Radthera

• Immobilization devices


  Devices commonly fabricated from plaster of paris or thermoplastic immobilization shells used during treatment to prevent movement and provided a reproducible geometry from one treatment to the next
• Immobilization devices can be viewed as torture devices by young patients
• Caliper


  Used to measure the thickness of the patient
• Patient Positioning and Immobilization Devices

  • Total Body Cast
  • Large bag filled with Styrofoam
  • Bite Block
  • Head Rest - most commonly used
  • Thermoplastic Cast
  • Thermoplastic Mask - connected to head rest
• Thermoplastic


  A polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently
• Most thermoplastics are high molecular-weight polymers whose chains associate through weak Van der Waals forces, stronger dipole-dipole interactions and hydrogen bonding, or even stacking of aromatic rings
• Bolus


  In radiation therapy, a tissue equivalent substance placed on the patient's skin in order to achieve the required dose distribution to the target site and surrounding healthy tissues
• Cerrobend Block


  Also called Aka Wood's Metal, Lipowitz Alloy. A eutectic, fusible alloy with a melting point of approximately 70 degrees Celsius (158 F). An alloy of 50% bismuth, 26.7% lead, 13.3% tin, and 10% cadmium by weight
• Head of Machine


  The portion of the machine directly above the patient, containing the source of radiation
• Collimator


  Directly above the patient and protruding from the head, used to help determine the limits of the treatment field
• Gantry


  Dial that indicates the angle of the head relative to the patient, capable of rotating a full 360 degrees around the patient
• dmax


  Depth of maximum delivered dose from a single radiation field, at this depth the number of collisions is at maximum
• Build-up Region


  Region between the body surface and dmax
• Types of X-ray


  • Diagnostic (25-150 kV)
  • Superficial (50-200 kV)
  • Orthovoltage (200-500 kV)
  • Supervoltage (500-1000 kV)
  • Megavoltage (1-25 MV)
  • Low-energy Machines (up to 300 kVp)
• Grenz Rays


  10-15 kVp, almost entirely absorbed in the first 2μm of skin and have a useful depth dose range of about 0.5μm. Used for treatment of inflammatory disorder, Bowen's disease, patchy stage mycosis fungoides, herpes simplex
• Contact Therapy


  Superficial skin lesions, treatment unit comes in contact with patient. Also used for endocavitary treatment for curative (rectal) purposes. Advantages: preserves sphincter, confined to bowel wall, maximum tumor sizes 3x5cm. Used for treatment of 3000 cGy each, separated by a 2 week interval. SSD 4cm, 1μm aluminum filtration. Typically operates at 40-50 kVp and 2-5 mA
• Superficial Equipment


  50-150 kVp and 5-10 mA, 1 - 6μm aluminum filtration, cones 2-5cm diameter, Pb cutouts, SSD 15-20cm, Typical HVLs 1.0 to 8.0mm Al. dmax is on the surface and falls off rapidly with depth, used to treat skin CA and tumors not deeper than 0.5cm
• Orthovoltage/ Deep Machines


  150-500 kVp and 10-20 mA, HVL 1-4mm Cu, SSD 50-70cm, dmax occurs at very close to the skin surface falling to about 90% at approximately 2cm of depth. Used to treat skin, mouth, and cervical carcinoma, experience limitation in the treatment of lesions deeper than 2-3 cm
• Kilovoltage Voltages


  First type of treatment machine available, disadvantage: large skin doses delivered relative to the tumor doses, dmax is the skin surface
• Super Voltages


  500-1000 kV, one type is a resonant transformer in which the voltage is stepped up in a very efficient manner
• Megavoltage


  Energy 1 MV or greater, the effective energy of the primary radiation is higher than kilovoltage equipment, dmax is therefore not at the skin surface but at some depth below the skin. Examples are accelerators such as Van de Graaff generator, linear accelerator, betatron and microtron, and teletherapy units such as cobalt-60
• Cobalt-60


  The first form of radiation therapy machine that was introduced to take place of kilovoltage equipment, contained high activity gamma-emitting sources, the source is stored in a well-shielded position when the machine is in the "off" position, the usual shielding material is either Uranium or Lead
• Prior to 1951 teletherapy using isotope were made for use with radium
• Cobalt-60 source


  A double encapsulated cylinder filled with discs or pellets of the isotope, the double steel capsule is sealed by welding to prevent escape of radioactive material, the source is 1.5cm diameter x 1.5cm high, shielded by an alloy of Pb, U, W
• Cobalt-60 source emits radiation constantly thus the source must be shielded when the machine is in the "off" position
• Isotope machine


  Consists of a lead filling container which the radioactive source is placed near the center, with an opening provided for the radiation beam to exit when the machine is in the "on" position
• If the power fails when the source is in the "on" position, the source will automatically move to the "off" position via fail-safe system
• Source head


  A steel field with lead (may be up to 2ft in diameter) that houses the cobalt-60 source, uses a counterweight to balance the lead shielding
• The cobalt-60 source shall be replaced about 1 or 2 half-lives that is 5-10 years
• Cobalt-60 gamma ray energy

  1.17 MeV and 1.33 MeV, average energy 1.25 MeV
• Radium-226 was the first radioactive substance to produce gamma rays
• Cobalt-60 skin sparing


  100% dose is not located in skin, skin is spared from high dose radiation, 100% dose is located at a certain depth of machine, dmax = 5mm (0.5cm) - 5mm below skin surface, SSD (source-to-skin distance) = 80 cm
• Cobalt-60 sources have an activity of 750-9000 Ci, typically 3000-9000 Ci used in radiation therapy
• Umbra


  The innermost and darkest part of the shadow, where the light source is completely blocked by the occluding body
• Penumbra


  The area at the edge of the radiation beam at which the dose rate changes rapidly as a function of distance from the beam axis, the larger the source size, the larger the penumbra
• Antumbra


  The region from which the occluding body appears entirely contained within the disc of the light source
• Dose Maximum (dmax)


  The depth at which the greater percentage of dose occurs below the skin surface, the maximum dose received by the tumor, also called Electron Equilibrium
• Cesium-137


  Energy of gamma ray = 66 KeV (0.0662 MeV), half-life = 30 years, SSD = 20-30 cm, did not become popular because gamma rays are not penetrating
• Main Parts of Cobalt Machine

  • Gantry
  • Control Console
  • PSA or Patient Support Assembly/Patient Couch
• Photon Beam


  4 MV to 22 MV, single beam/dual beam