Radiation Physics Terminology Flashcards
what is conventional sim
- Fluoroscopy X-ray require
2. Doctor draws field borders on x-ray images
What is CT sim
- requires CT scanner and virtual sim workstation
- patient CT scan is required
- relative points or other landmarks marked at sim
- field setup are created virtually
why are bbs used at simulation
BBs used to indicate where marked isocenter is located
why are solder wires used at simulation
solder wires can be used to mark the anatomic locations of structures (scar, border, breast, etc), to serve as a marker that is visible to the dosimetrist on the CT
what point(s) are marked during simulation
marked isocenter(s)
what is marked isocenter
- marked at simulation
2. in Pinnacle, you can only localize to one marked iso
what is final isocenter
- placed during planning by dosimetrist or physician
what is final isocenter
- placed during planning by dosimetrist or physician
2. usually if you have a final iso, there will be an iso shift
what is iso shift
directions for therapist indicating how to get from the marked isocenter to the final isocenter by adjusting the treatment chair
what is calc point
placed by dosimetrist during treatment planning if the marked or final iso does not follow rules of calc point
in which orientation are CT slides obtained
axial/ transverse
define image segmentation
slide by slide delineation of anatomic regions of interest
define critical structures
non target structures that we want to monitor how much radiation dose they receive
examples of critical structures
lung, heart, rectum, etc,…
define normal tissue and list examples
non target structures. could be critical (rectum) or non critical (fat)
define tolerance
each critical structure has a maximum dose. if the critical structure receives more dose than the tolerance, will have negative consequences
what are the consequences for exceeding tolerance
depends upon type of critical structures
pneumonitis for lungs
cataracts for eyes
GTV (ICRU 50)
gross tumor volume
gross demonstrable extent and location of the disease
CTV (ICRU 50)
clinical target volume
demonstrated tumor if present and any other tissue with presumed tumor
PTV (ICRU 50)
planning target volume
includes CTV and a setup margin compensate for patient movement and setup uncertainties
IM (ICRU 62)
internal margin
added to CTV to compensate for internal physiologic movement during treatment
ITV (ICRU 62)
internal target volume
volume that includes CTV with internal margin
SM ( ICRU 62)
setup margin
margin for patient movement and setup uncertainty
OR (ICRU 62)
organ at risk
PRV (ICRU 62)
planning organ at risk volume
margins to compensate for internal movement and setup variability of organ at risk
what is absorbed dose
energy deposited by ionizing radiation per unit mass (Gy)
1 Gy= 1 J/kg
what is a fraction
each time radiation is administered equals to one fraction
if the patient comes once a day, it’s 1 fx per day
if the patient comes twice a day, it’s two fx per day
what is the current unit for prescribed radiation dose
Gy or cGy
What is the previous unit for prescribed radiation dose
Rad (radiation absorbed dose)
what is the conversion unit between the older radiation unit and the current radiation unit
100 rad= 1 Gy
What is the approved dose calculation algorithm at MDACC
CC Convolution
What happens when an x-ray beam passes through a material
when an x-ray (or gamma) passes through a medium, photons and matter interact with the result that energy is transferred to medium
what is the first step in the interaction between the photon and the matter
ejection of electrons from the atoms of the absorbing medium, called secondary electrons
what do the high speed electrons that are produced do
- high speed electrons transfer energy by absorbing ionization and excitation of the atoms along their path
- if the absorbed medium consists of body tissues, sufficient energy may be deposited within cells, destroying their reproductive capacity
what happens to most of the absorbed energy
converted into heat, producing no biological effect
what happens when a photon interacts with the electrons in the material
a part or all energy of photons convert into kinetic energy of electron
describe photon scattering in the medium
- if only part of photon energy is given to electron, then photon is scattered with reduced energy
- the scattered photon may interact again with a partial or complete transfer of energy to the electron
- photon may experience one or multiple interaction in which energy lost by the photon is converted into kinetic energy of electrons
what is bremsstrahlung or braking radiation
- high speed electron passes near a nucleus
- electron is accelerated and deflected from its path by the attractive Coulomb force of the nucleus.
- The deflected electron will lose all part or all of its energy as electromagnetic radiation
what is the maximum energy of the resulting bremsstrahlung photon
the interaction may result in partial or complete loss of electron energy, the resulting bremsstrahlung photon may have energy up to the initial energy of electron
define buildup region
area from surface to dmax
define buildup region
area from surface to dmax
why cant we place a calc point in the build up
not correctly modeled in Pinnacle
not adequate tissue to allow for complete buildup
what is attenuation
gradual loss of intensity of x-ray through some material
what causes attenuation
- coherent scattering
- photoelectric effect
- Compton effect
- pair production
- annihilation radiation
define photoelectric effect
incident photon strike an electron bound to an atom
what happens to the energy in photoelectric effect
the entire energy of photon is transferred to the electron, which is ejected as photoelectron.
the incident photon vanishes, giving an energy equal to incident photon energy- Eb to the electron
define Compton effect
an incident photon strikes a weakly bound electron
what happens to the energy in Compton effect
a portion of the photon’s energy is absorbed by the electron which is emitted at an angle (theta). the photon is scattered with reduced energy
the photon leaves the site in a direction from that of the original photon (scattered at an angle phi)
what happens to the energy in Compton effect
a portion of the photon’s energy is absorbed by the electron which is emitted at an angle (theta). the photon is scattered with reduced energy
the photon leaves the site in a direction from that of the original photon (scattered at an angle phi)
define pair production
a photon is absorbed in the vicinity of the nucleus. If disappears, and gives rise to an electron and position
define annihilation radiation
a slowly moving position (created as a result of pair production) combines with a free electron and produces two photons. The photons are ejected opposite directions of each other.
an example of converting mass into energy
annihilation radiation
define penumbra
region near the edge of the field margin where the dose falls rapidly
the region, at the edge of a radiation beam, over which the dose rate changes rapidly as a function of distance from the beam axis.
