2. • 3 D CRT is based on 3-D
anatomic information and use
dose distributions that conform
as closely as possible to the
target volume in terms of
adequate dose to the tumor and
minimum possible dose to
adjacent normal tissue
3.
4. Overwiew
• Main distinction between treatment planning
of 3-D CRT and conventional radiation therapy
– it requires
: 3-D anatomic information
: treatment-planning system that
allows optimization of dose distribution which
meets the clinical objectives.
8. • Preplanning process
• proposed treatment position of the patient is
determined.
• immobilization device is fabricated.
• It is an Important step--Errors may occur if
patients are inadequately immobilized, with
resultant treatment fields inaccurately aligned
from treatment to treatment .
11. • Image acquisition is the foundation of 3 D
panning.
• The anatomic information is usually obtained
in the form of closely spaced transverse
images, which can be processed to reconstruct
anatomy in any plane, or in three dimensions
13. CT Image- most commonly used
• CT image -reconstructed from a
matrix of relative linear
attenuation coefficients
measured by the CT scanner.
14. CT SIMULATOR
• Images are acquired on a
dedicated CT machine called
CT simulator with following
features
– A large bore (75-85cm) to
accommodate various treatment
positions along with treatment
accessories.
– A flat couch insert to simulate
treatment machine couch.
– A laser system consisting of
• Inner laser
• External moving laser to
position patients for
imaging & for marking
- A graphic work station
15. • CT is done with patient in the treatment position with
immobilization device
• Radio opaque fiducial are placed . Intention is to place these
initial marks as close to final isocentre as possible
• These fiducial assist in any coordinate transformation needed as
a result of 3D planning and eventual plan implementation.
• The planning CT data set is transferred to a 3D-TPS or
workstation via a computer network.
19. • The term image registration -- a process of
correlating different image data sets to identify
corresponding structures or regions.
• Allows full voxel to voxel intensity match
• Image Fusion automatically correlates thousands
of points from two image sets, providing true
volumetric fusion of anatomical data sets.
20. • For example--, mapping of structures seen in MRI onto
the CT images.
• Various registration techniques include
– Point-to-point fitting,
– Line or curve matching
– Surface or topography matching
– Volume matching
23. Segmentation
• slice-by-slice delineation of anatomic regions of
interest-- external contours, targets, critical
normal structures, anatomic landmarks, etc.
• The radiation oncologist draws the target
volumes in each slice with appropriate margins to
include visible tumor, the suspected tumor
spread, and patient motion uncertainties.
• The segmented regions can be rendered in
different colors
24. • one of the most laborious but important
processes in treatment planning.
• It requires clinical judgment, which cannot be
automated or completely image based.
• It should not be delegated to personnel other
than the physician in charge of the case, the
radiation oncologist.
27. • volumes defined prior to treatment planning
– Gross tumor volume (GTV).
– Clinical target volume (CTV).
• Defined during the treatment planning process
– Planning target volume (PTV).
– Organs at risk.
• As a result of treatment planning, volumes described.
– Treated volume (TV).
– Irradiated volume (IRV).
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43. Beam aperture design
aided by
• the BEV ( Beam’s eye view)capability of the 3-
D treatment-planning system
• DRR
44. Digitally Reconstructed Radiograph-DRR
• A synthetic radiograph produced by tracing ray-lines from a
virtual source position through the CT data to a virtual film
plane .
• It is analogous to conventional simulation radiographs.
45. • DRR is used
– for treatment portal design
– for verification of treatment
portal by comparison with port
films or electronic portal
images
46. Beam Eye View-BEV
• In BEV observer’s viewing point is
at the source of radiation looking
out along axis of radiation beam.
• Targets and critical normal
structures visible in different
colors through segmentation can
be viewed from different
directions in planes perpendicular
to the beam's central axis.
– Demonstrates geometric coverage of
target volume by the beam
– Shielding & MLCs are designed on
BEV
– Useful in identifying best gantry,
collimator, and couch angles to
irradiate target & avoid adjacent
normal structures
47.
48. Beam apertures can be designed
• Automatically - the user sets a uniform
margin around the PTV.
• Manual- when its needed to draw a
nonuniform margin
• Generally, a 2-cm margin between the PTV
and the field edge ensures better than 95%
isodose coverage of the PTV
50. • For planning, the 3D TPS must have the capability to simulate
each of the treatment machine motion functions, including
– Gantry angle,
– Collimator length, width & angle,
– MLC leaf settings,
– Couch latitude, longitude, height & angle
51. FORWARD PLANNING
• For 3D CRT forward planning is used.
• Beam arrangement is selected.
• Using BEV, beam aperture is designed
• Dose is prescribed.
• 3D dose distribution is calculated.
• Then plan is evaluated.
• Plan is modified based on dose distribution evaluation, using
various combinations of
– Beam , collimator & couch angle,
– Beam weights &
– Beam modifying devices (wedges, compensators) to get desired dose
distribution.
52. • Three-dimensional treatment planning
encourages the use of multiple fields because
targets and critical structures can be viewed in
the BEV configuration individually for each field.
