part viii:medical exposures in radiotherapy lecture 6: determination of dose to the patient in...

Part VIII:Medical Exposures in Part VIII:Medical Exposures in RadiotherapyRadiotherapy

Lecture 6: Determination of dose to the patient in Radiotherapy II

IAEA Post Graduate Educational Course on Radiation Protection and Safe Use of Radiation Sources

Module 3 Optimization of Protection for Medical Exposures

Part VIII.3.6 Determination of dose to the patient in Radiotherapy -II

Slide 2

Scope Scope

• To understand the use of the factors for To understand the use of the factors for estimation of treatment timeestimation of treatment time

• To familiarize with the calculation of To familiarize with the calculation of treatment time for a single field treatment time for a single field

• To familiarize with the calculation of treatment To familiarize with the calculation of treatment time for multiple beams and for rotation beamtime for multiple beams and for rotation beam

• To understand the dose prescription and To understand the dose prescription and reporting guidelines of ICRUreporting guidelines of ICRU


Slide 3

This lecture covers………This lecture covers………

• Corrections to be applied to the calibration dose rate Corrections to be applied to the calibration dose rate to estimate the dose rate for the treatment conditionto estimate the dose rate for the treatment condition

• Calculation of treatment time / MU for single fieldCalculation of treatment time / MU for single field

• Calculation of treatment time / MU for multiple fieldsCalculation of treatment time / MU for multiple fields

• Use of TPR and PDD for estimation of treatment Use of TPR and PDD for estimation of treatment timetime

• Treatment time calculation for extended SSDTreatment time calculation for extended SSD

• Recommendations of ICRU for reporting doseRecommendations of ICRU for reporting dose


Slide 4

Put your Plan into actionPut your Plan into action

How long should the beam be ON?How long should the beam be ON?

How to determine the Treatment time?How to determine the Treatment time?

The Basic equation isThe Basic equation is

Treatment Time = Treatment Time = Dose per Beam per fraction

Dose Rate at that point


Slide 5

There are two basic quantities to be There are two basic quantities to be determineddetermined

1. Dose per beam per fraction1. Dose per beam per fraction

2. Dose rate at the point for that beam2. Dose rate at the point for that beam

Where do you get these from?Where do you get these from?

1. Dose Prescription1. Dose Prescription

2. Treatment plan2. Treatment plan

3. Calibration conditions of your 3. Calibration conditions of your teletherapy unitteletherapy unit


Slide 6

PrescriptionPrescription

Example:Example:

Tumour dose 6000 cGy in 30 fractions to Tumour dose 6000 cGy in 30 fractions to be delivered to:be delivered to:

a point, usually the centre of the tumour a point, usually the centre of the tumour or the isocentre (100%)or the isocentre (100%)

OROR

an isodose line covering the tumour an isodose line covering the tumour volume e.g. 95%volume e.g. 95%


Slide 7

Treatment Plan - Many ways to planTreatment Plan - Many ways to plan

• Technique :- SSD or SADTechnique :- SSD or SAD• Number of Beams Number of Beams

– single, parallel opposed, three field, etcsingle, parallel opposed, three field, etc

• Beam weightBeam weight– Weighted at isocentre or at DWeighted at isocentre or at Dmaxmax (given (given

dose)dose)

• Beam modifiersBeam modifiers– Wedges, shielding, compensators etc.Wedges, shielding, compensators etc.


Slide 8

Calibration - different methods / Calibration - different methods / protocols protocols

• Output stated at DOutput stated at Dmax max for a for a

reference field size (10 x 10 reference field size (10 x 10 cmcm22))

• Output stated at a reference Output stated at a reference depth ddepth drefref for reference field for reference field

size (10 x 10 cmsize (10 x 10 cm22))


Slide 9

How to get the dose per beam per How to get the dose per beam per fraction?fraction?

First part of the question;First part of the question;

How much dose per fraction?How much dose per fraction?

