clinical radiation generators Flashcards
what are kilovoltage units?
-x-ray therapy was performed with kilovoltage units until the 1950s
-x-ray tubes producing x-rays in the keV energy range
-superficial therapy: x-rays produced at potentials between 50 and 150kV
-orthovoltage therapy: x-rays produced at potentials between 150 and 500 kV
what is the x-rays spectra?
-spectra describe the x-ray beam produced by accelerating electron
-a graph of the relative number of photons (or relative energy in each energy interval) as a function of photon energy
-continuous spectrum with discrete sharp spikes superimposed
what are depth dose curves compared to x-ray spectra?
a graph of dose (as percentage of max dose) as a function of depth into the material (eg. patient)
-while x-ray spectra describe the photons composing an x-ray beam, depth dose curves describe the deposition of energy within a material
-depth dose curves provide information about how the photon beam deposits dose in a patient
what do depth dose curves look like?
a=<20kV tube voltage
b= 40-50 kV tube voltage
c= superficial therapy unit
d=orthovoltage unit
e=cobalt therapy unit
what does depth curves describe?
-describe dose to different depths
skin (first 2 mm)
other organs at varying depths
tumors at varying depths
-cross-sectional imaging (eg CT) is therefore important to determine depth of organs/tumors
what is the maximum dose (kilovoltage unit)?
maximum dose is at or very near the surface
-for orthovoltage (curve d), the dose at 3cm depth is about 80% of the maximum dose
-to treat lesions deeper than 3cm, the surface dose (skin dose) would be very high
what are cobalt 60 units?
-has half-life of 5.26 years
-99.88% of disintegrations are emission of an electron with maximum energy of 0.313 MeV
-this leads to an excited state of nickel, which loses its excess energy by emitting two gamma rays with energies of 1.17 and 1.33 MeV
-ln 0.12% of disintegrations, an electron with max energy of 1.486 MeV is emitted, leading to the lowest excited state of nickel and a gamma ray of 1.332 MeV
what is the maximum dose of cobalt 60 units?
is at about 5mm
-dose at 0 depth (skin) is about 40% of max dose
-for cobalt 60 (curve e), the dose at 5 cm depth is still about 80% of the max dose
-when treating deeper lesions, the surface dose (skin dose) would still be less than the dose to the tumor
why are MeV photons so important?
-MeV photons are considered “skin-sparing” and are used to treat deeper tumors
-another important advantage of megavoltage photons is that most interactions occur by compton scatter, and therefore all tissues receive about the same dose (independent of Z)
what is the depth dose curve of cobalt 60 and linacs?
what does a cobalt 60 machine graph look like?
how do we produce cobalt?
60Coproduced by irradiating stable 59Co with neutrons in a nuclear reactor
-the electrons (Beta minus) are reabsorbed in the source capsule
-treatment beam is composed of 2 gamma ray energies, 1.17 and 1.33 MeV (in contrast to continuous x-ray spectrum)
-decay of the source affects treatment time
-the source size is 1-2cm
what is penumbra?
disadvantage of cobalt 60
-refers to the region at the edge of the photon beam over which the dose drops off rapidly
-it is not desirable in a therapy machine
-directly related to the size of the radiation source
what is the graph of a penumbra?
what are linear accelerators?
-use high-frequency microwaves to accelerate electrons to high energies (4 to 25 MeV range)
-the electrons are directed at a tungsten target and bremsstrahlung x-rays are created
-the focal spot size (x-ray source size) is 2 to 3 mm, resulting in a much smaller penumbra than a cobalt 60 unit
what are the basic components of a linear accelerator?
-accelerator tube= in which electrons are accelerated by microwaves
-modulator= that delivers high voltage pulses to the magnetron or klystron and to the electron gun
-magnetron or klystron= the source of the microwaves
-wave guide= delivers the microwaves to the accelerator tube
-electron gun= injects the electrons into the accelerator tube
-treatment head= includes the target, beam collimators and an ion chamber
what is in linear accelerators?
linac treatment head
-the target is composed of tungsten
-the primary collimators are composed of solid lead and limit the beam to its maximum diameter
-the ion chamber monitors dose to the patient
-the secondary collimators are composed of lead and shape the beam
what does the graph of linear accelerators and photons?
what is the linear accelerators and electrons?
-some skin sparing
-rapid dose fall-off compared to photons
what are strontium applicators?
strontium is a pure beta (electron) emitter
-average energy 0.80 MeV
-human ophthalmic applicators are used for veterinary patients to treat corneal diseases and superficial tumors, such as pannus and cutaneous mast cell tumors in cats
-useful for treating lesions 3 to 5 mm in depth
