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Photon Beam Photon Beam Monitor-Unit Monitor-Unit Calculations Calculations Introduction to Medical Introduction to Medical Physics III: Physics III: Therapy Therapy Steve Kirsner, MS Steve Kirsner, MS
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Page 1: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

Photon Beam Monitor-Photon Beam Monitor-Unit CalculationsUnit Calculations

Introduction to Medical Physics III:Introduction to Medical Physics III:

TherapyTherapySteve Kirsner, MSSteve Kirsner, MS

Page 2: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

OverviewOverview

IntroductionIntroduction General Formalism for MU CalculationsGeneral Formalism for MU Calculations

Linear Accelerator MU CalculationsLinear Accelerator MU Calculations SSD Formalism- EquationsSSD Formalism- Equations SAD Formalism - EquationsSAD Formalism - Equations Important FactsImportant Facts

Summary: Equations SSD and SADSummary: Equations SSD and SAD ExamplesExamples

Page 3: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

IntroductionIntroduction

Standard calibration geometryStandard calibration geometry Linear Accelerators are calibrated under standard Linear Accelerators are calibrated under standard

conditions. These standard conditions enable us to conditions. These standard conditions enable us to know the absolute dose under these conditionsknow the absolute dose under these conditions At MD Anderson, and commonly elsewhere, this point At MD Anderson, and commonly elsewhere, this point

is located at is located at ddmaxmax in a water phantom, in a water phantom, 100 cm SSD100 cm SSD, ,

along the along the central axiscentral axis of an of an openopen 10x1010x10 field. Other field. Other option is defined at 100 cm SAD.option is defined at 100 cm SAD.

At this point, the unit is calibrated such that 1 monitor At this point, the unit is calibrated such that 1 monitor unit (MU) is equal to 1.0 cGy muscleunit (MU) is equal to 1.0 cGy muscle

Page 4: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

IntroductionIntroduction

Corrections Needed if not at Standard GeometryCorrections Needed if not at Standard Geometry DepthsDepths other than dmax, and other than dmax, and SSDsSSDs other than 100 other than 100

cm, and for cm, and for field sizesfield sizes other 10x10, and points off the other 10x10, and points off the central axiscentral axis, corrections become necessary, corrections become necessary These corrections are found in an institutions clinical These corrections are found in an institutions clinical

data tables. Where these relationships to the standard data tables. Where these relationships to the standard geometry have been establishedgeometry have been established

Corrections are also necessary to account for anything Corrections are also necessary to account for anything that is placed in the beam that will attenuate the that is placed in the beam that will attenuate the radiation. Wedges, compensators, blocks.radiation. Wedges, compensators, blocks.

Page 5: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

Corrections to standard geometryCorrections to standard geometry

DepthDepth corrections corrections Field-sizeField-size corrections corrections

DistanceDistance corrections corrections Off-axisOff-axis corrections corrections AttenuationAttenuation

correctionscorrections

PDD, TAR, TMR,TPRPDD, TAR, TMR,TPR Output (scatter factors)Output (scatter factors)

SSTT, S, SPP, S, SCC

Inv. Sq.Inv. Sq. ““SAD Factor”SAD Factor”

OAFs-wedge and openOAFs-wedge and open

WFs, TFs, compensator WFs, TFs, compensator factorsfactors

Page 6: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

FormalismFormalism

In general, the dose (In general, the dose (DD) at any point in a water ) at any point in a water phantom can be calculated using the following phantom can be calculated using the following formalism:formalism:

Where:Where: MUMU = monitor-unit setting for given conditions = monitor-unit setting for given conditions OO = calibrated output (cGy/MU) for standard conditions = calibrated output (cGy/MU) for standard conditions OFOF = output (scatter) factor(s): S = output (scatter) factor(s): SCC, S, SPP, S, STT

ISqISq = inverse-square correction (as needed) depending on = inverse-square correction (as needed) depending on calibration conditions and treatment conditions.calibration conditions and treatment conditions.

