Intensity Modulated Radiation Therapy (IMRT) is an advanced mode of high-precision radiotherapy that uses computer-controlled linear accelerators to deliver precise radiation doses to a malignant tumor or specific areas within the tumor by reducing radiation dose to the nearby normal tissues.

2.  Conventional to Conformal
 2D Conventional Radiotherapy
 3D Conformal Radiotherapy
 Intensity Modulation
 IMRT – Definition and Principle
 Conventional Radiotherapy vs. IMRT
 Benefits to the Patient
 Clinical Implementation of IMRT
 IMRT – Treatment Planning
 Different IMRT Techniques
 IMRT with Multileaf Collimator
 IMRT Quality Assurance Program
 Draw Backs of IMRT
 Risk of IMRT

3. Conventional Radiotherapy

4. Conformal Radiotherapy

5. Intensity Modulated Radiotherapy (IMRT)

6. Conventional Radiotherapy
Conventional to Conformal

7. Conventional to Conformal
Conformal Radiotherapy without
Intensity Modulation (CRT)

8. Conventional to Conformal
Conformal Radiotherapy with
Intensity Modulation (IMRT)

9. It is based on standardized treatment
techniques applied to classes of patients
thought to be similar
X-ray simulator and 2D computer
treatment planning system are used
This process is limited to generating
dose distributions in a single, or a few
planes of the patient’s target volume
2D Conventional Radiotherapy

10. It is based on standardized treatment
techniques applied to classes of patients
thought to be similar
X-ray simulator and 2D computer
treatment planning system are used
This process is limited to generating
dose distributions in a single, or a few
planes of the patient’s target volume
Testing the Modulex 2D Radiation
Treatment Planning System, circa
1980
2D Conventional Radiotherapy

11. 2D Conventional Radiotherapy
It is based on standardized treatment
techniques applied to classes of patients
thought to be similar
X-ray simulator and 2D computer
treatment planning system are used
This process is limited to generating
dose distributions in a single, or a few
planes of the patient’s target volume
Testing the Modulex 2D Radiation
Treatment Planning System, circa
1980

12. 2D Conventional Radiotherapy
2D transmission images of human
body provided unprecedented imagery
of bony landmarks, allowing
radiologists to deduce the location of
internal organs
Additional blocks placed daily to
match marks on the patient’s skin
Using planar radiographs,
radiologists planned cancer
treatments by collimating
rectangular fields encompassing
the presumed tumor location

13. 2D Conventional Radiotherapy
2D transmission images of human
body provided unprecedented imagery
of bony landmarks, allowing
radiologists to deduce the location of
internal organs
Additional blocks placed daily to
match marks on the patient’s skin
Using planar radiographs,
radiologists planned cancer
treatments by collimating
rectangular fields encompassing
the presumed tumor location

14. 2D Conventional Radiotherapy
2D transmission images of human
body provided unprecedented imagery
of bony landmarks, allowing
radiologists to deduce the location of
internal organs
Additional blocks placed daily to
match marks on the patient’s skin
Using planar radiographs,
radiologists planned cancer
treatments by collimating
rectangular fields encompassing
the presumed tumor location

15. 3D Conformal Radiotherapy
It allows to increase the doses of
radiation delivered to the tumor
without increasing damage to nearby
tissues.
Before treatment is begun, digital
images of the individual’s target are
prepared and compiled into virtual 3D
models of how the target will “look” to
the accelerator from all angles.
Then the accelerator shapes the beam
to match those “beam’s eye views”
(insets), thus reducing the amount of
radiation hitting the organs at risk or
other unintended targets.

16. 3D Conformal Radiotherapy
It allows to increase the doses of
radiation delivered to the tumor
without increasing damage to nearby
tissues.
Before treatment is begun, digital
images of the individual’s target are
prepared and compiled into virtual 3D
models of how the target will “look” to
the accelerator from all angles.
Then the accelerator shapes the beam
to match those “beam’s eye views”
(insets), thus reducing the amount of
radiation hitting the organs at risk or
other unintended targets.

17. 3D Conformal Radiotherapy
It allows to increase the doses of
radiation delivered to the tumor
without increasing damage to nearby
tissues.
Before treatment is begun, digital
images of the individual’s target are
prepared and compiled into virtual 3D
models of how the target will “look” to
the accelerator from all angles.
Then the accelerator shapes the beam
to match those “beam’s eye views”
(insets), thus reducing the amount of
radiation hitting the organs at risk or
other unintended targets.

