Radiation Oncology/Physics/Radiation Interactions

Radiation Interactions

Atomic Physics

Radiation Types

• Electromagnetic rays (photons)
  • Duality of a particle (photon) and a wave (mutually perpendicular electric and magnetic waves)
  • X-rays and Gamma-rays are identical in physical characteristics and biological effect, but differ in their origin:
    • X-rays - originate outside of nucleus, typically as excess energy shed by an incoming electron when bending around a nucleus
    • Gamma-rays - originate within the nucleus during radioactive decay or fluorescence
  • Ionizing radiation is that with photon energies > 1 keV
  • Shielding by dense high-Z metals (lead, depleted uranium)

• Electrons, Beta-rays, Positrons
  • Very light, in tissue do not travel in straight lines but are deflected by coulombic repulsions from atomic orbital electrons
  • Lose on average 50% of their energy on interaction
  • As a result, have a well-defined penetration "range" in tissue, even though their path is not straight but zig-zaggy = CSDA (continuous slowing down approximation) range
  • For 12 MeV electrons, this is couple centimeters

• Protons
  • Generated by cyclotron beams (e.g. MGH, Loma Linda)
  • Because they are heavier than electrons, travel mainly in a straight line by "boring" a path through atomic electron clouds
  • Toward end of their path rapidly increase interaction with incident atoms, and rapidly slow down, resulting in Bragg Peak
  • Shielding by medium-Z metals to provide electrons to slow the protons down. High-Z materials should be avoided, as they produce secondary x-rays and neutrons

• Alpha Particles
  • Produced by radioactive decay of heavy radionuclides
  • At same energy, travel much more slowly than protons (due to much higher mass), with effective range of couple mm
  • Well absorbed by skin (dead layer of cells) but dangerous in mucous membranes (ingestion, inhalation)
  • Experimentally used in RT
  • Shielding by low-Z materials

• Neutrons
  • Interact by colliding with protons (Hydrogen nucleus mainly)
  • Shielding by combination of highly hydrogenous (concrete) and highly absorbing (boron-10) materials

Interaction of X-rays / Gamma-rays with Tissue

• Excitation
  • Inner shell electron is imparted with sufficient energy to "jump up" to a higher energy shell, but not enough to separate from the nucleus
  • It then immediately "jumps down" to its original shell to fill the vacancy, and in the process sheds the excess energy as EM radiation (photon)
  • This radiation is characteristic for a given element (since it is a function of the difference in shell energy levels), and is called characteristic radiation
  • The chemistry of the atom is ultimately unchanged, but biological changes may be induced by disturbances of the electron cloud geometry

• Ionization
  • Inner shell electron is imparted with sufficient energy to separate from the nucleus (> binding energy)
  • Higher shell electron "jumps down" to fill the vacancy, and in the process sheds its excess energy as EM radiation. The EM energy is the same as the energy difference between the shells
  • In large atoms, yet higher shell electrons may then "jump down" in a cascade, resulting in multiple EM emissions
  • Once a valence shell electron "jumps down", the atomic is an electron short, and becomes a free radical
  • The emitted x-ray(s) are called characteristic x-rays, and are unique to each element
  • Grand average of binding energies across tissue elements and across different shells = 33 KeV (meaning, on average it takes 33 KeV to eject an electron from an atom, and thus ionize the atom)

X-rays/Gamma-rays interact with tissue in 5 different ways:

• Coherent Scattering
  • Only significant at lowest diagnostic x-ray energies (<5% interactions)
  • Incoming photon is deflected (absorbed and immediately re-emitted), with minimal direction and energy change
  • Can cause problems with contrast in mammography, but not relevant in treatment energies

• Photoelectric Absorption
  • Strongly enhanced in high-Z tissues such as bone, or at lower EM energies (thus not very important at treatment energies)
  • Incoming photon "sees" innermost shell electrons (85% K-shell, 14% L-shell, 1% rest), and is absorbed by them
  • Depending on the absorbed energy, this can cause the electron to be excitated to a higher shell or ejected from the atom in ionization. If ejected, it is called photo-electron and causes projectile damage in tissue
  • The vacancy is filled (see above in Ionization) from higher shells, leading to characterist x-rays
  • These characteristic x-rays may be immediately re-absorbed by the same atom's electrons (common). If they have sufficient energy, may excite or eject another electron in a second ionization event. If ejected, it is called Auger electron and causes projectile damage in tissues
  • The remaining atom may become a free radical, if valence electrons are affected

