Our clinic uses these for eye plaques
Monday, March 31, 2008
Linac based stereotactic treatment procedures
Linac based stereotactic treatment procedures are:
a) single plane transverse rotation (Gamma Knife)
b) multiple noncoplanar converging arcs
c) single arc dynamic rotation
a) single plane transverse rotation (Gamma Knife)
b) multiple noncoplanar converging arcs
c) single arc dynamic rotation
Cylindrical tertiary collimators (cones)
Use of cylindrical tertiary collimators in stereotactic radiotherapy
a) provides more precise collimation
b) gives rise to a spherical dose distribution surrounding the target volume
c) gives sharper dose fall off compared to secondary collimators
a) provides more precise collimation
b) gives rise to a spherical dose distribution surrounding the target volume
c) gives sharper dose fall off compared to secondary collimators
Gamma Knife
Gamma Knife radiosurgery
201 Co-60 sources distributed isotropically in a hemisphere
Collimator diameters vary between 4 and 18 mm.
201 Co-60 sources distributed isotropically in a hemisphere
Collimator diameters vary between 4 and 18 mm.
Stereotactic with 6 MV
For a 6MV photon beam used for stereotactic radiosurgery, the minimum beam size diamter (in mm) required for electronic equilibrium is about 30 mm.
Dosimetry of small stereotactic fields
Dosimetry of small stereotactic fields involves:
The problem of lack of charged particle equilibrium be given special consideration and also that TMR and off axis ratios be measured for small fields.
With Co-60 beam treatment, one requires about 5 mm of tissue material for charged particle equilibrium. So a field size of less than about 1 cm will give reduced output due to lack of CPE. Similarly for a 6MV beam, the minimum field size for CPE is about 3 cm; though in practice, the condition may hold slightly smaller field sizes as well since the buildup curve is very steep.
So dosimetric functions must be measured for very small field sizes, for treatment purposes. Also such small fields require microchambers for measurement.
A Farmer type chamber has typical dimensions of about 16 to 18 mm diameter by about 22 mm length, which is not suitable for the dosimetry of small fields.
The solution is a PTW pinpoint chamber (SEE OR INSERT DIAGRAM)
The problem of lack of charged particle equilibrium be given special consideration and also that TMR and off axis ratios be measured for small fields.
With Co-60 beam treatment, one requires about 5 mm of tissue material for charged particle equilibrium. So a field size of less than about 1 cm will give reduced output due to lack of CPE. Similarly for a 6MV beam, the minimum field size for CPE is about 3 cm; though in practice, the condition may hold slightly smaller field sizes as well since the buildup curve is very steep.
So dosimetric functions must be measured for very small field sizes, for treatment purposes. Also such small fields require microchambers for measurement.
A Farmer type chamber has typical dimensions of about 16 to 18 mm diameter by about 22 mm length, which is not suitable for the dosimetry of small fields.
The solution is a PTW pinpoint chamber (SEE OR INSERT DIAGRAM)
Sunday, March 30, 2008
Adjacent photon fields abutting at depth of dose specification
In a treatment involving adjacent photon fields abutting at depth of dose specification, the overlap region (below the junction gets overdosed and the region above the junction gets overdosed).
In order to make the dose more uniform:
1) The gap on the surface can be increased or decreased in successive fractions
2) the two fields can be angled in opposite directions to make the beam edges parallel at the junction
3) The two fields can be treated by half fields (using half beam blocks or asymmetric jaws), the central axes becoming the matching edges of the two adjacent fields
4) A matching pair of POP fields can be used.
FOLLOWUP
Adjacent fields abut at the depth where the dose is required to be uniform (over the full length of the two abutting fields).
This however, creates high and low dose regions around the abutting junction. The high dose region is created by the adjacent fields diverging into each other. In order to accept this plan for treatment, the low end high dose regions must be free of tumor and organ at risk, respectively.
The "hot" and "cold" regions can be avoided by the various methods mentioned in this question, namely making the abutting field borders parallel to each other.
In order to make the dose more uniform:
1) The gap on the surface can be increased or decreased in successive fractions
2) the two fields can be angled in opposite directions to make the beam edges parallel at the junction
3) The two fields can be treated by half fields (using half beam blocks or asymmetric jaws), the central axes becoming the matching edges of the two adjacent fields
4) A matching pair of POP fields can be used.
FOLLOWUP
Adjacent fields abut at the depth where the dose is required to be uniform (over the full length of the two abutting fields).
This however, creates high and low dose regions around the abutting junction. The high dose region is created by the adjacent fields diverging into each other. In order to accept this plan for treatment, the low end high dose regions must be free of tumor and organ at risk, respectively.
The "hot" and "cold" regions can be avoided by the various methods mentioned in this question, namely making the abutting field borders parallel to each other.
