Physics 2: IMRT in Cervix Cancer

1
Physics 2 IMRT in Cervix Cancer

• Tomas Kron, PhD
• Peter MacCallum Cancer Centre
• AUSTRALIA

2
Preface

• Despite the availability of IMRT in
  approximately 30 of radiotherapy centres in the
  western world, IMRT is rarely used for treatment
  of cervix cancer.

3
Objectives of the lecture

• Discuss the need for imaging in the assessment of
  target volumes in external beam RT of cervix
  cancer
• Introduce the concept of inverse treatment
  planning in IMRT
• Discuss pro- and cons of IMRT
• Compare IMRT and brachytherapy dose distributions
  for cervix radiotherapy

4
Some anatomy
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is it constant?
From Huh, SJ et al Radiother. Oncol. 71 (2004)
73 2 MRI T2 weighted images of the same patient 4
weeks and 35Gy apart
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Some anatomy lymph nodes
7
External beam radiotherapy for cervix cancer

• Typically 40 to 45Gy in fractions lt2Gy (eg 25fx
  of 1.8Gy)
• Two field (AP/PA) or four field box technique
• Also two lateral arcs possible

8
Cervix Plan
if inguinal lymph nodes need to be covered there
is typically no advantage in adding lateral
fields
9
Conventional treatment
Region 2 45Gy (para-aortic LN)
Region 1 45Gy 5.4Gy EBT brachytherapy boost
Mutic S et al IJROBP 55 (2003) 28
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Role of imaging for target definition

• Patterns of Care Study 1988/89 in US Ling et al
  IJROBP 1996
• Fairly uniform approach
• CT scans 11
• MRI none
• Target volume outline 14
• Small bowel outline lt1

From ICRU report 38 based on G Fletchers work
11
The impact of patient positioning on the adequate
coverage of the uterus in the primary irradiation
of cervical carcinoma a prospective analysis
using magnetic resonance imaging. Weiss E et al
Radiother. Oncol. 63 (2002) 83
Results Standard portals ie 4 field box did
not completely cover the uterus in supine
position in 7/21 (33), in prone position with
belly board in 7/21 (33) and without belly board
in 5/21 (24). Insufficient uterine coverage was
found only in the anteroposterior direction. The
mean distance ( standard deviation) between the
field borders of the lateral portals and the
uterus was in supine position anteriorly 3.4 cm
(2.2 cm) and posteriorly 1.8 cm (1.3 cm), in
prone position with belly board anteriorly 2.2 cm
(2.7 cm) and posteriorly 2.6 cm (1.6 cm), prone
without belly board anteriorly 3.3 cm (2.4 cm)
and posteriorly 1.9 cm (1.1 cm). The difference
was statistically significant between supine and
prone position with belly board and between prone
position with and without belly board. Repeated
MRI controls during therapy showed no significant
changes compared to the MRIs at the beginning of
therapy. Conclusions The use of standard
radiation fields results in a high percentage of
geographical misfits. Three-dimensional treatment
planning is a prerequisite for adequate uterus
coverage.
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what has changed in 10 years?

• Patterns of Care Study 1996-99 in US Eifel et
  al IJROBP 2004
• 1/3 stage IIIA - IVA
• CT most common
• 92.4 radical patients had brachytherapy
• 1999 63 had concurrent chemotherapy
• Small centres (less than about 4 cervix patients
  per year) tend to provide worse treatment (lt80Gy
  pt A, gt70d total treatment time)

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Role of imaging for target definition

• CT
• Treatment planning
• Nodal assessment
• MRI
• Extra cervical spread
• Design of lateral portals
• PET
• Lymph node involvement
• US/TRUS

Mutic S et al IJROBP 55 (2003) 28
14
What can IMRT do ?

• Reduction of dose to normal structures -
  conformal avoidance
• Deliver multiple dose levels at one time
• simultaneous in-field boost
• mimicking brachytherapy distributions

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Radiotherapy treatment planning
Patient information
Treatment unit data
Planning

• Inverse planning
• define what is ok
• tell the computer
• iterative optimization

Treatment plan
Treatment
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Inverse planning process

• CT scan - 3D, large volume, small slices
• Outlining of ALL (!) relevant structures (targets
  and critical organs)
• DICOM transfer of CT data sets and structures to
  planning system
• Definition of dose constraints
• Computer optimization
• Verification

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Eg Tomotherapy planning station interface
Everything of interest MUST be outlined The
system does not care about anything else.
18
Need for customisation?
Courtesy A Fyles
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scope for customisation
IMRT beneficial
Collage courtesy S Van Dyk, K Narayan
20
What are the target outlines?
IMRT difficult, if not impossible
Prior to Txt
After chemoradiation (40Gy)
K Narayan and Quinn 2003
21
Prescription panel
Three ways to guide the optimisation 1.
Precedence, 2. Importance, 3. Dose penalty
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A good dose calculation algorithm is required
to avoid steering the optimization into a false
minimum (Here Superposition Convolution)
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Inverse treatment planning

• Many automatic optimisation algorithms are in use
• gradient based
• iterative least square minimisation
• simulated annealing
• Do not necessarily find the best solution (local
  minima!)
• Can only be as good as the specified constraints
• Very computer and time consuming

Tomotherapy 30processor
24
Planning as part of a network
Issues reliability, compatibility, security
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What can IMRT do ?