DDFDDF = depth-dose factors (PDD, TMR, TPR, TAR) = depth-dose factors (PDD, TMR, TPR, TAR) OAFOAF = off-axis factors, open and wedge = off-axis factors, open and wedge TFTF = transmission factors-attenuation = transmission factors-attenuation

TFOAFDDFISqOFOMUD

Page 7: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

SSD Treatments and CalibrationSSD Treatments and Calibration

When the treatment unit is calibrated in a “SSD” When the treatment unit is calibrated in a “SSD” geometry, then for “SSD” beams, the formalism geometry, then for “SSD” beams, the formalism becomes:becomes:

where it is assumed that output (scatter) factors are where it is assumed that output (scatter) factors are given by given by SSCC and and SSPP, and where it is also assumed that the , and where it is also assumed that the calibrated output = calibrated output = 1.0 cGy/MU1.0 cGy/MU for a 10 x 10 field at for a 10 x 10 field at dmaxdmax

Note that no inverse-square term is needed since the Note that no inverse-square term is needed since the distance to the point of dose normalization (distance to the point of dose normalization (SSD + dSSD + dmaxmax) ) is equal to the distance to the point of dose calibration. is equal to the distance to the point of dose calibration. This is true unless treating at extended ssd. Then inverse This is true unless treating at extended ssd. Then inverse square is given by: (scd/(ssd +dmax))square is given by: (scd/(ssd +dmax))22

TFOAFPDDSSMUD PC

Page 8: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

SAD Setup-SSD CalibrationSAD Setup-SSD Calibration

When the treatment unit is calibrated in a “SSD” When the treatment unit is calibrated in a “SSD” geometry, then for “SAD” (isocentric) beams, the geometry, then for “SAD” (isocentric) beams, the formalism becomes:formalism becomes:

where the inverse-square factor accounts for the change where the inverse-square factor accounts for the change in output produced by the differences in the distances in output produced by the differences in the distances between the between the ssource and the point of ource and the point of ccalibration (alibration (SCDSCD) ) and between the and between the ssource and the ource and the ppoint of normalization oint of normalization ((SPDSPD):):

TFOAFTMRISqSSMUD PC

2SPDSCDISq

Page 9: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

Important Facts to RememberImportant Facts to Remember The inverse-square term of the SAD equation The inverse-square term of the SAD equation

accounts for the increased output that exists at the accounts for the increased output that exists at the isocenter distance relative to the output that exists at isocenter distance relative to the output that exists at “isocenter + “isocenter + ddmaxmax” (where the machine output is ” (where the machine output is 1 1

cGy/MUcGy/MU) due to calibration conditions chosen.) due to calibration conditions chosen. This inverse square factors is sometimes called the This inverse square factors is sometimes called the

“SAD Factor”, not to be confused with other inverse “SAD Factor”, not to be confused with other inverse square factor. This is solely adjust for calibration square factor. This is solely adjust for calibration conditions.conditions.

For 6 MV, the SAD Factor is:For 6 MV, the SAD Factor is:

030.11005.1100 22

SPDSCDFISq SAD

Page 10: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

More Important FactsMore Important Facts Field sizes, unless otherwise stated, represent Field sizes, unless otherwise stated, represent

collimator settingscollimator settings For most accelerators, field sizes are defined at 100 cm For most accelerators, field sizes are defined at 100 cm

(the distance from the source to isocenter)(the distance from the source to isocenter) For For SSD beamsSSD beams, , field sizes are defined at the surfacefield sizes are defined at the surface

(normally 100 cm SSD)(normally 100 cm SSD) For For SAD beamsSAD beams, , field sizes are defined atfield sizes are defined at the the depthdepth of of

dose calculation (normally 100 cm SAD)dose calculation (normally 100 cm SAD) For field sizes at distances other than 100 cm, field sizes For field sizes at distances other than 100 cm, field sizes

must be computed using triangulation:must be computed using triangulation:

