Khan Outline: Chapter 8: Measurement Of Absorbed Dose

Front 1

Define "Absorbed dose"

Back 1

The energy absorbed per unit mass of any material.

Front 2

What are the units for "Absorbed dose"

Back 2

SI = gray (Gy)
old unit is the rad

Front 3

Define "Kerma"

Back 3

The unit of kerma is the same as that of dose, (Gy)

The quantity kerma (K) is the kinetic energy released in the medium per unit mass.

Kerma pertains only to photon beams.

Kerma is proportional to photon energy fluence. It is maximum at the surface and decreases exponentially with depth.

Front 4

What is meant by IRAK in brachy and what does it stand for?

Back 4

?

Front 5

Define "Fmed factor"

Back 5

Fmed is the roentgen-to-cGy conversion factor for any medium

Front 6

What is the equation for "Fmed factor"

Back 6

insert equation

Front 7

Define "Nx"

Back 7

Nx is the exposure calibration factor for a cobalt-60 beam

Front 8

What is the "Bragg-Gray Cavity Theory"?

Back 8

The principle of Bragg-Gray cavity theory is that the ionization produced in a gas-filled cavity placed in a medium is related to the energy absorbed in the medium surrounding the cavity.

When the cavity is sufficiently small so that its introduction into the medium does not alter the number or distribution of the electrons that would exist in the medium without the cavity, then the following Bragg-Gray relationship is satisfied

Front 9

Define "Stopping Power"

Back 9

The term stopping power refers to the energy loss by electrons per unit path length of material.

Front 10

How is chamber volume determined?

Back 10

An indirect method of measuring the chamber volume is to make use of the exposure calibration of the chamber for 60Co y-ray beam. This in effect determines the chamber volume.

Front 11

What is the W/e factor?

Back 11

Work function

The average energy required to ionize air

Average value of 33.97 electron volts/ion pair

Front 12

What is the difference between Aion and Pion?

Back 12

Pion is a correction for collection efficiency (ionic recombination) and is therefore always greater than 1.

The Aion is the recombination measured by an Accredited Dosimetry Calibration Laboratory and is determined under different conditions and will not have the same value as the Pion.

Front 13

What is the purpose of TG-51?

Back 13

TG-51 requires that the calibration of photon and electron beams be performed in a water phantom.

Front 14

What is the R50 in electron beams?

Back 14

Th R50 is specified by the depth of 50% dose in water for electron beam calibration.

Front 15

What is "Rp" in electron beams?

Back 15

?

Front 16

How is a chamber waterproofed for beam scanning?

Back 16

A cylindrical ion chamber may be waterproffed using a thing (<_1 mm thick) acrylic sleeve. There should be little resistance and minimal air gaps.

A latex condom but without any talcum powder.

Front 17

Review Photon beam calibration

Back 17

?

Front 18

Review Electron beam calibration

Back 18

?

Front 19

Define "Calorimetry"

Back 19

Calorimetry is based on the principle that the energy absorbed in a medium from radiation appears ultimately as heat energy while a small amount may appear in the form of a chemical change.

This results in a small increase in temperature of the absorbing medium which, if measured accurately, can be related to the energy abosrbed per unit mass or the absorbed dose.

Front 20

What is chemical dosimetry?

Back 20

The energy absorbed from ionizing radiation may produce a chemical change, and if this change can be determined, it can be used as a measure of absorbed dose.

Many systems of chemical dosimetry have been proposed, but the ferrous sulfate or the Fricke dosimeter is considered to be the most developed system for the precision measurement of absorbed dose.

Front 21

What is in the typical TLD and what is the measurement process?

Back 21

The most commonly used TLD consists of lithium fluoride with trace amounts of magnesium.

When the material is irradiated, some of the electrons in the valence band (ground state) receive sufficient energy to be raised to the conduction band. The vacancy thus created in the valence band is called a positive hole. The electron and the hole move independently through their respective bands until they recombine (electron returning to the ground state) or until they fall into a trap (metastable state). If there is instantaneous emission of light owing to these transitions, the phenomenon is called fluorescence. If an electron in the trap requires energy to get out of the trap and fall to the valence band, the emission of light in this case is called phosphorescence (delayed fluorescence). If phosphorescence at room temperature is very slow, but can be speeded up significantly with a moderate amount of heating (~300°C), the phenomenon is called thermoluminescence.

Front 22

What are Diodes and what affects them?

Back 22

A dosimetry diode consists of a silicon crystal that is mixed or doped with impurities to make p and n-type silicon. When a diode is irradiated, electron-hole pairs are produced within the depletion zone. They are immediately separated and swept out by the existing electric field in the depletion zone. This gives rise to a radiation-induced current. The current is further augmented by the diffusion of electrons and holes produced outside the depletion zone within a diffusion length.

Diodes exhibit energy, angular and temperature dependence.

They can also suffer permanent damage when irradiated by ultrahigh doses of ionizing radiation.

Front 23

What is radiochromic film?

Back 23

RadioChromic Films consists of an ultrathin (7-23 um thick), colorless, radio-sensitive leuco dye bonded onto a 100-um thick Mylar base.

The unexposed film is colorless and changes to shades of blue as a result of a polymerizationprocess induced by ionizing radiation.

No physical, chemical, or thermal processing is required to bring out or stabilize this color

Measured with a spectrophotometer using a narrow spectral wavelength (nominal 610-670 nm)

Radiochromic films are almost tissue equivalent with effective Z of 6.0 to 6.5.

Advantages include: Tissue equivalence, High spatial resolution, Large dynamic range, Relatively low spectral sensitivity variation or energy dependence, Insensitivity to visible light, No need for chemical processing.