Collimator scatter provides...

Collimator scatter factor Sc(r), provides the

a) Output factor measured in air
b) influence of head scatter on beam outpu with increasing collimator field size.

TMR Data are generally obtained from...

TMR Data are generally obtained from
1) measurements
2) PDD data

TMR Depends on...

TMR Depends on the following...

Depth
Field Size
Beam quality

But not SSD.

SOURCE
2007 Therapy Part II (attached)
Related to Tg-51

Simple question 11
11. TG51: What's the upper limit for Pion?

DRR answer: Per pg 1853 of TG51, I get 1.05, or 5% would be the answer. Above this it states that if the correction for the chamber is larger than this it is unacceptable.

Question 12 TG51. Where is the cylindrical chambers center placed for photon beam calibration?

DRR answer: On central axis of the chamber and placed at the reference dpeth when measuring dose at an individual point.

Question 13. To cross calibrate a parallel plate chamber what should one us?

DRR answer: Since Co-60 cal factors for plane parallel chambersare sensitive to their construction, they should be calibrated against already calibrated cylindrical chambers in a high energy electron beam.

NOTE
Questions 14, 15, 16 aren't as well formulated, or in some cases the choices listed aren't complete so we'll have to come up with a better worded question for these.

Enjoy,
Dave

Pitch and patient dose.

OBJECTIVE. With single-slice helical CT, an increased pitch can decrease the radiation dose to the patient if all other parameters are constant. The purpose of this study was to determine whether the same relationship holds for a particular multislice helical CT system (Somatom Plus 4 VZ multislice helical CT scanner, version A11A) in our department.
CONCLUSION. The measured radiation dose to the phantom was identical for all pitch selections on the multislice helical CT system we tested. This unexpected result was because of an automatic proportionate increase in the tube current when the pitch selection was increased. Radiologists and physicists should exercise caution when extrapolating dose reduction strategies from single-slice to multislice helical CT systems, and they must acquire a detailed understanding of the multislice helical CT scanner of their chosen manufacturer.

For multislice helical CT scanners, manufacturers use different definitions of pitch, which has resulted in much confusion [9]. For the multislice CT scanner we described, the manufacturer defines "pitch" as the ratio of table movement per 360° rotation to single section thickness (P). We chose to use the definition of pitch [9] as table increment per 360° rotation divided by the total beam width (P'). This definition is applicable to both single- and multislice helical CT scanners, as shown in Table 1. Using slice combinations of 4 x 1 mm and 4 x 2.5 mm, the test volume was scanned on the multislice helical CT scanner at the manufacturer's defined pitch selections of 2, 4, and 8 (P' = 0.5, 1, 2). At a slice width of 3 mm, the same volume was scanned at pitch selections of 0.5, 1, and 2, respectively, on the single-slice helical CT system for comparison. Three dose measurements were recorded and averaged for each pitch setting we tested.

http://www.ajronline.org/cgi/content/full/177/6/1273

Gamma emissions

Emission of gamma does not change the identity of the radionuclide.

Gamma emissions take the nuclides only from excited states to the ground state.

Beta Decay

Neutron rich radionuclides are responsible for beta decay.

Output factor

Output factor is the ratio of the dose rate of a given field size to the dose rate of the reference field size.

the output fator is usually normalized or referenced to a 10x10cm field size, so there it will be 1.0

For fields sizes smaller than 10x10, the output factor is <1.0 because of a decrease in scatter as the collimator setting is decreased.

For field sizes >10x10 the output factor is >1.0 because of an increase in scatter as the collimator setting is increased.

Farmer Chamber Diagram

Outer electrode
Central electrode
Insulator
Aluminium
Graphite
PTCFE
Dural
FIG. Basic design of a cylindrical Farmer type ionization chamber.

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.

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

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.

Flatness

Where is flatness defined?

Per TG-40 flatness is defined at 10 cm depth.

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!

MEDICALPHYSICIST.VOX.COM

This is migrated or merged with www.medicalphysicist.vox.com

I might keep both running since this one allows co-authorship.

The other one allows photos and videos.

Either way it is tough work to keep the blogging up everyday until it becomes a habit.

www.medicalphysicist.vox.com