GoodPractice for treatment Planning

8/12/2019 GoodPractice for treatment Planning

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Radiation Protection in

Radiotherapy

Part 10

Good Practice including Radiation

Protection in EBTLecture 3: Radiotherapy Treatment Planning

IAEA Training Material on Radiation Protection in Radiotherapy


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Radiation Protection in Radiotherapy Part 10, lecture 3: Radiotherapy treatment planning 2

In BSS Treatment Planning is

part of Clinical Dosimetry BSS appendix II.20. Registrants and

licensees shall ensure that the following

items be determined and documented:...(b) for each patient treated with external beam

radiotherapy equipment, the maximum and minimum

absorbed doses to the planning target volumetogether with the absorbed dose to a relevant point

such as the centre of the planning target volume, plus

the dose to other relevant points selected by the

medical practitioner prescribing the treatment;


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and BSS appendix II.21

In radiotherapeutic treatments, registrants

and licensees shall ensure, within the ranges

achievable by good clinical practice and

optimized functioning of equipment, that:(a) the prescribed absorbed dose at the

prescribed beam quality be delivered to the

planning target volume; and

(b) doses to other tissues and organs beminimized.


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Treatment planning is the task to make

sure a prescription is put into practice

in an optimized way

Prescription

Planning

Treatment


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Objectives

Understand the general principles of radiotherapy

treatment planning

Appreciate different dose calculation algorithms

Understand the need for testing the treatment planagainst a set of measurements

Be able to apply the concepts of optimization of

medical exposure throughout the treatment planning

process Appreciate the need for quality assurance in

radiotherapy treatment planning


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Contents of the lecture

A. Radiotherapy treatment planning

concepts

B. Computerized treatment planning

C. Treatment Planning commissioning

and QA


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The need to understand

treatment planning IAEA Safety Report Series 17 Lessons

learned from accidental exposures in

radiotherapy (Vienna 2000):About 1/3 of problems directly related to

treatment planning!

May affect individual patient or cohort ofpatients


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A. Basic Radiotherapy Treatment

Planning Conceptsi. Planning process overview

ii. Patient data required for planning

iii. Machine data required for planning

iv. Basic dose calculation


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i. Planning process overview

Combine machine parameters and individual patient

data to customize and optimize treatment

Requires machine data, input of patient data,

calculation algorithm

Produces output of data in a form which can be used

for treatment (the treatment plan)

Patient information

Planning

Treatment unit data

Treatment plan


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ii. Patient information required

Radiotherapy is a localized treatment of

cancer - one needs to know not only the dose

but also the accurate volume where it hasbeen delivered to.

This applies to tumour as well as normal

structures - the irradiation of the latter can

cause intolerable complications. Again, bothvolume and dose are important.


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One needs to know

Target location

Target volume and shape

Secondary targets - potential tumourspread

Location of critical structures

Volume and shape of critical structures Radiobiology of structures


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It all comes down to the correct

dose to the correct volume

Dose Volume Histograms are a way to

summarize this information


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Dose Volume Histograms

0

20

40

60

80

100

120

0 20 40 60 80

Dose (Gy)

Volume(%)

Comparison of

three different

treatment

techniques (red,

blue and green)in terms of dose to

the target and a

critical structure

Target dose

Critical

organ


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The ideal DVH

Tumour:

High dose to all

Homogenous dose

Critical organ

Low dose to most of

the structure

100%

dose

100%

dose

volume volume


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Need to keep in mind

Always a 3D problem

Different organs may respond differently

to different dose patterns.

Question: Is a bit of dose to all the

organ better than a high dose to a small

part of the organ?


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Organ types

Serial organs - e.g.

spinal cord

Parallel organ - e.g.

lung

High

dose

region

High

dose

region

What difference in response

would you expect?

Serial

organ

Parallel

organ


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In practice not

always that clear cut ICRU report 62

Need to understand

anatomy andphysiology

A clinical decision


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*Int. J. Radiat. Oncol. Biol. Phys., 1998; 41:84-92.

In many organs, dose and volume

effects are linked - e.g.

Dose

(Gy)

Rectal

volume %

>65 40

>70 30

>75 5

Boersma*et al.,

classified the

following(Dose,Volume) regions

to be regions of high

risk for developing

rectal bleeding:


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In EBT practice

Need to know

where to direct beam to, and

how large the beam must be and how it

should be shaped


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Target design and reference

images In radiotherapy practice the target is

localized using diagnostic tools:

Diagnostic procedures - palpation, X Ray,ultrasound

Diagnostic procedures - MRI, PET, SPECT

Diagnostic procedures - CT scan, simulatorradiograph


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BSS appendix II.18.

Therapeutic exposure:

Registrants and licensees shall

ensure that:(a) exposure of normal tissue during

radiotherapy be kept as low as reasonably

achievable consistent with delivering the

required dose to the planning target

volume, and organ shielding be used when

feasible and appropriate ...


