Transcript IAEA Training Material on Radiation Protection in Radiotherapy

IAEA Training Material on Radiation Protection in Radiotherapy
Radiation Protection in
Radiotherapy
Part 10
Good Practice including Radiation
Protection in EBT
Lecture 4: Treatment verification and reporting
The Problems:
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The correct dose of radiation shall be
delivered just to the target.
The dose to surrounding structures shall be
as low as possible.
Must be achieved on many occasions
(typically >30 treatment fractions)
It must be verifiable
Must be documented in a way that allows
others to understand all important factors of
the treatment performed
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Objectives
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To understand the different frames of reference used
in radiotherapy
To be familiar with techniques which allow to verify
that the treatment is delivered to the appropriate
location
To appreciate the need for reporting dose AND
volume in prescription and treatment reporting in
radiotherapy
To be aware of the reports of the ICRU regarding
reporting of radiotherapy treatments
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Contents of lecture 4 in part
10
1. Sources of uncertainty
2. Methods to verify dose delivery
portal films
 in vivo dosimetry
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3. Prescription and reporting
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1. Sources of uncertainty
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Patient localization
Organ motion
Imaging (resolution, distortions,…)
Definition of anatomy (outlines,…)
Beam geometry
Dose calculation
Dose display and plan evaluation
Plan implementation
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Patient localization
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The patient should be
positioned identically
during diagnostics (CT),
in simulation and 30+
times during treatment
Sources of uncertainty:
 motion
 reliability of marks on
the skin
 couch sag
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Organ motion
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Affects most organs - particularly noticeable
for lung cancer, liver, prostate and other pelvic
malignancies
Shown here is the difference in CT scan
between inhale and exhale position
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Imaging issues
Partial volume effects
 Distortion (MR)
 Limited spatial resolution (PET)
 More discussion on these issues in the
companion course on diagnostic
radiology
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Target definition, outlining of
organs
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Decide where the
organ is and what
extend it has.
Are the seminal
vesicles really
where they are
drawn here?
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Beam geometry and positioning
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Is the block exactly
where it should be?
Is there gantry sag?
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Dose calculation
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There are many
different dose
calculation algorithms
All have limitations (and
be it the long time
required to calculate the
dose)
Must know what to trust
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Plan display and evaluation
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Comparison of competing plans...
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Implementation of the plan
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Transfer of data between
units
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2. Verification of dose delivery
The plan looks great...
 However, one must ensure that during
treatment everything is matching the
treatment plan
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ADAC
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In practice there are many
systems...
Patient
Diagnostic tools
Treatment
planning
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Treatment unit
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Ensure different co-ordinate
systems match...
Patient
Diagnostic tools
Treatment
planning
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Treatment unit
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Good practice
Verify data transfer
 Use critical approach
 Verify treatment by comparing an image
taking during treatment with a reference
image taken either during simulation or
taken from the treatment planning
system
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H u m an errors in d ata tran sfer d u rin g th e p rep aration
an d d elivery of rad iation treatm en t affectin g th e fin al
resu lt: " garb age in , garb age ou t"
L eu n en s, G ; V erstra ete, J ; V a n d en B og a ert, W ; V a n D a m , J ; D u treix, A ; va n d er S ch u eren , E
D ep artm en t o f R ad io th erapy , U n iv ersity H o sp ital, S t. R afaël, L euv en , B elg iu m
A b stract
D u e to th e larg e n u m b er o f step s an d th e n u m b er o f p erso n s in v o lv ed in th e p rep aratio n o f a rad iatio n
treatm en t, th e tran sfer o f in fo rm atio n fro m o n e step to th e n ex t is a v ery critical po in t. E rro rs du e to
