Intrafractional Junction Shifts
Utilizing Multileaf Collimation: A
Novel CSI Planning Technique
Rodney Hood RT(R)(T)CMD
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Happy Father’s Day!
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Quiet Room
Beam me up Scotty!
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What is CSI?
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CSI-DURHAM!
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CSI
• Craniospinal irradiation (CSI) is a technique used in
radiation therapy to deliver a prescribed amount of
radiation to the entire cranial-spinal axis to achieve
curative measures in the treatment of intracranial
tumors.
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Why is CSI Necessary?
• Curative surgical excision of malignant intracranial
neoplasms is rarely possible
• This tumor spreads malignant cells throughout the
subarachnoid space via the cerebrospinal fluid (CSF)
• Entire central nervous system must be treated (CSF)
Evans, A.E. et al. The Treatment of Medulloblastoma. J. Neurosurg. 72:572-583; 1990
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CSI
• Craniospinal Irradiation
– Treats anywhere CSF flows
– Treatment fields typically include the brain to
the thecal sac
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CSF
• Cerebrospinal fluid
– Clear bodily fluid found in the brain and spine
– Produced by choroid plexus of the brain
– Provides mechanical and immunological
protection for brain
– Cerebral auto regulation of cerebral blood flow
– Occupies the subarachnoid space between
arachnoid and pia mater
– Constitutes the contents of the cisterns, sulci
and brain ventricles as well as the central
canal of the spinal cord.
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Cerebrospinal Fluid
(cont.)
Brain produces
around 500ml of CSF
daily
Fluid is constantly
reabsorbed
Usually about 100160ml present at any
one time
CSF turns over
around 4 times a day
As always, all info
ripped directly from
Wikipedia
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Craniospinal irradiation (CSI),
combined with chemotherapy and
surgery, is the current choice of
treatment for malignant brain tumors
•
- medulloblastoma
- ependymoma
- Germinoma (prophylactic)
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CT Simulation
• Setup and Scanning
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Wedge and sponge combination to
extend mandible
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Styrofoam pad used to build up patient to
reduce cervical and thoracic spine curvature
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Long Orfit mask to immobilize head and
shoulders
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CT zero marked along midline
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No Styrofoam pad to build up spine
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Styrofoam pad to build up spine
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Why do we feather the junction?
• Uncertainty
• Possible setup error
• Possible patient movement
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Can you identify the pathology?
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According to a study performed at
Harvard Medical School that assessed
the accuracy and precision of patient
setup during treatment with a plan
involving feathered fields, the magnitude
of the spatial stochastic and systematic
setup error was determined to be
approximately 3 and 2 millimeters
respectively.
Holupka, E.J. et al. Effect of Set-Up Error on the Dose Across the Junction of Matching Cranial-Spinal Fields in the Treatment of
Medulloblastoma. Int. J. Rad. Onc. Biol. Phys. 27:345-352; 1993
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•Moving the junction (feathering) is a common technique
utilized in radiotherapy to smooth out any dose
inhomogeneity across the junction of the abutting fields
•Doses along a border of two fields
- too much dose (a hot spot) due to an overlap
- under dosing (a cold spot) from too wide a gap
•The gap must be “feathered” in order to decrease the
possibility of such hot or cold spots from occurring
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Feathering – 2 Types
In order to evenly distribute dose along field
junctions the junction/gap is moved
2 ways to feather
• Inter-fractional
• Intra-fractional
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Difference between inter and intra
The prefix inter means
between
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The prefix intra means
within
Inter-fractional
• Shifting the junction in-between treatments
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Intra-fractional
• The fields decrease or increase during the treatment
to move the field junction
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Dosimetric Advantages
• Utilizing Intrafractional shifts is superior dosimetrically
because it is much more forgiving for setup error and
patient movement.
• When errors are introduced, hot and cold spots are
reduced utilizing this technique
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Treatment Advantages
• In conventional CSI treatments each junction shift
requires a new setup
• Intrafractional treatments use the same plan
throughout the treatment so the long initial setup only
needs to be done on the first day
• Amount of imaging is reduced greatly
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Field Setup
Given the entire spine and brain must be treated, multiple fields
must be used to encompass the entire volume comprising the
brain and the spinal cord
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Traditional method
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FiF (Field-in-Field)
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Brain field creation
•
Set iso as low as possible while
maintaining flash on vertex of
skull (y2=20)
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Brain field creation
•
I try to set iso around c3
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Brain field creation
•
Set couch to match
divergence
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Brain field creation
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Brain field creation
•
Add MLC
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Upper spine field
creation
•
Ensure no exit through
mandible
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Upper spine field
creation
•
Keep same x & y-coordinate
( left, right and ant/post ) as
brain field
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Upper spine field
creation
•
I like to set lower border at
bottom of L2
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Upper spine field
creation
•
•
Collimator of brain field is set using upper jaw size
18/100=.18(inv_tan)=10.2
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Lower spine field
creation
•
I like to half beam this field
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Lower spine field
creation
•
Set iso maintaining same x&y
coordinates
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Lower spine field
creation
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•
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Kick table to 270°
Rotate gantry to match
divergence of upper spine field
Length of upper spine field will
determine gantry angle
Matching divergence is
dosimetrically advantageous
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Angled Lower Spine Field
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Angled lower spine vs. Pa low spine
PA fields converging
anterior to cord
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Post field matched to
upper spine field
divergence
• Don’t forget the “Golden Rule”
• ALARA
• As Low As Rodney Achieves!
