8.14 Radiation Therapy
315
of the DNA and some effects of radicals, the longer-term damage to cells depends
non-linearly on the intensity of the radiation exposure. Radiation may also induce
apoptosis (cell suicide).
If a cancer is concentrated in a tumor, then radiation can be focused at the tumor
location from a set of angles, reducing damage to healthy tissue. For tumors on
the skin, ‘superficial X-ray’ therapy can be employed, using low energy X-rays
(30–60 keV) which do not penetrate far below the skin (less than 4 mm). Lowenergy X-rays can be produced by X-ray tubes (using Bremsstrahlung in a metal
anode). ‘Orthovoltage X-rays’ (also called deep X-rays), having photon energies
200–500 keV, will penetrate up to 6 cm below the skin. Orthovoltage X-rays can
also be produced in X-ray tubes, but with special construction to isolate the high
voltages used. X-rays produced by the inelastic scattering of electrons accelerated
to millions of electron volts are referred to as ‘megavoltage X-rays’, and are used to
reach tumors on internal organs.
Localized radiation dosages range from 20 to 80 Gy, often ‘fractionalized’ over
a series of lower dose sessions.
8.14.2 Particle Beam Therapy
Cancer therapy using heavy-particle beams have a distinct advantage over X-ray
beams in that the energy loss due to ionization by a charged heavy particle has a
peak near the end of the particles motion (when it is slowed so much that it no
longer moves forward), at a location in dE/dx as a function of x called the Bragg
peak introduced in Sect. 8.13.3.
A common form of particle-beam therapy uses protons. Heavier particle accelerators, such as for 12 C nuclei, are more expensive, but have advantages over protons:
They have a stronger Bragg peak and the beam is not scattered from its direct line
as much. All such heavy-particle beams have the risk of producing new cancer in
healthy tissue along the beam track.
8.14.3 Systemic Radiation Therapy
Systemic radiation therapy refers to targeted radioactive-substance ingestion or
infusion.
Cancer of the thyroid can be treated by the injection or ingestion of a high dose
of iodine-131, which is partly taken up by the thyroid. Iodine-131 is radioactive,
with a physical half-life of 8.02 days, emitting an electron of maximum energy
either 333.8 keV (7.27% of the decays) or 606.3 keV (89.8% of the decays) and
then a gamma ray (of energy either 637.0 or 364.5 keV, resulting in xenon-131. It
is the electron that does most of the damage to nearby tissue. Longtime exposure
to low doses of iodine-131 can induce cancers. For patients with a fully functional
315
of the DNA and some effects of radicals, the longer-term damage to cells depends
non-linearly on the intensity of the radiation exposure. Radiation may also induce
apoptosis (cell suicide).
If a cancer is concentrated in a tumor, then radiation can be focused at the tumor
location from a set of angles, reducing damage to healthy tissue. For tumors on
the skin, ‘superficial X-ray’ therapy can be employed, using low energy X-rays
(30–60 keV) which do not penetrate far below the skin (less than 4 mm). Lowenergy X-rays can be produced by X-ray tubes (using Bremsstrahlung in a metal
anode). ‘Orthovoltage X-rays’ (also called deep X-rays), having photon energies
200–500 keV, will penetrate up to 6 cm below the skin. Orthovoltage X-rays can
also be produced in X-ray tubes, but with special construction to isolate the high
voltages used. X-rays produced by the inelastic scattering of electrons accelerated
to millions of electron volts are referred to as ‘megavoltage X-rays’, and are used to
reach tumors on internal organs.
Localized radiation dosages range from 20 to 80 Gy, often ‘fractionalized’ over
a series of lower dose sessions.
8.14.2 Particle Beam Therapy
Cancer therapy using heavy-particle beams have a distinct advantage over X-ray
beams in that the energy loss due to ionization by a charged heavy particle has a
peak near the end of the particles motion (when it is slowed so much that it no
longer moves forward), at a location in dE/dx as a function of x called the Bragg
peak introduced in Sect. 8.13.3.
A common form of particle-beam therapy uses protons. Heavier particle accelerators, such as for 12 C nuclei, are more expensive, but have advantages over protons:
They have a stronger Bragg peak and the beam is not scattered from its direct line
as much. All such heavy-particle beams have the risk of producing new cancer in
healthy tissue along the beam track.
8.14.3 Systemic Radiation Therapy
Systemic radiation therapy refers to targeted radioactive-substance ingestion or
infusion.
Cancer of the thyroid can be treated by the injection or ingestion of a high dose
of iodine-131, which is partly taken up by the thyroid. Iodine-131 is radioactive,
with a physical half-life of 8.02 days, emitting an electron of maximum energy
either 333.8 keV (7.27% of the decays) or 606.3 keV (89.8% of the decays) and
then a gamma ray (of energy either 637.0 or 364.5 keV, resulting in xenon-131. It
is the electron that does most of the damage to nearby tissue. Longtime exposure
to low doses of iodine-131 can induce cancers. For patients with a fully functional