Radiation Therapy Part I

Penumbra

a dose transition region near the borders of the field. Penumbras are of three kinds: geometric, transmission, and physical

free-air ionization chamber

a standard ion chamber that can measure exposure for relatively low-energy photon beams in which electronic equilibrium can be achieved in free air. It measures exposure in accordance with its definition.

Ion recombination correction

depends on chamber design, bias voltage, beam type (photons or electrons), beam intensity, and whether the beam is pulsed, pulse scanned, or continuous (as in cobalt-60).

Half Value Layer

the thickness of an absorber required to attenuate the intensity of the beam to half its original value. • HVL and attenuation coefficient are related by: HVL = 0.693/μ.

T+P Correction factor

- For a chamber that is open to the outside temperature and pressure of air, the reading must be corrected by a factor given by C(T,P)=(760/P)x(273.2+t / 295.2) - This correction factor is relative to the environmental conditions specified at the standards laboratory.

good geometry

- HVL must be measured under these conditions - a narrow beam and a large distance between absorber and detector in order to avoid measurement of scattered radiation

Polarity effect

- If the chamber reading changes with change in polarity of its bias voltage - should be less than 0.5% for a well-designed chamber.

photoelectric effect

- Photoelectric effect involves complete absorption of photon energy by the atom and transferring that energy to an orbital electron, which is ejected. - The process results in the emission of photoelectron, characteristic x-rays (fluorescent radiation), and Auger electrons. • Photoelectric probability varies as 1/E3 and Z3. • Photoelectric effect in water (or soft tissue) is predominant for photon energies of 10 to 25 keV (average energies of x-ray beams generated at 30-75 kVp).

electrometer

- The electrometer is a charge-measuring device. - The most commonly used electrometers are negative-feedback operational amplifiers. They are connected to the ion chamber through a shielded cable.

Photodisintegration

- involves a photon creating a nuclear reaction - In most cases it results in the emission of a neutron. The process is only important at high photon energies and is responsible for neutron contamination of therapy beams of energy greater than 10 MV.

Coherent scattering

- involves no net loss of energy. - This type of interaction is probable with low-energy photons and high-Z materials—not important for radiation therapy beams interacting with body tissues, which have low Z.

Activity

A of a radioactive element is the rate of disintegration or decay and is given by: A = A0 e-lt, where A is activity at time t, A0 is activity at the start of time t, and λ is the disintegration constant. • The SI unit for activity is the becquerel (Bq). 1 Bq = 1 disintegration/sec (dps). • A practical unit of activity is the curie (Ci). 1 Ci = 3.7 × 1010 dps. • Activity of 1g of radium is 0.975 Ci.

secondary chambers

Chambers that require calibration by a standard chamber (e.g., condenser chambers, Farmer chambers).

peak V on x-ray tube

Peak voltage on an x-ray tube = √2. line voltage · transformer turn ratio. • Rectifiers conduct electrons in one direction only and can withstand reverse voltage up to a certain magnitude. Full-wave rectification increases effective tube current.

photon beam attenuation

characterized by attenuation coefficient μ. For a narrow monoenergetic beam, the attenuation is given by: I(x) = I0e^(-mx).

μen

energy absorption coefficient - the product of energy transfer coefficient and (1 - g) where g is the fraction of the energy of secondary charged particles that is lost to bremsstrahlung in the material

Exposure calibration factor

factor of a secondary ion chamber that converts its reading (corrected for conditions different than those specified in the chamber calibration certificate) into exposure in roentgens in dry air in the absence of a chamber.

μtr

the energy transfer coefficient - the fraction of photon energy transferred into kinetic energy of charged particles per unit thickness of absorber

Elements series

• All of the 92 naturally occurring elements (Z = 1 to 92) have been grouped together into three series: uranium, actinium, and thorium. The rest (Z = 93 to 117) are produced artificially

focal spot

• Apparent side of the focal spot size α is given by: α = A sin u, where A is the side of the actual focal spot presented at an angle u with respect to the perpendicular to the direction of electron beam - Apparent focal spot size ranges from 0.1 × 0.1 mm to 2 × 2 mm for imaging, and 5 × 5 mm to 7 × 7 mm for orthovoltage therapy tubes.