• Compton Scattering
  • Most dominant interaction in soft tissues at treatment energies
  • Incoming photon "sees" individual outermost electrons, and bounces off them like a billiard ball. It is then called Compton photon, and can undergo additional interactions
  • In the process, it ejects the electron from the orbit in an ionization event, which causes projectile damage in the tissue. Accounts for ~75% of radiation damage
  • The atom becomes a free radical, causing biological damage in the tissue. Accounts for ~25% of radiation damage
  • Probability of a Compton interaction is inversely proportional to energy of the incoming photon.
  • It is independent of atomic number, so at treatment energies, bone and soft-tissue interfaces are barely distinguishable (= poor contrast)
  • At diagnostic x-ray energies, Compton Scattering direction is fairly random; at treatment x-ray energies, it is forward-peaked
  • Energy loss at interaction <20%, so incoming electron can interact numerous times before losing sufficient energy to be absorbed via photoelectric absorption (below)
  • This is the main tissue effect at energies 30 keV - 30 MeV

• Pair Production
  • Occurs only at high photon energies (>1.02 MeV) and preferentially in high-Z tissues
  • Incoming photon (energy) is converted to mass (electron and positron) in the vicinity of atomic nucleus via E=mc2
  • Additional energy over 1.02 MeV is converted to kinetic energy, and divided between the 2 particles
  • Positron then interacts with some electron (not necessarily from same atom), and is converted back into 2 EM photon at 0.51 MeV each
  • Electron causes projectile damage in the tissue
  • Significant pair production can be seen in blocking of the oncoming beam, since blocks are high-Z materials (for lead, this is the main effect at energies >5 MeV)

• Photodisintegration
  • Occurs only at very high photon energies (>7 MeV)
  • Incoming photon deposits so much energy, that the nucleus disintegrates
  • Source of low-level neutron production (important for radiation shielding)

• Interaction Summary at Therapy Energies
  • Coherent Scattering - no real effect
  • Photoelectric Absorption - smaller effect: 1) free electron (photo-electron), 2) +/- free electron (Auger electron), 3) free radical atom
  • Compton Scattering - main effect: 1) free electron (Compton electron), 2) free radical atom
  • Pair Production - smaller effect: 1) free electron, 2) two 0.51 MeV photons
  • Photodisintegration - no real effect

Interaction of Electrons with Tissue

• Same process as described above in "Generating X-rays"
• Two fundamental interactions:
  • Radiation (Bremsstrahlung) - bending of electrons around nucleus => shedding of energy as EM x-rays
  • Ionization (Characteristic X-rays) - impact with orbital electron => electron release => vacancy fill => shedding of energy as Characteristic x-rays
• Any given electron can in a single interaction lose a very small or very large fraction of its energy, and be deflected by a very small or very largy amount. This leads to large variation among incoming electrons in their path (and distance) into the tissue (range-straggling)

Interaction of Protons with Tissue

• Incoming protons also lose energy mainly by interacting with orbital electrons; however, since they are much heavier (~1800x), they only lose very small fraction of their kinetic energy with each interaction, and thus scatter only minimally
• The interactions (and thus energy loss) become more frequent at slower energies. Thus the slower the proton moves, the more energy it loses to the tissue electrons, in a feed-forward loop, until it abruptly loses all energy. This region of rapid energy loss (and its deposition into the tissue) is called the Bragg peak.
• The distance at which Bragg peak occurs, and the energy is deposited, can be calculated very precisely (unlike electrons). The rapid drop-off in dose make it ideal for delivering dose precisely to the tumor, and not to the healty tissue beyond the tumor.
• Incoming protons also rarely interact with the nucleus, and may enhance cell kill by ~10% (PMID 472125 [No abstract] - "The determination of absorbed dose in a proton beam for purposes of charged-particle radiation therapy." Verhey LJ, Radiat Res. 1979 Jul;79(1):34-54.)

Interaction of Heavy Ions with Tissue

• Several heavy ions (helium, argon nuclei) have been tested clinically
• They interact with tissue similarly to protons, but since they are heavier still, they scatter less initially, and have a faster dose fall-off (Bragg peak) at the end
• They also interact with nuclei more often, and may lose a large amount of their large kinetic energy in a single interaction. This high energy deposition is much more efficient per unit dose than x-rays in killing cells (high relative biologic effectiveness = RBE)

Interactions of Neutrons with Tissue

• Because neutrons are heavy and neutral, they only lose energy by interacting with nuclei. Similarly to heavy ions, this leads to a sudden large loss (deposition) of energy in a single event. They also have a high relative biologic effectiveness (RBE)
• Similarly to x-rays, the fall-off in dose is exponential (because the interactions are random, unlike proton/ion interactions with increase with slowing velocity). In tissues, 50% of dose is at ~10 cm