This is obtained from the prescriptionThis is obtained from the prescription

Prescription (e.g.):Prescription (e.g.):

Tumour dose (TD) 6000 cGy in 30 fractions (N) to Tumour dose (TD) 6000 cGy in 30 fractions (N) to

1. 1. a point, may be the centre of tumour or isocentre i.e a point, may be the centre of tumour or isocentre i.e to 100% (Recommended by ICRU)to 100% (Recommended by ICRU)

2. an isodose line covering the tumor e.g. 95%2. an isodose line covering the tumor e.g. 95%


Slide 10

Dose per fractionDose per fraction

Case 1- To a point at a depthCase 1- To a point at a depth

DoseDose(f)(f) = = == = = 200cGy200cGy

Case 2 – To an isodose line covering the tumourCase 2 – To an isodose line covering the tumour • DoseDose(f)(f)= = == = = 210.5cGy210.5cGy

– TD is the Tumour DoseTD is the Tumour Dose

TD

N

6000cGy

30

TD

N x % isodose

6000 x 100

30 x 95


Slide 11

How to get the dose per beam per How to get the dose per beam per fraction?fraction?

Second part of the question;Second part of the question;

How much dose per beam?How much dose per beam?

This is obtained from the beam weightsThis is obtained from the beam weights

DoseDose(b,f) (b,f) ==Total Weight(W)

Dose (f) x w(b)


Slide 12

Dose rate at the point for the beamDose rate at the point for the beamCalibration condition to treatment conditionCalibration condition to treatment condition

1. Correct for Field size1. Correct for Field size

D(dref,Sref)

dref

Ref Field Size (Sref=10 x 10 cm2) Treatment Field Size (S=15 x 15cm2)

dref

D(dref,s)

D(dref,S) =D(dref, S ref) x RDF( S)


Slide 13

Calibration condition to treatment conditionCalibration condition to treatment condition 2. Calibration depth to Treatment depth2. Calibration depth to Treatment depth

If calibration is at reference depth use TPR...If calibration is at reference depth use TPR...

D(dref,Sref)

dref

Ref Field Size (Sref=10 x 10 cm2) Treatment Field Size (s)

d

D(d,s)

D(dref,S) =D(dref, S ref) x RDF( S) x TPR(d,s,Q)


Slide 14

Calibration condition to treatment conditionCalibration condition to treatment condition 2. Calibration depth to Treatment depth2. Calibration depth to Treatment depth

If calibration is at DIf calibration is at Dmaxmax depth use TMR…. depth use TMR….

D(dref,Sref)

dref

Ref Field Size (Sref=10 x 10 cm2) Treatment Field Size (s)

d

D(d,s)

D(dref,S) =D(dref, S ref) x RDF( S) x TMR(d,s,Q)


Slide 15

Correction for beam modifiersCorrection for beam modifiers

3. Correction for shielding 3. Correction for shielding (if applicable)(if applicable)

D(dref,Sref)

dref

Ref Field Size (Sref=10 x 10 cm2)Treatment Field Size (S)

d

D(d,s)

D(dref,S) =D(dref, S ref) x RDF( S) x TPR(d,S,Q) x Tf

Shielding block Tray

Where Tf is the shielding tray factor and S is the equivalent square of the shielded field


Slide 16

Correction for beam modifiersCorrection for beam modifiers

3. Correction for Wedge 3. Correction for Wedge (if applicable)(if applicable)

D(dref,Sref)

dref

Ref Field Size (Sref=10 x 10 cm2)

Treatment Field Size (s)

d

D(d,s)

D(dref,S) =D(dref, S ref) x RDF( S) x TPR(d,S,Q) x Tf x Wf

Shielding block Tray

Wedge

Where Wf is the Wedge factor


Slide 17

Treatment at extended SSDTreatment at extended SSD

• Use of large field sizes require Use of large field sizes require treatment at extended SSD.treatment at extended SSD.

• Points to rememberPoints to remember– Output decreases - inverse square law Output decreases - inverse square law

correction to be appliedcorrection to be applied– Field size corrected by similar triangleField size corrected by similar triangle


Slide 18

Extended SSD treatmentExtended SSD treatment

D(dref,Sref)

dref

Ref Field Size (Sref=10 x 10 cm2)

Treatment Field Size (s)

d

D(d,s)

D(dref,S) =D(dref, S ref) x RDF( S) x TPR(d,s,Q) x Tf x Wf x (f/f1)2

SAD=f=100cm

SAD=f1=120cm


Slide 19

And thus the basic equation...And thus the basic equation...

