This document discusses the principles and utility of 3D conformal radiation therapy (3DCRT). It begins by explaining the goals of radiotherapy to maximize dose to the tumor while minimizing dose to normal tissues. It then describes some disadvantages of conventional 2D planning, including lack of 3D visualization and irradiation of large normal tissue volumes. The document goes on to define 3DCRT as radiotherapy that closely conforms the high dose volume to the target while sparing critical tissues. It discusses the history and development of 3DCRT and provides details on target volume definition, treatment planning workflow including imaging, contouring, planning and evaluation.

1. PRINCIPLES OF
CONFORMAL
RADIATION AND
UTILITY OF 3DCRT
Dr.G.Lakshmi Deepthi

2. Radiotherapy :
 Principles :
maximum dose to the tumor
least normal tissue toxicity

4. Conventional planning :

5. Disadvantages of conventional
planning :
 Lack of 3D visualization of the tumor
 Irradiation of large volumes of normal tissue along
with the tumor
 Higher toxicity and side effects.
 2D planning of 3D tumor.

6. Conformal radiation therapy
It is described as radiotherapy treatment that
creates a high dose volume that is shaped to
closely “Conform” to the desired target volumes
while minimizing the dose to critical normal
tissues..

7. History :

8.  CT - Goitein and co workers used CT based
imaging –high quality BEV displays and display
radiographic images from CT –DRR’s
 End of 1980’s –3D planning systems were
developed.
 1990’s – 3DTPS became commercially available

9. Features of conformal
radiotherapy :
 Target volumes are defined in three dimensions
 Multiple beam directions are used to crossfire on the
targets.
 Individual beams are shaped or intensity modulated to
create a dose distribution that conforms to the target
volume and desired dose levels.
 Use of image guidance , accurate patient setup
,immobilization and management of motion to ensure
accurate delivery of the planned dose distributions

10. Types Of Conformal
Radiotherapy :
3DCRT :Techniques
aiming to employ
geometric field shaping
alone
IMRT :Techniques to
modulate the intensity of
fluence across the
geometrically-shaped
field.

11. What is 3DCRT :
 To plan & deliver treatment based on 3D anatomic
information. such that resultant dose distribution
conforms to the target volume closely in terms of
◦ Adequate dose to tumor &
◦ Minimum dose to normal tissues.

13. Conformation : Automated
Manual

14. Workflow of Conformal
Radiotherapy :

15. Step 1 : Positioning :
 comfortable and reproducible.
 Suitable for beam entry with minimum accessories
in beam path.
 Positioning devices are ancillary devices used to
maintain the patient in a non standard treatment
position.

16. Step 1 :Immobilization :
An immobilization device is any device that helps to
establish and maintain the patient in a fixed, well-
defined position from treatment to treatment over a
course of radiotherapy-reproduce the treatment
everyday.

17. Step 2 : Image Acquisition
 It provides foundation for treatment planning
 Usually more than one imaging modalities are
required for better delineation of target volume
 Images are acquired for :
◦ Treatment planning
◦ Image guidance and/or treatment verification
◦ Follow-up studies (during & after treatment)

18. Step 2: Imaging modalities
 Anatomic images of high quality are needed to
accurately delineate target volumes and normal
structures
 Modalities :
CT
MRI
US
SPECT
PET

19. CT Imaging :
 Advantages :
 reconstruction of images in
plane other than that of
original transverse image.
 Bony structures
 Easily available ,
inexpensive
 4D imaging can be done
 Gives quantitative data in form of CT no. (electron
density) to account for tissue heterogeneities while
computing dose distribution

20. MRI Imaging :
 depend on proton density
distribution
 They can be used alone or in
conjunction with CT

21. Advantages :
 directly generates scan in axial , sagittal, coronal
planes.
 No radiation dose to patient
 Superior to CT in soft tissue delineation such as
CNS, head and neck ,sarcoma , prostrate, lymph
nodes.
Disadvantages :
 Insensitive to calcification
and bony structures.
 longer time
 artifacts

