Physics 6C Photons Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB.
Physics Training Class MU Photons
Transcript of Physics Training Class MU Photons
Monitor Unit Calculations - Photons
Outline
• XiO Weighting Methods
• General Photon Monitor Unit Equation
• Equivalence to Traditional Monitor Unit Calculations
• Source Index Review for Photons
Weight Point
• Affects the dose delivered by that beam to the specified point.• Can be a “true” dose or a “relative” dose delivered to this weight
point.• Can initially be defined as relative values and then scaled to
achieve prescription.• Is the point used for the MU calculation for that beam.• Should always be defined in regions of high relative dose and
low dose gradient (e.g. Avoid penumbra, heterogeneity borders, buildup region, etc.)
Weighting Mode
Relative Weighting Mode
– Weights are relative values.
– Are related to dose through a prescription to a point or an isodose line.
– Intended for use in simple treatment setups where:
• All individual beam fractions are equal
• Prescribed dose is per plan
Absolute Weighting Mode
– Weights are doses.
– Can be scaled to achieve a prescribed dose at a specified point or isodose line.
– Intended for more complex treatment setups where:
• Beams can be individually fractionated
• Dose can be prescribed for individual beams or groups of beams
Advantages of Absolute Weighting
Individually weight and fractionate per beam (composite planning).
Weight beams based on absolute dose contribution to weight point.
Use straight-forward and intuitive beam weighting, avoiding multiple options for beam weight “normalization” modes.
MU based on exact beam weight, not interpolated isocurve value
Monitor unit weighting
Isocurves viewed in absolute dose
Interest point doses displayed
Prescribe sub-groups of beams within a plan
Use of biological modeling tools in Isoplan
Inv. Planning/IMRT available
Source Index Header Information
Weighting Mode
Source Index Overview
Beam Setup Information
Monitor Unit Calculation Terms
Appearance of these headers depends on whether they pertain to at least one beam in the plan. At a minimum, the fields shown will appear.
SSD
(1)
(3)(2)
(1) Phantom Setup: SAD beam, normal incidence, 20x20 field, 6 MV, depth = 20 cm. Weight at isocenter at 100 cm.
Source Index: SSD/Wt fan SSD (cm) = 80/80 Depth; skin (cm) = 20
(1) Phantom Setup: SAD beam, normal incidence, 20x20 field, 6 MV, depth = 20 cm. Weight at isocenter at 100 cm.
Source Index: SSD/Wt fan SSD (cm) = 80/80 Depth; skin (cm) = 20
(2) Phantom Setup: SAD beam, 30 degree gantry angle, 20x20 field, 6 MV. Weight at isocenter at 100 cm.
Source Index: SSD/Wt fan SSD (cm) = 76.9/76.9 Depth; skin (cm) = 23.1
(2) Phantom Setup: SAD beam, 30 degree gantry angle, 20x20 field, 6 MV. Weight at isocenter at 100 cm.
Source Index: SSD/Wt fan SSD (cm) = 76.9/76.9 Depth; skin (cm) = 23.1
(3) Phantom Setup: Same as (2) above with weight point at (-5,0,0)
Source Index: SSD/Wt fan SSD (cm) = 76.9/78.8 Depth; skin (cm) = 23.7
(3) Phantom Setup: Same as (2) above with weight point at (-5,0,0)
Source Index: SSD/Wt fan SSD (cm) = 76.9/78.8 Depth; skin (cm) = 23.7
Source Index SSD
Off-axis weight Central Axis Weight
SSD = distance from source to skin, along central axis
Wt fan SSD = distance from source to skin, along the fan containing the weight point, projected to the central axis
Depth = depth from skin to weight point, computed by projecting the fan containing the weight point to the central axis
Effective (depth) = density corrected depth from the skin to the weight point, computed by projecting the fan containing the weight point to the central axis
Source Index Field Size
Be aware of the field size definition distance and how it relates to scaling adjustments that might need to be made for monitor unit calculations.
SAD BeamsIsocenterSkinReference Depth
SSD BeamsSkinNominal SSDReference Depth
Rotational Beams
Isocenter
Source Index Collimator Equivalent Square
Calculated using 4AOP:
( 2 * Length * Width ) / (Length + Width)
Source Index Blocked Equivalent Square (ES)
Not a geometric conceptIs a calculated value
– During SFM validation:• Convolution calculates Sp at 10
cm depth, for a number of beams of varying field size, incident on a flat, homogeneous phantom.
– During Isoplan dose calculation:• Convolution calculates Sp in the
patient, along the weight fan at a depth of 10 cm.
