3-D conformal radiation therapy (3-D CRT) aims to conform the high radiation dose region to the target volume while reducing dose to surrounding normal tissues. The process involves immobilizing the patient, obtaining images like CT and MRI to define targets and organs at risk, delineating these structures, and planning radiation beams individually shaped to the target only. Key volumes defined include the gross tumor, clinical target, planning target, and organ at risk volumes.

1. THREE-DIMENSIONAL CONFORMAL
RADIATION THERAPY
P Sathish Kumar
RNT MEDICAL COLLEGE
UDAIPUR

2. INTRODUCTION
 The aims of 3-D CRT are to achieve conformity of the
high dose region to the target volume and consequently to
reduce the dose to the surrounding normal tissues.
 3-D conformal radiotherapy (3-D CRT) is the term used to
describe the design and delivery of radiotherapy treatment
plans based on 3-D image data with treatment fields
individually shaped to treat only the target tissue.

3. A typical 3-D CRT Process

5. CLINICAL IMPLEMENTATION OF 3-D
CONFORMAL RADIOTHERAPY
 PATIENT ASSESSMENT AND DECISION TO TREAT
WITH RADIATION.
 IMMOBILIZATION AND PATIENT POSITIONING.
 IMAGE ACQUISITION AND TARGET
LOCALIZATION.
CT imaging
MRI and other imaging modalities
• SEGMENTATION OF STRUCTURES.
• TREATMENT PLANNING FOR 3-D CONFORMAL
RADIOTHERAPY.

6. PATIENT ASSESSMENT AND DECISION TO
TREAT WITH RADIATION
 The first step in the process is patient assessment and
deciding how the patient should be treated. During
assessment various diagnostic and investigative
procedures are undertaken to define the state of the
disease.
 This involves imaging, biochemical testing and review
of pathologic information to identify the type, stage and
grade of the cancer. The decision to treat the patient with
radiation should be made by a team of clinicians

7. IMMOBILIZATION AND PATIENT
POSITIONING
Ra.diation treatment accuracy can be divided into two
separate but highly interrelated issues:
Dosimetric accuracy- deal with issues such as radiation
beam calibration and dose calculations.
Geometric accuracy- It covers issues related to patient
positioning and immobilization that have a strong effect on
how well we can accurately cover a specified anatomic
volume with a desired radiation dose

8.  An accurately set up laser alignment system
is an essential requirement for accurate
radiotherapy.
 This should consist of at least three lasers
to provide two lateral crosses and a sagittal
line which can be used in conjunction with
appropriately placed tattoos to ensure the
patient is not rotated.
 Special immobilization systems are
available for immobilizing different parts of
the body

9. For example, knee supports and ankle
stocks are used for pelvic and abdominal
immobilization, adjustable breast boards
are used for breast and vacuum
immobilization bags or alpha cradles are
used for chest, thermoplastic masks are
used for head and neck treatment, and
relocatable stereotactic frames are used for
brain tumours.

10. Head Rest
head rest- transparent headrest
Polyurethane

11. H &N Molds
FLATTENED AT 70° C IN WATER
HARDENS IN FEW MINUTES
USED FOR HEAD & NECK AND PELVIS CASES
EASILY FIXED TO THE COUCH
REUSABLE
CAN BE CUT AT STRATEGIC PLACES FOR
BEAM ENTRANCE
LASTS LONG WITHOUT DETERIORATION

12. Patient is placed on the couch in the same position as
was during simulation using the same immobilization
devices

13. Image acquisition and target Localization
 CT imaging
 For many tumour sites CT scanning provides
the optimal method of tumour localization. All
CT planning must be carried out under
conditions as nearly identical as possible to
those in the treatment room, including the
patient support system (couch top), laser
positioning lights and any patient positioning
aids.

14. LASER CROSS HAIR SYSTEM

15.  For conformal therapy a slice separation and
thickness of between 3 mm and 5 mm is
recommended for CT scanning.
 For head and neck and Central Nervous
System (CNS) planning this may be reduced
to between 2 mm and 3 mm.
 The CT scanner couch top must be flat,
securely fitted and compatible with the
therapy machine couch. Transverse and
longitudinal lasers with additional laser
positioning lights are needed in the CT room
identical to those in the treatment room to
ensure exact positioning of the patient.

16. Digitally reconstructed radiographs
 One of the important features of 3-D treatment planning is
the ability to reconstruct images in planes other than that of
the original transverse image. These are called the digitally
reconstructed radiographs (DRRs).
 DRRs are produced by tracing ray lines from a virtual
source position through the CT data of the patient to a
virtual film plane. The sum of the attenuation coefficients
along any one ray line gives a quantity analogous to optical
density (OD) on a radiographic film.

