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Electron therapyElectron therapy Class 3: MU calculationsClass 3: MU calculations
Laurence [email protected]
Laurence CourtLaurence [email protected]@mdanderson.org
Reference: Faiz M. Khan, The Physics of Radiation Therapy
Slide acknowledgements: Karl Prado, Rebecca Howell, Kent Gifford, and Khan’s book
Some review questions
•
What is a pencil beam?
•
Explain the concept of a pencil beam algorithm for electrons
•
How does it account for field shape?
•
How does it account for heterogeneities?
••
What is a pencil beam?What is a pencil beam?
••
Explain the concept of a pencil beam Explain the concept of a pencil beam algorithm for electronsalgorithm for electrons
••
How does it account for field shape?How does it account for field shape?
••
How does it account for heterogeneities?How does it account for heterogeneities?
Monte Carlo review questionws
•
What is Macro Monte Carlo?•
What is the effect of smoothing the dose distribution from a MMC calculation?
•
How accurate can we expect MMC MU calculations to be for fairly simple situations?
•
What clinical scenarios are often not included in commercial TPS electron calculations?
•
What clinical scenarios are ones where we might bring concerns about the accuracy of dose calculation to the radiation oncoligists
attention?
••
What is Macro Monte Carlo?What is Macro Monte Carlo?••
What is the effect of smoothing the dose What is the effect of smoothing the dose distribution from a MMC calculation?distribution from a MMC calculation?
••
How accurate can we expect MMC MU How accurate can we expect MMC MU calculations to be for fairly simple situations? calculations to be for fairly simple situations?
••
What clinical scenarios are often not included in What clinical scenarios are often not included in commercial TPS electron calculations?commercial TPS electron calculations?
••
What clinical scenarios are ones where we might What clinical scenarios are ones where we might bring concerns about the accuracy of dose bring concerns about the accuracy of dose calculation to the radiation calculation to the radiation oncoligistsoncoligists
attention?attention?
A few more review questions
•
What is the thickness of Pb
required to stop a 16MeV electron beam?
•
What is the approximate average energy of a 16MeV electron beam after 4cm of water?
•
What is the primary cause of:–
Dose beyond Rp+2cm?
–
Buildup, first few mm?–
Buildup, first few mm?
–
Slow falloff beyond R90?–
Finite energy range
••
What is the thickness of What is the thickness of PbPb
required to stop a required to stop a 16MeV electron beam?16MeV electron beam?
••
What is the approximate average energy of a What is the approximate average energy of a 16MeV electron beam after 4cm of water?16MeV electron beam after 4cm of water?
••
What is the primary cause of:What is the primary cause of:––
Dose beyond Rp+2cm?Dose beyond Rp+2cm?
––
Buildup, first few mm?Buildup, first few mm?––
Buildup, first few mm?Buildup, first few mm?
––
Slow falloff beyond R90?Slow falloff beyond R90?––
Finite energy rangeFinite energy range
MU calculation
ODDMU
%
EOFSCSEOFFSCSEO cal ,,,,
D Prescribed dose%D Prescribed %O Dose output at R100
Output depends on energy, applicator, field size, SSD and skin collimation.
First consider different cones and field sizes:
Note: All at R100 , which is dependent of field size and energy
),(100100 FSERR
WxWLxLLxW OFOFOF
Example output chart for 9MeV electron (Varian)
Irregular field shapes
•
Measure•
Analytical algorithm or Monte Carlo code
•
Equivalent rectangles–
Applicator insert field shape
–
Max dose in broadest region of field–
Distant regions do not usually contribute significantly to output
–
Not sufficiently accurate for highly irregular fields –
measure these!
••
MeasureMeasure••
Analytical algorithm or Monte Carlo codeAnalytical algorithm or Monte Carlo code
••
Equivalent rectanglesEquivalent rectangles––
Applicator insert field shapeApplicator insert field shape
––
Max dose in broadest region of fieldMax dose in broadest region of field––
Distant regions do not usually contribute Distant regions do not usually contribute significantly to outputsignificantly to output
––
Not sufficiently accurate for highly irregular Not sufficiently accurate for highly irregular fields fields ––
measure these!measure these!
