IAEA Training Material on Radiation Protection in Radiotherapy
Radiation Protection in
Radiotherapy
Part 10
Good Practice including Radiation
Protection in EBT
Lecture 3: Radiotherapy Treatment Planning
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 volume
together 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 be
minimized.
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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 plan
against 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 of
patients

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A. Basic Radiotherapy Treatment
Planning Concepts
i. 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
Treatment unit data
Planning
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 has
been delivered to.
This applies to tumour as well as normal
structures - the irradiation of the latter can
cause intolerable complications. Again, both
volume and dose are important.
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One needs to know
Target location
 Target volume and shape
 Secondary targets - potential tumour
spread
 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
Dose Volume Histograms
Comparison of
three different
treatment
techniques (red,
blue and green)
in terms of dose to
the target and a
critical structure
120
Volume (%)
100
80
60
Critical
organ
40
20
Target dose
0
0
20
40
60
80
Dose (Gy)
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The ideal DVH

Tumour:



High dose to all
Homogenous dose
volume
Critical organ

Low dose to most of
the structure
volume
100%
100%
dose
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dose
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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
Parallel
organ
Serial
organ
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What difference in response
would you expect?
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In practice not
always that clear cut



ICRU report 62
Need to understand
anatomy and
physiology
A clinical decision
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In many organs, dose and volume
effects are linked - e.g.
Boersma*
et al.,
classified the
following
(Dose,Volume) regions
to be regions of high
risk for developing
rectal bleeding:
*Int.
Dose
(Gy)
Rectal
volume(%)
>65
40
>70
30
>75
5
J. Radiat. Oncol. Biol. Phys., 1998; 41:84-92.
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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, simulator
radiograph

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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 in
practice = 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



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Ruler, calipers, many homemade jigs…
CT scanner, MRI, PET scanner, US,…
Simulator including laser system, optical
distance 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
Rotating
gantry
Diagnostic
X Ray tube
Radiation beam
defining system
Simulator couch
Nucletron/Oldelft
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Image intensifier
and X Ray film
holder
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Radiotherapy simulator

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Obtain images and
mark beam entry
points on the patient
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Patient marking
Marks on shell

Create relation
between patient
coordinates and
beam coordinates
Tattoos
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Skin markers
31
Beam placement and shaping
DRR with
conformal shielding
simulator film
with block
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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
Choice of best
beam angle
beam
beam
target
patient
target
patient
wedge
target
Use of a beam
modifier
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Beam number and weighting
beam
Beam 1
100%
50%
50%
target
patient
Beam 2
patient
40%
30%
10%
20%
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A note on weighting of beams
Different approaches are
possible:
1. Weighting of beams as
to how much they contribute
to the dose at the target
2. Weighting of beams as
to how much dose is
incident on the patient
These are NOT the same
25%
40%
25%
25%
30%
10%
20%
25%
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Use of wedges
Wedged pair
 Three field
techniques

Isodose lines
patient
patient
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Typical isodose lines
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Beam placement and shaping





Entry point
Field size
Blocks
Wedges
Compensators
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a two-dimensional
approach?
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Beam placement and shaping





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 for
Treatment Verification:
Simulator Film, Digitally Reconstructed Radiograph
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Simulator image


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During ‘verification
session’ the treatment is
set-up on the simulator
exactly like it would be
on the treatment unit.
A verification film is
taken in ‘treatment’
geometry
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Simulator Film




Field defining wires
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Shows relevant
anatomy
Indicates field
placement and size
Indicates shielding
Can be used as
reference image for
treatment
verification
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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 the
preparation of beam data for the
planning process in your center?
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 completely
independent set of data
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Machine data availability
Hardcopy (isodose charts, output factor
tables, wedge factors,…)
 Treatment planning computer (as above
or 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?
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 absolute
dose should be given to a relative dose
value in a treatment plan - e.g. deliver
2Gy per fraction to the 90% isodose
 Often the reason for misunderstanding
 Should follow recommendation of
international bodies (compare e.g. ICRU
reports 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 point
is not on central axis

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Variation of percentage depth
dose with field size
120
FS 5
FS 10
FS 20
FS 30
FS 40
100
80
60
40
10MV photons
20
0
0
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5
10
15
20
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25
30
67
Variation of percentage depth
dose with FSD
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Dose calculation


Scatter corrections for field
size changes with blocking
Attenuation factors for
wedges and trays


difference between physical
and dynamic wedges
the thicker the wedge, the
higher the attenuation at
central axis
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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
3
2
60 Gy
4
Beam weighting must be factored in !!!
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