Clinical Radiation Generators
C. NEGATIVE PIONS
The existence of pi mesons was theoretically predicted by Yukawa in 1935 when he postulated that protons and neutrons in the nucleus are held together by a mutual exchange of pi mesons.
A pi meson (or pion) has a mass 273 times that of electron and may have a positive charge, a negative charge, or may be neutral. The charged pions decay into mu mesons and neutrinos with a mean life of 2.54 × 10−8 seconds and the neutral pions decay into pairs of photons with a mean life of about 10−16 seconds.
10 20
Range in water (cm)
Proton energy (MeV)
30 00
20 40 60 80 100 120 140 160 180 200
Figure 4.19. range–energy relationship for protons. (From raju Mr. Heavy Particle Radiotherapy. New York, NY: academic Press; 1980, with permission.)
56 Part I Basic Physics
pSmn pSmv
p0Shn1hn2 Only negative pions have been used for radiation therapy.
Beams of negative pions can be produced in a nuclear reaction. Protons of energy in the range of 400 to 800 MeV, produced in a cyclotron or a linear accelerator, are usually used for pion beam production for radiotherapy. Beryllium is a suitable target material. Pions of positive, negative, and zero charge with a spectrum of energies are produced and negative pions of suit- able energy are extracted from the target using bending and focusing magnets. Pions of energy close to 100 MeV are of interest in radiation therapy, providing a range in water of about 24 cm.
The Bragg peak exhibited by pions is more pronounced than other heavy particles because of the additional effect of nuclear disintegration by π− capture. This phenomenon, commonly known as star formation, occurs when a pion is captured by a nucleus in the medium near the end of its range. A pion capture results in the release of several other particles such as protons, neutrons, and a particles.
Although pion beams have attractive radiobiologic properties, they suffer from the problems of low dose rates, beam contamination, and high cost.
• Kilovoltage, supervoltage, Van de Graaff, betatrons, and cobalt-60 units have been largely replaced by linear accelerators. A few of these machines, however, are still in use, e.g., endocavitary x-rays (for rectal cancers), superficial x-rays (for skin cancers), and cobalt-60 γ rays (for head and neck cancers).
• Linear accelerator:
• Energized by microwaves of frequency ~3,000 MHz.
• Major components: power supply, modulator (pulse-forming network), hydrogen thyratron (switch tube), magnetron (microwave generator) or klystron (microwave amplifier), waveguide system (to conduct microwaves), electron gun, accelerator structure, circulator (to prevent reflected microwaves from reaching the microwave power source—magnetron or klystron), focusing coils, bending magnets, automatic frequency control (AFC), and treatment head.
• Treatment head:
• Shielded by lead, tungsten, or lead–tungsten alloy.
• Tungsten target (in position for the x-ray mode). Focal spot size ~2 to 3 mm in diam- eter.
• Dual scattering foil (in position for the electron mode). The function of scattering foil is to spread the electron beam as well as make it uniform in cross section.
• Flattening filter (in position for the x-ray mode). The function of the flattening filter is to make the x-ray beam intensity uniform across the field.
• Primary collimator provides a fixed maximum aperture for the x-ray beam.
• Secondary collimators (x-ray jaws) are movable and provide variable rectangular field sizes.
• Multileaf collimators provide irregularly shaped fields as well as intensity modula- tion of the beam in the IMRT mode.
• Monitor chambers (dual flat ion chambers) monitor dose delivery (when calibrated) and beam flatness.
• Electron applicators (in the electron mode) collimate electron beam close to the patient surface (~5 cm away). They are interlocked for the choice of electron mode as well electron beam energy.
K E Y P O I N t S
(continued)
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CHaPtEr 4 Clinical radiation Generators 57
• Microtron:
• Microtron combines the principle of linear accelerator and cyclotron.
• Beam characteristics are similar to linear accelerator. The difference is primarily in the electron beam acceleration technology and electron transport. Treatment heads are similarly equipped.
• Penumbra: a dose transition region near the borders of the field. Penumbras are of three kinds:
• Geometric penumbra is due to the finite dimensions of the source (or focal spot). Its width is proportional to source diameter. It increases with increase in SSD and depth and decreases with increase in SDD.
• Transmission penumbra is caused by variable transmission of beam through nondivergent collimator edge.
• Physical penumbra is the spread of dose distribution near field borders and is usually specified by the lateral width of isodose levels (e.g., 90% to 20%). It is influenced by geometric penumbra, beam energy, and the lateral transport of electrons in the tissues.
• Neutron beams are generated in D–T generators (deuterons bombarding tritium target) or cyclotrons (deuterons bombarding beryllium target).
• Proton, negative pion, and heavy particle beams are produced in cyclotrons or linear accelerators by bombarding appropriate targets with appropriate particles.