100100, dSSDFSFS dSSD

Page 11: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

Points to RememberPoints to Remember

Depth Dose and Scatter FactorsDepth Dose and Scatter Factors SSCC is a function of the collimator setting is a function of the collimator setting

SSPP is a function of the size of the field: is a function of the size of the field: at the phantom surface for SSD beamsat the phantom surface for SSD beams at the depth of calculation for SAD beamsat the depth of calculation for SAD beams

Depth-dose factors are a function ofDepth-dose factors are a function of:: field size at the phantom surface for SSD beamsfield size at the phantom surface for SSD beams field size at depth for SAD beams field size at depth for SAD beams

Page 12: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

Prescription DosePrescription Dose Calculate Monitor units per field for a given Calculate Monitor units per field for a given

Prescription dose.Prescription dose. This dose is “prescribed” by the Physician.This dose is “prescribed” by the Physician. Value must be known Value must be known at the pointat the point of calculation. of calculation.

With multiple fields, the dose per field is calculated With multiple fields, the dose per field is calculated using the beam weights.using the beam weights. If a dose If a dose DDRxRx is prescribed through multiple fields is prescribed through multiple fields ii

each having a relative weight each having a relative weight wtwtii, then the dose , then the dose DDii from each field is:from each field is:

i

iRxiwt

wtDD

Page 13: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

Prescription DosePrescription Dose

If the physician then prescribes the dose If the physician then prescribes the dose to a specific isodose line. The dose per to a specific isodose line. The dose per field then becomes:field then becomes:

DDii/IDL where IDL is the isodose line that is /IDL where IDL is the isodose line that is

prescribed to.prescribed to.

Page 14: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

Calculation EquationsCalculation Equations

For SSD beams: Inverse square if extended For SSD beams: Inverse square if extended SSD not shown. If extended SSD add inverse SSD not shown. If extended SSD add inverse square. square. (scd/(ssd +dmax))(scd/(ssd +dmax))22

For SAD beams:For SAD beams:

TFOAFPDDSS

DMU

PC

ii

TFOAFTMRISqSS

DMU

PC

ii

Page 15: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

A patient is planned to deliver a four field A patient is planned to deliver a four field box. The weightings of the beams are as box. The weightings of the beams are as follows:follows:

AP=25%, PA = 20%, Rt lat=25%, Lt. Lat= AP=25%, PA = 20%, Rt lat=25%, Lt. Lat= 30% 30%

What is the dose per field if the Physician What is the dose per field if the Physician prescribes 180 cGy to the 95 % isodose prescribes 180 cGy to the 95 % isodose line.line.

Page 16: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

AP and Rt Lateral – ((180 x .25)/(0.95))=47.4 cGyAP and Rt Lateral – ((180 x .25)/(0.95))=47.4 cGy

PA = ((180 x .30)/(.95)) = 56.8 cGyPA = ((180 x .30)/(.95)) = 56.8 cGy

Lt. Lateral = ((180 x .20)/(0.95)) = 37.9 cGyLt. Lateral = ((180 x .20)/(0.95)) = 37.9 cGy

Page 17: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

A patient is to be treated with parallel A patient is to be treated with parallel opposed fields that are weighted 3:2 opposed fields that are weighted 3:2 Anterior to Posterior. The prescription Anterior to Posterior. The prescription dose is 200 cGy to isocenter. What is the dose is 200 cGy to isocenter. What is the dose per field?dose per field?

Page 18: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Total weight is 5.Total weight is 5. Dose from anterior is 200 x 3/5 = 120 cGyDose from anterior is 200 x 3/5 = 120 cGy

Dose from posterior is 200 x 2/5 = 80 cGyDose from posterior is 200 x 2/5 = 80 cGy

Page 19: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

What monitor-unit setting is necessary to What monitor-unit setting is necessary to deliver 200 cGy to a point at 5 cm depth in deliver 200 cGy to a point at 5 cm depth in a water phantom. The field size is 12 x 20. a water phantom. The field size is 12 x 20. Energy used is 6 MV. 100 cm SSD is set Energy used is 6 MV. 100 cm SSD is set to the surface of the phantom.to the surface of the phantom.

