Intensity-modulated radiotherapy

1. Intensity Modulated Radiotherapy
PRESENTOR :- DR.VIJAY.P.RATURI

2. • To achieve Goal of Radiotherapy.
• During past decades, with advances in radiological imaging
and computer technology.
Shift from Conventional to conformal radiotherapy

3. • Conventional RT:- Irradiation of pt with few beam direction, all
beam are aimed at single point denoted as isocenter.
• Radiation shape from each beam are manually drawn on the
projected 2D images taken from an Xray Simulator.
• Difference in conventional and conformal is planning CT is used to
to define tumor volume and to design treatment portal accordingly
• Conformal RT: 3DCRT & IMRT
• Transition from 3DCRT to 4DRT.

4. • IMRT was Proposed over 50 years ago by Takahashi in Japan
(Takahashi et al).
• 1st attempted in the late 1960s by Hellman and colleagues .
• In mid-1990s, IMRT began to be used in different academic
centers.
• Not till the late 1990s with the availability of commercial
treatment planning systems did IMRT start to become widely
available

5. • In the 2002 survey, 32%of radiation oncologists were using
IMRT.“(Mell LK, Roeske JC, Mundt AJ. Cancer2003;204-211)
• In the 2004 survey, this percentage increased to 74%.
(Mell LK, MehrotraAK, Mundt AJ. Cancer2005;104:1296)
• While commonly available, it is being used to treat only a subset
of patients at most centers.

6. • Process of changing beam intensity profile to achieve
composite plan is called intensity modulation.
INTENSITY MODULATORS
- Compensators
- Wedges
- Dynamic Multileaf collimators

7. Intensity modulated radiotherapy:-
• It’s a technique in which non uniform fluence is delivered to the
patient from any given position of the treatment beam to optimize
the composite dose distribution.
• It assigns non uniform intensity/weights to tiny subdivision of
beams ( a.k.a rays or beamlets)
• Permits greatly increased control over the radiation fluence
,enabling custom design of optimum dose distributions.

8. The clinical implementation of IMRT requires at least two
systems:
• Treatment planning computer system that can calculate non
uniform fluence maps for multiple beams directed from
different directions to maximize dose to the target volume while
minimizing dose to the critical normal structures.
• System of delivering the non uniform fluences as planned.

9. IMRT system consist of :-
• Optimization
• Dynamic mlc conversion program
• Dosimetry verification tool.

10. FORWARD PLANNING
Planner manually selects no of beams, direction & shape
inclusion and exclusion of wedges , weighing of each beam
(manual optimization)
Computer calculates resultant dose distribution from beam
parameters
Planner can make adjustment to improve the plan

11. INVERSE PLANNING (PROPOSED BY BRAHME IN 1988)
Define orientation & energies of all the beams
Planner specify desired dose limits(or constraint ) for PTV
& all region of interest
computer optimization algorithm first divides each beam into many
small “beamlets” (or “rays” or “pencil beams”)
Alteration of beamlets until the composite 3-D dose distribution best
conforms to the specified dose objectives.
Computer then calculate the sequence of MLC leaf motions

12. IMRT PLANING
Positioning &
immobilization
Image
acquisition
(CT,MRI )
Structure
segmentation
IMRT t/t & planing
File transfer &
management
Plan validation
Position verificationIMRT t/t & delivery

13. POSITIONING AND IMMOBLIZATION
• Highly conformal nature of the IMRT.
• Techniques to reduce or follow the internal organ movement.
• Patient comfort.

17. IMAGE ACQUISITION
• Goal of treatment should be discussed.
• For Inverse planning a CT scan is required .
• Slice spacing not more than 5 mm.
• Different Imaging modality may required to identify the correct
target volume and OAR (CECT,MRI,PET, etc) called as image
augmentation
• Should be compatible with the available treatment planning
system.

