Unfiled

Isodose changes with small electron fields What is the effect of a reduced field size on the isodoses of a clinical 12MeV electron beam?
[in this case, the use of diagram is very helpful. the diagram should be constructed to demonstrate that you can draw a normal curve first! You have the knowledge to to this … ACCURATELY thank you! The field edge corresponds to the 50% line, Dmax can be calculated by MeV/4, and D90% by MeV/3, and D10% by MeV/2. You just have to know the surface dose for each beam - no formulae help! But for a 12MeV e- beam the surface dose is just over 90%. So having drawn the pattern on the left, the smaller field on the right can be constructed, but very loosely now because you want to demosntrate to me (as the examiner! isn't that a nice role to have?!) that you know how things change. The changes are listed and pointed to here, AND use the required terms that get examiners feel gooey inside - constriction, ballooning - oh, it's all so clear! Don't you think?]

Notes_unfiled - the radiation oncology BLOGDraw an accurate diagram of the isodose curves of a 12MeV electron beam, indicating the salient features of the curve.
[here is another occasion where drawing accurately helps an awful lot. And let's face it, if you are going to draw, you should do so as to reduce your workload at examination time. Now just as a word to the wise, it takes time to develop the skill of good drawing AND to do it quickly but it is a very good skill to have. It is the pictorial equivalent of the written example! Anyway, enough coaching! This answer requires that the curves are drawn accurately as you know where the isodose depths will be situated - well, at least you should know otherwise you are going to have some difficulty soon! ]

Exposure

This is a difficult concept, firstly because the word has so many other meanings! Revealing oneself, publicity, the effects of being left out in the cold … sometimes the English language is too constrained. Anyway, when a beam of ionizing radiation passes through air, it IONIZES the air particles. Now that must seem like a revelation - an ionizing beam ionizes - but then, you know, physics is simple! Anyway, moving right along, the ionization event has to follow all the conservation laws of physics (mass, energy, momentum, charge), so for every +ve particle there will be a -ve particle also. And that is why the particles OF ONE CHARGE is chosen, otherwise the number of charged particles is twice the number of photon-air interactions. As with most physics unit, everything is at STP - standard temperature and pressure - as well as a whole lot of other standard things such as air volume (usually one metre cubed - ah, the good old metric system devised by those naughty french in a moment of 'equalite' and 'fraternite'!

So EXPOSURE is defined as "amount of charged particles of one charge produced by an incident beam in a defined quantity of air under standard conditions", and as every good physicist knows, when there is a definition in the offing …. don't you ever let a chance go by … write an equation to make it really complicated and daunting! This is how physicist establish their rite of passage - you can join us if you can represent the world in equations! The unit of Exposure is Roentgen - yes, Wilhelm Conrad has his names up in lights which is a problem because the pronunciation is so …. german! And unfortunate because we hardly ever talk about Roentgens like we talk about Ohms, Curies, Becquerels, Grays or Fractions. What's that you say? There was no Dr Fraction? Yes, I'm just testing you to see if you are awake and I'm being silly because I am the Grand Master of this blogsphere [maniacal laugh 2-3 times!]!

E=dQ/dm where [E=exposure I=charge, m=mass of air]
this equated to approximately 2.08×10^9 ion pairs in a reference volume.

In SI units, 1 R = 2.58×10−4 C/kg.

Dose
total energy (from all interactions) deposited into matter by the ionizing radiation beam. It is more than KERMA

D=dE/dm
[D=dose E=energy m=mass]

In SI units, 1 Gy = 1 J/kg

Equivalent Dose
descibes the relative biological effectiveness of a measured radiation dose for a specific radiation beam, where the differing biological effectiveness is reflected in different LETs and RBEs of beams.

ED=D.Q

[ED=equivalent dose, D=received dose, Q=quality or equivalence weighting factor for radiation type used (photon/electron=1, SXR/ortho=1.2, proton/neutron=5, hadron=20, alpha=120]

units are Sv (Sievert) or eGy

Activity
disintegrations undergone by a radionuclide in a unit time

A=N/t

[A=Activity, N=number of disintegrations, t=time]

SI unit is the Becquerel (1 Bq = 1 disintegration/second). Original unit was the Curie which represented the amount of disintegrations occurring in a 1 g sample of pure Ra each second - because of its very long half-life of 1640 years, this figure was relatively fixed for the life time of any individual. 1 curie = 37GBq (3.7 x 10^10 dps)

Wedge Factor
(this is NOT the wedge angle)
ratio of dose to a field with and without a wedge in place. It is measured at depth and is applied so that isodose plots have a 100%, when in actual fact the maximum dose with a wedge is less than that of the open field. The wedge attenuates AND shapes the beam.

WF=D(wedged field)/D(open field)

There are no units, it is a ratio.

Linear Stopping Power
This measure is dependent on describing the medium being considered (water, soft tissue, lead), and represents the energy lost by a charged particle (i.e., an electron or proton, NOT photon - that has a linear attenuation!) passing through a substance per unit length of path, whether by radiative or collisional losses. For electrons, there is a 50/50 loss ratio of about 2 MeV/cm in water (hence the depth of the electron plateau!).