• Multiple fields removes the need for using ultra-
high-energy beams (>10 MV), which are required
when treating thoracic or pelvic tumors with only
two parallel opposed fields
53. • Using a large number of fields (greater than
four) creates the problem of
- designing an excessive number of beam-
shaping blocks
- requiring longer setup times
-Carrying so many heavy blocks creates a
nuisance for therapists who have to guard
against dropping a block accidentally or using
a wrong block.
54. • A good alternative to multiple
field blocking is the use of a
multileaf collimator (MLC)
• A field drawn on a BEV printout
can be digitized to set the MLC
setting.
• BEV field outlines can also be
transmitted electronically to the
accelerator to program the MLC.
56. Plan Optimisation
• Optimisation refers to the technique of finding
the best physical and technically possible
treatment plan to fulfill the specified physical
and clinical criteria
• An optimal plan should deliver tumoricidal
dose to the entire tumor and spare all the
normal tissues.
57. PLAN EVALUATION
• Tools used in the evaluation of the planned
dose distribution:
• Isodose lines
• Color wash
• DVHs (Dose volume histograms )
– Dose distribution statistics
58. Isodose curves
• Dose distributions of
competing plans are
evaluated by viewing
isodose curves in individual
slices, orthogonal planes
(e.g., transverse, sagittal,
and coronal), or 3-D isodose
surfaces.
59. Colour wash
• Spectrum of colors superimposed on
the anatomic information represented
by modulation of intensity
– Gives quick over view of dose
distribution
– Easy to assess overdosage in
normal tissue that are not
contoured.
– To assess dose heterogeneity
inside PTV
• Slice by slice evaluation of dose
distribution can be done
60. Dose volume histograms
• DVHs summarize the information contained in
the 3-D dose distribution & quantitatively
evaluates treatment plans.
• DVHs are usually displayed in the form of ‘per
cent volume of total volume’ against dose.
• The DVH may be represented in two forms:
– Cumulative integral DVH
– Differential DVH.
61. CUMULATIVE DVH-more useful
• It is plot of volume of a given
structure receiving a certain
dose.
• Any point on the cumulative
DVH curve shows the volume
of a given structure that receives
the indicated dose or higher.
• It start at 100% of the volume
for zero dose, since all of the
volume receives at least more
than zero Gy.
62. DIFFERENTIAL DVH
• The direct or differential DVH is
a plot of volume receiving a dose
within a specified dose interval
(or dose bin) as a function of
dose.
• It shows extent of dose variation
within a given structure.
• The ideal DVH for a target
volume would be a single column
indicating that 100% of volume
receives prescribed dose.
• For a critical structure, the DVH
may contain several peaks
indicating that different parts of
the organ receive different doses.
DVH - target vol.
DVH - OAR
63. 3-D DOSE CLOUD
• Map isodoses in three
dimensions and
overlay the resulting
isosurface on a 3-D
display with surface
renderings of target
& other contoured
organs.
64. Dose statistics
• It provide quantitative information on the volume of the target or critical
structure and on the dose received by that volume.
• These include:
– The minimum dose to the volume
– The maximum dose to the volume
– The mean dose to the volume
• Useful in dose reporting.
65.
66.
67.
68. PLAN IMPLEMENTATION
• Once the treatment plan has been evaluated &
approved, documentation for plan implementation
must be generated.
• It includes
– beam parameter settings transferred to the treatment
machine’s record and verify system,
– MLC parameters communicated to computer system
that controls MLC system of the treatment machine,
– DRR generation & printing or transfer to an image
database.
69. Plan implementation
• After Physician contuors target volumes and determination
of treatment isocentre is done
• Couch shifts ( distances in three directions ) between
reference marks drawn on CT Scanner and treatment
centre are then calculated
• On first day of treatment , patient is first positioned to
initial refernce marks and then shifted to treatment
isocentre using the calculated shifts .
• The treatment isocentre is then marked on the patient
70. Position verification
• Patient position is verified and thus corrected using
EPID ( electronic portal imaging device)
• Both the field and the bony anatomy are matched
sequentially to give an estimate of error.
71. Online and offline corrections
• refer to whether the patient is on the
treatment couch while the verification is being
done and whether the correction would be
applied to the same or subsequent sessions
72. Offline corrections
• images acquired before treatment and matched
to the reference image at a later time point.
• Aims to determine the individual systematic
setup error and thus reduce it.
• When combined with setup data of other
patients treated under the same protocol, it helps
define the population standard error for that
treatment in that institution.
• PTV margins in an institution depend on these
determinations of individual and population
systematic errors
73. Online corrections
• Acquisition of images and their verification and
correction prior to the day’s treatment.
• Aims to reduce both random and systematic errors.
• The treatment site and the expected magnitude of
error may determine the frequency of online imaging.
• Sites where large daily shifts are anticipated (abdomen,
pelvis, and thorax) or where even slight shifts will alter
the dose distribution within adjacent critical structures
(paraspinal tumors, intracranial tumors in close
proximity to optic structures) are best managed with
daily imaging
74.
75. Dose calculation
• Dose calculation algorithms ….three broad
categories:
(a) correction based,
(b) model based, and
(c) direct Monte Carlo.
• Direct Monte Carlo - most accurate method
for treatment planning.