Off-axis fields
•
Often neglect this
•
Flatness < 3%
•
Can assume radial OAR
••
Often neglect thisOften neglect this
••
Flatness < 3%Flatness < 3%
••
Can assume radial OARCan assume radial OAR
Irregular field inserts: Equivalent rectangles
Treating at extended SSD
•
Electron output dose not follow a simple conventional inverse-square relationship
–
Lots of attenuation, scatter in air, collimation, foils, etc…
•
Distance corrections take two forms:–
Use of an “effective SSD”
that can be used in an
inverse-square fashion. –
Use of an “air-gap factor”
that can be used in
addition to (on instead of) a conventional inverse-square factor
••
Electron output dose not follow a simple Electron output dose not follow a simple conventional inverseconventional inverse--square relationshipsquare relationship––
Lots of attenuation, scatter in air, collimation, Lots of attenuation, scatter in air, collimation, foils, etcfoils, etc……
••
Distance corrections take two forms:Distance corrections take two forms:––
Use of an Use of an ““effective SSDeffective SSD””
that can be used in an that can be used in an
inverseinverse--square fashion.square fashion.––
Use of an Use of an ““airair--gap factorgap factor””
that can be used in that can be used in
addition to (on instead of) a conventional addition to (on instead of) a conventional inverseinverse--square factorsquare factor
Electron source
•
Virtual Source –
An intersection point of the back-projections along the most probable directions of electron motion at the patient surface.
••
Virtual Source Virtual Source ––
An An intersection point of the intersection point of the backback--projections along the projections along the most probable directions most probable directions of electron motion at the of electron motion at the patient surface.patient surface.
2
maxEffective
maxEffective
gdSSDdSSDCorrection Gap
Effective SSD is a function of field size (cone) and energy
Effective SSD ValuesEffective SSD Values6 x 66 x 6 10 x 1010 x 10 15 x 1515 x 15 20 x 2020 x 20 25 x 2525 x 25
6MeV6MeV 118.90118.90 82.1682.16 88.6088.60 90.0390.03 90.9990.99
9MeV9MeV 77.5677.56 86.3486.34 88.7588.75 90.7690.76 91.4691.46
12MeV12MeV 117.13117.13 86.3486.34 89.2889.28 90.5390.53 91.4891.48
16MeV16MeV 84.6684.66 87.1487.14 89.6689.66 91.7991.79 92.8792.87
20MeV20MeV 80.1780.17 83.7283.72 88.6388.63 91.5991.59 92.9192.91
Air-gap factor
May be combined into a single fair
• fair is a function of energy and field size• Use square-root method for rectangular fields:
Example air-gap factors for 9MeV beam
Beware of different approaches
ODDMU
%
ODCFDMU
%
Approach 1:
Approach 3:
ODDMU
%
Approach 2:
Output tabulated as O(E,CS,FS,SSD)
CF(E,FS,SSD) – corrects for SSDOutput is O(E,CS,FS)
Approach 2: Output factors and SSD effects included in a single factor
1.0
1.2
1.4
1.6
1.8
2.0
2.2
0 5 10 15 20 25
Eqs (cm)
CF
6MeV9MeV12MeV15MeV18MeV
After eqs~5cm, CF has steep gradients
For eqs>10cm, CF is fairly flat at ~1.23
For 5<eqs<10cm, CF has shallow gradient
Approach 3: Field-size dependent CF
Beware of different approaches
ODDMU
%
ODCFDMU
%
Approach 1:
Approach 3:
ODDMU
%
Approach 2:
Output tabulated as O(E,CS,FS,SSD)
CF(E,FS,SSD) – corrects for SSDOutput is O(E,CS,FS)
Calculations to include bolus
•
When bolus is used, the depth-dose curve shifts “upstream”
by a distance equal to the bolus
thickness (e.g. if 1 cm bolus is used, the depth of dmax
shifts by a distance of 1 cm toward the skin surface)
•
–The output at this shorter distance is:
••
When bolus is used, the depthWhen bolus is used, the depth--dose curve shifts dose curve shifts ““upstreamupstream””
by a distance equal to the bolus by a distance equal to the bolus
thickness (e.g. if 1 cm bolus is used, the depth of thickness (e.g. if 1 cm bolus is used, the depth of dd
maxmax
shifts by a distance of 1 cm toward the skin shifts by a distance of 1 cm toward the skin surface)surface)
••
––The output at this shorter distance is:The output at this shorter distance is:
Where b is the bolus thickness in CD, and SSD is the nominal SSD
Tissue Inhomogeneities
•
Electron beam dose distribution can be significantly altered in the presence of tissue inhomogeneities; bone, lung, air cavities
•
Difficult to determine dose distribution within and around small inhomogeneities
because of
enhanced scattering effects.
•
For LARGE and UNIFORM slabs, dose distribution beyond an inhomogeneity
using the
coefficient of equivalent thickness (CET) method.
••
Electron beam dose distribution can be Electron beam dose distribution can be significantly altered in the presence of tissue significantly altered in the presence of tissue inhomogeneitiesinhomogeneities; bone, lung, air cavities; bone, lung, air cavities
••
Difficult to determine dose distribution within Difficult to determine dose distribution within and around small and around small inhomogeneitiesinhomogeneities
because of because of
enhanced scattering effects.enhanced scattering effects.