• SFM validation table is used to find the field size corresponding to this calculated Sp value. (ES)
ES will vary with– Position of weight point in field– Heterogeneities– Surface irregularities
Used to determine wedge factor
Convolution calculation, 10 x 10 port, central axis weight point
Convolution calculation, 10 x 10 port, off-axis weight point
Source Index Blocked Equivalent Square (ES)
• The first thing to note is that in our algorithm implementation the ES is NOT a geometric concept.
• Most physicists and dosimetrists, when performing a manual calculation, will approximate the ES geometrically (i.e. they look at the port and determine the unblocked area).
• They then use this information to look up the TPR/TAR and Sp/BSF to be used in the manual calculation.
• Ultimately, the user desires to obtain the ES which will yield the correct values for these quantities.
• XiO always starts with the TPR or Sp calculated for the blocked situation and deduces the ES.
• The system determines the ES differently depending on the calculation algorithm selected and the computed ES value may vary with algorithm.
Source Index Blocked Equivalent Square (ES)
• In Clarkson algorithm, the system performs a separate calculation for each port along the weight fan at a physical depth of 20 cm on a flat homogeneous phantom to determine the TPR there.
• The system then takes this and searches the computational TPR table for the field size that has this TPR value at a depth of 20 cm.
• This field size is reported as the ES field size on the source index.
• Why do we employ a depth of 20 cm? Because at shallower depths the relationship between TPRs and field size may not be monotonic (i.e. an increase in field size may not correlate with a larger TPR value) and, therefore, there may be two different field sizes that have the same TPR.
• We take advantage of the fact that at deeper depths the TPRs are usually monotonic and, therefore, every TPR at the depth of 20 cm will, ideally, correlate to only one field size.
• Even at a depth of 20 cm this may not be the case and when the system finds multiple field sizes with the same TPR it will report the one closest to the collimator equivalent square (EC) and indicate its confusion by placing a double asterisk next to the value on the source index page.
Source Index Blocked Equivalent Square (ES)
• For FFT or Superposition, we perform a point dose calculation for each beam on the patient at a physical depth of 10 cm along the weight fan to determine Sp at that point.
• This number is then compared to a table of Sp vs. field size. (This table is generated during the validation process by placing a series of beams onto a flat, homogeneous phantom and calculating Sp on CAX at a depth of 10 cm.)
• The system then finds the field size in the table that corresponds to the value of Sp calculated on the patient and reports that field size as the ES.
• Because ES is determined through Sp in the specific patient, it includes:
a) the effects of surface irregularities, including “spill”
b) the effects of heterogeneities if the user is performing a heterogeneous calculation.
Source Index Blocked Equivalent Square (ES)
The ES is only used in XiO calculations to determine:
• the appropriate wedge factor
• (in the case of FFT/Superposition only) to determine the electron contamination
Source Index Customized Ports and MLC
Source Index– Indication of Customized Port or
MLC:• Presence of Blk. Eq.Square
(cm)• Presence of Customized Port
ID or “MLC” under Tx Aids heading
Dose Calculation
– TPR includes the effects of blocking– MLCs restrict collimator scatter if
they serve as upper or lower jaw replacements
Sc (ec) ? or Sc (bl) ?
Multileaf Collimators
Upper Jaw Replaced (Elekta)
Lower Jaw Replaced (Siemens)
Tertiary Collimation (Varian)
Slide courtesy of John P. Gibbons, Ph.D. Time/MU Refresher Course, AAPM 2001
Slide courtesy of John P. Gibbons, Ph.D. Time/MU Refresher Course, AAPM 2001
Upper Jaw Replacement:
– Palta found Sc best described by MLC field
Lower Jaw Replacement:
– Das found Sc best described by MLC field
Tertiary Collimator:
– Klein found Sc best described by collimator jaws
Collimator Scatter for MLCsCollimator Scatter for MLCs
Source Index Collimator Scatter
To minimize errors on collimator replacement MLCs, conform collimator to port opening
ecSblSD cp 0Factor Output
For MLCs serving as tertiary collimators:
For MLCs serving as upper or lower collimators:
blSblSD cp 0Factor Output
Phantom Scatter
Source Index– Three values of PSCF are
relevant• PSCF(es)
– For the blocked equiv sq field at the weight point distance
• PSCF(ec)– For the open
collimator field at the calibration distance
• PSCF(0)– For the 0x0 field
Dose Calculation– PSCF(es) and PSCF(ec) are
calculated by the FFT algorithm.