18. 3D CRT : Treatment Planning Process
Imaging Data Aquisition
To accurately delineate target volume and normal structures
CT is the most commonly used procedure, even other modalities offer
special advantages in imaging certain types of tumors and locations
CT MRI PET SPECT

19. 3D CRT : Treatment Planning Process
Image Registration
It is a process of correlating
different image data sets to
identify corresponding structures
or regions
Image fusion is the seamless
mixing up of two image sets of the
same patient; it may be
- Two different image modalities
- Same modality in which image
sets are taken at different point
of time

20. 3D CRT : Treatment Planning Process
It refers to slice-by-slice
delineation of anatomic
regions of interest
The segmented regions can
be rendered in different
and can be viewed in
beam’s eye view (BEV)
configuration or in other
planes using digital
reconstructed radiographs

21. 3D CRT : Treatment Planning Process
Designing beam aperture is aided by the BEV capability of the 3D
treatment planning system

22. 3D CRT : Treatment Planning Process
Combination of multileaf collimators
and independent jaws provides almost
unlimited capability of designing
multiple fields of any shape
Targets and critical structures
can be viewed in the BEV
configuration individually for
each field

23. 3D CRT : Treatment Planning Process
An optimal plan should deliver tumoricidal dose to the entire tumor
and spare all the normal tissues.
Isodose Curves and Surfaces
Dose distributions of competing
plans are evaluated by viewing
isodose curves in individual slices,
orthogonal planes or 3D isodose
surfaces

24. 3D CRT : Treatment Planning Process
Dose Volume Histograms (DVHs)
Display of dose distribution in
the form of isodose curves or
surfaces is useful because their
anatomic location and extent.
This information is supplemented by dose volume histograms
(DVHs).
DVH summarizes the entire dose distribution into a single curve
for each anatomic structure of interest.

25. 3D CRT : Treatment Planning Process
Electronic Portal Imaging (EPI)
Patient position at the time of
treatment should be verifiable via
electronic portal imaging (EPI)
Tools should exist to quantify
discrepancies between treatment
position and planned position and
to evaluate consequences and
make corrections

26. 3D CRT

27. Intensity Modulation
• Conventional radiotherapy treatments are delivered with radiation
beams that are of uniform intensity across the field (within the flatness
specification limits)
• Wedges or compensators are used to modify the intensity profile to
offset contour in irregularities and produce more uniform composite
dose distributions such as in techniques using wedges
• This process of changing beam intensity profile to meet the goals of a
composite plan is called intensity modulation

28. Intensity Modulation
• Conventional radiotherapy treatments are delivered with radiation
beams that are of uniform intensity across the field (within the flatness
specification limits)
• Wedges or compensators are used to modify the intensity profile to
offset contour in irregularities and produce more uniform composite
dose distributions such as in techniques using wedges
• This process of changing beam intensity profile to meet the goals of a
composite plan is called intensity modulation

29. Intensity Modulation
• Conventional radiotherapy treatments are delivered with radiation
beams that are of uniform intensity across the field (within the flatness
specification limits)
• Wedges or compensators are used to modify the intensity profile to
offset contour in irregularities and produce more uniform composite
dose distributions such as in techniques using wedges
• This process of changing beam intensity profile to meet the goals of a
composite plan is called

30. IMRT refers to a radiation therapy technique in
which nonuniform fluence is delivered to the
patient from any given position of the
treatment beam to optimize the composite
dose distribution
Definition and Principle of IMRT

31. The fluence files thus generated are electronically
transmitted to the linear accelerator, which is
computer controlled, to deliver intensity modulated
beams (IMBs) as calculated
IMRT refers to a radiation therapy technique in
which nonuniform fluence is delivered to the
patient from any given position of the
treatment beam to optimize the composite
dose distribution
The optimal fluence profiles for a given set of beam
directions are determined through
Definition and Principle of IMRT

32. Definition and Principle of IMRT
The fluence files thus generated are electronically
transmitted to the linear accelerator, which is
computer controlled, to deliver intensity modulated
beams (IMBs) as calculated
IMRT refers to a radiation therapy technique in
which nonuniform fluence is delivered to the
patient from any given position of the
treatment beam to optimize the composite
dose distribution
The optimal fluence profiles for a given set of beam
directions are determined through inverse planning

33. IMRT is especially useful when
the target volume has a concavity in its surface
and/or
closely juxtaposes OARs
Definition and Principle of IMRT
IMRT is an approach to conformal therapy that not only
conforms (high) dose to the target volume
but also
conforms (low) dose to sensitive structures

34. IMRT is especially useful when
the target volume has a concavity in its surface
and/or
closely juxtaposes OARs
Definition and Principle of IMRT
IMRT is an approach to conformal therapy that not only
conforms (high) dose to the target volume
but also
conforms (low) dose to sensitive structures

35. Definition and Principle of IMRT
A breast cancer, metastatic to T7, previously
treated with a full course of spinal radiation.
The patient continued with severe pain in the
thoracic spine and was referred for palliative
radiation. She was to receive 18 Gy in nine
fractions to the target volume shown on the
above. The delivered dose distribution is
shown to the below. Note the achievement of
a concave high-dose volume and protection of
the spinal cord. (From Carol 1997a.)

