Outer electrode
Central electrode
Insulator
Aluminium
Graphite
PTCFE
Dural
FIG. Basic design of a cylindrical Farmer type ionization chamber.
Wednesday, October 10, 2007
Spencer Attix review of how it differs
The Bragg–Gray cavity theory does not take into account the creation of
secondary (delta) electrons generated as a result of hard collisions in the
slowing down of the primary electrons in the sensitive volume of the dosimeter.
The Spencer–Attix cavity theory is a more general formulation that accounts
for the creation of these electrons that have sufficient energy to produce
further ionization on their own account. Some of these electrons released in the
gas cavity would have sufficient energy to escape from the cavity, carrying some
of their energy with them. This reduces the energy absorbed in the cavity and
requires modification of the stopping power of the gas. The Spencer–Attix
theory operates under the two Bragg–Gray conditions; however, these
conditions now even apply to the secondary particle fluence in addition to the
primary particle fluence.
secondary (delta) electrons generated as a result of hard collisions in the
slowing down of the primary electrons in the sensitive volume of the dosimeter.
The Spencer–Attix cavity theory is a more general formulation that accounts
for the creation of these electrons that have sufficient energy to produce
further ionization on their own account. Some of these electrons released in the
gas cavity would have sufficient energy to escape from the cavity, carrying some
of their energy with them. This reduces the energy absorbed in the cavity and
requires modification of the stopping power of the gas. The Spencer–Attix
theory operates under the two Bragg–Gray conditions; however, these
conditions now even apply to the secondary particle fluence in addition to the
primary particle fluence.
Two conditions for Bragg Gray
The Bragg–Gray cavity theory was the first cavity theory developed to
provide a relation between the absorbed dose in a dosimeter and the absorbed
dose in the medium containing the dosimeter.
The conditions for application of the Bragg–Gray cavity theory are:
(a) The cavity must be small when compared with the range of charged
particles incident on it, so that its presence does not perturb the fluence
charged particles in the medium;
(b) The absorbed dose in the cavity is deposited solely by charged particles
crossing it (i.e. photon interactions in the cavity are assumed negligible
and thus ignored).
The result of condition (a) is that the electron fluences in Eq. (2.22)
the same and equal to the equilibrium fluence established in the surrounding
medium. This condition can only be valid in regions of CPE or TCPE.
addition, the presence of a cavity always causes some degree of fluence perturbation
that requires the introduction of a fluence perturbation correction
factor.
Condition (b) implies that all electrons depositing the dose inside
cavity are produced outside the cavity and completely cross the cavity.
secondary electrons are therefore produced inside the cavity and no electrons
stop within the cavity.
Courtesy Pgorsak
provide a relation between the absorbed dose in a dosimeter and the absorbed
dose in the medium containing the dosimeter.
The conditions for application of the Bragg–Gray cavity theory are:
(a) The cavity must be small when compared with the range of charged
particles incident on it, so that its presence does not perturb the fluence
charged particles in the medium;
(b) The absorbed dose in the cavity is deposited solely by charged particles
crossing it (i.e. photon interactions in the cavity are assumed negligible
and thus ignored).
The result of condition (a) is that the electron fluences in Eq. (2.22)
the same and equal to the equilibrium fluence established in the surrounding
medium. This condition can only be valid in regions of CPE or TCPE.
addition, the presence of a cavity always causes some degree of fluence perturbation
that requires the introduction of a fluence perturbation correction
factor.
Condition (b) implies that all electrons depositing the dose inside
cavity are produced outside the cavity and completely cross the cavity.
secondary electrons are therefore produced inside the cavity and no electrons
stop within the cavity.
Courtesy Pgorsak
Dose Difference and Distance to Agreement
Dose Difference and Distance (courtesy Yeo et al Procedural Method for Film Dosimetry, Medical Physics Publishing)
to Agreement Analysis
There exist other methods of analysis that account
for the limitation of alignment. Van Dyk et al. (1993)
introduced the idea of dividing the evaluation into
two groups depending on the magnitude of dose gradient:
high- and low-gradient regions each with a
different acceptance criterion. The idea is based on
the fact that dose difference in a high-dose-gradient
region can be extremely higher than that in a lowerdose-
gradient region because of imperfect
alignment. This approach may provide exceedingly
simplistic analysis for an IMRT field, where a
diverse degree of a dose gradient typically exists.
To overcome this limitation, therefore, the simultaneous
use of a distance-to-agreement (DTA) and
a percent dose difference (DD) is proposed. These
parameters can help evaluate the agreement of the
two distributions in terms of misalignment and
difference, respectively. DTA is defined as the nearest
distance from a point of a reference dose to the
point of the same amount of dose on the compared
(or quarried) dose distribution. If the former is
selected in the measured distribution, then select the
latter in the calculated distribution. DTA, thus, is an
indicator of how good the alignment of the two distributions
is, provided that the difference is zero. The
percent dose difference is defined as the difference
in percent, implicitly assuming that the alignment of
the two distributions is perfect. In reality, as the dose
difference as well as the misalignment contribute to
the difference of the two clinical distributions, use of
the two independent parameters together will be
necessary. By providing an acceptance criterion,
respectively for a dose difference and a DTA, the
acceptability of the comparison can be determined.
to Agreement Analysis
There exist other methods of analysis that account
for the limitation of alignment. Van Dyk et al. (1993)
introduced the idea of dividing the evaluation into
two groups depending on the magnitude of dose gradient:
high- and low-gradient regions each with a
different acceptance criterion. The idea is based on
the fact that dose difference in a high-dose-gradient
region can be extremely higher than that in a lowerdose-
gradient region because of imperfect
alignment. This approach may provide exceedingly
simplistic analysis for an IMRT field, where a
diverse degree of a dose gradient typically exists.
To overcome this limitation, therefore, the simultaneous
use of a distance-to-agreement (DTA) and
a percent dose difference (DD) is proposed. These
parameters can help evaluate the agreement of the
two distributions in terms of misalignment and
difference, respectively. DTA is defined as the nearest
distance from a point of a reference dose to the
point of the same amount of dose on the compared
(or quarried) dose distribution. If the former is
selected in the measured distribution, then select the
latter in the calculated distribution. DTA, thus, is an
indicator of how good the alignment of the two distributions
is, provided that the difference is zero. The
percent dose difference is defined as the difference
in percent, implicitly assuming that the alignment of
the two distributions is perfect. In reality, as the dose
difference as well as the misalignment contribute to
the difference of the two clinical distributions, use of
the two independent parameters together will be
necessary. By providing an acceptance criterion,
respectively for a dose difference and a DTA, the
acceptability of the comparison can be determined.
Tuesday, October 9, 2007
Tuesday, October 2, 2007
Annual QA
On the other blog I started to document the Annual QA process. I have to admit, I like how the text looks better on this site though, however vox has some nice features for audio and video.
In the end, looks like I'll use both.
And although I feel bad about updating so infrequently, I've still done better than some of my contemporaries. Take a look at medicalphysicist.blogger.com, and medicalphysics.bloggger.com. They were no doubt started by someone with good intentions to continue them, but have had the same difficulties I've had at updating them!
In the end, looks like I'll use both.
And although I feel bad about updating so infrequently, I've still done better than some of my contemporaries. Take a look at medicalphysicist.blogger.com, and medicalphysics.bloggger.com. They were no doubt started by someone with good intentions to continue them, but have had the same difficulties I've had at updating them!
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