– PSCF(0) is looked up from the tabular values entered in SFM
PSCF(es) =
[ dose calc at ref depth of (es)] /
[ dose calc at ref depth of ref field size ]
So, TPR * PSCF(es) = dose calc at wt depth of
es in weighting conditions.
Source Index Phantom Scatter
PSCF(es) for the treatment field
sizePSCF(0) for the 0x0 field size
PSCF(ec) for the field size at the calibration distance
Source Index Compensating Filter
Source Index– Indication of compensating filter
• CF Factor = (Dose at wt pt w/comp) /
(Dose at the wt pt w/o comp)• High point SSD (cm) = point at which
the uncompensated dose at the compensation plane is the lowest.
Dose Calculation• CF factor is calculated directly in FFT
Convolution/Superposition algorithms by doing a calculation with the modeled compensator and a calculation without, then taking the ratio of the computed dose values.
• CF factor includes effects of:• Patient scatter• Beam hardening in the comp filter
Source Index Tray Factor
Source Index
– Display of three Tray Factors if defined:
• Comp: Composite tray factor
• Blk: Customized Port tray factor
• CF: Compensating Filter tray factor
Dose Calculation
– Composite tray factor is used in the dose calculation
Beam Utilities Page
Source Index
Source Index Wedge Factor
Source Index– Indication of wedge
• Wedge ID/Orientation• Wedge Factor/Normalization
Dose Calculation– For each measured wedge factor field
size, algorithm calculates a correction factor: i.e. (WF measured) / (WF calculated for calibration setup)
– WF calculated by the algorithm for the treatment setup at the weight point
– WF = (WF for Tx setup) *
(WF measured) / (WF for calibration setup)
When treatment setup = calibration setup, will be able to reproduce measured Wedge Factors
Includes hardening
Source Index Wedge Factor
XiO supports multiple wedge types– Fixed– Motorized– Enhanced Dynamic Wedges
(EDW)– Virtual Wedges (VW)
Dose Calculation– Fixed, EDW, VW:
• Monitor unit calculation returns one value with wedge correction
– Motorized:• Monitor unit calculation
returns two values:• open field portion of
the treatment• wedged field portion
of the treatment
TAR / TPR
Source Index– XiO returns the (photon) dose
delivered to the weight point as either:
• TAR
• TPR * (PSCF (es)
/ PSCF (0))
– If TAR is used, BSF also listed
– If TPR * (PSCF (es) / PSCF (0)) is used, (PSCF (0) / PSCF (ec)) is also listed to ensure that the monitor unit equation is equivalent to traditional monitor unit calculations.
Dose Calculation– Two values are returned on the
Source Index, TAR or TPR:• At depth• At effective
– At depth• Table look-up value based
on the blocked field and physical depth to the weight point. NOT TO BE USED IN MU CALC.
– At effective• Value calculated by the
algorithm for the blocked field and effective depth to the weight point. USED FOR MU CALC.
Source Index TAR / TPR
TPR value used in the monitor unit calculation
Source Index Inverse Square
Dose Output for the collimator field size at the source to calibration distance.
Inverse Square terms to project dose output at the calibration point to dose output at the weight point.
TPR Calculations Hand Calculations
TFWFCFOARSTPRD
Dose
pcSWDSCD
MU,
20
where:
factorTray
factor Wedge
factorfilter ngCompensati
ratio axis-Off
factor scatter Total
factorscatter Phantom
factorscatter Collimator
distancepoint weight toSource
distance rate dosen calibratio toSource
depth effectiveat ratio phantom toTissue
rate dosen Calibratio
,
0
TF
WF
CF
OAR
SSS
S
S
SWD
SCD
TPR
D
pcpc
p
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TPR Calculations Derivation of XiO Calculation
TFWFCFOutput
Dose
TFWFCFOutput
Dose
TFWFCFECSECSD
Dose
TFWFCFOARESSECSTPRD
DoseTFWFCFOARSSTPRD
DoseTFWFCFOARSTPRD
DoseMU
ECPSCFPSCF
PSCFESPSCFTPR
SWDSCD
ECS
S
S
ESSTPR
SWDSCD
ECS
S
S
ESSTPR
SWDSCD
cp
S
S
ECS
ECS
pcSWDSCD
pcSWDSCD
pcSWDSCD
p
p
p
p
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p
p
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0
0
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SPSCF
ECSECSDOutput
ES
ES
where
Square Equivalent Collimator
Square Equivalent Blocked
:
0
TPR Calculations XiO Source Index
TFWFCFECPSCF
PSCFPSCF
ECPSCFTPRSWDSCD
Output
DoseMU
0
0
2