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.
Sunday, December 30, 2007
TMR Data are generally obtained from...
TMR Data are generally obtained from
1) measurements
2) PDD data
1) measurements
2) PDD data
Thursday, December 27, 2007
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
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
Wednesday, December 26, 2007
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
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.
Gamma emissions take the nuclides only from excited states to the ground state.
Saturday, December 15, 2007
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.
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.
Wednesday, October 10, 2007
Farmer Chamber Diagram
Outer electrode
Central electrode
Insulator
Aluminium
Graphite
PTCFE
Dural
FIG. Basic design of a cylindrical Farmer type ionization chamber.
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.
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!
Saturday, March 24, 2007
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
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
Wednesday, February 14, 2007
Sterlings approximation
There are two methods to calculate equivalent square.
One is Sterlings approximation, this is better found from 4*Area/Perimeter of the field.
The other method is Days method. This was published in the British Journal of Radiology and is the common method of interpolation between field sizes and involves lookup in a table. This is definitely preferred for more rectangular fields (ex 12x36 or the like) as Sterlings approximation gives a improper equivalent square size under those conditions.
One is Sterlings approximation, this is better found from 4*Area/Perimeter of the field.
The other method is Days method. This was published in the British Journal of Radiology and is the common method of interpolation between field sizes and involves lookup in a table. This is definitely preferred for more rectangular fields (ex 12x36 or the like) as Sterlings approximation gives a improper equivalent square size under those conditions.
Saturday, February 10, 2007
Update
Work demands have made it a bit difficult to keep up the habit of blogging daily.
I am considering also migrating this to my own personal website. However the main hope is that it looks better in that format.
I am considering also migrating this to my own personal website. However the main hope is that it looks better in that format.
Monday, January 29, 2007
Stopping Power
Stopping power is defined as the rate of energy loss per unit path length as a charged particle travels through a material.
The total energy loss of the particle can be separated into collisional and radiative components:
S/dtot= (S/d) collision+(S/d) radiative
Where d is the density of the material transversed and S/d represents the mass stopping power.
Stopping power is greater for low Z material than for High Z because High Z materials have fewer electrons per gram and have more tightly bound inner electrons.
The total energy loss of the particle can be separated into collisional and radiative components:
S/dtot= (S/d) collision+(S/d) radiative
Where d is the density of the material transversed and S/d represents the mass stopping power.
Stopping power is greater for low Z material than for High Z because High Z materials have fewer electrons per gram and have more tightly bound inner electrons.
Mantle field
What are the factors involved in setting up Mantle and Inverted Y fields? (From Rosemarck)
Draw typical fields
Does matching the fields require a gap?
Are there multiple computer dosimetry points?
What concern do you have for the gonads? How can you protect the gonads? What can be done to protect the gonads of a young female? (Move them surgically)
Draw typical fields
Does matching the fields require a gap?
Are there multiple computer dosimetry points?
What concern do you have for the gonads? How can you protect the gonads? What can be done to protect the gonads of a young female? (Move them surgically)
Wednesday, January 24, 2007
Converting to depth dose
What are the factors required to convert electron depth ionization to depth dose?
Wednesday, January 17, 2007
Tuesday, January 16, 2007
Thursday, January 11, 2007
Stereotactic question 1/10 07, 3d Conformal question
1/10 07 RADIOSURGERY
How do you setup a linac for radiosurgery?
How many beams do you need?
Which energy is best?
What relative dose can you get outside the target?
What was the original stereotactic unit and how many beams did it use?
Which medical conditions is it used for?
1/11/07 3d Conformal
What are the advantages of 3d Conformal therapy?
What changes in design made conformal therapy possible?
What considerations made conformal therapy necessary?
Have the clinical results been improved?
How do you setup a linac for radiosurgery?
How many beams do you need?
Which energy is best?
What relative dose can you get outside the target?
What was the original stereotactic unit and how many beams did it use?
Which medical conditions is it used for?
1/11/07 3d Conformal
What are the advantages of 3d Conformal therapy?
What changes in design made conformal therapy possible?
What considerations made conformal therapy necessary?
Have the clinical results been improved?
Tuesday, January 9, 2007
Monday, January 8, 2007
Neutron detectors
What types of neutron detectors are available?
Followup:
Are they sensitive to spectra?
What is the energy of neutrons generated in a linear accelerator?
Followup:
Are they sensitive to spectra?
What is the energy of neutrons generated in a linear accelerator?
Thursday, January 4, 2007
Shielding variables
Define the major variables used in a shielding calculation. What are typical values for each.
DRR writes, per NCRP reports No.49 and 51 we consider the workload (W), the use factor (U) the occupancy factor (T), the distance to the barrier (d) and the transmission factor of the shielding factor (B).
For a primary barrier this results in the following formula:
P=(W U T /distance squared)x B
DRR writes, per NCRP reports No.49 and 51 we consider the workload (W), the use factor (U) the occupancy factor (T), the distance to the barrier (d) and the transmission factor of the shielding factor (B).
