Extended Introduction
From MedPhysWiki
- The study and use of ionizing radiation in medicine started with three important discoveries:(Figure 5.1.1)
- X rays by Wilhelm Roentgen in 1895.
- Natural radioactivity by Henri Becquerel in 1896.
- Radium-226 by Pierre and Marie Curie in 1898.
- Immediately upon the discovery of x rays and natural radioactivity, ionizing radiation has played an important role in:
- Atomic and nuclear physics from the basic physics point of view.
- In medicine providing an impetus for development of radiology and radiotherapy as medical specialties and medical physics as a specialty of physics.
- In industry offering many non-destructive measurement techni-ques and special techniques used in evaluation of oil fields.
- In agriculture providing food sterilization and pest control.
- During the first 50 years of radiation medicine the techno-logical progress was aimed mainly towards:
- Development of analog imaging techniques.
- Optimization of image quality with concurrent minimization of dose.
- Ever increasing energies and beam intensities.
- During the past two decades most developments in radia-tion medicine were related to:
- Integration of computers in imaging
- Development of digital imaging techniques
- Incorporation of computers into therapeutic dose delivery with high energy linear accelerators (linacs).
- Roentgen discovered x rays in 1895 while experimenting with a Crookes “cold cathode” tube. (Figure 5.1.2)
- Crookes tube is a sealed glass cylinder with two embedded electrodes operated with rarefied gas.
- The potential difference between the two electrodes produces discharge in the rarefied gas causing ionization of gas molecules.
- Electrons (cathode rays) are accelerated toward the positive electrode producing x rays upon striking it.
- Coolidge in 1913 designed a “hot cathode” x ray tube and his design is still in use today.(Figure 5.1.3)
- The main characteristics of the Coolidge tube are its high vacuum and its use of heated filament (cathode).
- The heated filament emits electrons through thermionic emission.
- X rays are produced in the target (anode) through radiation losses of electrons producing characteristic and bremsstrahlung photons.
- The maximum photon energy produced in the target equals the kinetic energy of electrons striking the target.
- The invention of the cobalt-60 teletherapy machine by Harold E. Johns in Canada in the early 1950s provided a tremendous boost in the quest for higher photon energies and placed the cobalt unit at the forefront of radiotherapy for a number of years.
- Most modern cobalt therapy machines are arranged on a gantry so that the source may rotate about a horizontal axis referred to as the machine isocentre axis.
- The source-axis distance (SAD) is either 80 or 100 cm.
- Cobalt-60 isocentric teletherapy machine built in the 1970s and 1980s by Atomic Energy of Canada, Ltd. (Figure 5.1.4)
- Source-axis distance = 80 cm
- Field size: from 5x5 cm2 to 35x35 cm2.
- At about the same time as cobalt machines clinical linacs were developed. They allowed even higher x-ray energies, eventually eclipsed the cobalt machines and became the most widely used radiation source in modern radiotherapy.
- With its compact and efficient design, linac offers excellent versatility for use in radiotherapy through isocentric mounting and provides either electron or x-ray therapy with megavoltage beam energies.
- Standard machines used for modern radiotherapy:
- X-ray machine:
- Superficial x-ray machine: 50 - 80 kVp
- Orthovoltage x-ray machine: 80 - 350 kVp
- Cobalt-60 teletherapy machine
- Linear accelerator (linac):
- Megavoltage x rays: 6 - 25 MV
- Electrons: 6 - 30 MeV
- X-ray machine:
- Specialized machines used for modern radiotherapy:
- Microtron: megavoltage x rays and electrons
- Betatron: megavoltage x rays and electrons
- Neutron machines
- Neutron generator: (d,t) machine producing 14 MeV neutrons
- Cyclotron accelerating protons
- Proton machines
- Cyclotron
- Synchrotron