CSI treatments
In CSI treatments, the cranial field on the abutting side can be made nondiverging by using a half beam block. The size of the abutting spinal field is 40cm x 6 cm. SSD is 100 cm.
If Pheta is the angle of rotation of the collimator required for the abutting of the cranial and spinal fields, Tan Pheta is given by
In this particular example, the abutting border has no divergence. So collimator rotation to match the divergence of the spinal field would be enough to match the two orthogonal fiedlds.
The inferior border of the brain field in this case, will not diverge into the abutting border of the spine field.
If Pheta is the angle of rotation of the collimator required for the abutting of the cranial and spinal fields, Tan Pheta is given by
In this particular example, the abutting border has no divergence. So collimator rotation to match the divergence of the spinal field would be enough to match the two orthogonal fiedlds.
The inferior border of the brain field in this case, will not diverge into the abutting border of the spine field.
Friday, March 28, 2008
TBI Treatment
In TBI treatment, the
1) prescribed dose is about 1200 cGy, generally delivered using POP of fields
2) dose is delivered as a single fraction
3) dose is delivered as multiple fractions, 10 to 15 cGy per fraction
High dose TBI is delivered in a single fraction or in a small number of fractions of 200 cGy/fraction, 6 fractions for a total 1200 cGy.
Low dose TBI is delivered in 10 to 15 fractions of 10 to 15 cGy per fraction. For further study see Podgorsak.
1) prescribed dose is about 1200 cGy, generally delivered using POP of fields
2) dose is delivered as a single fraction
3) dose is delivered as multiple fractions, 10 to 15 cGy per fraction
High dose TBI is delivered in a single fraction or in a small number of fractions of 200 cGy/fraction, 6 fractions for a total 1200 cGy.
Low dose TBI is delivered in 10 to 15 fractions of 10 to 15 cGy per fraction. For further study see Podgorsak.
Total body irradiation
Total body irradiation involves:
Irradiation of the whole body of the patient
Delivery of dose in single or multiple fractions
Use of Co-60 beams or accelerator photon beams
NOTE +/- 10% accuracy for both TBI and TSET
Irradiation of the whole body of the patient
Delivery of dose in single or multiple fractions
Use of Co-60 beams or accelerator photon beams
NOTE +/- 10% accuracy for both TBI and TSET
Tuesday, March 25, 2008
Gamma Analysis
Gamma Analysis
Although the use of the two factors provides the
independent evaluation of a dose difference and misalignment,
the gamma offers a composite analysis
with the two variables collapsed into one parameter
(Harms et al. 1998; Low et al. 1998c; Dupuydt,Van
Esch, and Huyskens 2002). The gamma is defined as
the square root of a linear quadratic addition of the
two factors, while they are provided in relative magnitude
to their acceptance criteria (CDTA and CDD), as
shown in equation (4.1).
Parallel Plate Chamber Design (Govinda Rajan)
PP chamber design is very simple.
1) Take two circular perspex plates of about 1 mm thickness and about 4or 5 cm in diameter.
2)Coat the inside of one disk with graphite(aquadaq).
3) Take the other disk and keep a thin ring of less than or equal to one mm thickness and 1 cm in diameter and keep it concentric with the disk centre and now coat with graphite. You now have the central collecting electrode and the outer guard with a smallinsulating gap in between.
4) Take a perspex ring of anout 5 cm diameterand about 2 mm thickness and fix the two disks on the ring with the graphite coated portion on the inside. you now have the high tension electrode, the central electrode and guard.
5)Now you have to take the connections. HT wall is the entry window. If you want dose at smaller depths you must make this wall thinner. If you want the cavity thickness to be smaller than 2 mm you must use a ring of smaller thickness as spacer between the two electrodes. If you want a larger electrode you must use a larger inner ring before coating withgraphite.If you need a larger guard, the diameter of the perspex disk chosen must be larger.
If you see the protocol. you will find PP chambers of different sizes because of changes in the parameters thatwe discussed. How the characteristics will depend on these dimensions are given in the protocol.If you can make the spacer ring whose thickess can be varied, you have an extrapolation chamber. If you make the wall materials interchangeable you can study wall effects.
You need some expertise to attach wires to the central electrode,guard and HT wall and take out and connect to cable.Hope this gives you some idea of PP chamber
Govinda Rajan.K.N.Govinda Rajan.K.N.
1) Take two circular perspex plates of about 1 mm thickness and about 4or 5 cm in diameter.
2)Coat the inside of one disk with graphite(aquadaq).
3) Take the other disk and keep a thin ring of less than or equal to one mm thickness and 1 cm in diameter and keep it concentric with the disk centre and now coat with graphite. You now have the central collecting electrode and the outer guard with a smallinsulating gap in between.
4) Take a perspex ring of anout 5 cm diameterand about 2 mm thickness and fix the two disks on the ring with the graphite coated portion on the inside. you now have the high tension electrode, the central electrode and guard.
