Week8 physics

• Radiotherapy


  Using ionizing radiation to kill tumors

  
• How radiotherapy can treat cancer

  1. Deliver lethal doses of radiation to malignant tumor cells

  2. Healthy tissues are also exposed to high doses along with the tumors

  3. Radiation therapy strives to achieve total extinction of tumor cells

  
• Radiotherapy

  • Malignant tumor cells are rapidly reproducing cells, easily affected by radiation
  • Destroying enough of a tumor to prevent its regrowth is still a difficult task
  • Cancer cells can be spread throughout the body and infiltrate vital organs

  
• Treating a cancer


  Often involves treating an entire region surrounded by healthy tissue
• There is an appreciable overlap between the dose-response curves for tumor cells and healthy cells, making the choice of dose size difficult
• Dose-response curve


  • There is a threshold dose for cell killing
  • The fraction of cells killed increases with dose via a characteristic S-shaped curve
  • There is a dose at which 100% of the cells are killed

  
• If the threshold for the healthy tissue exceeds that for the tumor, treatments can use a dose that kills most of the tumor cells and affects relatively few of the healthy cells

  
• The dose-response curve depends on the tissue, type of ionizing radiation used and its energy, and the rate at which the radiation dose is delivered

  
• Fractionation


  1. Dividing the high dose required to kill tumor cells into many fractions

  2. Smaller doses are delivered with a pause of a day or so between fractions

  3. This gives time to healthy cells to recover and repopulate

  4. Using fractions also damages tumor cells by changing the point in the cell cycle at which the tumor cells are irradiated

  
• The 4 R's of fractionation

  • Repair (few hours)
  • Redistribution (few hours-days)
  • Re-oxygenation (few hours-days)
  • Repopulation (few weeks)
• Fractionation schemes


  • Conventional (Dose/fraction: 1.8 -2.2 Gy, Fractions/week: 5, Total dose: 45.0 – 50.4 Gy)
  • Hyper-fractionation (Dose/fraction: 1.1 - 1.3 Gy, Fractions/week: 10, Total dose: 70 - 80 Gy)
  • Accelerated fractionation (Dose/fraction: 1.4 – 2.5 Gy, Fractions/week: 10, Total dose: 40 – 50 Gy)
  • Hypo-fractionation (Dose/fraction: above 2.5 Gy, Fractions/week: 1-5, Total dose: 20 – 55 Gy)
• Factors controlled in radiotherapy

  • Means of delivering the radiation
  • Radiation type
  • Total dose
  • Fractions into which the total dose is divided
  • Time between fractions

  
• The type of particle used and its energy are essential issues in radiation therapy
• Alternative approaches for delivering ionizing radiation for therapy

  • Beam sources external to the body
  • Brachytherapy sources
  • Unsealed sources

  
• Beam sources


  • Deliver a uniform, well-defined, and stable beam
  • The particles are pre-selected for correct penetration depth as per the needs of each individual case
• In the last 25 years, computerization and intensity modulated beam delivery has become important in radiotherapy treatment machines
• Beam sources


  • Produce a beam of ionizing radiation aimed at the tumor(s) during treatment sessions

  
• Brachytherapy sources


  • Sealed radioactive sources are placed in proximity with the tumor
• Unsealed sources


  • Radionuclides are taken into the body in liquid form, either by injection or swallowing
• Beam sources are the most widely used type
• Beam source


  • Delivers a uniform, well-defined, and stable beam
  • The particles are pre-selected for correct penetration depth as per the needs of each individual case
  • This ensures that the beam reaches only the regions intended and delivers the desired dose
• Since the beginning of radiotherapy, the aim was to produce ever higher photon and electron beam energies and intensities

  
• In the last 25 years, computerization and intensity modulated beam delivery has become important
• During the first 50 years of radiotherapy the technological progress was relatively slow
• The invention of the 60Co teletherapy unit in the early 1950s provided a tremendous boost
• This placed the cobalt unit at the forefront of radiotherapy for a number of years
• The parallel developed medical LINACS, soon eclipsed cobalt units and became the most widely used radiation source
• LINAC


  Linear Accelerator
• LINAC


  • Offers excellent versatility and provides either electron or megavoltage X ray therapy with a wide range of energies
• Other types of accelerator used for electron and X ray radiotherapy

  • Betatrons
  • Microtrons
• More exotic particles, like protons, neutrons and heavy ions are produced by special accelerators and are also sometimes used for radiotherapy, however, most contemporary radiotherapy is carried out with LINACS or teletherapy cobalt units
• Superficial X rays


  50 to 150 kV
• Orthovoltage X rays


  150 to 500 kV
• Radiotherapeutic X ray machine


  • Main components: X ray tube, ceiling or floor mount for the X ray tube, target cooling system, control console, and an X ray power generator
• Teletherapy machine


  • Main components: a radioactive source, a source housing with beam collimator and source movement mechanism, a gantry and stand or a housing support assembly in stand-alone machines, a patient support assembly and a machine console
• Teletherapy source


  Most widely used is 60Co radionuclides contained inside a cylindrical stainless steel capsule and sealed by welding
• Teletherapy source


  • Standard source capsules have been developed to enable swapping of sources from one teletherapy machine to another and from one isotope production facility to another
  • Typical diameter is between 1 and 2 cm with a height of about 2.5 cm
  • Smaller diameter results in smaller physical penumbra but is more expensive
• Typical teletherapy source activities

  5000–10 000 Ci (185–370 TBq)
• Typical dose rate at 80 cm from the teletherapy source

  1–2 Gy/min
• Teletherapy sources are usually replaced within one half-life after they are installed, but financial considerations often result in longer source usage