17. Digitally reconstructed
radiographs

18.  In treatment planning, MRI images may be used
alone or in conjunction with CT images. In general,
MRI is considered superior to CT in soft-tissue
discrimination such as central nervous system
tumors and abnormalities in the brain.
 Also, MRI is well suited to imaging head and neck
cancers, sarcomas, the prostate gland, and lymph
nodes. On the other hand, it is insensitive to
calcification and bony structures, which are best
imaged with CT.
 Although important differences exist between CT and
MRI image characteristics, the two are considered
complementary in their roles in treatment planning.

19.  The most basic difference between CT and MRI is
that the former is related to electron density and
atomic number (actually representing x-ray linear
attenuation coefficients), while the latter shows
proton density distribution.
 One of the most important requirements in
treatment planning is the geometric accuracy. Of
all the imaging modalities, CT provides the best
geometric accuracy and, therefore, CT images are
considered a reference for anatomic landmarks,
when compared with the other modality images.

20. SEGMENTATION OF STRUCTURES
 The term image segmentation in treatment planning
refers to slice-by-slice delineation of anatomic
regions of interest, for example, external contours,
targets, critical normal structures, anatomic
landmarks, etc.
 The segmented regions can be rendered in
different colors and can be viewed in BEV
configuration or in other planes using DRRs.
Segmentation is also essential for calculating dose
volume histograms (DVHs) for the selected regions
of interest.

21.  Image segmentation is one of the most laborious but
important processes in treatment planning.
 Although the process can be aided for automatic
delineation based on image contrast near the
boundaries of structures, target delineation requires
clinical judgment, which cannot be automated or
completely image based.

22. TREATMENT PLANNING FOR 3-D CONFORMAL RADIOTHERAPY
 CT: attenuation coefficients (m), can be converted to
electron density,
 used for treatment planning/dose calculation.
 Spatial resolution ~1mm in X/Y directions,
 variable (1-10 mm) in Z-direction, which affects the
quality of DRR
 MRI: proton density, better soft tissue delineation
(brain, head/neck, prostate), but insensitive to
calcification and bony structures.
 Spatial resolution ~1mm in all directions.
 PET: functional image

23. DEFINITION OF VOLUMES
 Volume definition is a prerequisite for meaningful 3-D
treatment planning and for accurate dose reporting.
ICRU Reports 62 and 83 define and describe several
target and critical structure volumes that aid in the
treatment planning process and provide a basis for
comparison of treatment outcomes
 Gross tumor volume or GTV
 Clinical target volume or CTV
 Planning target volume or PTV
 Organ at risk or OAR
 Planning organ-at-risk volume or PRV
 Internal target volume or ITV
 Treated volume or TV
 Remaining volume at risk or RVR

24.  The Gross Tumor Volume (GTV) is the
gross palpable or visible/ demonstrable
extent and location of malignant growth.
 The GTV is usually based on information
obtained from a combination of imaging
modalities (computed tomography (CT),
magnetic resonance imaging (MRI),
ultrasound, etc.), diagnostic modalities
(pathology and histological reports, etc.)
and clinical examination.

25.  Clinical target volume.
 The clinical target volume (CTV) is the tissue volume that
contains a demonstrable GTV and/or sub-clinical
microscopic malignant disease, which has to be
eliminated. This volume thus has to be treated
adequately in order to achieve the aim of therapy, cure or
palliation.
 The CTV often includes the area directly surrounding the
GTV, which may contain microscopic disease and other
areas considered to be at risk and requiring treatment
(e.g. positive lymph nodes).
 The CTV is usually stated as a fixed or variable margin
around the GTV (e.g. CTV = GTV + 1 cm margin), but in
some cases it is the same as the GTV (e.g. prostate

26. Internal target volume
 The ITV consists of the CTV plus an internal margin.
The internal margin is designed to take into account
the variations in the size and position of the CTV
relative to the patient’s reference frame (usually
defined by the bony anatomy); that is, variations due
to organ motions such as breathing and bladder or
rectal contents.

27. Planning target volume
 The planning target volume (PTV) is a
geometrical concept, and it is defined to select
appropriate beam arrangements, taking into
consideration the net effect of all possible
geometrical variations, in order to ensure that the
prescribed dose is actually absorbed in the CTV.
 The PTV includes the internal target margin
(ICRU Report No. 62) and an additional margin
for set-up uncertainties, machine tolerances and
intratreatment variations. The PTV is linked to the
reference frame of the treatment machine and is
often described as the CTV plus a fixed or
variable margin (e.g. PTV = CTV + 1 cm).

28.  The PTV depends on the precision of such tools
as immobilization devices and lasers, but does
not include a margin for the dosimetric
characteristics of the radiation beam (i.e.
penumbral areas and buildup region), as these
will require an additional margin during treatment
planning and shielding design.