36. Conventional Radiotherapy vs. IMRT
Conventional Radiotherapy
“Blocks” are used to shape the
beam to the target and to
avoid dose from areas outside
the target
FIELD 1
Normal Tissue
Tumor

37. Conventional Radiotherapy
Conventional Radiotherapy vs. IMRT

38. Conventional Radiotherapy
FIELD 2
Conventional Radiotherapy vs. IMRT

39. Conventional Radiotherapy
Conventional Radiotherapy vs. IMRT

40. Conventional Radiotherapy
FIELD 3
Conventional Radiotherapy vs. IMRT

41. Conventional Radiotherapy
Conventional Radiotherapy vs. IMRT

42. IMRT
Intensity of radiation is
varied across the beam
depending on the shape of
the target and the presence
of sensitive structures within
its envelope
FIELD 1
Conventional Radiotherapy vs. IMRT
Normal Tissue
Tumor

43. IMRT
Conventional Radiotherapy vs. IMRT

44. IMRT
FIELD 2
Conventional Radiotherapy vs. IMRT

45. IMRT
Conventional Radiotherapy vs. IMRT

46. IMRT
FIELD 3
Conventional Radiotherapy vs. IMRT

47. IMRT
Conventional Radiotherapy vs. IMRT

48. Tumor and Normal tissues are
irradiated with UNIFORM DOSE..!
Conventional Radiotherapy IMRT
Tumor and Normal tissues are irradiated
with MODULATED INTENSITY BEAMS..!
TARGET
Conventional Radiotherapy vs. IMRT

49. Benefits to the Patient
 Better normal tissue sparing
– Less toxicity
 Possibly higher dose to the target
– Higher chance of cure
 More dose in a fraction
– Fewer fractions

50. Clinical implementation of IMRT requires two systems, they
are :
1. A treatment planning computer system that can
calculate nonuniform fluence maps for multiple beams
directed from different directions to maximize dose to
the target volume while minimizing dose to the critical
normal structures
2. A system of delivering the nonuniform fluence as
planned
Clinical Implementation of IMRT

51. IMRT Treatment Planning
Divides each beam into a large
number of beamlets
Determines optimum
setting of their
fluences or weights
The optimization process
involves inverse planning in
which beamlet weights or
intensities are adjusted to
satisfy predefined dose
distribution criteria for the
composite plan

52. IMRT Treatment Planning
Once the tumor and OAR are contoured,
the treatment planner must take a decision
on the number, energy and direction of
treatment beams

53. It is common practice to select a fixed set
of five, seven or nine equally spaced non
opposing coplanar beams
IMRT Treatment Planning
A large number of
beams (e.g. nine vs.
five) may produce a
more conformal plan

54. For deep seated targets, the dose to both PTV and OAR does not depend
significantly on the number of beams and energy. Instead, the main difference
occurs in regions far from the PTV.
In these regions, the dose is significantly increased for both a smaller number of
beams and for lower energy.
Nine beams or more, the energy dependence far from the PTV was negligible
∴ for a five field plan, one may want to consider using higher energy beams (15
MV), but if one prefers to use nine fields, then a 6 MV beam could be used with
the same result.
IMRT Treatment Planning

55. For deep seated targets, the dose to both PTV and OAR does not depend
significantly on the number of beams and energy. Instead, the main difference
occurs in regions far from the PTV.
In these regions, the dose is significantly increased for both a smaller number of
beams and for lower energy.
Nine beams or more, the energy dependence far from the PTV was negligible
∴ for a five field plan, one may want to consider using higher energy beams (15
MV), but if one prefers to use nine fields, then a 6 MV beam could be used with
the same result.
IMRT Treatment Planning

56. For deep seated targets, the dose to both PTV and OAR does not depend
significantly on the number of beams and energy. Instead, the main difference
occurs in regions far from the PTV.
In these regions, the dose is significantly increased for both a smaller number of
beams and for lower energy.
Nine beams or more, the energy dependence far from the PTV was negligible
∴ for a five field plan, one may want to consider using higher energy beams (15
MV), but if one prefers to use nine fields, then a 6 MV beam could be used with
the same result.
IMRT Treatment Planning

57. For deep seated targets, the dose to both PTV and OAR does not depend
significantly on the number of beams and energy. Instead, the main difference
occurs in regions far from the PTV.
In these regions, the dose is significantly increased for both a smaller number of
beams and for lower energy.
Nine beams or more, the energy dependence far from the PTV was negligible
∴ for a five field plan, one may want to consider using higher energy beams (15
MV), but if one prefers to use nine fields, then a 6 MV beam could be used with
the same result.
IMRT Treatment Planning