5)Now you have to take the connections. HT wall is the entry window. If you want dose at smaller depths you must make this wall thinner. If you want the cavity thickness to be smaller than 2 mm you must use a ring of smaller thickness as spacer between the two electrodes. If you want a larger electrode you must use a larger inner ring before coating withgraphite.If you need a larger guard, the diameter of the perspex disk chosen must be larger.
If you see the protocol. you will find PP chambers of different sizes because of changes in the parameters thatwe discussed. How the characteristics will depend on these dimensions are given in the protocol.If you can make the spacer ring whose thickess can be varied, you have an extrapolation chamber. If you make the wall materials interchangeable you can study wall effects.
You need some expertise to attach wires to the central electrode,guard and HT wall and take out and connect to cable.Hope this gives you some idea of PP chamber
Govinda Rajan.K.N.Govinda Rajan.K.N.
Shielding notes
1. Shielding Design Goal
The shielding design goals (P) are 0.02 mSv/week (1mSv/yr) for uncontrolled areas and 0.1 mSv/week (5mSv/yr) for controlled areas.
The maximum dose equivalent in any one hour (TADR) is 0.02 mSv (20µSv).
SOURCE: Shielding Calculation Report
Varian iX
The shielding design goals (P) are 0.02 mSv/week (1mSv/yr) for uncontrolled areas and 0.1 mSv/week (5mSv/yr) for controlled areas.
The maximum dose equivalent in any one hour (TADR) is 0.02 mSv (20µSv).
SOURCE: Shielding Calculation Report
Varian iX
Monday, March 24, 2008
TSEI Total skin electron irradiation
Total skin electron irradiation involves:
1) treating large areas of skin and it's underlying layers
2) use of low energy (about 3 to 6 MeV) electron beams
3) measurement of beam uniformity and output for the treatment geometry
4) an accuracy of about +/ 10%
1) treating large areas of skin and it's underlying layers
2) use of low energy (about 3 to 6 MeV) electron beams
3) measurement of beam uniformity and output for the treatment geometry
4) an accuracy of about +/ 10%
Electron arc therapy
Electron arc therapy
1) it is used to treat superificial layers of skin that are not curved
2) it is used to create superficial layers that have curvature as well, ex the chest wall
3) it requires proper dosimetric procedures to be utilized
1) it is used to treat superificial layers of skin that are not curved
2) it is used to create superficial layers that have curvature as well, ex the chest wall
3) it requires proper dosimetric procedures to be utilized
Clarksons method
Using Clarksons method:
1) dose at any point in a patient can be computed for an irregular field
2) scatter and primary components of dose at any point in a patient can be evaluated.
1) dose at any point in a patient can be computed for an irregular field
2) scatter and primary components of dose at any point in a patient can be evaluated.
Blocking
When a part of the open beam is blocked for reducing dose to an organ at risk,
Primary dose in the patient under the unblocked region remains almost the same.
Moderate blocking does not affect the effective primary reaching the phantom. Scatter dose under the unblocked region decreases since the shadow region woudl scatter very little radiation.
The dose under the block comprises the primary leakage radiaton and scatter going into the shadow region from unblocked regions.
Primary dose in the patient under the unblocked region remains almost the same.
Moderate blocking does not affect the effective primary reaching the phantom. Scatter dose under the unblocked region decreases since the shadow region woudl scatter very little radiation.
The dose under the block comprises the primary leakage radiaton and scatter going into the shadow region from unblocked regions.
LDR, MDR, HDR
DOSE RATE Dose Rate at the dose specification point
Low dose rate (LDR) 0.4-2 Gy/h
Medium Dose Rate (MDR) 2-12 Gy/h
High Dose Rate (HDR) >12 Gy/h
Low dose rate (LDR) 0.4-2 Gy/h
Medium Dose Rate (MDR) 2-12 Gy/h
High Dose Rate (HDR) >12 Gy/h
Sunday, March 23, 2008
CT Numbers
Because CT numbers bear a linear relationship with the attenuation coefficients it is possible to infer electron density (electrons cm-3). Although CT numbers can be correlated with electron density, the relationship is not linear in the entire range of tissue densities. The nonlinearity is caused by change in atomic number of tissues, which affects the proportion of beam attenuation by Compton versus photoelectric interactions. Khan (see Figure on pg 233)
To ensure accurate dose calculation, the CT numbers must be converted
to electron densities and scattering powers. The conversion of CT numbers to
electron density and scattering power is usually performed with a user defined
look-up table, which in turn is generated using a water equivalent circular
phantom containing various inserts of known densities simulating normal body
tissues such as bone and lung. Pdgorsak.