22. PET CT Imaging :
 enables the collection of both anatomical & biological
information simultaneously
 Advantages :
Earlier diagnosis of tumour
Accurate staging
Precise treatment
Monitoring of response
to treatment
 Disadvantages :
 Poor resolution
 Costly

23. Simulation :
 Virtual simulation is a process in which the
physician uses the digital CT data to define normal
tissue and target volume contours to reconstruct
the patient in three dimensions on a video display
terminal.
 Images are obtained on a CT simulator as it
provides the best geometric accuracy

24. CT Simulator :
• 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

25. Requisites Of A Planning CT :
 Ct couch should be flat
 Same position
 Immobilization
 Fiducial pointers
 Planning CT protocols are tumor site dependent
and typically 2-5mm thickness and 50-200 slices

26. Image Acquisition :
Patient made to lie in treatment position.
Immobilization devices are used
Radio opaque fiducial are placed
Topogram is generated and VOI is selected
Using site dependent protocols images are
generated
Transfer of images to a 3DTPS or workstation

27. Step 3 :Image Registration :
 Process of correlating different image data sets to
identify corresponding structures or regions.
 It provides accurate geometric model of the patient
,as well as the electron density information needed
for the calculation of the 3D dose distribution that
takes into account tissue heterogeneities

28. MRI IMAGE
CT IMAGE
Contouring On fused
Image
POINT TO POINT MATCHING
IMAGE FUSION

29. Applications Of Image
Registration :
 Visualizing CNS structures more clearly seen on MRI
and mapping them to CT image for planning-fusion
 Combining functional or biochemical signals from
emission tomography onto CT scans for planning
purposes.
 For organ motion studies
 Image guidance
 For follow-up studies
 Image registration allows computation of cumulative
doses from multiple plans done on different image
sets for same patient

30. Step 4 :Image segmentation :
 It is the slice by slice delineation of anatomic
regions of interest.
 CT is the principal source of imaging data used for
defining the structures.
 Problems with CT :
1.in case of GTV –appropriate CT window
and level settings– maximum dimension of gross
disease.
2.organ motion

31. Volume specification :
 Volume definition is prerequisite
for 3-D treatment planning.
 To aid in the treatment planning
process & provide a basis for
comparison of treatment
outcomes.
 ICRU reports50 & 62 define &
describe target & critical structure
volumes.

32. Volume specification :
 ICRU 29—1978
Target volume
Treated volume
Irradiated volume
Organ at risk
Hot spot

33. Target volume :
 Definition : volume containing those tissues that are
to be irradiated to a specified absorbed dose
according to a specified time dose pattern.
 Did not address the issue of coordinate system and
no definite margin added for different type of
uncertainties

34.  Treatment volume : volume enclosed by the
isodose surface representing minimal target dose.
 Irradiated volume : volume that receives a dose
considered significant in relation to normal tissue
tolerance (eg:50% isodose surface).
 OAR : radiosensitive organs in or near the target
volume whose presence influences treatment
planning or prescribed dose.
 Hot spot :tissues outside the target area receiving
dose higher than 100% of the specified target
dose(at least 2cm2 in section)

35. VOLUMES :
 Gross target volume
 Clinical target volume
 Planning target volume
 Organs at risk
 Treated volume
 Irradiated volume
Defined prior
to T/t planning
During T/t
planning
Depends on the
T/t technique
ICRU 50

36. ICRU 50-1993
 Well suited for conformal therapy
TARGET VOLUME :
PTV
CTV
GTV

38. GTV-Gross Target Volume
 Gross demonstrable extent and location of the
malignant growth.
 It consists of :
 Primary tumor(GTV primary)
 Metastatic lymphadenopathy(GTV nodal)
 Other metastasis(GTV M)
 If the tumor has been removed prior to
radiotherapy then no GTV can be defined.