in ad eq u ate tran sfer o f in form atio n w ill b e reflected in ev ery n ex t step and can seriou sly affect th e fin al
resu lt o f th e treatm en t. W e stu d ied th e freq u en cy and th e so u rces o f th e tran sfer erro rs. A to tal n u m b er o f
4 6 4 n ew treatm en ts h as b een check ed o v er a p erio d of 9 m o n th s (Janu ary to O cto b er 1 9 90 ). E rron eo u s d ata
tran sfer h as b een d etected in 1 3 9 /2 4 ,12 8 (less th an 1 % ) o f th e tran sferred p aram eters; th ey affected 26 %
(1 1 9 /46 4 ) o f th e ch eck ed treatm en ts. T w en ty-fiv e o f th ese d ev iation s co u ld h av e led to larg e g eog raph ical
m iss o r im p o rtan t o v er- o r u n d erd o sag e (m u ch m o re th an 5 % ) o f th e o rg an s in th e irrad iated vo lu m e, th u s
in creasin g th e com p licatio n s o r d ecreasin g th e tu m ou r con tro l pro b ab ility , if no t co rrected . S u ch m ajo r
d ev iatio n s, o n ly o ccurrin g in 0 .1 % o f th e tran sferred p aram eters, affected 5 % (25 /4 6 4) o f th e n ew
treatm en ts. T h e so u rces of th ese larg e d ev iatio n s w ere n early alw ay s hu m an m istak es, w h ereas a
co n sid erab le nu m b er o f th e sm aller d ev iatio n s w ere, in fact, co n scio u sly tak en d ecisio n s to d ev iate from th e
in ten d ed treatm en t. N early h alf o f th e m ajor d ev iation s w ere in trod u ced d u ring in pu t o f th e d ata in th e
ch eck -and -con firm sy stem , d em o n stratin g th at a sy stem aim ed to p rev en t accid en tal erro rs, can lead to a
co n sid erab le nu m b er o f sy stem atic erro rs if u sed as an u n con tro lled set-u p sy stem . T h e resu lts o f th is stu d y
sh o w th at h u m an m istak es can serio u sly affect th e ou tco m e o f p atien t treatm en ts.(A B S T R A C T
T R U N C A T E D A T 2 50 W O R D S ) [Jou rn al A rticle; In E n g lish ; N eth erland s]
Green Journal 1992: > 50 occasions of data transfer
from one point to another for each patient!
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Most important comparison
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Reference from
planning
• Simulator film
• DRR
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Check film during
treatment
• Port film
• EPID
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Portal films
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Taken during (or
directly before)
treatment with:
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beam from the
treatment unit
patient in treatment
position
shielding in place
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Port films
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Usually taken before
or after treatment
If the field itself does
not show enough
anatomy, a double
exposure technique
can be used:
Expose treatment field
Open collimators and expose the same film again
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In radiotherapy practice
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The risk from the additional exposure in
dual exposure technique portal images
is negligible compared to the risk not
treating the correct area in the patient...
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Electronic Portal Images
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A film less way to
verify field location
Mounted on the
linac
Different systems:
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ion chamber
fluoroscopic screen
semiconductor
arrays
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Electronic Portal Imaging
Devices - EPIDs
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Offered by all major
manufacturers
Has several
advantages:
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Easy use and
positioning of the
system
Allows on line
verification
Multiple images (‘cine’)
can be taken during one
treatment
Images available in
digital format
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Verification films/images
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Two different aims:
Verify correct shielding
 Verify patient location
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Verify block shape and position
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should be done for every treatment field
at least once per course of radiotherapy
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Verify patient location
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Should be done using two images which give
adequate information on the location of the
target in respect to the treatment beam
geometry.
These do not necessarily need to be
treatment fields (e.g. in case of IMRT or arcs)
This verification should be repeated every
week during treatment for radical treatments
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Verify dose delivered to the
patient
May be done in
customized
anthropomorphic
phantoms
 More likely using in
vivo dosimetry
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Breasts modeled on a particular
patient to verify skin dose using
TLDs
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IN VIVO DOSIMETRY
ICRU report 24 (1976):
“An ultimate check of the actual
treatment given can only be made by
using in vivo dosimetry.”