To ensure accurate dose calculation, the CT numbers must be converted
to electron densities and scattering powers. The conversion of CT numbers to
electron density and scattering power is usually performed with a user defined
look-up table, which in turn is generated using a water equivalent circular
phantom containing various inserts of known densities simulating normal body
tissues such as bone and lung. Pdgorsak.
DRR
Digitally reconstructed radiographs
DRRs are produced by tracing ray lines from a virtual source position
through the CT data of the patient to a virtual film plane. The sum of the
attenuation coefficients along any one ray line gives a quantity analogous to
optical density (OD) on a radiographic film. If the sums along all ray lines from
a single virtual source position are then displayed on to their appropriate
positions on the virtual film plane, the result is a synthetic radiographic image
based wholly on the 3-D CT data set that can be used for treatment planning.
Figure 7.10 provides an example of a typical DRR.
Pdgorsak
DRRs are produced by tracing ray lines from a virtual source position
through the CT data of the patient to a virtual film plane. The sum of the
attenuation coefficients along any one ray line gives a quantity analogous to
optical density (OD) on a radiographic film. If the sums along all ray lines from
a single virtual source position are then displayed on to their appropriate
positions on the virtual film plane, the result is a synthetic radiographic image
based wholly on the 3-D CT data set that can be used for treatment planning.
Figure 7.10 provides an example of a typical DRR.
Pdgorsak
Friday, March 21, 2008
Depth shifting ionization curves for gradient effects (TG-51)
Figure 1: Effect of shifting depth-ionization data measured with cylindrical chambers upstream by 0.6 for photon beams (panel a) and 0.5 for electron beams (panel b) (with = 1.0 cm). The raw data are shown by curve I (long dashes) in both cases and the shifted data, which are taken as the depth-ionization curve, are shown by curve II (solid line). The value of the % ionization at point A (10 cm depth) in the photon beam gives and the depth at point B (solid curve, 50% ionization) in the electron beam gives from which can be determined (see section VIII.C). For the photon beams, curve II is effectively the percentage depth-dose curve. For the electron beams, curve II must be further corrected (see section X.D) to obtain the percentage depth-dose curve shown (short dashes - but this is not needed for application of the protocol).
Thursday, March 20, 2008
Craniospinal irradiation (CSI)
While treating a patient for craniospinal irradiation (CSI), two orthogonal fields must abut at the desired depth in the patient. If both fields are divergent, for the abutting of lateral and spinal fields:
Both collimator and couch must be turned through the angle of divergence of abutting spinal and cranial fields respectively.
See a figure in the Dosimetry Review Book.
The figure shows the rotation of the collimator by an angle to make the inferior border of the lateral field tangent to the P/A spinal field for the abutting of two orthogonal fields.
Both collimator and couch must be turned through the angle of divergence of abutting spinal and cranial fields respectively.
See a figure in the Dosimetry Review Book.
The figure shows the rotation of the collimator by an angle to make the inferior border of the lateral field tangent to the P/A spinal field for the abutting of two orthogonal fields.
Blocking and collimator scatter factor (Sc)
Example
In a radiation therapy treatment, using a Co-60 beam, the open field, 12 cmx12 cm requires some blocking at the periphery to protect normal structures around PTV. The blocked field was 8cm x 8 cm in size so:
Sc of blocked field = Sc of unblocked field
BECAUSE, blocks are below the area of collimator scatter.
In a radiation therapy treatment, using a Co-60 beam, the open field, 12 cmx12 cm requires some blocking at the periphery to protect normal structures around PTV. The blocked field was 8cm x 8 cm in size so:
Sc of blocked field = Sc of unblocked field
BECAUSE, blocks are below the area of collimator scatter.
Partial blocking
When a blocked field is not a significant fraction of an unblocked or open field;
1) the effective primary reaching the patient is altered to a negligible amount
2) the collimator scatter factor Sc remains the same as that of the unblocked field
Blocking does change scatter volume and hence the phantom scatter factor (Sp) changes.
The output factor also changes relative to the unblocked field.
1) the effective primary reaching the patient is altered to a negligible amount
2) the collimator scatter factor Sc remains the same as that of the unblocked field
Blocking does change scatter volume and hence the phantom scatter factor (Sp) changes.
The output factor also changes relative to the unblocked field.
Wednesday, March 19, 2008
SAR (scatter air ratio)
Scatter Air Ratio (SAR) is obtained by subtracting zero field TAR from TAR of the given field. It is generally required for the dosimetry of irregular fields.
It dose depend on field size since scatter volume increases with an increase in field size.
It dose depend on field size since scatter volume increases with an increase in field size.
Photon beam dose profile
The photon beam dose profile at depth of clinical interest is fairly uniform across the field size for Co-60 and linac beams.
It is nonuniform across field size for linac photon beams due to flattening filter effects.
It is nonuniform across field size for linac photon beams due to flattening filter effects.