Page 20: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

First calculate the equivalent square it is First calculate the equivalent square it is 15.0 .15.0 .

Next determine which formula to use Next determine which formula to use based on SSD or SAD set-up.based on SSD or SAD set-up.

This is an ssd set-up so pdd will be used This is an ssd set-up so pdd will be used and the ssd equation.and the ssd equation.

Page 21: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Sc for 15 = 1.021Sc for 15 = 1.021 Sp for 15 = 1.013Sp for 15 = 1.013 PDD for 15 at 5cm = 0.87PDD for 15 at 5cm = 0.87 Dose = 200 cGyDose = 200 cGy MU = (200)/(1.021 x 1.013 x 0.87)) = 222MU = (200)/(1.021 x 1.013 x 0.87)) = 222

TFOAFPDDSS

DMU

PC

ii

Page 22: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

What monitor-unit setting is necessary to What monitor-unit setting is necessary to deliver 200 cGy to a point at mid-deliver 200 cGy to a point at mid-seperation in a phantom 10 cm thick. The seperation in a phantom 10 cm thick. The phantom is irradiated with parallel phantom is irradiated with parallel opposed fields with a collimator setting of opposed fields with a collimator setting of 12 x 20 cm. Fields are blocked to a 10 x 12 x 20 cm. Fields are blocked to a 10 x 16 cm field using MLC. A 6 MV beam is 16 cm field using MLC. A 6 MV beam is used. Fields are weighted 3 to 1 and the used. Fields are weighted 3 to 1 and the dose is prescribed to the 98% idl.dose is prescribed to the 98% idl.

Page 23: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

First calculate the equivalent squares:First calculate the equivalent squares: 12 x 20 = 15 ; 10 x 16 = 12.312 x 20 = 15 ; 10 x 16 = 12.3 Next determine from type of set-up which Next determine from type of set-up which

equation will apply.equation will apply. Next determine which field size is used to Next determine which field size is used to

look up each factor.look up each factor. Calculate dose per field.Calculate dose per field.

Page 24: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

First Field: Dose = ((200 x ¾)/(0.98) = 153.1First Field: Dose = ((200 x ¾)/(0.98) = 153.1

Second Field : Dose = ((200 x ¼)/(0.98) = 51.0Second Field : Dose = ((200 x ¼)/(0.98) = 51.0

Sc for 15 = 1.021Sc for 15 = 1.021 Sp for 12.3 = 1.007Sp for 12.3 = 1.007 DD at 5cm for 12.3 = .8664DD at 5cm for 12.3 = .8664

Page 25: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

MU field 1 = ((153.1)/ (1.021 x 1.007 x.8664)) = 172MU field 1 = ((153.1)/ (1.021 x 1.007 x.8664)) = 172

MU field 2 = ((51) / (1.021 x 1.007 x .8664)) = 57MU field 2 = ((51) / (1.021 x 1.007 x .8664)) = 57

TFOAFPDDSS

DMU

PC

ii

Page 26: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Recalculate the monitor units necessary Recalculate the monitor units necessary from the previous problem. If now the from the previous problem. If now the blocking is done with cerrobend. The blocking is done with cerrobend. The prescription point at mid seperation is now prescription point at mid seperation is now 5 cm off-axis. 5 cm off-axis.

Only difference is need to look up off axis Only difference is need to look up off axis factor for 5 cm off axis at 5 cm depth and factor for 5 cm off axis at 5 cm depth and include tray factor for blocks.include tray factor for blocks.

