Mod 4

• 3 commonly used radiotherapy technologies

  • External beam radiation therapy (EBRT or XRT or teletherapy)
  • Brachytherapy or sealed source radiation therapy
  • Systemic radioisotope therapy or unsealed source radiotherapy
• Radioactivity


  A process whereby a nucleus that contains an excess energy undergoes a transformation to a more stable state by emitting energy in the form of particles or electromagnetic radiation
• Radioactive half-life


  The time interval in which the original activity level, tabulated as disintegrations per second (dps) decreases by one half
• Radionuclide/Radioisotopes and their half-lives

  • Polonium-215 (0.0018 sec)
  • Bismuth-212 (60.5 sec)
  • Sodium-24 (15 hrs)
  • Iodine-131 (8.07 days)
  • Cobalt-60 (5.26 yrs)
  • Radium-226 (1,600 yrs)
  • Uranium-238 (4.5 billion yrs)
• Superficial X-Ray tube (Philips RT 100)

  • X-ray produced at 50-150 kV
  • Varying thickness of filtration (usually 1-6 mm Al) are added to harden the beam
  • Superficial treatments are usually given with the help of applicators or cones
  • SSD range 15 to 20 cm
  • Dose is highly dependent on source-skin distance, filtration and applicator area
  • Usually operated at 5-8 mA
  • Useful for irradiating tumor confined to about 5 mm depth

  
• Issues with superficial radiotherapy

  • Short focus to skin distance (FSD) and hence high output and large influence of inverse square law
  • Calibration difficult due to strong dose gradient i.e. dose fall off and electron contamination
• Low-energy machines

  • Superficial Equipment
  • Orthovoltage Units
• Grenz rays


  • 10-15 kVp
  • Treatment of inflammatory disorders (Langerhans' cells), Bowen's disease, patchy stage mycosis fungoides, herpes simplex
• Contact therapy


  • Superficial skin lesions
  • Endocavitary treatments for curative intent (rectal)
• Superficial equipment


  • 50-150 kVp
  • Skin cancer and tumors no deeper than 0.5 cm
• Orthovoltage machines


  • 150-500 kVp
  • Skin, mouth, and cervical carcinoma
  • Experience limitation in the treatment of lesions deeper than 2 to 3 cm
• Orthovoltage units


  • Uses "conventional" X-Ray tube with electrons accelerated by an electric field
  • Stationary anode (in contrast to diagnostic tubes which have a rotating anode)
  • Filtration important

  
• Limitations of low energy machines

  • Can not reach deep-seated tumors with an adequate dosage of radiation
  • Do not spare skin and normal tissues
• Differences between superficial and orthovoltage

  • Superficial: 50 to 150kVp, small skin lesions, maximum applicator size typically < 7cm, typical FSD < 30cm, beam quality measured in HVL aluminium (0.5 to 8mm)
  • Orthovoltage: 150 to 500kVp, skin lesions, bone metastases, applicators or diaphragm, FSD 30 to 60cm, beam quality in HVL copper (0.2 to 5mm)
• Orthovoltage units


  • Uses conventional X-ray tube
  • Energy range 150- 500 kV X-rays
  • Mostly used around 250 - 300 kVp
  • Applicators are used in superficial therapy
  • Treatment depths of around 20 mm
  • Penetration sufficient for palliative treatment of bone lesions relatively close to the surface (ribs, spinal cord)
• Disadvantages of deep X-ray

  • Higher dose to bone - photoelectric absorption
  • Maximum dose on the surface hence higher skin dose
  • Treatment to a depth of only a few centimeters possible
  • High scattered radiation and larger penumbra
• Orthovoltage X-ray filters


  • Copper and sometimes Tin in addition to Aluminum
  • Aluminum is placed distal to copper to remove soft secondary radiation
  • Copper is placed distal to Tin to remove soft secondary radiation when beam interacts with Tin
• Orthovoltage X-ray operation


  • Usually operates at SSD (Source to surface distance) 50 - 70 cm with or without cone
  • Has moveable lead shields
  • Low dose rate due to long SSD and heavy filtration of beam
• Superficial machine or kilovoltage units

  • Operating with X-rays tubes at an accelerating potential < 50 kV
  • Superficial therapy refers to potentials of 50-150
  • Low/poor penetrating ability
  • Additional filters in the form of aluminum, tin and copper are added for beam hardening
  • Used in treatment of superficial lesions
  • Cone is directed to skin surface of patient
  • Lead shielding to be placed directly to shield surrounding areas
  • Treatment distance is usually 15 - 20 cm to decrease depth dose
  • Higher patient dose due to distance
  • Backscatter is high due to low energy and increases with increased field distance
• Orthovoltage therapy or deep therapy

  • Energy: 200 - 300 kV
  • Tube current: 10 - 20 mA
  • HVLs: 1 - 4 mm Cu
  • Cones or movable diaphragm (continuous adjustable field size)
  • SSD: 50 cm
  • Application: tumor located < 2-3 cm in depth
• Limitations of orthovoltage therapy