Field blocking
Field blocking
1) reduces scatter volume
2) reduces scatter dose at the depth of interest
3) leaves the primary dose reaching the phantom along the beam central axis relatively unaffected (assuming that blocking is neither along central axis nor a larger fraction of open field)
1) reduces scatter volume
2) reduces scatter dose at the depth of interest
3) leaves the primary dose reaching the phantom along the beam central axis relatively unaffected (assuming that blocking is neither along central axis nor a larger fraction of open field)
Scatter dose
The scatter dose at any point in a phantom can be determined
1) by the evaluation of the scatter air ratio (SAR)
2) by the evaluation of the scatter maximum ratio
1) by the evaluation of the scatter air ratio (SAR)
2) by the evaluation of the scatter maximum ratio
Equivalent square field of an irregular field
The equivalent square field of any irregular field for any dose function can be determined by
CLARKSONS sector integration method
CLARKSONS sector integration method
Scatter Air Ratio
Scatter Air Ratio gives the scatter component of TAR
It is the difference between TAR and zero field TAR
The concept aids in separating the scatter and primary dose at a point in the patient
It is the difference between TAR and zero field TAR
The concept aids in separating the scatter and primary dose at a point in the patient
Independent jaw movement
Independent jaws in a linac aid in
1) Field blocking
2) Field splitting
3) Field matching
4) creating dynamic wedge fields
1) Field blocking
2) Field splitting
3) Field matching
4) creating dynamic wedge fields
Lead Thickness and electrons
The thickness of the lead cutout you would use with a 10 MeV electron is about:
5mm
Electron absorption is about 2 MeV/CM of unit density material (water) and 10 times less for lead so 2MeV/mm for lead.
5mm
Electron absorption is about 2 MeV/CM of unit density material (water) and 10 times less for lead so 2MeV/mm for lead.
Stereotactic Procedure
Image Transfer
Fixer (to remove unneeded segments below and above)
Localize the CT (selecting the rods) first before fusing. Fuse first CT axial to MR axial, then CT axial to MR coronal.
Save PTV in terms of it's contour as PTVaxial and PTV coronal to avoid confusion.
Create body contour and trim.
Plan by manipulating percentages to the target of interest and organs at risk
Fixer (to remove unneeded segments below and above)
Localize the CT (selecting the rods) first before fusing. Fuse first CT axial to MR axial, then CT axial to MR coronal.
Save PTV in terms of it's contour as PTVaxial and PTV coronal to avoid confusion.
Create body contour and trim.
Plan by manipulating percentages to the target of interest and organs at risk
Monday, March 17, 2008
Wedged treatment pairs
When using a pair of wedged treatment fields, the wedge angle is much smaller than the ideal one. The anterior region of overlap will lead to
a high dose region.
Conversely, when using a pair of wedged treatment fields and the wedge angle is much larger than the ideal one, the anterior region of overlap will lead to a low dose region
a high dose region.
Conversely, when using a pair of wedged treatment fields and the wedge angle is much larger than the ideal one, the anterior region of overlap will lead to a low dose region
Hinge angle and wedge angle PART II
In a treatment involving a pair of wedged fields, the hinge angle is 90 degrees. What wedge angle must be chosen to get uniform dose to the region of overlap?
90 - FI/2, where FI is the hinge angle.
90 - FI/2, where FI is the hinge angle.
Wedge Angle and Hinge Angle
Making dose in an overlap region uniform:
This can occur by wedging the two fields, choosing the proper wedge angle that would make the isodose lines parallel to one another in the overlap region.
Finally this can be performed by correcting for surface obliquity if or after wedging the fields.
NOTE:
For a given hinge angle, there is a wedge angle for a wedge pair of fields that would make the dose uniform in the beam overlap region.
This assumes that the incident surface is normal to the central axis. If the patient surface is sloping, either compensator should be made use of or the wedge angle, derived from hinge angle should be adjusted to take into account the hinge angle.
This can occur by wedging the two fields, choosing the proper wedge angle that would make the isodose lines parallel to one another in the overlap region.
Finally this can be performed by correcting for surface obliquity if or after wedging the fields.
NOTE:
For a given hinge angle, there is a wedge angle for a wedge pair of fields that would make the dose uniform in the beam overlap region.
This assumes that the incident surface is normal to the central axis. If the patient surface is sloping, either compensator should be made use of or the wedge angle, derived from hinge angle should be adjusted to take into account the hinge angle.
Sunday, March 16, 2008
Bolus
The use of bolus in electron beam therapy is obviously used to enhance the skin dose, but also can be used for depth compensation as well (for example, if there is great variation in the chest wall thickness).
Lead vs Cerrobend
The thickness of lead required for making a field block for a clinical photon beam is about 8 cm. If Cerrobend material is used, the required thickness would be:
ABOUT 20% MORE. This is because cerrobend is 20% less dense than lead.