39. GTV Identification :

40. GTV Identification--CT

42. CTV – Clinical Target Volume
2 types of
Subclinical
extension:-
Around the GTV-
CTV I
At a distance
(Regional lymph
nodes)-CTV II
To account for uncertainties in microscopic tumor spread

44.  The PTV is a static geometrical concept defined
to select appropriate beam sizes and beam
arrangements.
 It considers the net effect of the geometrical
variations to ensure that the prescribed dose is
actually absorbed in the CTV.
PTV-Planning Target Volume :

45. Treated volume : (previously treatment volume )
volume enclosed by any isodose surface, selected and
specified by the radiation oncologist as being
appropriate to achieve the purpose of treatment.
Irradiated volume : tissue volume that receives a dose
that is considered significant
in relation to normal
tissue tolerance.
Hot spot :
2cm2  1.5cm2

46. ICRU 50
Irradiated
Volume
Treated Volume
PTV
CTV
GTV

47. ICRU 62- 1999
 Supplement ICRU report 50 –conformal therapy
 different margins to account for Anatomical &
Geometrical uncertainties – internal and setup
margin.
 Introduces concept of reference points &
coordinate systems.
 Introduces the concept of conformity index.
 Classifies Organs at Risk.
 Introduces planning organ at risk volume.
 Gives additional recommendations on reporting
doses, not only in a single patient but also in a
series of patients.

48. SM
IM
CT
V
Internal Target Volume
Planning Target Volume
SM-setup margin
CTV-Clinical target volum
IM-internal margin

49. PTV :
 Based on published clinical experience
 Van Herk and colleagues –systemic and random errors
on the required margins to account for setup error and
organ motion –
PTV margin = 2.5 Σ + 0.7σ
 Asymmetric nature of positional uncertainties eg:
prostate
 Beam portals– additional margin beyond PTV required
to obtain dose coverage because of beam penumbra
and treatment techniques.

50.  Coplanar treatment techniques –margins across
plane of treatment and margins orthogonal will be
different .
 PTV overlapping with a contoured normal structure
–
 PTV contour extending outside the skin –delineate
PTV 5cm below the skin surface.
 PTV margin can be reduced if more frequent
imaging or other technical innovation is used to
reduce geometric uncertainties.

52. Organs At Risk Classification :
Normal tissue whose radiation sensitivity may
significantly influence treatment planning
&prescribed dose.
 Class I Organs: radiation lesions are fatal or result
in severe morbidity.
 Class II Organs: mild to moderate radiation
morbidity
 Class III Organs: mild, transient, reversible, not
significant radiation morbidity
ICRU-50

53. Classification Of Organs At Risk
Serial – whole organ is a continuous unit and damage at one point
will cause complete damage of the organ (spinal cord, digestive
system). So even point dose is significant
Parallel – organ consists of several functional units and if one part
is damaged, the rest of the
organ makes up for the loss (lung, bladder). Dose delivered to a
given volume or average/mean dose is considered.
Serial-parallel – kidney (glomerulus-
parallel, tubules- serial), heart
(myocardium- parallel, coronary arteries
- serial).
ICRU-62

56. Step 5 :Dose prescription :
 Based on published data and clinical experience
 Prescription is specified as a dose at or near the
center of PTV as a dose covering certain
percentage of PTV.

57. Step 6 : Conformal Planning
 Forward Planning :In this places beams into a
radiotherapy treatment planning system which can
deliver sufficient radiation to a tumour while both
sparing critical organs and minimising the dose to
healthy tissue and later modification is done
 Inverse Planning :this approach starts with
desired result (a uniform target dose) & works
backward toward incident beam intensities.

58. Step 6 :Forward Planning
 Beam arrangement – ability to orient beams in 3D
allows one to develop plans using non coplanar
beams.
 Field shaping – BEV and DRR display allow to
view target volume and OAR ,hence shielding
blocks and MLC can be placed accordingly .