29. Organ at risk
 The organ at risk is an organ whose sensitivity to
radiation is such that the dose received from a
treatment plan may be significant compared with its
tolerance, possibly requiring a change in the beam
arrangement or a change in the dose.
 Specific attention should be paid to organs that,
although not immediately adjacent to the CTV, have a
very low tolerance dose (e.g. the eye lens during
nasopharyngeal or brain tumor treatments).
 Organs with a radiation tolerance that depends on the
fractionation scheme should be outlined completely to
prevent biasing during treatment plan evaluation.

30. Planning Organ at Risk Volume
 As is the case with the PTV, uncertainties and
variations in the position of the OAR during
treatment must be considered to avoid serious
complications. For this reason, margins have to be
added to the OARs to compensate for these
uncertainties and variations, using similar principles
as for the PTV.
 A margin around an OAR with a serial-like structure
(e.g., spinal cord) is more clinically relevant than that
around an OAR with a parallel-like structure (e.g.,
liver, lung, parotid).

31. Treated Volume
 Because of the limitations of irradiation
techniques, the volume receiving the prescribed
absorbed dose might be different than the PTV; it
might be larger (sometimes much larger) or
smaller, and in general more simply shaped (less
so with IMRT than with conventional or three
dimensional radiation therapy).

32. The treatment planning process
 Once the target volume, organs at risk, and
the required doses have been defined, the
treatment plan will be produced by a person
trained in 3-D planning.
 The aim of the treatment planning process is
to achieve the dose objectives to the target
and critical structures and to produce a dose
distribution that is “optimal”.

33.  The beam angles can be chosen using standard
templates such as a six field prostate plan or by
using a beam’s-eye-view display to maximize PTV
coverage and to minimize irradiation of critical
structures.
 When a beam aperture is defined, an additional
margin of about 7 to 8 mm needs to be added
beyond the PTV in all directions in the transverse
plane to obtain the desired dose coverage to the
PTV.

34.  In the superior inferior directions one needs to add
about 12 to 15 mm margin because of beam
divergence effects.
 In its most basic implementation conformal
radiotherapy may consist of coplanar static beams in
a standard geometric configuration with MLCs or
conformal blocks used to achieve the required
conformal shape.

35. Dose-volume Histograms
 Display of dose distribution in the form of isodose
curves or surfaces is useful because it shows not only
regions of uniform dose, high dose, or low dose, but
also their anatomic location and extent.
 In 3-D treatment planning, this information is essential
but should be supplemented by DVHs for the
segmented structures, for example, targets, critical
structures, etc.
 A DVH not only provides quantitative information with
regard to how much dose is absorbed in how much
volume, but also summarizes the entire dose
distribution into a single curve for each anatomic
structure of interest. It is, therefore, a great tool for
evaluating a given plan or comparing competing plans

36.  The DVH may be represented in two forms:
the cumulative integral DVH and the
differential DVH.
 The cumulative DVH is a plot of the volume
of a given structure receiving a certain dose
or higher as a function of dose .
 Any point on the cumulative DVH curve
shows the volume that receives the
indicated dose or higher. The differential
DVH is a plot of volume receiving a dose
within a specified dose interval (or dose
bin) as a function of dose

37. 100
75
50
25
0
Volume (%)
target
bladder
rectum
0 25 50 75 100
Dose (Gy)
femurs
D=77Gy
V=90%
D=75Gy
V=30%
D=72Gy
DVH for a prostate plan

38. Dose Computation Algorithms
 Dose calculation algorithms for computerized
treatment planning have been evolving since the
middle of the 1950s. In broad terms the algorithms
fall into three categories.
 a) correction based,
 (b) model based,
 (c) direct Monte Carlo
 Either one of the methods can be used for 3-D
treatment planning, although with a varying degree of
accuracy and speed. However, the model-based
algorithms and the direct Monte Carlo are becoming
more and more the algorithms of the future.

39. Correction-based Algorithms
 These algorithms are semiempirical. They are based
primarily on measured data (e.g., percent depth doses
and cross-beam profiles, etc.) obtained in a cubic water
phantom.
 The corrections typically consist of
 (a) attenuation corrections for contour irregularity;
 (b) scatter corrections as a function of scattering volume,
field size, shape, and radial distance;
 (c) geometric corrections for source to point of calculation
distance based on inverse square law;
 (d) attenuation corrections for beam intensity modifiers
such as wedge filters, compensators, blocks, etc.;
 (e) attenuation corrections for tissue heterogeneities
based on radiologic path length

40. Model-based Algorithms
 Convolution-superposition is currently the most
accurate model-based algorithm.
 Direct Monte Carlo is the most accurate method for
treatment planning, but currently it is not feasible
because it requires prohibitively long computational
times. However, with the continuing advancement of
computer technology, it is possible that direct Monte
Carlo will be used routinely for treatment planning in
the not too distant future.

41. Thank
you