ABOUT 20% MORE. This is because cerrobend is 20% less dense than lead.
Blocks vs MLC
Disadvantages in using custom blocks are:
Small errors in block position can adversely affetct treatment and fabrication, alignment, etc take alot of time.
This keys in to the advantages of MLC, which also include the fact that treatment delivery is faster and they are equally convenient for any angled field.
Small errors in block position can adversely affetct treatment and fabrication, alignment, etc take alot of time.
This keys in to the advantages of MLC, which also include the fact that treatment delivery is faster and they are equally convenient for any angled field.
Thickness of blocks
The thickness of blocks used for field blocking (in HVL) is about 5 HVL's.
To reduce the dose in the shielding region to less than 5% of the open field dose, the shield thickness must be greater than 5HVL's
To reduce the dose in the shielding region to less than 5% of the open field dose, the shield thickness must be greater than 5HVL's
Primary transmission for a block
The primary transmission for a block used to shield a portion of the field is usually about
3% to 5%
The dose under a block in a patient will be MORE than 3% to 5% and depends on how much patient scatter the shadow region of the patient receives.
3% to 5%
The dose under a block in a patient will be MORE than 3% to 5% and depends on how much patient scatter the shadow region of the patient receives.
Errors in custom field blocking
Errors in custom field blocking can arise due to
1) incorrect source to film distance (SFD)
2) incorrect source to tray distance
3) incorrect positioning of the block on the tray.
Incorrect SFD will change the size of the block fabricated
Incorrect source to tray distance or incorrect repositioning of the block or tray will influence the region blocked in patient.
1) incorrect source to film distance (SFD)
2) incorrect source to tray distance
3) incorrect positioning of the block on the tray.
Incorrect SFD will change the size of the block fabricated
Incorrect source to tray distance or incorrect repositioning of the block or tray will influence the region blocked in patient.
Physical Penumbra
The physical penumbra width is defined as the lateral distance between two specified isodose curves at a specified depth (e.g., lateral distance between 90% and 20% isodose lines at the depth of dmax).
Geometric penumbra
Geometric penumbra, which exists both inside and outside the geometrical boundaries of the beam depends on:
Source size
Distance from the source and
Source to diaphragm distance.
Source size
Distance from the source and
Source to diaphragm distance.
"HORNS"
Linac x-ray beams exhibit areas of high dose known as horns, near the surface in the periphery in the field.
These horns are created by the flattening filter, which is usually designed to overcompensate near the surface in order to obtain flat isodose curves at great depths.
These horns are created by the flattening filter, which is usually designed to overcompensate near the surface in order to obtain flat isodose curves at great depths.
Saturday, March 15, 2008
Monday, March 10, 2008
Imaging book
Radiology Review: Radiologic Physics (Paperback)by Edward L. Nickoloff (Author), Naveed Ahmad (Author)Price: $56.95
Will take a look for this at med school bookstore. Someone on yahoo group said reviews recommended it over Huda.
Will take a look for this at med school bookstore. Someone on yahoo group said reviews recommended it over Huda.
Custom blocks
Custom blocks used for shielding critical structures in the treatment of MV photon beams must be diverging.
THEY ARE NOT KEPT TO CLOSE TO THE PATIENT SKIN BECAUSE THAT WOULD INCREASE THE ELECTRON CONTAMINATION REACHING THE PATIENT
However, also consider this: Another reason not to keep it close to the patient is that if the block is closer to the patient, the larger size and weight of the block needes to shield the same volume will be a disadvantage. This occurs due to the beam divergence.
On the other hand, the transmission penumbra would be larger for a larger block to skin distance. A block to skin distance of about 15 to 20 cm is a good compromise for positioning custom blocks in patient treatment.
THEY ARE NOT KEPT TO CLOSE TO THE PATIENT SKIN BECAUSE THAT WOULD INCREASE THE ELECTRON CONTAMINATION REACHING THE PATIENT
However, also consider this: Another reason not to keep it close to the patient is that if the block is closer to the patient, the larger size and weight of the block needes to shield the same volume will be a disadvantage. This occurs due to the beam divergence.
On the other hand, the transmission penumbra would be larger for a larger block to skin distance. A block to skin distance of about 15 to 20 cm is a good compromise for positioning custom blocks in patient treatment.
Field weighting
Field weighting
1) gives the relative contribution of beams to dose at target center or dmax point
2) improves dose uniformity across the target
3) reduces dose to normal tissues or critical structures
Field weighting is used:
When contribution from any of the fields needs to be reduced or increased with respect to another field.
1) gives the relative contribution of beams to dose at target center or dmax point
2) improves dose uniformity across the target
3) reduces dose to normal tissues or critical structures
Field weighting is used:
When contribution from any of the fields needs to be reduced or increased with respect to another field.