• Reduction of dose to normal structures -
  conformal avoidance
• Deliver multiple dose levels at one time
• simultaneous in-field boost
• mimicking brachytherapy distributions

Lujan et al IJROBP 57 (2003) 516
26
What can IMRT do ?

• Reduction of dose to normal structures -
  conformal avoidance
• Deliver multiple dose levels at one time
• simultaneous in-field boost
• mimicking brachytherapy distributions

Mutic et al IJROBP 55 (2003) 28
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IMRT to mimic Brachytherapy
HDR brachy
HDR brachy
7 field IMRT
7 field IMRT
Schefter et al. Med Dosim 27 (2002) 177
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The first issue of a new journal
(Elsevier)Brachytherapy 1 (2002) 191

• Point/Counterpoint Can IMRT replace
  brachytherapy in the management of cervical
  cancer?

• K Alektiar (New York) Brachy-therapy
• A Mundt, J Roeske (Chicago) IMRT

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K Alektiar

• Brachytherapy is more suitable
• Can give 80-90Gy to point A safely (even higher
  to cervix point)
• Target volume difficult to define for EBRT
  (parametrium particularly)
• Organ motion likely to be larger than in prostate

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Inter-fraction Organ Motion
7 July 03
21 July 03
Courtesy A Fyles
5 Aug 03
14 July 03
31
Some comments

• Optimisation of HDR applicators and stepping
  source pattern will further improve
• Experience is very important in brachytherapy
• Must consider overall treatment time when using
  external beam and brachytherapy combination

Dose distributions from four different HDR source
movements as determined using film
Nucletron
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A Mundt and J Roeske
IMRT is a revolution in the treatment of cancer
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Role of IMRT in cervix cancer

• For pelvic treatment sparing of normal structures
  (bone marrow, intestines)
• Potentially replace brachytherapy (80Gy possible
  with 0.5cm margin) - alternatively applicator
  based IMRT (Low et al 2002)
• Simultaneous integrated boost

Lujan 2003
...IMRT may one day rival and perhaps replace
brachytherapy... Mundt and Roeske 2002
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What can IMRT do ?

• Reduction of dose to normal structures -
  conformal avoidance
• Deliver multiple dose levels at one time
• simultaneous in-field boost
• mimicking brachytherapy distributions

Unlikely
Ahmed et al IJROBP 60 (2004) 550
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Considering IMRT

• And also
• Leakage
• Integral dose, dose dumping
• Treatment time
• Dose rate
• Resources required for set-up, maintenance and QA

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Consequences for radiation safety

• More beam on time means more radiation leakage -
  assume up to 10 times more mu
• Secondary barriers may need to be increased
• If high energy photons are used, neutrons may be
  a problem

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More mu per Gy

• Imperfections of the system multiply
• Dosimetry becomes more important in particular if
  small fields are used

Linac mounted MLC
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The ideal cumulative DVH

• Tumor
• High dose to all
• Homogenous dose

• Critical organ
• Low dose to most of the structure

100
100
dose
dose
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Dose Volume Histograms
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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Documentation of the treatment

• More is required than beam direction, beam energy
  and beam on time
• IMRT requires many MLC leaf configurations
• A tomotherapy treatment is characterized by some
  60000 individual leaf opening times depending on
  gantry angle...

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Green Journal 1992 gt 50 occasions of data
transfer from one point to another for each
patient!
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Two final comments...
Small bowel dose with limited arc technique

• Positioning of the patient is important
• Imaging is not all high cost

Adli et al IJROBP 57 (2003) 230
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Prone position with belly board improved small
bowel irradiation

• but was not superior to prone position without
  belly board in terms of target caverage using
  standard fields
• The impact of patient positioning on the adequate
  coverage of the uterus in the primary irradiation
  of cervical carcinoma a prospective analysis
  using magnetic resonance imaging. Weiss E et al
  Radiother. Oncol. 63 (2002) 83

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Summary (personal opinion)

• Cervix cancer radiotherapy is likely to include
  brachytherapy in years to come
• Promising imaging techniques because of soft
  tissue contrast are MRI and US
• IMRT is likely to play a role in
• optimising conventional part of external beam
  delivery
• allow for simultaneous boost of involved lymph
  nodes

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Any questions?
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Thank you

• Acknowledgements
• A Fyles
• K Narayan
• S Van Dyk