18. STRUCTURE SEGMENTATION or Image segmentation
• Most important and critical step
• The success of IMRT is closely tied to the accuracy of the target and
OAR delineation
• Department should develop a protocol.
• Should follow the guidelines (RTOG, relevant atlas ).
• ICRU 50,62 should be used for the nomenclature, Prescribing,
Recording, and Reporting.

20. TREATMENT PLANNING
• Beam energy , No of beams and orientation
• Isocenter placement (ICRU Reference point)
• Min Dose, Max Dose, Dose volume ( Biological Constrains)
• Optimization – Fluence Modulation
• Deliverability ( Reasonable MU, Treatment time)
• Final dose calculation( Dose grid size, algorithm)

21. PLAN EVALUATION
• IMRT/VMAT plans needs to evaluated very carefully
• Inspecting and Comparing DVH is useful.
• Planner needs to be inspect the Isodose in each slice.
• Is the dose uniformity in the target acceptable?
• Are the stated plan goals for the hot spots and target coverage
satisfied?
• Are the stated plan goals for normal tissue sparing satisfied?
• Is the organs contoured entirely or not ?

22. Characteristic of MLCs
The design system of MLC differ in many ways with respect to
their manufacturer:-
a> Location of MLC relative to the conventional Jaws
b> Single focused or double focused MLCs( Elekta & Varian are
single foccused)
c> Physical characteristic or shape of the leaves
d> Leaf movement restrictions
e> Maximum achievable field size

23. Advantage : Smaller and lighter MLC design with physical
clearance between gantry head and patient

26. • Most MLCs have cutting tongue and groove pattern on the
side of leaves
• It reduces the radiation leakage

27. Leaf end transmission
Convex leaf edge
maintain a geometric
pneumbra over all
field size

29. GENERATION OF LEAF MOTION FILES :-
• “Leaf sequencer” is an algorithm that translates the beam intensity
pattern produced by the IP system into an instruction set describing
how to move the MLC leaves during beam delivery.
• Result of optimization may be either continuous 2-D intensity
profiles, or discrete 2-D intensity matrices with discrete resolution.
• DMLC delivery (referred to as a sliding window) delivers a
continuous 2-D intensity profile, whereas STATIC delivery (referred
to as step & shoot) results in “discretized” intensity patterns.

30. ADVANTAGE OF STATIC MLC :-
• Advantages of static, or step-and-shoot, IMRT are that it is
conceptually simple, there are no requirements to control the
individual leaf speeds (thus simplifying the MLC control system) or
delivery dose rate.
• Interrupted treatment is easy to resume, easy to verify an intensity
pattern for each field, and fewer MUs are required in comparison
with DMLC delivery.
DISADVANTAGE OF STATIC MLC :-
• Require prolonged treatment time, particularly for complex
intensity patterns

31. ADVANTAGE OF DYNAMIC MLC :-
• The advantage of DMLC delivery is that it can deliver a smoothly
varying intensity profile.
DISADVANTAGE OF DYNAMIC MLC :-
• Delivery mechanism is rather complicated, involving leaf-speed
and dose-rate modulation
• Requires the precise control of individual leaf speeds; and small
errors in the calibration of leaf position could (for very highly
modulated fields) result in significant errors in delivered doses

32. • IMRT is becoming a mature technology, and is widely applied
to many cancer sites.
• Many treatment-planning comparison studies have
demonstrated the clear dosimetric advantages of IMRT.
• Clinical results from the past decade have shown the
improvement of local tumor control and reduction of
treatment toxicities for prostate cancer, head and neck
cancer, and other cancers.
IMRT Utilization

33. Site %
Prostate 85%
Head and neck 80%
CNS 64%
Gynecology 35%
Breast 28%
GI 26%
Sarcoma 20%
Lung 22%
Pediatrics 16%
Lymphoma 12%
IMRT PRACTICE SURVEY (2004) TOP TREATED SITES
Mell LK, Mundt AJ. Survey of IMRT Use in the USA-2004 Cancer 2005;104:1296

