of penumbra.
  • The penumbra region is the region near the edges of the field where the dose rate decreases rapidly as a function of lateral distance from the beam axis.
    • The physical penumbra is a term used to define the lateral distance between the 90% and the 20% isodose curves at a depth of D_max.
    • The field size is defined as the lateral distance between the 50% isodose lines at a reference depth.
  • Several types of penumbra contribute to the penumbra region:
    • The geometric penumbra is the penumbra that is caused by the physical dimensions of the source/machine and the treatment specifications.
      • The source size (s): the target that generates photons is not a point source. Thus, photons can be generated at any point in the finite dimensions of the target. A larger source allows for photons to travel in a more oblique angle.
      • Source-to-skin distance (SSD): as the phantom is moved farther from the source, the region of the geometric penumbra will increase. This idea is analogous to holding a lit flashlight close to the wall. The dim light along the outskirts of the projected light is minimal compared to if the flashlight was projected meters away from the wall.
      • Source-to-diaphragm distance/source-to-collimator distance (SDD/SCD): the flashlight analogy will also apply to the collimator blocks. Placing blocks on the patient will yield the sharpest penumbra, but the skin sparing effect will be lost due to electron contamination from the block. For this reason, the blocks for photons must be kept about 20 CM away from the patient.
      • The geometric penumbra is given by the equation: P_geometric = s * (SSD + d – SDD) / SDD.
    • The transmission penumbra is the penumbra that is caused by the divergence passing through the edges of the collimator blocks (including MLC leaves). The original design of secondary collimator blocks did not match the divergence of the beam resulting in differential attenuation/variable transmission of the beam. The inner surface of the blocks were eventually made parallel to the divergence of the beam by either:
      • Using divergent collimators that moved along the circumference of a circle to match the beam divergence based on the open field size.
      • Tilting a small portion of the inner surface of each jaw to match the beam divergence while restricting collimator movement to a single plane perpendicular to the beam axis.
    • The scatter penumbra is the penumbra that is caused by scatter from inside the field depositing into the penumbra region.
      • In orthovoltage beams, the low isodose lines (5% and 10%) were greatly distended due to less lateral constriction, and so the dose to tissue outside the treatment region was increased. Penumbra trimmers were used to reduce the penumbra to attenuate the beam in the penumbra region.
      • In megavoltage beams, the electrons scatter predominantly in the forward direction, and so scatter seen outside the field is minimal relative to that of orthovoltage beams. Thus, megavoltage beams have a greater isodose sharpness at depth or a thinner penumbra width. The laterally scattered electrons have a greater range, and so the lateral dose variation near the field edges are made more gradual (field sharpness).
        • When comparing between megavoltage beams, e.g. 6 MV vs 20 MV, the penumbra width of the higher energy (20 MV) will be greater as the scattered electrons will have a greater range.
    • Outside the geometric limits of the field and the penumbra width, the dose variation is the result of scatter from inside the field and both leakage and scatter from the collimator system. Beyond the collimators, the dose deposited is by the lateral scatter from the medium and leakage from the head of the machine.
      • A thick shell of high-density shielding material surrounds the treatment head to provide sufficient shielding against leakage radiation.
• Lastly, the beam will pass through the multileaf collimators (MLCs) to allow for more precise blocking of irregular shaped target volumes and for IMRT.
  • The radiation field size can be viewable on a phantom surface by the light localizing system comprising of a light source and a mirror located between the ion chambers and secondary collimator jaws.
• Collimation used for photons in the secondary collimator and MLCs are not used for blocking in electron treatments. Electrons scatter readily in air, and so the beam collimation must be close to the skin surface. Commonly seen are electron applicators that attach onto the gantry head, and various cones, positioned near the skin surface, are equipped by the applicator.

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