Bremsstrahlung

• Bremsstrahlung x-rays have a spectrum of energies. The maximum energy is numerically equal to the peak voltage. Average energy is about one third of the maximum energy.

Characteristic x-rays

• Characteristic x-rays have discrete energies, corresponding to the energy level difference between shells involved in the electron transition.

Direct/indirect ionizing radiation

• Charged particles are directly ionizing radiation. Uncharged particles such as neutrons and photons are indirectly ionizing—they liberate directly ionizing particles that are responsible for producing ionization and excitation of atoms.

Charged particle interactions

• Charged particles interact primarily by ionization and excitation. Radiative collisions (bremsstrahlung) are possible but more likely for electrons than heavier charged particles. • All charged particles exhibit Bragg peak near the end of their range. Bragg peak is not observed in electron beams because of excessive scattering and smearing of the Bragg peaks.

Compton Scattering

• Compton effect involves photon interaction with a "free electron" (loosely bound electron—binding energy much less than the incident photon energy). • Compton interaction probability in water increases with photon energy from 10 to 150 keV. It then decreases with further increase in energy. However, it is the predominant mode of interaction in water for 30 keV to 24 MeV. That includes all x-ray beams used in radiation therapy. • Compton probability is almost independent of Z. It depends on electron density (number of electrons per cm3). • Maximum energy of a photon scattered at 90 degrees is 0.511 MeV, and at 180 degrees it is 0.255 MeV.

Chamber Types

• Condenser chambers (e.g., Victoreen R meters) may be used for exposure measurements up to cobalt-60. They are not suitable for higher-energy beams. • Farmer or Farmer-type chambers can be used to calibrate all beam energies used in therapy. • Extrapolation and parallel-plate (or plane-parallel) chambers are suitable for measuring surface dose or dose in the buildup region where dose gradients are high

Linac: Dual scattering foil

• Dual scattering foil (in position for the electron mode). Function of scattering foil is to spread the electron beam as well as make it uniform in cross section.

Equivalent energy of x-ray beam

• Effective or equivalent energy of an x-ray beam is the energy of a monoenergetic photon beam that has the same HVL as the given beam. • Energy spectrum of an x-ray beam can be measured by scintillation spectrometry. The spectrum may be displayed in terms of photon fluence per unit energy interval as a function of photon energy.

Electron Capture

• Electron capture is a process in which a nucleus captures an orbital electron, thus transforming one of its protons into a neutron • Electron capture creates a vacancy in the electron orbit involved, which, when filled by an outer orbit electron, gives rise to characteristic x-rays (fluorescent radiation) and Auger electrons. The process is likened to "internal photoelectric effect."

radioactivity

• Emission of radiation from a nucleus in the form of particles, γ rays, or both

Exposure

• Exposure is a measure of ionization in air produced by photons. • The unit of exposure is the roentgen (R): 1R = 2.58 × 10-4C/kg of air.

Linac: Flattening filter

• Flattening filter (in position for the x-ray mode). Function of flattening filter is to make the x-ray beam intensity uniform across the field.

Fluorescent yield

• Fluorescent yield is Z dependent, increasing from lower Z to higher Z - ratio of characteristic x-rays produced to x-rays + Auger electrons

Geometric penumbra

• 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.

Half/Mean Life

• Half-life T1/2 and λ are related by: T1/2 = 0.693/λ. • Average or mean life Ta = 1/λ = 1.44 T1/2.

heat absorption in anode

• Heat generated in the target must be removed to prevent target damage (e.g., using copper anode to conduct heat away, rotating anode, fans, and oil bath around the tube). The function of the oil bath is to provide electrical insulation as well as heat absorption.