Treatment Time =Treatment Time =

becomesbecomes

Trt. Time = =Trt. Time = =

Dose per Beam per fraction


D(dref, S ref) x RDF( S) x TPR(d,s,Q) x Sf x WfD(d,S)

Dose(b,f)Dose (f) x w(b) /Total Wt


Slide 20

Treatment time calculation with Treatment time calculation with Percentage depth dose Percentage depth dose


becomesbecomes

Trt. TimeTrt. Time = = = =

Dose per Beam per fraction


D(dref, S ref) x RDF( S) x PDD(d,f,s,Q) x Sf x Wf x Total WtDose(dref,S)

Dose(b,f) Dose (f) x100 x w(b)/ Total Wt

Remember, PPD should be corrected for Extended SSD


Slide 21

Try this exampleTry this example

Dose per beam per fraction = 3000/15=200cGyDose per beam per fraction = 3000/15=200cGyDose rate at 4cm = 150 x 1.024 x 1 x .965 x 1.06 Dose rate at 4cm = 150 x 1.024 x 1 x .965 x 1.06

=157.2 cGy/min=157.2 cGy/minTreatment Time = 200/157.2 = 1.29 minutes Treatment Time = 200/157.2 = 1.29 minutes

Dose per beam per fraction

Dose rate at that point

Calculate Treatment time to deliver 3000 cGy to 4 cm depth in15 # byCalculate Treatment time to deliver 3000 cGy to 4 cm depth in15 # by

single direct field 15 x 15 cmsingle direct field 15 x 15 cm22,, no wedge but shielding includedno wedge but shielding included

(RDF=1.024, W(RDF=1.024, Wff=1, S=1, Sff=.965, TPR=.965, TPR(4cm)(4cm)=1.06, D=1.06, D(ref)(ref)=150cGy/minute)=150cGy/minute)



Slide 22

Try this now - one step furtherTry this now - one step further• Calculate the treatment time to deliver 5000cGy to a tumour volume Calculate the treatment time to deliver 5000cGy to a tumour volume

covered by a 95% isodose line by SAD technique with covered by a 95% isodose line by SAD technique with 6060Co beam Co beam for the plan shown here. Beam output 150cGy/min at Dfor the plan shown here. Beam output 150cGy/min at Dmaxmax

10cm

16cm16cm

6 x 10 cm2

Beam Wt. 40%

6 x 10 cm2

30o Wedge

Beam Wt 30%

6 x 10 cm2

30o Wedge

Beam Wt 30%


Slide 23

Points to notePoints to note

• Technique used : SADTechnique used : SAD– To use TPR or TMRTo use TPR or TMR

• Dose calibrated at DDose calibrated at Dmaxmax

– TMR to be used for calculationTMR to be used for calculation

• Collect the data and tabulateCollect the data and tabulate


Slide 24

Treatment time tableTreatment time tableParameter Beam 1 Beam 2 Beam 3 Field size (cm2) 6 x 10 6 x 10 6 x 10 RDF 0.98 0.98 0.98 Depth (cm) 10cm 16cm 16cm TMR 0.707 0.467 0.467 Wedge Nil 30o 30o

Wedge Factor Wf 1 0.62 0.62 Shielding Nil Nil Nil Shielding tray factor Sf 1 1 1 Dose Rate at Depth (RDF x TMR x Wf x Sf) (cGy/min)

103.9 43.4 43.4

Beam weight (wt.) 0.4 0.3 0.3 TD / beam (cGy) (Total TDxWt x isodose value /100)

2105 1579 1579

Fractions 25 25 25 TD / fraction (cGy) 84.2 63.16 63.16

Treatment time (minutes) 0.81 1.45 1.45


Slide 25

How to calculate the treatment time How to calculate the treatment time for rotational beam?for rotational beam?

• Points to Note:-Points to Note:-– Rotational or ARC techniques are Rotational or ARC techniques are

isocentricisocentric– TPR (TMR) is used for calculation TPR (TMR) is used for calculation – For a rotational beam or arc technique, the For a rotational beam or arc technique, the

depth of treatment varies constantlydepth of treatment varies constantly– Average TPR (TMR) is to be determinedAverage TPR (TMR) is to be determined


Slide 26

Determination of average Determination of average TPR(TMR)TPR(TMR)

• Obtain the depth for Obtain the depth for every 10 or 20 degree every 10 or 20 degree intervalinterval

• Obtain the TPR for each Obtain the TPR for each depthdepth

• Estimate the average Estimate the average TPR or TMRTPR or TMR


Slide 27

And thus the basic equation for And thus the basic equation for rotation therapy is...rotation therapy is...