There is no formal unit as the LSP describes the loss as MeV.cm^-1 (i.e., energy per centimetre)

Inverse Square Law
(the energy of the beam does not change - e.g., 4MV > 1MV - the fluence of beams (or photon density, or dose/cm^2) changes.
A description of the geometric change in intensity of beams associated with an aperture or point receiving radiation from a point source. As distance increases, the aperture area for the radiation beam increases, and so fluence (photons/cm^2) decreases although the total number of photons remains unchanged.

F prop 1/d^2
[F=fluence, d=distance from pointsource]

Has no units associated.

Dose Build Up
(answer requires more than saying DBU area is area where dose builds up – there should be something about how it occurs and KERMA changes in the area)
the area of dose in-equilibrium within the superficial portion of an irradiated medium. Its approximate depth closely correlates with the maximum track length (and therefore kinetic energy) of an electron released by the particular particle employed.

unit is cm (depth)

Field Size
(surprisingly poorly done by a significant number)
Determination of the aperture size used in radiotherapy.
The assigned field size corresponding to an aperture setting is determined by the geometric extent of the 50% isodose measured at the level of dmax (the depth at which the 100% isodose appears). This is measures perpendicularly to a perpendicular field incident on a phantom.

Anisotropy factor
(poorly done by most, but others had good knowledge)
A description of the degree of deviation of a dose distribution from a spherical shape. In the case of brachytherapy sources, the isodoses along the axis of the source are closer than those extending laterally because of the source’s inherent ability to filtration radiation along that axis.

Penumbra

Definition of Penumbra

Definition of Penumbra

Types of Penumbra

PhysicalPenumbra.png

Geometric Penumbra

Dosimetric Penumbra

Exposure amount of charged particles of one charge produced by an incident beam in a defined quantity of air under standard conditions E=dQ/dm [E=exposure I=charge, m=mass of air] approximately 2.08×10^9 ion pairs. In SI units, 1 R = 2.58×10−4 C/kg. Dose
total energy (from all interactions) deposited into matter by the ionizing radiation beam. It is more than KERMA D=dE/dm [D=dose E=energy m=mass] In SI units, 1 Gy = 1 J/kg Equivalent Dose
descibes the relative biological effectiveness of a measured radiation dose for a specific radiation beam, where the differing biological effectiveness is reflected in different LETs and RBEs of beams. ED=D.Q [ED=equivalent dose, D=received dose, Q=quality or equivalence weighting factor for radiation type used (photon/electron=1, SXR/ortho=1.2, proton/neutron=5, hadron=20, alpha=120] units are Sv (Sievert) or eGy Activity
disintegrations undergone by a radionuclide in a unit time A=N/t [A=Activity, N=number of disintegrations, t=time] SI unit is the Becquerel (1 Bq = 1 disintegration/second). Original unit was the Curie which represented the amount of disintegrations occurring in a 1 g sample of pure Ra each second - because of its very long half-life of 1640 years, this figure was relatively fixed for the life time of any individual. 1 curie = 37GBq (3.7 x 10^10 dps) Wedge Factor
ratio of dose to a field with and without a wedge in place. It is measured at depth and is applied so that isodose plots have a 100%, when in actual fact the maximum dose with a wedge is less than that of the open field. The wedge attenuates AND shapes the beam. WF=D(wedged field)/D(open field) There are no units, it is a ratio. Linear Stopping Power
This measure is dependent on describing the medium being considered (water, soft tissue, lead), and represents the energy lost by a charged particle (i.e., an electron or proton, NOT photon - that has a linear attenuation!) passing through a substance per unit length of path, whether by radiative or collisional losses. For electrons, there is a 50/50 loss ratio of about 2 MeV/cm in water (hence the depth of the electron plateau!). There is no formal unit as the LSP describes the loss as MeV.cm^-1 (i.e., energy per centimetre) Inverse Square Law
A description of the geometric change in intensity of beams associated with an aperture or point receiving radiation from a point source. As distance increases, the aperture area for the radiation beam increases, and so fluence (photons/cm^2) decreases although the total number of photons remains unchanged. F prop 1/d^2 [F=fluence, d=distance from pointsource] Has no units associated. Dose Build Up
the area of dose in-equilibrium within the superficial portion of an irradiated medium. Its approximate depth closely correlates with the maximum track length (and therefore kinetic energy) of an electron released by the particular particle employed. unit is cm (depth) Field Size
Determination of the aperture size used in radiotherapy. The assigned field size corresponding to an aperture setting is determined by the geometric extent of the 50% isodose measured at the level of dmax (the depth at which the 100% isodose appears). This is measures perpendicularly to a perpendicular field incident on a phantom. Anisotropy factor
A description of the degree of deviation of a dose distribution from a spherical shape. In the case of brachytherapy sources, the isodoses along the axis of the source are closer than those extending laterally because of the source's inherent ability to filtration radiation along that axis.

page revision: 1, last edited: 31 Dec 2007 02:41