Why do in vivo dosimetry
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Quality Assurance – Treatment Verification
Measure because we don’t know
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Limitations of dose planning
Patient movement
Verify dose for the record
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Critical organs
Legal aspects
Clinical trials
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Challenges for in vivo dosimetry
Typically low dose where the detectors
can be located
 Variation of detector response with
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Temperature
 Radiation Quality
 Direction of radiation
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Need to not interfere with therapeutic
objective
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Potential detectors
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Needed: High sensitivity, tissue
equivalence, high spatial resolution
TLD
Semiconductors (diodes, MOSFETs)
Radiochromic film
Others (alanine, gel dosimetry, …)
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In vivo exit dosimetry
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calculate exit
fluence
determine what the
portfilm/EPID should
look like
verify dose by
projecting back
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3. Prescription and reporting
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Prescription may be at the discretion of
individual clinicians, depending on equipment
available, experience and training
Reporting must be uniform - any adequately
educated person must be able to understand
what happened to the patient in case of:
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need for a different clinician to continue treatment
re-treatment of the patient
clinical trials
potential litigation
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Recommendations by the ICRU
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International
Commission on
Radiation Units and
Measurements
ICRU reports provide
guidance on
prescribing, recording
and reporting
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The required dose information
Should describe the treatment
concisely and unambiguously
 Gets more and more complex as
treatment approaches become more
complex
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The required dose information
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1950: Patient X: x
Gy in n fractions to y
tumour
1993: ICRU 50,
appendix I
1999: ICRU 62 …plus
the same for OAR...
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Important
Information includes dose AND volume
 Information is provided for target AND
normal structure
 Information includes dose AND
fractionation
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Dose Volume Histograms
Dose and
Volume
information
 Example
from ICRU
report 62:
prostate
cancer
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Normal tissue information
rectum
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bladder
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Normal tissue information
rectum
bladder
femoral heads
Quick Question: Why is there a ‘kink’ in the DVH for
the femoral heads at about half of the dose to the
prostate?
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Answer
1
3
60 Gy
2
4
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In a ‘four field box’ treatment two of the
four fields go through the femoral
heads...
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Record keeping required...
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BSS.appendix II.31 “… in radiation
therapy, a description of the planning
target volume, the dose to the centre of
the planning target volume and the
maximum and minimum doses
delivered to the planning target volume,
the doses to other relevant organs, the
dose fractionation, and the overall
treatment time; …”
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A final point
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The best localization is
void if it is the wrong
patient!!!
Always check patient
identification prior to
treatment
In practice this is a surprisingly
common problem - there are
many patients called ‘Mr Smith’
and patients may be confused
when presenting for treatment.
A photo in the treatment sheet
may also be useful
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Summary
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Optimization of radiotherapy includes the
optimization of radiation beam design
Treatment verification using portal imaging is
essential
In vivo dosimetry is a useful complementation of
portal imaging
A large amount of information - including dose AND
volume - is required to describe a radiotherapy
treatment concisely
Radiotherapy treatments should be reported
following international conventions
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Where to Get More Information
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International Commission on Radiation Units and
Measurements. ICRU report 50: Prescribing,
recording, and reporting photon beam therapy.
Bethesda. 1993.
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International Commission on Radiation Units and
Measurements. ICRU report 62: Prescribing,
recording, and reporting photon beam therapy
(Supplement to ICRU report 50). Bethesda 2000.
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Any questions?
Question:
Please discuss the advantages (and
disadvantages) of electronic portal imaging as
compared to portal films for verification of patient
positioning during radiotherapy.
The answer should include:
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Advantages
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No additional dose
required for most images
Easy positioning of the
system
Allows on line verification
Multiple images (‘cine’)
can be taken during one
treatment
Images available in
digital format
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Disadvantages
 High
initial investment
costs
 Dual exposures may be
more difficult
 Image quality of many
systems inferior to film
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