• Protons and heavier charged particles exhibit the Bragg peak. The Bragg peak for negative pions is accentuated because of pion capture by nuclei—a process called star formation.
K E Y P O I N t S
( c o n t i n u e d )R e f e r e n c e s
1. Paterson R. The Treatment of Malignant Dis- ease by Radium and X-Rays. Baltimore, MD:
Williams & Wilkins; 1963.
2. National Council on Radiation Protection and Measurements. Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies up to 10 MeV. Report No. 49. Washington, DC: National Council on Radiation Protection and Measurements; 1976.
3. Karzmark CJ, Nunan CS, Tanabe E. Medical Electron Accelerators. New York, NY: McGraw- Hill; 1993.
4. Podgorsak EB, Rawlinson JA, Johns HE.
X-ray depth doses from linear accelera- tors in the energy range from 10 to 32 MeV.
Am J Roentgenol Radium Ther Nucl Med.
1975;123:182.
5. Veksler VJ. A new method for acceleration of relativistic particles. Dokl Akad Nauk SSSR.
1944;43:329.
6. Reistad D, Brahme A. The microtron, a new accelerator for radiation therapy. In: The Third ICMP Executive Committee, ed. Digest of the
3rd International Conference on Medical Physics.
Götenborg, Sweden: Chalmers University of Technology; 1972:23.5.
7. Svensson H, Johnsson L, Larsson LG, et al.
A 22 MeV microtron for radiation therapy.
Acta Radiol Ther Phys Biol. 1977;16:145.
8. Rosander S, Sedlacek M, Werholm O. The 50 MeV racetrack microtron at the Royal Institute of Technology, Stockholm. Nucl Inst Meth. 1982;204:1-20.
9. Cormack DV, Johns HE. Spectral distribution of scattered radiation from a kilocurie cobalt 60 unit. Br J Radiol. 1958;31:497.
10. Johns HE, Cunningham JR. The Physics of Radiology. 3rd ed. Springfield, IL: Charles C Thomas; 1969:120.
11. International Commission on Radiation Units.
Determination of Absorbed Dose in a Patient Irradiated by Beams of x- or Gamma Rays in Radiotherapy Procedures. ICRU Report 24.
Washington, DC: International Commis- sion on Radiation Units and Measurements;
1976:54.
58
W
hen an x- or γ-ray beam passes through a medium, interaction between photons and matter can take place with the result that energy is transferred to the medium. The initial step in the energy transfer involves the ejection of electrons from the atoms of the absorbing medium.These high-speed electrons transfer their energy by producing ionization and excitation of the atoms along their paths. If the absorbing medium consists of body tissues, sufficient energy may be deposited within the cells, destroying their reproductive capacity. However, most of the absorbed energy is converted into heat, producing no biologic effect.
5.1. IONIZATION
Ionization is the process by which a neutral atom acquires a positive or a negative charge. Ion- izing radiations can strip electrons from atoms as they travel through media. An atom from which electron has been removed is a positive ion. In some cases, the stripped electron may sub- sequently combine with a neutral atom to form a negative ion. The combination of a positively charged ion and a negatively charged ion (usually a free electron) is called an ion pair.
Charged particles such as electrons, protons, and a-particles are known as directly ionizing radiation provided they have sufficient kinetic energy to produce ionization by collision1 as they penetrate matter. The energy of the incident particle is lost in a large number of small increments along the ionization track in the medium, with an occasional interaction in which the ejected electron receives sufficient energy to produce a secondary track of its own, known as a d ray. If, on the other hand, the energy lost by the incident particle is not sufficient to eject an electron from the atom but is used to raise the electrons to higher-energy levels, the process is termed excitation.
Uncharged particles such as neutrons and photons are indirectly ionizing radiation because they liberate directly ionizing particles from matter when they interact with matter. Ionizing pho- tons interact with the atoms of a material or absorber to produce high-speed electrons by three major processes: photoelectric effect, Compton effect, and pair production. Before considering each process in detail, we shall discuss the mathematical aspects of radiation absorption.
5.2. PHOTON BEAM DESCRIPTION
An x-ray beam emitted from a target or a g-ray beam emitted from a radioactive source consists of a large number of photons, usually with a variety of energies. A beam of photons can be described by many terms, some of which are defined as follows:
1. The fluence (Φ) of photons is the quotient dN by da, where dN is the number of photons that enter an imaginary sphere of cross-sectional area da:
dN
da (5.1)
2. Fluence rate or flux density (f) is the fluence per unit time:
fd
dt (5.2)
where dt is the time interval.
1 The process of collision is an interaction between the electromagnetic fields associated with the colliding particle and orbital electron. Actual physical contact between the particles is not required.