Midterm Flashcards
During___, px is positioned on table with appropriate treatment accessory & the table is raised & rotated under gantry.
Simulation
a diagnostic x-ray machine
mounted on a rotating gantry which provides geometries
identical with those found on megavoltage therapy machines. can be used in either a radiographic or fluoroscopic mode to provide diagnostic quality images on film or a television I mc nitor respectively
Simulator
- deals with the measurements of the absorbed dose resulting from interaction of lonizing radiation with matter.
Radiation Dosimetry
- total nur iber of particles or photons per unit area of the interacting material
Fluence (w)
Amount of lonization per mass of air due to x and gamma rays
Exposure ( x)
Conventional unit & SI unit of Exposure
Roentgen ( R)
Coulomb /kg ( C/kg)
1R=
2.58 ×10^-4 c/kg
Amount of energy Imparted by radiation per mass
Absorbed Dose ( D)
Conventional unit & SI unit of Absorbed dose
rad, Gy
1 rad=
10 mGy
100 rad
1 gy
1 j/kg
1Gy
A measure of radiation specific biological damage in man
Dose Equivalent ( H)
Conventional unit & SI UNIT OF Dose Equivalent ( H)
rem
Sv
1 rem=
100 rem
10 mSv
1 Sv
Amount of radioactive material expressed as, the nuclear transformation rate
Conventional unit:
SI UNIT:
Activity ( A)
Ci
Bq
1 Ci=
3.7 x10^10 Bq
4 absorbed dose measurements methods
Calorimetry
Fricked dosimetry
Film dosimetry
Ionization method
Involves 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. For water, 1 Gy produces a temperature rise of 2.4x 10-4 Calories/gram which can be measured usina a sensitive device called thermistors.
CALORIMETRY
based on chemical changes caused by
radiation.
FRICKED DOSIMETRY
When x-ray film is exposed to ionizing/ radiation. the exposed siver bromide crystals 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-
2 chamber use for photon beam
Thimble and farmer
chamber-used or electron beam
Markus
4 Dosimetry Parameters
Dose
Dose rate
Monitor unit
Isocenter
a general term used to refer the effect on a material when exposed on radiation
Dose
the radiation dose delivered per unit time
Dose rate
unit used in linear accelerator to determine the those most linear accelerator is calibrated to 1MU= cGy
Monitor unit
a point in space where the x y and z axis intersect
Isocenter
4 Modification to the Radiation Field
Bolus
Compensation
Thermoluminescence dosimetry
Wedges
-tissue equivalent material that have Electron Density, Physical Density, 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
is intended to compensate for some topographical deficit. A balus placed in the tissue deficit of the patient is the simplest way to compensate, but since it diminishes the skin sparing effect, retracted tissue compensators are usually employed.
Compensator
-constructed of brass, steel, or lead and when placed in the beam, progressively decreases in field intensity. A wedge bends dose distribution at a specified depth
Wedges
an irradiated crystal stores a small fraction of energy in the crystal lattice. This energy can be recovered later as visible light when heated. The phenomena of releasing visible photon by thermal means is coined Thermoluminescence
Thermoluminescence Dosimetry-
4 field arrangement in EBRT
SINGLE FIELD
PARALLEL OPPOSED FIELD
MULTI- FIELD ARRANGEMENT
MOVING FIELD TECHNIQUE
-simplest treatment and the dose distribution in tissue is represented in an isodose chart for the energy and field used
- Electron is usually the radiation used to treat shallow tumors, where a rapid drop in dose beyond the 57 depths of the tumor
Single Field
-pair of fields directed along the same axis of opposite sides of the treatment volume. Relatively easy to set up and to reproduce from day to day Isodose line is within the entire treatment volume.
Parallel Opposed Field
ADVANTAGES of parallel field
-Simplicity and reproducibility of setup
- Homogenous dose to tumor
-Less geometrical miss chance
DISADVANTAGES of parallel
*
Excessive dose to normal tissues critical organs above and below tumors
-directed at target volume requires that the beam entrance and exit of each field be aimed at different angles/segments. Reducing dose to adjacent tissues.
-the___ technique is the famous multi-field arrangement, often used in pelvic malignancies. Composed of opposed posterior and anterior fields, and opposed right lateral and left lateral fields
Multi-field Arrangement
box
technique in which the axis of the therapy machine’s rotation, the isocenter, is in the center of the target volume. Radiation source is moved around the Px through are, often in a complete circle. It yields a high dose in the target volume where the dose outside falls off rapidly
Moving Field Technique-
-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
Lipowit’s metal _%
Tin-
Bismuth
Lead& cadmium
Density- cm ( 20 celcius)
13.3
50
26.7
10
9.48
The chemical radiation dosimeter most commonly used in Fricked Dosimetry is____ which it is oxidized by radiation into ferric sulfate.
Ferric Ion concentration is measured by absorption spectrometry at 224
nm and 304
Ferrous Sulfate
For water, 1 Gy produces a temperature rise of___ which can be measured usina a sensitive device called thermistors.
2.4x 10-4 Calories/gram
