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.


(3) Phantom Setup: Same as (2) above with weight point at (-5,0,0)


(3) Phantom Setup: Same as (2) above with weight point at (-5,0,0)


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


• 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.


• 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.


• 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.


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

c

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

p

p

p

p

p

p

p

p

00

2

0

0

2

0

0

20

0

020

20

,2

0

p

cp

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

physics training class mu photons

Documents

monitor unit

convolution

ssdwt fan

dose delivered

field size

homogeneous

relative values

dose calc