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Setup Field Junctions
•
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First decide # of junctions
needed
This is dependent on total dose
We feather every 9Gy or once a
week
So a 27Gy course would require
2 junction shifts
Jaws must be opened to largest
aperture size because we will
be merging fields to create a
step and shoot delivery
Utilizing 1cm junctions
feathering would require 2cm of
field overlap
We use a 5mm safety gap so
the overlap region is 1.5cm at
both junctions
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1.5cm overlap
Setup Brain
Subfields
•
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First subfield is open
to largest aperture
size
For next subfield pull
two MLC’s to
decrease aperture
size by 1cm
For the last subfield
pull an additional 2
MLC’s to decrease
aperture by another
1cm
Fields are closed by
1cm to match the
larger MLC in upper
spine field
Dose is calculated
and subfields are
merged
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Subfields merged
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Setup Upper Spine
Fields
• Setup upper spine
field so the aperture
shrinks on both
superior and inferior
borders
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Setup Lower Spine
Fields
• Setup lower spine field
similar to upper but only
modify aperture
at superior
aspect
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Dose Calculation
• Dose is calculated to midplane of brain fields
• Dose to each spine field is normalized to a specific
depth by the Rad Onc by averaging depths along the
spinal axis.
• Two separate spine fields are desirable when spinal
canal depths vary greatly from upper spine and lower
spine segments
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Gap Calculations
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Upper/Lower spine
Junction verification
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During planning the PA upper
spine field as well as the PSO
lower spine fields are set to
gantry 0
The distance between the lower
border of the upper field and the
upper border of the lower field is
measured in the TPS
I also like to contour the location
of the BB’s so that DRR’s can
be created from the treatment
fields
This eliminates hand
calculations
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Calculated Gaps
Gap and Couch Position Report
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Single Spine Field
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Treatment
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First Day (Dry Run)
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Pt is setup identical to CT simulation.
Make indicated iso shifts
Start with the brain and work inferior
From this iso all shifts are longitudinal
only
Image brain (I prefer KV)
Adjust pt as needed-i.e. shifts or pt
adjustment
Make calculated inferior shift from brain
iso to upper spine field
Image and verify match
Make calculated shift from upper spine
iso to lower spine field
Image lower spine
No adjustment to spine iso’s are made
only pt postition
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Dry Run (cont.)
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After all initial kV imaging is complete an MV of each tx field is taken
After MV imaging of the brain a BB is placed on the center of the
inferior border
We now shift inferiorly to the upper spine field
The field is imaged to verify blocks and the position of the BB’s placed
on the inferior aspect of the brain field
This field is now rotated to 0 and the field is drawn onto the patient
A BB is placed at the inferior aspect of this field
We now shift to the lower spine field
This field is imaged to verify blocks and the location of the BB.
This field is rotated to 0 and drawn on
The anterior skin gap is measured between the upper and lower spine
field and verified against the calculated distance
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Junction Verification-Brain/Upper Spine
BB’s are placed on inferior border of brain field, portal image of upper spine field is
taken and BB distance is verified in off-line review.
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Anterior Skin Gap Verification
Rotate gantry to 0 and mark field edges on skin verify the gap on the
skin against the numbers instructed
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Upper/lower spine verification
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5mm gap
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Dosimetry
No gap
Dosimetry-evaluating junction doses
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Aligned/Misaligned Static 1cm overlap
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Aligned/Misaligned FiF 1cm overlap
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Introduced Errors
Perfect Alignment
3800
3750
Dose (cGy)
3700
3650
3600
FiF
Static
3550
3500
3450
3400
0
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1
2
3
Distance (cm)
4
5
6
Introduced Errors
5mm Gap
5000
4500
4000
Dose (cGy)
3500
3000
2500
FiF
2000
Static
1500
1000
500
0
0
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1
2
3
Distance (cm)
4
5
6
Introduced Errors
5mm Overlap
8000
7000
Dose (cGy)
6000
5000
4000
Static
FiF
3000
2000
1000
0
0
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1
2
3
Distance (cm)
4
5
6
Introduced Errors
1cm Overlap
9000
8000
7000
Dose (cGy)
6000
5000
FiF
4000
Static
3000
2000
1000
0
0
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1
2
3
Distance (cm)
4
5
6
Introduced Errors
1cm Gap
4500
4000
3500
Dose (cGy)
3000
2500
FiF
2000
Static
1500
1000
500
0
0
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1
2
3
Distance (cm)
4
5
6
TREATMENT VERIFICATION
Gotta Be Right!
74
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Delivery Verification-Portal Dosimetry
Brain
Upper Spine
Lower Spine
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Delivery verification
Be mindful of where MLC are parked
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Park MLC outside of patient or under jaw
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Brain/Spine Junction – Film Dosimetry
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Spine/Spine Junction – Film Dosimetry
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Conclusion
Shifting the junction intrafractionally reduces
treatment setup time by limiting the number of
setups needed. This technique is also superior
dosimetrically because it reduces the variation in
dose across the junction. Lastly, shifting in this
manner is more forgiving to setup error and
patient movement.
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Questions?
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Acknowledgements
Justus Adamson, Ph.D.
Taoran Li, Ph.D.
Irina Vergalasova, Ph.D.
Kevin Kirby, CMD
Scott Green, CMD
https://www.youtube.com/watch?v=
dui8noVkqBQ
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