Trt. Time = =Trt. Time = =

Where TPRWhere TPRAVGAVG = = nn TPR TPR(d(di,i,s,q)s,q)

D(dref, S ref) x RDF( S) x TPR(avg) D(d,S)

Dose(b,f)

i=1

Dose (f) x w(b)/ Total Wt


Slide 28

Dose Prescription/ReportingDose Prescription/Reporting

• Dose Reporting always to Dose Reporting always to • Dose at or near center of PTVDose at or near center of PTV• Max/Min dose in PTVMax/Min dose in PTV

• Prescribing dose to a reference Prescribing dose to a reference point point Minimum dose pointMinimum dose point

Maximum dose pointMaximum dose point

ICRU Reference point – generally the ICRU Reference point – generally the beam meeting point.beam meeting point.


Slide 29

Methods of prescribing and reporting Methods of prescribing and reporting dose in EBTdose in EBT

• Dose at the periphery of the PTV Dose at the periphery of the PTV e.g. isodose line or isodose e.g. isodose line or isodose surface encompassing the PTVsurface encompassing the PTV

• Average dose in the PTVAverage dose in the PTV• Dose in or at the central parts of Dose in or at the central parts of

PTV and on or close to the central PTV and on or close to the central axis of the beam(s) – ICRU axis of the beam(s) – ICRU reference pointreference point


Slide 30

ICRU Reference Point - significanceICRU Reference Point - significance

• PointPoint– Clinically relevant & representative of dose in PTVClinically relevant & representative of dose in PTV– Easy to define unambiguouslyEasy to define unambiguously– Where dose can be determined accuratelyWhere dose can be determined accurately– Where there is no steep dose gradientWhere there is no steep dose gradient

• LocatedLocated– At or near centre of PTV At or near centre of PTV – Near central axis of beam(s)Near central axis of beam(s)– Sometimes not possible at centre of PTVSometimes not possible at centre of PTV

• Select meaningful point Select meaningful point


Slide 31

ICRU recommendation on reporting ICRU recommendation on reporting dose variation within PTVdose variation within PTV

• ICRU recommends that as a minimum ICRU recommends that as a minimum requirement, the maximum and requirement, the maximum and minimum dose to the Planning Target minimum dose to the Planning Target Volume (PTV) shall be reported Volume (PTV) shall be reported together with the dose at the ICRU together with the dose at the ICRU reference point.reference point.


Slide 32

ICRU Reference Point – some ICRU Reference Point – some examplesexamples

Single field Parallel Opposed Pair Wedge Pair


Slide 33

ICRU Reference Point – some more ICRU Reference Point – some more examplesexamples

3 Field 4 Field4 Field


Slide 34

SummarySummary

• To obtain the dose rate during treatment To obtain the dose rate during treatment the output has to be corrected for field the output has to be corrected for field size, depth, attenuators and SSDsize, depth, attenuators and SSD

• TPR or TMR is used for calculation of TPR or TMR is used for calculation of treatment timetreatment time

• PDD could be used for treatment time PDD could be used for treatment time estimation estimation


Slide 35

Summary Summary

• For extended SSD treatments, the For extended SSD treatments, the output should be corrected for Inverse output should be corrected for Inverse square lawsquare law

• Dose prescription and reporting should Dose prescription and reporting should be to ICRU reference point where be to ICRU reference point where tumour cell density usually is hightumour cell density usually is high


Slide 36

Try these questionsTry these questions

• What are the basic parameters required for What are the basic parameters required for treatment time calculation?treatment time calculation?

• What are the corrections required for output What are the corrections required for output to correct from calibration condition to to correct from calibration condition to treatment condition?treatment condition?

• Why TMR/TPR is preferred for calculation of Why TMR/TPR is preferred for calculation of rotational treatments/rotational treatments/

• What are the significances of ICRU reference What are the significances of ICRU reference point?point?


Slide 37

Where to find more information?Where to find more information?

• The Physics of RadiologyThe Physics of Radiology– H.E Johns & J R CunninghamH.E Johns & J R Cunningham

• The Physics of Radio TherapyThe Physics of Radio Therapy– Faiz M KhanFaiz M Khan

• The Modern Technology of Radiation The Modern Technology of Radiation Oncology, edited by J. Van DykOncology, edited by J. Van Dyk

• International Commission on Radiation Units International Commission on Radiation Units and Measurements Report No. 50 and 62and Measurements Report No. 50 and 62

part viii:medical exposures in radiotherapy lecture 6: determination of dose to the patient in...

Documents

calibration dose rate

treatment depthif calibration

treatment condition1

treatment plan3

determination of dose

dose prescription2

tumour dose td

reference field size