Radioactive Equilibrium

• If half-life of the parent nuclide is larger than that of the daughter nuclide, a condition of equilibrium occurs after a certain amount of time. At equilibrium the ratio of daughter activity to parent activity becomes constant.

Internal Conversion

• Internal conversion is a process in which a nucleus in the excited state transfers its excess energy to one of the orbital electrons, causing it to be ejected from the orbit. The ejected electron creates a vacancy in the involved shell and, as mentioned in the electron capture process, causes the emission of characteristic x-rays (fluorescent radiation) or Auger electrons.

Isomeric transition

• Isomeric transition involves an excited nucleus in the metastable state decaying to the ground state Example: 99mTc decaying to 99Tc with a half-life of 6 hours.

Clinical Radiation Generators

• 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]).

LINAC

• Linear accelerators are energized by microwaves of frequency ∼3,000 MHz. • The major components are as follows: power supply, modulator (pulse-forming network), hydrogen thyratron (switch tube), magnetron (microwave generator) or klystron (microwave amplifier), wave guide 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, and treatment head.

Microtron

• Microtron combines the principle of linear accelerator and cyclotron. • Beam characteristics are similar to the linear accelerator. The difference is primarily in the electron beam acceleration technology and electron transport. Treatment heads are similarly equipped.

particle production

• 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.

Neutrons interactions

• Neutrons interact by ejecting recoil protons or producing nuclear disintegrations. • Lead is an efficient absorber of x-rays but not of neutrons. The most efficient absorber of neutrons is a hydrogenous material such as water, paraffin wax, and polyethylene

Nuclear Fission

• Nuclear fission is a process of splitting high Z nucleus into two lower Z nuclei. - The process results in the release of a large amount of energy. - Example: fission of 235U nucleus by bombarding it with thermal neutrons (i.e., neutrons of energy <0.025 eV). - A chain reaction is possible with a critical mass of fissionable material.

Nuclear fusion

• Nuclear fusion is the reverse of nuclear fission—lighter nuclei are fused together into heavier ones. Again, a large amount of energy is released in the process. • Fusion of hydrogen nuclei into helium nuclei is the source of our sun's energy.

Nuclear Reactions

• Nuclear reactions can be produced by bombarding heavier nuclides with lighter nuclides or particles. • Examples of bombarding particles are α particles, protons, neutrons, deuterons, and γ-ray photons. • Photodisintegration process is responsible for contamination of the high-energy x-ray beams generated by linear accelerators. • Radioactive sources used in radiation therapy are produced by bombarding nuclides in nuclear reactors or particle accelerators.

x-ray output

• Output (exposure rate) of an x-ray machine is very sensitive to the filament current. - With tube current it increases proportionally and with voltage it increases approximately as a square of the voltage.

Pair production

• Pair production involves a high-energy photon interaction with the electromagnetic field of a nucleus. Photon energy is all used up in creating a pair of electron (e-) and positron (e+) and providing it with kinetic energy. • The threshold energy for pair production is 1.02 MeV—just enough to create the electron-positron pair. • Pair production probability increases slowly with energy beyond 1.02 MeV. It increases from about 6% at 4 MeV to 20% at 7 MeV (average energies of 12-21 MV x-ray beams). • Pair production coefficient varies approximately as Z2 per atom, Z per electron, and Z per gram. • The reverse of pair production process is the electron-positron annihilation, giving rise to two photons each of 0.511 MeV ejected in opposite direction.

MV measuring

• Peak energy (MV) of a megavoltage x-ray beam can be measured directly by scintillation spectrometry or by photoactivation of appropriate foils (e.g., PAR method). Most commonly used methods, however, are indirect, such as comparing measured percent depth dose distribution in water with published data.

kVp measuring

• Peak voltage (kVp) applied to an x-ray generator can be measured directly (e.g., voltage divider, sphere-gap method) or indirectly (e.g., fluorescence, attenuation, or a penetrameter device such as an Adrian-Crooks cassette).

photon beam interactions

• Photon beams interact with matter through five major processes: coherent scattering, photoelectric effect, Compton effect, pair production, and photodisintegration.