Page 27: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

MU field 1 = ((153.1)/ (1.021 x 1.007 x.8664 x 1.019 x .97)) MU field 1 = ((153.1)/ (1.021 x 1.007 x.8664 x 1.019 x .97)) = 174= 174

MU field 2 = ((51) / (1.021 x 1.007 x .8664 x 1.019 x .97)) = MU field 2 = ((51) / (1.021 x 1.007 x .8664 x 1.019 x .97)) = 5858

TFOAFPDDSS

DMU

PC

ii

Page 28: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

A patient is to be treated with an isocentric A patient is to be treated with an isocentric wedged pair on a Varian Linac with 6 MV. wedged pair on a Varian Linac with 6 MV. Field 1 is 8 x 14 tht is blocked to a 6.5 x Field 1 is 8 x 14 tht is blocked to a 6.5 x 14. The SSD for this field is 94 cm. Field 2 14. The SSD for this field is 94 cm. Field 2 is 12 x 14 that is blocked to a 7 x 14, its is 12 x 14 that is blocked to a 7 x 14, its SSD is 88cm. Both fields use 30 degree SSD is 88cm. Both fields use 30 degree dynamic wedges. The prescribed dose is dynamic wedges. The prescribed dose is to isocenter and is 180 cGy, beams are to isocenter and is 180 cGy, beams are weighted 2 to 1.weighted 2 to 1.

Page 29: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Calculate Dose per field:Calculate Dose per field: Field 1: Dose = ((180 x 2/3)/(0.95)) = 126.3Field 1: Dose = ((180 x 2/3)/(0.95)) = 126.3

Field 2 : Dose = ((180 x 1/3)/(0.95) = 63.2Field 2 : Dose = ((180 x 1/3)/(0.95) = 63.2

Determine depths of treatment per field.Determine depths of treatment per field. Field 1: SSD= 94 therefore depth = 6 cmField 1: SSD= 94 therefore depth = 6 cm Field 2: SSD= 88 therefore depth = 12 cmField 2: SSD= 88 therefore depth = 12 cm

Page 30: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Calculate Equivalent Squares for each Calculate Equivalent Squares for each field.field.

Field 1: 8 x 14 = 10.1; 6.5 x 14 = 8.9Field 1: 8 x 14 = 10.1; 6.5 x 14 = 8.9

Field 2: 12 x 14 = 12.9 ; 7 x 14 = 9.3Field 2: 12 x 14 = 12.9 ; 7 x 14 = 9.3

Determine which depth factor to use: Determine which depth factor to use: isocentric set-up indicates TMR.isocentric set-up indicates TMR.

Page 31: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Field 1Field 1: : TMR at 6 cm for 8.9 field = ..8912TMR at 6 cm for 8.9 field = ..8912 Wedge factor for 8.9 field = 0.872Wedge factor for 8.9 field = 0.872 Sc for 10.1 field is 1.0Sc for 10.1 field is 1.0 Sp for 8.9 field is 0.996Sp for 8.9 field is 0.996 Inverse square = (101.5/100)Inverse square = (101.5/100)22 = 1.030 = 1.030 Tray factor for 8.9 field = 0.97Tray factor for 8.9 field = 0.97

Page 32: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Monitor Units from field Number 1Monitor Units from field Number 1

(126.3)/ (1.0 x .996 x 1.03 x .8912 x (0.97 x .872)) = 163(126.3)/ (1.0 x .996 x 1.03 x .8912 x (0.97 x .872)) = 163

TFOAFTMRISqSS

DMU

PC

ii

Page 33: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Field 2Field 2 TMR at 12 cm for 9.3 field = 0.7161TMR at 12 cm for 9.3 field = 0.7161 Wedge factor for 9.3 field = 0.865Wedge factor for 9.3 field = 0.865 Sc for 12.9 field = 1.025Sc for 12.9 field = 1.025 Sp for 9.3 field = 0.998Sp for 9.3 field = 0.998 Inverse square = 1.03Inverse square = 1.03 Tray factor for 9.3 field= 0.97Tray factor for 9.3 field= 0.97

Page 34: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Monitor Units for field number 2Monitor Units for field number 2

(63.2)/ (1.025 x 0.998 x 1.03 x 0.7161 x (0.97 x 0.865))= 100(63.2)/ (1.025 x 0.998 x 1.03 x 0.7161 x (0.97 x 0.865))= 100

TFOAFTMRISqSS

DMU

PC

ii

Page 35: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

A 30 x 30 x 30 cmA 30 x 30 x 30 cm33 water phantom is water phantom is centeredcentered at isocenter in a at isocenter in a pair of Varian 6 MV x-ray beams, a “right lateral” and a “left lateral”. pair of Varian 6 MV x-ray beams, a “right lateral” and a “left lateral”. Each field has a collimator setting of 12x18 and is further collimated Each field has a collimator setting of 12x18 and is further collimated to a 10x14 using the MLC.to a 10x14 using the MLC.