••
For LARGE and UNIFORM slabs, dose For LARGE and UNIFORM slabs, dose distribution beyond an distribution beyond an inhomogeneityinhomogeneity
using the using the
coefficient of equivalent thickness (CET)coefficient of equivalent thickness (CET) method.method.
•
The attenuation by a given thickness, z of the inhomogeneity
is equivalent to z x CET.
Where,
•
Dose at a point beyond the inhomogeneity
is determined by calculating effective depth, deff
, along the ray joining the point and the virtual source of the electrons.
•
May include correction because of ISF
••
The attenuation by a given thickness, z of the The attenuation by a given thickness, z of the inhomogeneityinhomogeneity
is equivalent to z x CET. is equivalent to z x CET.
Where, Where,
••
Dose at a point beyond the Dose at a point beyond the inhomogeneityinhomogeneity
is is determined by calculating effective depth, determined by calculating effective depth, dd
effeff
, , along the ray joining the point and the virtual along the ray joining the point and the virtual source of the electrons. source of the electrons.
••
May include correction because of ISFMay include correction because of ISF
Coefficient of Equivalent Thickness (CET)
OH
ityinhomogene
2DensityElectron
DensityElectron CET
Coefficient of Equivalent Thickness (CET)
•
Bone–
CETCompact
Bone
≈
1.65
–
CETSpongy
Bone ≈
1.1, can assume unity
–
CET method is in good agreement with in vivo measurements in patients for dose behind the mandible.
•
Lung–
Studies show considerable variation of CET with depth in the lung.
–
CET is only a rough approximation for lung inhomogeneity
••
BoneBone––
CETCET
CompactCompact
BoneBone
≈≈
1.651.65
––
CETCET
SpongySpongy
Bone Bone ≈≈
1.1, can assume unity1.1, can assume unity
––
CET method is in good agreement with in vivo measurements CET method is in good agreement with in vivo measurements in patients for dose behind the mandible.in patients for dose behind the mandible.
••
LungLung––
Studies show considerable variation of CET with depth in the Studies show considerable variation of CET with depth in the lung.lung.
––
CET is only a rough approximation for lung CET is only a rough approximation for lung inhomogeneityinhomogeneity
Question
•
We calculated 250 MU for an electron treatment (6Mev, 0.5cm bolus) to the scalp. The physician wants to know why we didn’t consider the bone in the calc, and if we did what effect would it have on the MU and PDD. What should we tell them?
••
We calculated 250 MU for an electron We calculated 250 MU for an electron treatment (6Mev, 0.5cm bolus) to the scalp. treatment (6Mev, 0.5cm bolus) to the scalp. The physician wants to know why we didnThe physician wants to know why we didn’’t t consider the bone in the calc, and if we did consider the bone in the calc, and if we did what effect would it have on the MU and what effect would it have on the MU and PDD. What should we tell them?PDD. What should we tell them?
Raphex Question: T61, 2002
••
An electron beam with a custom insert has a An electron beam with a custom insert has a measured OF measured OF ofof
0.954cGy/MU at 0.954cGy/MU at dd
maxmax
. If 200cGy . If 200cGy are prescribed to the 90% are prescribed to the 90% isodoseisodose, the MU setting , the MU setting is ___.is ___.
A.A.
117117B.B.
210210
C.C.
212212D.D.
222222
E.E.
233233
Raphex Question: T60, 1999
•
How much dose is delivered at the 90% PDD level from an electron beam in 100MU. OF=1.05cGy/MU.
A.
110cGy
B.
106cGyC.
100cGy
D.
95cGyE.
90cGy
••
How much dose is delivered at the 90% PDD level How much dose is delivered at the 90% PDD level from an electron beam in 100MU. OF=1.05cGy/MU.from an electron beam in 100MU. OF=1.05cGy/MU.
A.A.
110cGy110cGyB.B.
106cGy106cGy
C.C.
100cGy100cGyD.D.
95cGy95cGy
E.E.
90cGy90cGy
Raphex Question: T66, 2000
•
An electron cone has an output of 1.13cGy/MU. The MU setting to deliver 200cGy at the 93% isodose
level
is _____MU? A.
243
B.
215C.
190
D.
177E.
165
••
An electron cone has an output of 1.13cGy/MU. The An electron cone has an output of 1.13cGy/MU. The MU setting to deliver 200cGy at the 93% MU setting to deliver 200cGy at the 93% isodoseisodose
level level
is _____MU?is _____MU?A.A.
243243
B.B.
215215C.C.
190190
D.D.
177177E.E.
165165
Raphex Question: T48, 2001
•
A 12MeV electron field delivers 200cGy at the 90% isodose
level. The output of the cone and insert is
1.02cGy/MU. The MU setting is____.
A.
176B.
184
C.
218D.
227
E.