Dose rate and extended SSD
In the previous post, we have a patient with a 6 MV photon beam at an extended SSD of 125 cm. The calibration dose rate is given at Dcal (1.5, 10x10, 100) by 0.993 cGy/MU.
Calculate the reference dose rate in cGy/MU for the new SSD:
ANSWER
Obviously we know the dose rate in cGy/MU will decrease with a change to the extended SSD distance. The extent is given by
Calib dose rate x (100+dmax)/(extended SSD+ dmax) = 0.993 (100+1.5)/(125+1.5)
Calculate the reference dose rate in cGy/MU for the new SSD:
ANSWER
Obviously we know the dose rate in cGy/MU will decrease with a change to the extended SSD distance. The extent is given by
Calib dose rate x (100+dmax)/(extended SSD+ dmax) = 0.993 (100+1.5)/(125+1.5)
Treatment at extended SSD
A patient is to be treated with a 6MV photon beam for a field size of 12cmx12cm on the patient surface, at an extended SSD of 125 cm. (PLEASE SKETCH DIAGRAM). The depth of target center is 9 cm. In this case (compared to a standard SSD treatment):
1) The reference dose rate (or dose/MU) at the input port (dmax position) decreases relative to a shorter SSD
2) the PDD (9, 12x12, 125) increases due to the increased SSD
3) the PDD correction is approximately given by Mayneords F Factor
1) The reference dose rate (or dose/MU) at the input port (dmax position) decreases relative to a shorter SSD
2) the PDD (9, 12x12, 125) increases due to the increased SSD
3) the PDD correction is approximately given by Mayneords F Factor
Extended SSD treatments
Extended SSD treatments are used for:
1) Total Body irradiation
2) Large mantle fields
1) Total Body irradiation
2) Large mantle fields
Clarksons method
Clarksons method is useful for dose calculation of irregular fields. (i.e. Mantle cases, per example in Khan)
Entrance dose and exit dose
Entrance dose is defined as the dose at dmax for the incident field.
The exit dose is defined as the dose at dmax for the exiting field.
The exit dose is defined as the dose at dmax for the exiting field.
Collimator scatter
The collimator scatter (Sc) for accelerator photon beams can be measured in air with the Farmer chamber with a cap of appropriate build-up thickness.
Increase in field size
With an increase in field size, the (effective) primary radiation incident on a patient increases.
Minimizing gap between two adjacent fields
To minimize the gap between two adjacent fields:
A half beam block can be used.
A half beam block can be used.
Physical Penumbra
Physical penumbra depends on
Geometric penumbra
Collimator transmission
Lateral Photon Scatter (in the patient)
Lateral Electron Transport (in the patient)
Geometric penumbra
Collimator transmission
Lateral Photon Scatter (in the patient)
Lateral Electron Transport (in the patient)
Beam quality and Peak Scatter Factor (PSF)
Beam quality and PSF for a 15cmx15 cm field
a) Co-60 beam - 1.05
b) 6 MV- 1.015
c) 18 MV- 1.008
a) Co-60 beam - 1.05
b) 6 MV- 1.015
c) 18 MV- 1.008
Beam Quality and PDD (10x10, 10 deep)
Beam Quality and PDD (10x10, 10 deept), SSD=80 cm for Co-60 and 100 cm for accelerator photon beams.
BEAM PDD
Co-60 55.6
4 MV 64.8
10 MV 73
BEAM PDD
Co-60 55.6
4 MV 64.8
10 MV 73
Overall Uncertainty in Dose Delivery
Per Khan Ed 3, Table 17.4, pg 431.
Step Uncertainty (%)
Ion chamber calibration 1.6
Calibration procedure 2.0
Dose calc parameters and methods 3.0
Effective depth 2.0
SSD 2.0
Wedges 2.0
Blocking Trays 2.0
CUMULATIVE 5.6
Step Uncertainty (%)
Ion chamber calibration 1.6
Calibration procedure 2.0
Dose calc parameters and methods 3.0
Effective depth 2.0
SSD 2.0
Wedges 2.0
Blocking Trays 2.0
CUMULATIVE 5.6
Beam quality and dmax for a 10x10 field
Beam quality and depth of dmax for a 10x10 cm field
Kilovoltage- 0 cm
Co-60 - 0.5 cm
10 MV- 2.5 cm
25 MV- 3.5 cm
Kilovoltage- 0 cm
Co-60 - 0.5 cm
10 MV- 2.5 cm
25 MV- 3.5 cm
Surface dose for clinical electron beams
The surface dose for clinical electron beams is about 75% to 95%
Surface dose for electron beams is much larger compared to photon beams and can be measured with an extrapolation chamber.
Surface dose for electron beams is much larger compared to photon beams and can be measured with an extrapolation chamber.