Physical penumbra

• 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%-20%). It is influenced by geometric penumbra, beam energy, and the lateral transport of electrons in the tissues.

Linac: Collimators

• Primary collimators provide 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 modulation of the beam in the intensity modulated radiotherapy mode. • Monitor chambers (dual flat ion chambers) monitor dose delivery (when calibrated) and beam flatness. • Electron applicators (in the electron mode) collimate the electron beam close to the patient surface (about 5 cm away). They are interlocked for the choice of electron mode as well electron beam energy.

Bragg peak

• Protons and heavier charged particles exhibit Bragg peak. Bragg peak for negative pions is accentuated because of pion capture by nuclei—a process called star formation.

Secular Equilibrium

• Secular equilibrium occurs when the half-life of the parent is much longer than that of the daughter (e.g., decay of 226Ra to 222Rn). At secular equilibrium, A2 = A1.

x-ray production efficiency

• The higher the energy of electrons bombarding the target, the more forward is the direction of x-ray emission. • Efficiency of x-ray production is proportional to the atomic number (Z) of target and voltage applied to the tube. - Efficiency is less than 1% for x-ray tubes operating at 100 kVp (99% of input energy is converted into heat). - Efficiency improves considerably for high-energy accelerator beams (30%-95%, depending upon energy).

Linac Treatment head

• The treatment head is shielded by lead, tungsten, or lead-tungsten alloy.

x-ray tube

• The x-ray tube is highly evacuated to prevent electron interactions with air. • The choice of tungsten for filament (cathode) and target (anode) is based on its having a high melting point (3,370°C) and a high atomic number (Z = 74), which is needed to boost efficiency of x-ray production. • Function of the hooded anode (tungsten + copper shield around target) is to prevent stray electrons from striking the nontarget components of the tube and absorbing bremsstrahlung as a result of their interactions.

Transient equilibrium

• Transient equilibrium occurs when the half-life of the parent (T1) is not much longer than that of the daughter (T2) (e.g., decay of 99Mo to 99mTc). At transient equilibrium, the daughter activity A2 and the parent activity A1 are related by: A2 = A1 × T1/(T1 - T2).

Transmission penumbra

• Transmission penumbra is caused by variable transmission of beam through nondivergent collimator edge.

Linac Tungsten target

• Tungsten target (in position for the x-ray mode). Focal spot size is ∼2 to 3 mm in diameter.

x-ray production

• X-rays are produced by two different mechanisms: bremsstrahlung and characteristic x-ray emission. Useful x-ray beams in imaging and therapy are all bremsstrahlung.

Quality of x-ray beams

• is specified by kVp, filtration, and HVL (for diagnostic, superficial, and orthovoltage beams); and MV and percent depth dose in water (for megavoltage x-rays). • Quality of cobalt-60 beams is designated as cobalt-60 because it is known that they all have the same energy, namely γ rays of 1.17 and 1.33 MeV.

Modes of Decay

• α particles are helium nuclei and are emitted by high-atomic-number radionuclides (Z > 82). • β- particle is a negatively charged electron (negatron) emitted from a nucleus. • β+ particle is a positively charged electron (positron) emitted from a nucleus. • β particle does not exist as such in the nucleus but is emitted at the instant of a neutron or a proton decay in the nucleus:

β particles

• β particles are emitted with a spectrum of energies, ranging from zero to a maximum. They share the available kinetic energy with the accompanying neutrino. • The average energy of β particles is about one third of the maximum energy.

attenuation coefficeint

• μ, μtr, and μen are parameters that respectively characterize photon beam attenuation, energy transfer, and energy absorption as the beam traverses a medium.