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Optimization of protection

One part of the optimization of

radiotherapy

Strategies: Employ shielding where possible

Use best available radiation quality

Ensure that plan is actually followed inpractice = verification


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Selection of treatment approach

Requires training and experience

May differ from patient to patient

Requires good diagnostic tools

Requires accurate spatial information

May require information obtained from

different modalities


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Minimum patient data required for

external beam planning

Target location

Patient outline


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Diagnostic tools which could be

used for patient data acquisition Ruler, calipers, many homemade jigs

CT scanner, MRI, PET scanner, US,

Simulator including laser system, opticaldistance indicator (ODI)

Many functions of the simulator are also

available on treatment units as an alternative

- simulator needs the same QA! (compare

part 15)


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Simulator

Diagnostic

X Ray tube

Simulator couch

Rotating

gantry

Image intensifier

and X Ray film

holder

Radiation beam

defining system

Nucletron/Oldelft


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Radiotherapy simulator

Obtain images and

mark beam entry

points on the patient


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Patient marking

Create relation

between patient

coordinates and

beam coordinates

Tattoos Skin markers

Marks on shell


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Beam placement and shaping

simulator filmwith block

DRR withconformal shielding


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Tools for optimization of the

radiotherapy approach

Choice of radiation

quality

Entry point

Number of beams

Field size

Blocks Wedges

Compensators


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Optimization approaches

patient

target

beam

patienttarget

beam

patient

target

wedge

Choice of best

beam angle

Use of a beam

modifier


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Beam number and weighting

patient

target

beam100%

patient

Beam 150%

Beam 2

50%

30%

40%

10%

20%


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A note on weighting of beams

30%

40%

10%

20%

Different approaches are

possible:

1. Weighting of beams as

to how much they contribute

to the dose at the target2. Weighting of beams as

to how much dose is

incident on the patient

These are NOT the same

25%

25%

25%

25%


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Use of wedges

Wedged pair

Three field

techniques

patient

Isodose lines

patient Typical isodose lines


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Entry point

Field size

Blocks Wedges

Compensators

a two-dimensional

approach?


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Entry point

Field size

Blocks Wedges

Compensators

Multiple beams

Dynamic delivery

Non-coplanar Dose compensation

(IMRT) not just

missing tissue

Biological planning

This is actually a 3D approach


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Target Localization

Diagnostic procedures - palpation, X Ray,

ultrasound

Diagnostic procedures - MRI, PET, SPECT

Diagnostic procedures - CT scan,simulator radiograph

Allows the creation of Reference Images forTreatment Verification:

Simulator Film, Digitally Reconstructed Radiograph


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Simulator image

During verification

session the treatment

is set-up on the

simulator exactly like itwould be on the

treatment unit.

A verification film is

taken in treatmentgeometry


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Simulator Film Shows relevant

anatomy

Indicates field

placement and size Indicates shielding

Can be used as

reference image for

treatment

verificationField defining wires


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iii. Machine data requirements for

treatment planning Beam description (quality, energy)

Beam geometry (isocentre, gantry, table)

Field definition (source collimator distance,applicators, collimators, blocks, MLC)

Physical beam modifiers (wedges,compensator)

Dynamic beam modifiers (dynamic wedge,arcs, MLC IMRT)

Normalization of dose


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Machine data required for

planning Depends on

complexity of treatment

approaches

resources available for

data acquisition

May be from published

data or can be acquired MUST be verified...


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Quick Question:

Who is responsible for thepreparation of beam data for theplanning process in your center?


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Acquisition of machine data

from vendor or

publications (e.g.BJR 17

and 25) - this requires

verification!!!

Done by physicist

Some dosimetric equipment

must be available (water

phantom, ion chambers, film,

phantoms,)

Documentation essential


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Machine data availability

Hardcopy (isodose charts, output factor

tables, wedge factors,) - for emergencies

and computer break downs

Treatment planning computer (as above or

beam model) - as standard planning data

Independent checking device (e.g.MU

checks) - should be a completelyindependent set of data


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Machine data availability

Hardcopy (isodose charts, output factor

tables, wedge factors,)

Treatment planning computer (as aboveor beam model)

Independent checking device (eg. mu

checks)


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Machine data summary

Need to include all beams and options

(internal consistency, conventions, collision

protection, physical limitations)

Data can be made available for planning in

installments as required

Some data may be required for individual

patients only (e.g.special treatments) Only make available data which is verified


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Quick Question:

What data is available for physical

wedges in your center?


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iv. Basic dose calculation

Once one has the target volume, the

beam orientation and shape one has to

calculate how long a beam must be on(60-Co or kV X Ray units) or how many

monitor units must be given (linear

accelerator) to deliver the desired dose

at the target.


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Normalization

Specifies what absolutedose should be given to a relative dosevalue in a treatment plan - e.g.deliver2Gy per fraction to the 90% isodose

Often the reason for misunderstanding

Should follow recommendation of

international bodies (compare e.g.ICRUreports 39, 50, 58 and 62)


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Components of dose calculation

for a single beam Calibration method - what is the

reference condition?

Dose variation with depth and field size- covered in percentage depth dose or

TPR/TMR data

Off axis ratio - if the normalization pointis not on central axis


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Variation of percentage depth

dose with field size

0

20

40

60

80

100

120

0 5 10 15 20 25 30

FS 5

FS 10

FS 20

FS 30

FS 40

10MV photons


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Variation of percentage depth

dose with FSD


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From single to multiple beams

Mainly an issue

for megavoltage

photons where

we have

significant

contribution of

dose to the target

from many beams

1

4

3 260 Gy

Beam weighting must be factored in !!!

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