Electron energies (in MeV) and their ranges in water
Range in water for various electron energies
6 MeV ------- 3 cm
8 Mev ------- 4 cm
10 Mev------ 4.9 cm
20 MeV----- 9.2 cm
The range in cm is approximately related to the energy in MeV/2
6 MeV ------- 3 cm
8 Mev ------- 4 cm
10 Mev------ 4.9 cm
20 MeV----- 9.2 cm
The range in cm is approximately related to the energy in MeV/2
Sunday, March 9, 2008
Thursday, March 6, 2008
Mean Energy of the electron beam (Eo)
The mean energy of the electron beam, Eo, can be determined, fairly accurately, from the parameter
2.33 R50
2.33 R50
Corrections for ion chamber measurements
For accurate dosimetry, chamber measurements must be corrected for
a) saturation
b) polarity
c) temperature/pressure
a) saturation
b) polarity
c) temperature/pressure
Electron energy specified by the machine manufacturer (II)
The electron energy specified by the machine manufacturer refers to:
The most probable energy of the electrons. The manufacturer usually measures the practical range of the electron beam, which is related to the most probable energy of the electron beam.
The most probable energy of the electrons. The manufacturer usually measures the practical range of the electron beam, which is related to the most probable energy of the electron beam.
Electron energy specified by the manufacturer
The electron energy beam incident on a patient can be characterized by
a) most probable energy
b) mean energy
c) a maximum energy
a) most probable energy
b) mean energy
c) a maximum energy
Wednesday, March 5, 2008
1 mm Pb foil
1 mm Pb Foil is placed in the beam path during photon beam calibration because it:
introduces known electron contamination, facilitating the determination of PDD only due to photons, PDD (10)x, specifier of beam quality.
introduces known electron contamination, facilitating the determination of PDD only due to photons, PDD (10)x, specifier of beam quality.
Plane parallel chambers
Plane parallel chambers are recommended for the dosimetry of low-energy electron beams (<10 MeV) in place of Farmer type chambers, because:
Farmer type chambers give rise to significant electron fluence perturbation at low electron energies
Farmer type chambers are less accurrate for low-energy electrons.
Farmer type chambers give rise to significant electron fluence perturbation at low electron energies
Farmer type chambers are less accurrate for low-energy electrons.
Tuesday, March 4, 2008
TG51, Kq
Kq can vary from chamber to chamber by as much as 5%
http://www.irs.inms.nrc.ca/tg51/sld013.htm
http://www.irs.inms.nrc.ca/tg51/sld013.htm
Monday, March 3, 2008
AAPM TG-51 protocol
The AAPM TG-51 protocol is based on calibration in Co-60 beams in terms of
absorbed dose to water.
The reference phantom for beam calibration (AAPM TG-51) is water.
absorbed dose to water.
The reference phantom for beam calibration (AAPM TG-51) is water.
Recommended depth for photon beam calibration
The recommended depth for photon beam calibration is 10 cm.
The calibration depth must be much beyond the depth of dose maximum, ie. the transient equilibrium region, where the electron contamination does not reach.
A depth of 10 cm will satisfy this criterion for the whole range of clinically used photon beam qualities. Earlier recommendations gave depth of 5 cm, 7 cm and 10 cm for different beam qualities, but recent protocols recommend a single depth of 10 cm for all beam qualities.
The calibration depth must be much beyond the depth of dose maximum, ie. the transient equilibrium region, where the electron contamination does not reach.
A depth of 10 cm will satisfy this criterion for the whole range of clinically used photon beam qualities. Earlier recommendations gave depth of 5 cm, 7 cm and 10 cm for different beam qualities, but recent protocols recommend a single depth of 10 cm for all beam qualities.
Dmax for Co-60
The depth of dose maximum in a patient for a relatively clean Co-60 beam is
5 mm (0.5cm)
5 mm (0.5cm)
Surface or buildup region doses and measurement
Surface or buildup region doses must be measured using an extrapolation chamber.
Yes- The entry window thickness becomes the thickness of the dead layer of the skin and the chamber would measure the dose just below the dead layer of the skin.
Yes- The entry window thickness becomes the thickness of the dead layer of the skin and the chamber would measure the dose just below the dead layer of the skin.
Field size dependence of the PDD of an accelerator photon beam
The field size dependence of the PDD of an accelerator photon beam is very much influenced by the electron contamination in the beam.
This is because of electron and scatter photon contamination of the beam.
This is because of electron and scatter photon contamination of the beam.
AAPM TG-51 and electron beam quality
In AAPM TG-51 protocol, the electron beam quality is specified by the beam penetration parameter, R50,D which is the depth where the electron beam depth dose drops to 50% of the peak value. This is more accurate than the parameter, Eo, the mean energy of the electron beam used in the earlier protocols (as derived from R50 using an empirical equation)
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