midterms Flashcards
Parts of Linear Accelerator
wave guide
bending magnet
target
scattering foil
ion chamber
electron gun
magnetron
beam flattening filter
collimators
vacuum tube where the electrons are accelerated
wave guide
Directs the electron beam to the target or scattering foil.
Bending Magnet
where the electrons interact to produce x-rays
target
placed along the path of the electron beam to scattered the pencil thin electron beam.
scattering foil
hollow tube with gas usually air where the process of ionization occurs. It controls and switches off the equipment for an adequate radiation to be used.
ion chamber
it produces electron to be accelerated
electron gun
Magnetron device that produces microwaves.It functions as a high power oscillation generating microwave pulses of several microsecond during and with a repetition rate of several hundred pulses per second.
placed along the path of the x-ray beam in order to attain homogenous intensity of the X-ray beam in the entire field.
Beam Flattening Filter
restrict the beam to desired beam size or field size
collimators
patient is positioned on the table using appropriate treatment accessory devices and the table is raised and rotated under the gantry. The treatment area is determined based upon review of any related diagnostic studies, operative reports, CT scan, X-ray, MRI or isotope studies, palpation of the site and tumor visualization.
simulation
The radiation therapy simulator is a diagnostic x-ray machine mounted on a rotating gantry which provides geometries identical with those found on megavoltage therapy machines. Simulators can be used in either a radiographic or fluoroscopic mode to provide diagnostic quality images on film or a television I monitor respectively
simulator
deals with the measurements of the absorbed dose resulting from interaction of ionizing radiation with matter.
Radiation Dosimetry
total number of particles or photons per unit area of the interacting material
Fluence (w)
ABSORBED DOSE MEASUREMENT METHODS
calorimetry
fricked dosimetry
film dosimetry
ionization method
involves the measurement of radiation based on the change in the thermal energy per unit mass of the medium. It is based on the fact that almost all the energy deposited in the medium by the radiation beam eventually appears as heat within the medium.
calorimetry
For water, 1 Gy produces a temperature rise of ___ Calories/gram which can be measured using a sensitive device called
2.4x 10-4
thermistors
based on chemical changes caused by radiation.
fricked dosimetry
The chemical radiation dosimeter most commonly used is ____ in which it is oxidized by radiation into ____
Ferrous Sulfate
Ferric Ion concentration is measured by absorption spectrometry at
224 nm and 304
When x-ray film is exposed to ionizing radiation. the exposed siver bromide crystaIs form a latent image. The degree of blackening of the film is proportional to the energy absorbed and is measured by determining the optical density with a densitometer.
film dosimetry
measurement of radiation energy based on ion pairs produced in a given mass of medium.
ionization method
used for photon beam
thimble chamber
used for photon beam
farmer chamber
used for electron beam
markus chamber
when a crystal is irradiated, a very minute fraction of the absorbed energy is stored in the crystal lattice. Some of this energy can be recovered later as visible light if the material is heated.
thermoluminescence dosimetry
phenomenon of the released of visible photon by thermal means is known as
Thermoluminescence
DOSIMETRY PARAMETERS
dose
dose rate
monitor unit
isocenter
general term used to refer to the effect on a material which is exposed to radiation
Ex. 180cGy
dose
radiation dose delivered per unit time
Ex. 180cGy/min for radioactive materials
180MU/min
dose rate
unit used in linear accelerator to determine the dose. Most linear accelerator is calibrated to 1 MU = 1cGy
monitor unit
point in space where the x, y and z axis intersect.
isocenter
MODIFICATION TO THE RADIATION FIELD
bolus
compensator
wedges
blocks
tissue equivalent material that have electron density, physical density, and atomic number similar to that of tissue or water. It increases the dose to the skin. ex. Paraffin Wax, Gauze soaked in water, “Super- Slab“ , Elastomeric Polymer.
bolus
intended to compensate for some topographical deficit. A bolus placed in the tissue deficit of the patient is the simplest way to compensate, but because it diminishes the skin sparing effect, retracted tissue compensators are usually employed.
compensator
generally constructed of brass, steel, or lead and when placed in the beam, they progressively decrease intensity across the field. A wedge bends the dose distribution at some specified depth.
wedges
used to shield organs at risk, made of lead or Lipowit’s metal (cerrobend) which consists of 13.3% Tin, 50% Bismuth, 26.7% Lead, and 10% Cadmium. The physical density of Lipowit’s metal at 20° Celsius is 9.4 8/cm’, compared with 11.3g/cm
blocks
FIELD ARRANGEMENT IN EBRT
single field
parallel opposed field
multi-field arrangement
moving technique field
Treatment through one single field arrangement is the simplest treatment and the dose distribution in tissue is essentially as represented on an isodose chart for the particular energy and the field used.
single field
with single field arrangement, Electron is usually the type of radiation used to treat shallow tumors where a rapid drop in dose beyond the ___ depth of the tumor.
57
- pair of fields directed along the same axis from opposite sides of the treatment volume.
- are relatively easy to set-up and to reproduced from day to day.
- 100% isodose line is with in the entire treatment volume.
Parallel opposed fields
Advantages of Parallel Opposed Field
Simplicity and reproducibility of set-up
Homogenous dose to the tumor
Less chances of geometrical miss
Disadvantage of Parallel Opposed Field
Excessive dose to normal tissues critical organs above and below tumors
use of multi-fields directed at the target volume requires that the beam entrance and exit of each field be aimed at different angles/segments, thereby reducing the dose to adjacent tissues.
multi-field arrangement
most popular multi-field arrangement is the “____”, which is often used in the treatment of pelvic malignancies. The technique consists of opposed anterior and posterior fields and opposed right lateral and left lateral field.
box Technique
technique in which the axis of the rotation of the therapy machine, the isocenter, is positioned in the center of the target volume. The radiation source is moved around the patient through are, often a complete circle. It yields a high dose in the target volume where the dose outside this volume falls off rapidly.
moving field technique
IMMOBILIZATION DEVICES
- ALPHA CRADLE
- THERMOPLASTIC MASK
- BREAST BOARD (BREAST STEP)
- PRO STEP
- VACUUM BAG
- HEAD REST
