A Novel Sequencing Algorithm for Step and Shoot IMRT and IMAT

1
A Novel Sequencing Algorithm for Step and Shoot
IMRT and IMAT

• Daliang Cao, Matthew A. Earl, David M. Shepard
• University of Maryland at Baltimore
• Department of Radiation Oncology

2
Introduction

• Current leaf sequencing algorithms for
  step-and-shoot IMRT cluster the ideal intensity
  maps into discrete levels. This results in a
  loss of plan quality even before the leaf
  sequencer is applied.
• The efficiency of the resulting plans also vary
  significantly depending upon which
  leaf-sequencing algorithm is used.
• A new leaf sequencing algorithm named Continuous
  Intensity Map Optimization (CIMO) has been
  developed that provides significant improvements
  in both the plan quality and the efficiency of
  the sequenced plans.
• The CIMO algorithm also serves as the first
  arc-sequencing algorithm that can produce
  efficient IMAT plans.

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Features of CIMO algorithm

• This algorithm eliminates the need to cluster the
  intensities before sequencing because it can work
  directly with continuous intensity levels.
• This algorithm optimizes the aperture shapes and
  weights simultaneously instead of simply applying
  a set of rules to define the aperture shapes.
• With a user defined error tolerance, the
  algorithm determines the minimum number of
  apertures needed for each field in the plan.

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A head neck case using Varian MLC
Transverse
Coronal
There are three separate targets in this clinical
case
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Comparison of Intensity Maps
A The original optimized Intensity Map from
Pinnacle
B The Sequenced Intensity Map obtained from
Pinnacle using a total of 28 segments
C The Sequenced Intensity Map using CIMO
algorithm with total of 12 segments
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Comparison the DVHs
PTV
GTV
left parotid
cord
right parotid
Thick solid line Optimized plan using 7
beams Thin solid line Pinnacle sequenced plan
using 169 segments Dashed line CIMO plan with
only 69 total segments
7
A pelvic case using Elekta MLC
Transverse
Coronal
This large irregular shaped PTV (purple
structure) covers prostate, seminal vesicles and
pelvis
8
Comparison of intensity maps
A The original optimized Intensity Map from
Pinnacle
B The Sequenced Intensity Map obtained from
Pinnacle using a total of 22 segments
C The Sequenced Intensity Map using CIMO
algorithm with total of 12 segments
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Comparison of the DVHs
PTV
rectum
bladder
bone
Thick solid line Optimized plan using 9 beams
Thin solid line Pinnacle sequenced plan using
206 segments Dashed line CIMO plan with only 84
total segments
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Detailed Comparison between plans using CIMO
algorithm and Pinnacle software
On average, CIMO algorithm reduces the number of
segments by 57, while providing a significant
improvement in the agreement between the
optimized and sequenced plans.
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Intensity Modulated Arc Therapy

• Rotational IMRT is delivered on a standard
  treatment unit using a conventional MLC.
• A dynamic arcing approach is used where the shape
  of the MLC changes during the arced beam
  delivery.
• Multiple overlapping arcs are used to produce a
  modulated intensity pattern from each beam angle.

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IMAT constraint

• Since each arc is approximated with a series of
  static beams separated by 10 degrees, leaf motion
  between adjacent angles is limited by leaf travel
  speed and gantry rotation speed. For example, if
  the gantry speed is 10 degree/sec and the leaf
  travel speed is 3 cm/sec, then the maximum leaf
  travel distance between two adjacent angles is 3
  cm.
• In order to keep a reasonable aperture
  interpolation between adjacent angles, a leaf
  cannot travel too fast even if the speed is
  allowed by the treatment machine. So we currently
  use 3 cm as the maximum leaf travel distance
  between adjacent angles.

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A 6-arc IMAT plan for a prostate case
Transverse
Sagittal
The Blue structure is the PTV the pink structure
is bladder the purple structure is rectum. Note
that the 90 isodose lines (green) is highly
conformal.
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Comparison of DVHs
GTV
PTV
bladder
rectum
Dashed lines a 36-beam step and shoot IMRT plan
with 411 total segments Solid lines a 6-arc
IMAT plan (solid lines).
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IMAT aperture shapes
110
120
130
140
180
150
160
170
3cm
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Another case a lung cancer with a 2-arc IMAT plan
Transverse
Coronal
The green structure is the PTV the pink
structure is spinal cord. Note the 95 isodose
lines (red) cover the target very well.
17
IMAT vs. Step and Shoot IMRT for a pancreas case
PTV
right kidney
left kidney
liver
Solid lines 7-field IMRT plan Dashed lines
5-arc IMAT plan IMAT plan clearly shows a better
sparing to the critical structures while keeping
a similar coverage to the PTV.
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Prostate Example
PTV
GTV
bladder
rectum
normal tissue
Solid lines 7-field IMRT plan Dashed lines
4-arc IMAT plan providing a similar PTV coverage
and critical structure sparing, the IMAT plan
delivers lower dose to the normal tissue.
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Comparison of the dose distribution
4-arc IMAT plan
7-field IMRT plan
Note that hot streaks near the skin seen in the
7-field IMRT plan are eliminated in the IMAT plan
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Summary

• A new leaf-sequencing approach called Continuous
  Intensity Map Optimization (CIMO) has been
  developed that uses a simulated annealing
  algorithm to simultaneously optimize the aperture
  shapes and weights.
• The CIMO algorithm eliminates the need to divide
  the pencil beam intensities into a small number
  of discrete levels.
• Results for step-and-shoot IMRT demonstrate that
  the CIMO algorithm reduces the number of
  segments, on average, by 57 as compared to plans
  sequenced in Pinnacle.
• The CIMO algorithm also serves as an efficient
  IMAT sequencer. Results demonstrate that highly
  conformal plans can be obtained using between 2-8
  IMAT arcs.