250
••
A 12MeV electron field delivers 200cGy at the 90% A 12MeV electron field delivers 200cGy at the 90% isodoseisodose
level. The output of the cone and insert is level. The output of the cone and insert is
1.02cGy/MU. The MU setting is____.1.02cGy/MU. The MU setting is____.
A.A.
176176B.B.
184184
C.C.
218218D.D.
227227
E.E.
250250
Raphex Question: T49, 2001
•
A lesion of a maximum depth of 3cm is treated with 12MeV electrons (PDD data below). 1 cm of bolus is placed on the skin. If 200cGy is delivered to the distal edge of the tumor, the skin dose and maximum tissue doses are?
•
Use the PDD Data below:
Depth (cm)
0 1 2 3 4 5 6
PDD
90
95
98
100
80
40
5
••
A lesion of a maximum depth of 3cm is treated with A lesion of a maximum depth of 3cm is treated with 12MeV electrons (PDD data below). 1 cm of bolus is 12MeV electrons (PDD data below). 1 cm of bolus is placed on the skin. If 200cGy is delivered to the distal placed on the skin. If 200cGy is delivered to the distal edge of the tumor, the skin dose and maximum tissue edge of the tumor, the skin dose and maximum tissue doses are?doses are?
••
Use the PDD Data below:Use the PDD Data below:
Depth (cm)Depth (cm)
00 11
22
33
44
55
66
PDDPDD
9090
9595
9898
100100
8080
4040
55
A bonus question
•
We decide to add 1cm bolus to a 6MeV electron field. Which of the following is not true (if we do not change the MUs)?
a)
The position of dmax
moves upstream by 1cmb)
The dose at 2cm depth in the tissue goes down
c)
The skin dose increases to almost dmax
d)
The maximum dose increases by 2%e)
The maximum dose decreases by 2%
••
We decide to add 1cm bolus to a 6MeV We decide to add 1cm bolus to a 6MeV electron field. Which of the following is electron field. Which of the following is not true (if we do not change the not true (if we do not change the MUsMUs)?)?
a)a)
The position of The position of dmaxdmax
moves upstream by 1cmmoves upstream by 1cmb)b)
The dose at 2cm depth in the tissue goes The dose at 2cm depth in the tissue goes downdown
c)c)
The skin dose increases to almost The skin dose increases to almost dd
maxmax
d)d)
The maximum dose increases by 2%The maximum dose increases by 2%e)e)
The maximum dose decreases by 2%The maximum dose decreases by 2%
Extended SSD, rectangular field
•
Dose prescription: 180 cGy
to 100% isodose
•
Electron energy: 9 MeV
•
Applicator: 15x15 cm2
•
Field size: 5x12 cm2
•
SSD: 110 cm
••
Dose prescription: 180 Dose prescription: 180 cGycGy
to 100% to 100% isodoseisodose
••
Electron energy: 9 Electron energy: 9 MeVMeV
••
Applicator: 15x15 cmApplicator: 15x15 cm22
••
Field size: 5x12 cmField size: 5x12 cm22
••
SSD: 110 cmSSD: 110 cmOD
DMU
%
What if we:
•
Add 1cm bolus:
•
What would be the change in dose at Rmax if we added 4x11 cm2
skin collimation?
••
Add 1cm bolus:Add 1cm bolus:
••
What would be the change in dose at What would be the change in dose at RmaxRmax if we added 4x11 cmif we added 4x11 cm22
skin collimation?skin collimation?
Effect of Oblique Incidence on Dose Distribution
••
The broad electron The broad electron beam can be beam can be represented as a large represented as a large number of pencil number of pencil beams placed beams placed adjacent to each adjacent to each other. other.
When a beam is obliquely incident on the patient’s surface:– Points at shallow depths receive greater
side scatter from adjacent pencil beams, which have traversed a greater amount of the material.
– Points at greater depths receive less scatter.
Beam obliquity factor
More homework, I’m afraid
•
Feel free to create an Excel table
•
Please include all working (OF used, etc) so I can understand what happened if you don’t get the correct answer (partial points may be possible!)
•
Always use a sanity check: What answer do we expect for:
–
200cGy, 80%, 100cm SSD?
–
200cGy, 90%, 100cm SSD?
–
200cGy, 90%, 110cm SSD?
••
Feel free to create an Excel tableFeel free to create an Excel table
••
Please include all working (OF used, etc) so I can Please include all working (OF used, etc) so I can understand what happened if you donunderstand what happened if you don’’t get the t get the correct answer (partial points may be possible!)correct answer (partial points may be possible!)
••
Always use a sanity check: What answer do we Always use a sanity check: What answer do we expect for:expect for:––
200cGy, 80%, 100cm SSD?200cGy, 80%, 100cm SSD?
––
200cGy, 90%, 100cm SSD?200cGy, 90%, 100cm SSD?
––
200cGy, 90%, 110cm SSD?200cGy, 90%, 110cm SSD?