(a) What are the MU settings of each field if a (a) What are the MU settings of each field if a totaltotal dose of 200 cGy is dose of 200 cGy is to be delivered using a relative weighting of 2:1 with the right lateral to be delivered using a relative weighting of 2:1 with the right lateral having the higher weight?having the higher weight?

Make a picture!

Page 36: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

First compute the relative doses of the right- and First compute the relative doses of the right- and left-lateral fields:left-lateral fields:

Rt Lat (wt = 2):Rt Lat (wt = 2):

Lt Lat (wt = 1):Lt Lat (wt = 1):

13332200

i

iRxiwt

wtDD

6731200

i

iRxiwt

wtDD

Page 37: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Then compute the equivalent squares of the open Then compute the equivalent squares of the open and blocked fields:and blocked fields:

12x18:12x18:

10x14:10x14:

4.141812181222

WLLWEqSq

7.111410141022

WLLWEqSq

Page 38: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Determine equation (for “SAD” beams):Determine equation (for “SAD” beams):

SSCC (for 14.4) = 1.019(for 14.4) = 1.019

SSPP (for 11.7) = 1.005 (for 11.7) = 1.005

ISqISq = 1.030 = 1.030 TMR TMR (depth 15, for 11.7) = 0.651(depth 15, for 11.7) = 0.651 OAFOAF and and TFTF = 1.0 = 1.0

TFOAFTMRISqSS

DMU

PC

ii

Page 39: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Rt Lat:Rt Lat:

Lt Lat:Lt Lat:

1940.10.1651.0030.1005.1019.1

133

iMU

980.10.1651.0030.1005.1019.1

67

iMU

Page 40: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Calculate the Monitor Units Necessary to Calculate the Monitor Units Necessary to deliver a dose of 200 cGy to a depth of 8 deliver a dose of 200 cGy to a depth of 8 cm from parallel opposed fields equally cm from parallel opposed fields equally weighted. The field size is 15 x 15 blocked weighted. The field size is 15 x 15 blocked to an 8 x 8 field. The patient has to be to an 8 x 8 field. The patient has to be treated with an extended distance of 120 treated with an extended distance of 120 cm SSD. Assume that the field size given cm SSD. Assume that the field size given is defined at 120 cm. The energy that is is defined at 120 cm. The energy that is used is 6 MV.used is 6 MV.

Page 41: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Field size at 100 cm is 15 x (100/120)= 12.5Field size at 100 cm is 15 x (100/120)= 12.5 Sc for 12.5 = 1.023Sc for 12.5 = 1.023 Sp for 8 cm field = 0.993Sp for 8 cm field = 0.993 PDD for 8 x 8 field = .732 at 100 cm which PDD for 8 x 8 field = .732 at 100 cm which

equals .732 x 1.02 = .747 at 120 cm.equals .732 x 1.02 = .747 at 120 cm. Inverse square factor for extended SSD is Inverse square factor for extended SSD is

(101.5/121.5)(101.5/121.5)22 = 0.698 = 0.698 Dose per field is 200/2 = 100 cGyDose per field is 200/2 = 100 cGy

Page 42: Photon Beam Monitor- Unit Calculations Introduction to Medical Physics III: Therapy Steve Kirsner, MS.

ExamplesExamples

Monitor Units per fieldMonitor Units per field

(100/(1.023 x 0.993 x .747 x 0.698)) = 189(100/(1.023 x 0.993 x .747 x 0.698)) = 189

TFOAFPDDSS

DMU

PC

ii


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