For a primary barrier this results in the following formula:
P=(W U T /distance squared)x B
Wednesday, January 3, 2007
Radiation detection for rad safety
Briefly explain the types of equipment your clinic uses for radiation safety and the physics behind their operation. (submitted 1/03/07)
WMK remarks. Pogorsak (chapter 4) covers these in some detail.
WMK remarks. Pogorsak (chapter 4) covers these in some detail.
Tuesday, January 2, 2007
Collimator Scatter vs Phantom Scatter
Describe the difference between collimator scatter and phantom scatter. Tell how you would measure each clinically.
DRR
PROCEDURE FOR DETERMINING Sc
Total collimator scatter
This setup is in air and the chamber is placed on the ring stand 300 cm away from the treatment head. We have buildup cap over the chamber in this case to account for the needed buildup for the appropriate energy in air. Ex 6MV has its own buildup, 18MV has another.
The same field size as the phantom scatter are utilized.
PROCEDURE FOR DETERMINING St
TOTAL PHANTOM SCATTER
Total phantom scatter is determined for the annual calibration of each linear accelerator.
The setup requires the large water tank to develop full scatter conditions.
1) The chamber is placed on a ring stand at Dmax (the easiest way to do this is to tape the side of the box and measure carefully).
2) First the chamber is place even with the surface of the water.
3) Then this line is marked on tape and we measure the distance of Dmax (for SL-75 and 6 MV this is 1.5 cm) and mark this carefully on the tape.
4) Next we lower the chamber to this point and add an amount of water corresponding to Dmax
5) Now that the chamber is at dmax we plug in the electrometer and measure at various field sizes as defined in the worksheet.
SEE ATTACHED PHOTO OF SETUP
SEE SAMPLE SPREADSHEET
__________________________________________________________________
Phantom scatter is then obtained from knowing that St=Sc X Sp
NOTE THERE IS SOME THOUGHT THAT THE CHAMBER SHOULD BE SET AT 5 CM PER SOME READING WMK DID. THIS SEEMS TO MAKE MORE SENSE ANYWAYS AS WE WOULD BE PAST THE BUILDUP REGION.
DRR
PROCEDURE FOR DETERMINING Sc
Total collimator scatter
This setup is in air and the chamber is placed on the ring stand 300 cm away from the treatment head. We have buildup cap over the chamber in this case to account for the needed buildup for the appropriate energy in air. Ex 6MV has its own buildup, 18MV has another.
The same field size as the phantom scatter are utilized.
PROCEDURE FOR DETERMINING St
TOTAL PHANTOM SCATTER
Total phantom scatter is determined for the annual calibration of each linear accelerator.
The setup requires the large water tank to develop full scatter conditions.
1) The chamber is placed on a ring stand at Dmax (the easiest way to do this is to tape the side of the box and measure carefully).
2) First the chamber is place even with the surface of the water.
3) Then this line is marked on tape and we measure the distance of Dmax (for SL-75 and 6 MV this is 1.5 cm) and mark this carefully on the tape.
4) Next we lower the chamber to this point and add an amount of water corresponding to Dmax
5) Now that the chamber is at dmax we plug in the electrometer and measure at various field sizes as defined in the worksheet.
SEE ATTACHED PHOTO OF SETUP
SEE SAMPLE SPREADSHEET
__________________________________________________________________
Phantom scatter is then obtained from knowing that St=Sc X Sp
NOTE THERE IS SOME THOUGHT THAT THE CHAMBER SHOULD BE SET AT 5 CM PER SOME READING WMK DID. THIS SEEMS TO MAKE MORE SENSE ANYWAYS AS WE WOULD BE PAST THE BUILDUP REGION.
Monday, January 1, 2007
Plane parallel vs thimble ionization chamber
Explain the main differences between plane parallel chambers and thimble ionization chambers.
Give an example of when you would use each.
DRR
The parallel plate chamber is one in which voltage is applied to two thin, closely separated parallel plates that collect ions. The measurement positon is defined as just inside of the front electrode. (ref Berman). We call this the "pancake chamber" in our clinic and it is typically used for monthly output. The chamber is recommended for measurement of electron beams with E<10 MeV. (So we should research why do we use this for higher energies on the Varian?)
The thimble ionization chamber is a coaxial chamber with one electrode that forms a thimble shaped shell around the collecting volume and the oterh electrode is a narrow central rod. In our clinic we commonly call this the "Farmer" chamber. It can be used to measure higher energy radiation beams. (Research again the reasoning why we use such a chamber for our annual)
Give an example of when you would use each.
DRR
The parallel plate chamber is one in which voltage is applied to two thin, closely separated parallel plates that collect ions. The measurement positon is defined as just inside of the front electrode. (ref Berman). We call this the "pancake chamber" in our clinic and it is typically used for monthly output. The chamber is recommended for measurement of electron beams with E<10 MeV. (So we should research why do we use this for higher energies on the Varian?)
The thimble ionization chamber is a coaxial chamber with one electrode that forms a thimble shaped shell around the collecting volume and the oterh electrode is a narrow central rod. In our clinic we commonly call this the "Farmer" chamber. It can be used to measure higher energy radiation beams. (Research again the reasoning why we use such a chamber for our annual)
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