  • Skin dose
  • Depth dose distribution
  • Increase absorbed dose in bone
  • Increase scattering
• Contact X-ray brachytherapy


  • Also called "CXB", "electronic brachytherapy" or the "Papillon Technique"
  • A type of radiation therapy using X-rays applied close to the tumour to treat rectal cancer
• Megavoltage


  A clinical modality consisting of the administration of high energy (1 megavolt or greater) to a deep-seated cancer (e.g., prostate or brain cancer) or cerebrovascular defects by an MRT unit (e.g., linear accelerator or 60Co unit)
• Radioisotope therapy


  • Delivers radiation directly into the cancer cells, usually as a capsule, drink or injection into a vein
  • Cancer cells absorb the radioactive substance more than normal cells, receiving a higher dose of radiation and causing the cells to die
• Betatron


  • A cyclic accelerator that produces high-energy electrons for radiotherapy
  • The magnetic field of the betatron deflects electrons into a circular orbit, and an increasing magnetic orbital flux produces an induced circumferential electric field that accelerates them
  • Developed by KERST in 1941
  • Replaced by LINAC in 1950
• Disadvantages of betatron

  • Low dose
  • Limited field size
  • Needs large treatment room due to large size
  • Limited motion
• Van de Graaff generator


  • An electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials
  • Produces very high voltage direct current (DC) electricity at low current levels
  • 1931 (MIT)
  • 40 feet high Electrostatic device capable of operating at 5,000,000 volts
  • 2 MeV Clinical Van de Graaff X-ray machine
  • Invented by Robert van de Graaff
• Machines using isotopes


  • Teletherapy - Treatment in which source of radiation is at some distance from the patient
  • Brachytherapy - The source of radiation is very close to treated tissue
• Cobalt-60


  • On and off shield source
  • Emits two photons/disintegration: 1.17 and 1.33 Mev which is useful in radiation therapy
  • The dose rate is constantly decreasing and adjustment of treatment is done periodically
  • Large size: penumbra
  • LINAC: small focal spot
  • Cobalt: near: small source: activity and dose is reduced
• Cesium-137


  • Teletherapy isotope machine
  • Same with the shield source of Cobalt-60
  • 20 to 30 cm with 0.662 MeV
• Radioactive sources (γ ray equipment)

  • Cobalt 60
  • Cesium 137
• Telecurie units


  • Cs-131
  • Cobalt-60 Unit
• Features of a teletherapy source

  • High energy gamma ray emission
  • High specific air kerma rate constant
  • Simple means of production
  • Low cost
  • High specific activity
  • Long half-life
• Cesium-137


  • Photon energy 0.66MeV
  • Relatively large source to relatively low specific activity
  • Medium FSD (around 60cm)
  • No isocentric mounting - similar to orthovoltage equipment in set-up
  • Not sold anymore and should not be in use
• Natural cobalt (59Co)


  • Kobald, from the German for goblin or evil spirit
  • Discovered in 1735
  • Brittle hard metal similar to iron and nickel
  • Found in minerals and meteorites
  • Salts and glass oxides are deep blue in colour
• Differences between orthovoltage and telecobalt unit

  • Orthovoltage: 150-500 KV x-rays, maximum dose on the skin, treatment to a depth of few centimeters, higher absorption by bone, non uniform dose distribution, higher side scatter hence larger penumbra, mostly isocentric unit
  • Telecobalt: 1.25 MeV γ Photon, maximum dose at depth of 5 mm, relatively uniform dose absorption, higher penetration deep seated tumours, more of forward scatter, lesser penumbra
• Differences between linear accelerator and telecobalt unit

  • Linear Accelerator: 4 to 21 MV photon beams, maximum dose at higher depth with energy, no radioactive source, radiation only when the source is switched is ON, uniform dose absorption, 1mm source - nearly point source, small Penumbra, electron beam of various energies possible
  • Telecobalt: 1.25 MeV γ Photon, maximum dose at depth of 5 mm, source to be changed every 4 to 5 years, leakage radiation present even while the beam is off, relatively uniform distribution, 1-2 cm source diameter, larger penumbra, gamma Photon only
• Telecobalt unit

  • Introduced in the 1950's, being replaced by linacs
  • The first practical radiation therapy treatment unit to provide a significant dose below the skin surface and simultaneously spare the skin
  • Still used in developing countries: simpler design, cost, little tech support
• Properties of telecobalt unit

  • Photon energy around 1.25MeV
  • Specific activity large enough for FSD of 80cm or even 100cm
  • Therefore, isocentric set-up possible
  • Constantly emit radiation
  • 60Co source must be shielded in a protective housing (source head)
  • Source head is a steel shell filled with lead (may be up to 2 ft in diameter)
• Application of telecobalt unit

  • To treat cancers of the head and neck area, breast, spine, and extremities
  • Areas just below the skin surface