59.  Beam directions that create greater difference
between targets and critical structures are
preferred
 A 2cm margin between the PTV and field edge
ensures better than 95% isodose coverage of the
PTV.
 coplanar or non coplanar

60. Beam Designing: Beam
arrangement
Field multiplicity : less need for high energy beams
Disadvantage :
designing excess number of beams
shaping blocks
longer setup time
carrying of blocks – danger

62.  to get a practical idea about geometry of beam
placement .
 Simulates any arbitrary viewing location in
treatment room.
 Provides near time capability for evaluating the
location of hot and cold spots in a given dose
distribution.
Rooms Eye View :

63. Beam Designing : Field Shaping
 MLCs are used to shape the field around the PTV.
Placed using Beams eye view (BEV)
 BEV : The observer's viewing point is at the source
of radiation looking out along the axis of the
radiation beam in planes perpendicular to central
axis of the beam .
 Easily view the critical structure
volumes and the target volume
so that shielding blocks or MLC
defined apertures can be defined.

64. Beam Modeling :

65. DRR- Digitally Reconstructed
Radiograph
 After beam arrangement, DRR is generated.
 Used for treatment portal design.
 Verification of treatment delivery by comparison
with portal film.
 allow better visualization of organs of interest.

66. Step 7 :Dose distribution
calculation :
Rectilinear coordinate system affixed to the patient
3D CT image set is typically used for calculating the
dose distribution .
Horizontal axis
Vertical
axis of CT
couch motion

67.  CT numbers are not directly used in photon dose
calculations , electron density of the corresponding
tissue are used. Why ??
 Compton scattering is dominant for photon beam
used in radiotherapy and absorption and scattering
of photons in tissue depends on electron density
 Errors in CT numbers – errors in dose calculation .
<10% errors not significant.

68.  Once the parameters are defined, the Treatment
Planning Software generates the dose distribution
 Past – dose calculation algorithms were
traditionally based on dose distribution measured
in water phantoms and applying correction factors
for
non uniform surface/beam obliquity
tissue heterogeneities
beam modifiers
 Advanced models– superposition/convolution
method

69. Plan Optimization And
Evaluation :
 Iterative, interactive approach
 Beam arrangement is done based on review of
DVH and multilevel 2D display levels showing
isodose lines superimposed on CT images.
 REV view is used to display
dose clouds along with rendered PTV and OAR’S.
hot and cold spots can be seen

70. Plan Evaluation
The following tools are used in the evaluation of the
planned dose distribution:
• Methods of dose display
Isodose lines
Color wash
DVHs (Dose volume histograms )
• Dose distribution statistics

71. Colour Wash -
Spectrum of colours superimposed
on the anatomic information
represented by modulation of
intensity
Gives quick over view of dose
distribution
Easy to assess over dosage in
normal tissue that are not
contoured.
To asses dose heterogeneity
inside PTV
Slice by slice evaluation of dose distribution can be done.

72. Coverage By Slice
 Always verify the anatomical dose distribution slice
by slice, in order to identify where under dosage or
over dosage is occurring.
overdosage
underdosage

73. Problems With 3D Dose
Distribution:
 Huge amount of information to assess
 Difficult to quantify visually
 Difficult to understand relationship between dose
and 3d anatomy
DVH

74. DVH– Dose Volume Histogram
 gives an idea of the percentage volume receiving
the percentage dose both for the tumor and the
normal organs.
 2 types – Cumulative
Differential
 provides a complete summary of the entire 3D
dose matrix,
 it does not provide any spatial information. Thus,
the DVH can only complement, and not replace,
spatial dose-distribution displays.

75. Differential DVH :
 Is a plot of the volume of a given structure
receiving a dose within a specified dose interval(or
dose bin) as a function of dose.
 Shows the extent of dose variation within a given
structure.
 Useful to display dose to target volumes—to see
max ,min , mean
dose.