58. The dose gradient away from the PTV for a nine field IMRT plan is less
steep than is achieved with a well-designed static field (conventional) plan
If a high gradient is required at a certain critical interface, then careful
selection of beam angles is important
In a typical plan, the use of noncoplanar beam orientations resulted in a 15
– 25 % decrease in dose to the hottest portion of the rectum compared
with coplanar beam orientations
A seven field noncoplanar IMRT technique produced increased bladder
sparing compared with standard field arrangements
IMRT Treatment Planning

59. The dose gradient away from the PTV for a nine field IMRT plan is less
steep than is achieved with a well-designed static field (conventional) plan
If a high gradient is required at a certain critical interface, then careful
selection of beam angles is important
In a typical plan, the use of noncoplanar beam orientations resulted in a 15
– 25 % decrease in dose to the hottest portion of the rectum compared
with coplanar beam orientations
A seven field noncoplanar IMRT technique produced increased bladder
sparing compared with standard field arrangements
IMRT Treatment Planning

60. The dose gradient away from the PTV for a nine field IMRT plan is less
steep than is achieved with a well-designed static field (conventional) plan
If a high gradient is required at a certain critical interface, then careful
selection of beam angles is important
In a typical plan, the use of noncoplanar beam orientations resulted in a 15
– 25 % decrease in dose to the hottest portion of the rectum compared
with coplanar beam orientations
A seven field noncoplanar IMRT technique produced increased bladder
sparing compared with standard field arrangements
IMRT Treatment Planning

61. The dose gradient away from the PTV for a nine field IMRT plan is less
steep than is achieved with a well-designed static field (conventional) plan
If a high gradient is required at a certain critical interface, then careful
selection of beam angles is important
In a typical plan, the use of noncoplanar beam orientations resulted in a 15
– 25 % decrease in dose to the hottest portion of the rectum compared
with coplanar beam orientations
A seven field noncoplanar IMRT technique produced increased bladder
sparing compared with standard field arrangements
IMRT Treatment Planning

62. Different IMRT Techniques

63. Different IMRT Techniques

64. IMRT with Multileaf Collimator
Material of Multileaf Collimator : Tungsten alloy
• Highest density
• Hard
• Simple to fashion
• Reasonably inexpensive
• Low coefficient of thermal expansion

65. IMRT with Multileaf Collimator
It is a technique to construct IMBs using a
sequence of static MLC shaped fields in which
the shape changes between the delivery of
quanta of fluence
Multiple Static Field (MSF) Technique or
Static Multileaf Colliator (SMLC) Technique or
Step & Shoot Technique

66. IMRT with Multileaf Collimator
Dynamic Multileaf Collimator (DMLC) Technique
The leaves may define changing shapes with the radiation ON

67. IMRT Quality Assurance Program
Frequency Procedure Tolerance
Before first treatment Individual field
verification, plan
verification
3% (point dose), other
per clinical significance
Daily Dose to a test point in
each IMRT field
3%
Weekly Static field vs Sliding
window field dose
distribution as a function
of gantry and collimator
angles
3% in dose delivery

68. IMRT Quality Assurance Program
Frequency Procedure Tolerance
Annualy All commissioning procedures :
Stability of leaf speed,
Leaf acceleration and
deceleration,
MLC transmission,
Leaf Position accuracy,
Static field vs Sliding window field
dose distribution as a function of
gantry and collimator angles,
Standard plan verification
3% in dose delivery,
other per clinical
significance

69. Draw Backs of IMRT
• More complexity
• Need for new (and more accurate) equipment
• More need for QA
• Longer treatment times
• Higher risk of geographical miss

70. Risk of IMRT
There is an increased risk secondary malignancies in
patients treated with beam energies of >10 MV
owing to a higher neutron dose. However, the degree
of neutron production depends on the specific IMRT
plan parameters, including the number segments and
monitor units (MUs).
Moreover, the importance of considering secondary
malignancies from IMRT treatments for any energy
beam has been raised, especially for pediatric cases.
These aspects require further investigation and
scrutiny.

71. References
1. Steve Webb : “Intensity- Modulated Radiation Therapy”
(2001)
2. Arno J. Mundt. MD, John C. Roeske. PhD: “Intensity
Modulated Radiation Therapy – A Clinical Perspective” (2005)
3. Faiz M. Khan : “Treatment Planning in Radiation Oncology”
(2nd Ed)
4. Faiz M. Khan : “Physics of Radiation Therapy” (4th Ed)