76.  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 .
Cumulative DVH :

79. See 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.

80. Plan Approval :
 uniform dose is delivered to the target volume
(+7% to -5% of prescribed dose)
 Dose to critical structures below tolerance level
 Well within constraints for maximum ,median dose
or according to volume constraint.
 Acceptable dose distribution is one that differs from
desired dose distribution
within pre-set limits of dose and
only in regions where desired dose distribution
can’t be physically achieved.

81. Plan Implementation And
Treatment Verification :
 Once the plan is designed, evaluated, and
approved, documentation for plan implementation
is generated.
 This includes beam parameter settings and MLC
parameters communicated to the computer system
that controls the MLC of treatment machine, and
DRR generation and
printing or transfer to the
Image database .

82.  Transfer plan parameters into treatment machine
record-and-verify system
 Set up (register) the real patient according to plan
(verification simulation optional)
 Perform patient treatment quality assurance
checks including independent check of monitor
units.

83. Clinical Utility 3DCRT :
The possible benefits with 3D CRT in clinical practice
are as follows
 Improved local control
 Reduced acute and late morbidity
 Possibility of dose escalation

84. Patient Selection
Patients most likely to be benefited with this
technique are those who have :
 Tumors in sites with complex anatomy
 Irregularly shaped tumors
 Tumors adjacent to radiation sensitive normal
structures
 Small volume or high dose treatments

85. Clinical Areas
Following tumor sites have been extensively
treated with this technique :
 Lung Cancer
 Brain Tumors
 Prostate Cancer
 Partial Breast Irradiation
 Head and Neck Cancer
 Pancreatic Tumors
 Liver Tumors

86. Brain Tumors :

87. PTV
RT TEMPORAL
LT TEMPORAL
BRAINSTEM
LENS
OPTIC NERVE
SPINAL CORD

88. Ca Lung

89. Ca Cervix

91. Role In Prostate Cancer :
 Less bladder and rectum toxicity
 Dose escalation – better disease control

93.  3DCRT can drastically reduce rectal and bladder
dose (Perez et al)
Conv RT 3DCRT
Bladder > 65Gy ~60% ~34%
Rectum > 65Gy ~50% ~22%
Rectal and bladder complication rates are reduced
Conv RT 3DCRT IMRT
Gr II ~50-60% ~20-30% 5% (R) 15% (B)
Gr III ~3-4%
(20-30% with dose escalation)

94. Pelvis Treatments :
 Reduction in small bowel toxicity.
 Prevention of late term ano-rectal toxicity
 Escalation of dose to pelvic lymph nodes
 Better target coverage

95. Pediatric Tumors :

96. Results other sites :
 Breast : improves dose coverage and reduces
inhomogeneity, elimination of additional surgical
procedures, improves cosmesis, reduces fat
necrosis
 Anal canal : Conformal techniques provides an
opportunity to spare small bowel and the femoral
heads.
 GIT : 3D CRT can be validated because it improves
dose distribution , reduces dose to kidneys, liver
and cord.
 Head & neck :permits good coverage of the PTV
.There is low rate of acute toxicity which can be
explained by improving the dosimetric parameters
of organs of risk

97. Advantages :
 allows for the simulation of the patient's treatment
without their physical presence after the CT scan is
obtained.
 Treatment modifications.
 system allows for better dosimetric optimization above
that which is achievable with geometric optimization
alone.
Limitations :
• Knowledge of tumor extent
• CTV is often not fully discernible
• Patient motion
• Biologic response of tumor

98. Conclusion :
 Tightening of field margins around image based GTV
with little attention to occult disease, patient motion
is a misuse of 3DCRT ---should be avoided.
 3DCRT is not synonymous with better results
 Its superiority lies entirely on how accurate the PTV
is and how much better the dose distribution is…
hence i conclude by stating that it is a
superior tool for treatment planning with a potential of
achieving better results.

99. Thank you…