34. Volume modulated arc therapy
• Radiation dose is delivered to the tumor while simultaneously
moving the MLCs leaves and the gantry.
• Both the dose rate and gantry speed vary in this system.
• Advantage: a> excellent dose distribution in deep seated
tumors
b> faster delivery
c> Reduction of MUs ( decreasing the risk of secondary cancer)

35. Image guided radiotherapy
• IGRT = IMRT with image guidance
• Goal of IGRT is to eliminate or reduce the uncertanities
associated with target volume defination and it
intrafractional and interfractional organ motion
• 4DCT, 4DPETCT , 4D-CBCT are applied to R.T guidance and
verification
• Respiratory gated IMRT

36. RESPIRATORY GATING WITH IMRT
•Cancers that are subject to tumor motion during respiration benefit
from respiratory gating.
•Internal structures can move a significant amount- The diaphragm
and liver can move up to 3 cm and the pancreas, kidney and thorax
can move up to 2 cm
•Respiratory gating is used to minimize the increased margins of the
treatment volume that are directly related to respiratory movements

37. •Cancers that are subject to tumor motion during respiration benefits
from respiratory gating.
•These include lung, breast, pancreatic, stomach, liver, and prostate .
•Breast cancers are usually treated at the point of maximum
inhalation because this increases the lung volume, decreasing the
amount of lung tissue in the field.

38. TRACKING RESPIRATION
•Respiratory gating uses marker systems that “learn” the patient’s
respiratory pattern.
EXTERNAL MARKER INTERNAL MARKER
• RPM by varian.
• Reflective marker learns
patients breathing pattern.
• Camera system
- sends signal
- reflected off markers
- respiratory wave form
created
• Implanted gold seeds
• Use fluoroscopy to
locate

39. •The most commonly used marker is an external marker known as the
Real-time Position Management (RPM) system by Varian.
•It’s a little plastic box with two reflective markers normally placed
halfway between the patient’s xyphiod and
Navel in order to take advantage of the
Greatest external displacement.
•A camera located near the foot of the table sends a signal that is
reflected off the metallic markers and translated into a waveform whose
shape represents the patient’s movement with regard to the respiratory
cycle.
•Implanted gold seeds are internal markers that require a surgical
procedure for placement. During simulation and treatments they are
tracked in real-time using flouroscopy.
Real time positioning system

40. RESPIRATORY WAVEFORMS
inspiration
Expiration
phases
Beam off Beam off
Beam on Beam on
Irregular respiration
Beam on
System
automatically shuts
of till normal
respiration is back

41. •Treatment plans are formed using the respiratory waveform and the
CT images
•The treatment plan is designed to place the treatment field around the
target volume during a specific phase of the respiratory cycle.
•For most treatment plans, the beam on time correlates with
exhalation because the anatomy is in relatively the same position for
the greatest period of time.

42. RESPIRATORY GATING SYSTEM BY VARIAN

43. Dose calcuation algorithm
• More complex the intensity modulation, more will be
the M.U required
• One time monte carlo calculation of pencil beam
algorithm is used.

44. Quality assurance ( American association of
physicist in medicine report 82 ,2003
• Simple daily film QA
• EPID
• Dosimetric test

45. Frequency Procedures Tolerance
Before first t/t Individual field
verificaiton and plan
verification
3%
Daily Dose to test point in each
IMRT field
3%
Weekly Static Vs sliding window
field dose distribution
3%
Annually All commissioning
procedure: leaf stability,
speed , leaf acceleration,
MLCs , leaf position, static
Vs sliding window field
3%
IMRT Quality Assurance Program

46. IMRT IN HEAD AND NECK CANCER
• Variable numbers of co planar beams are used(5-15)
• Maintain target and nodes coverage as in conventional t/t
• Keep dose to cord and brainstem tolerable.
• Reduce dose to parotid as much as possible.
• Concomitant boost can be done in single plan ,there is no
need of subsequent electron boost.

47. • Eisbruch et al (1999): A mean parotid dose of < 26 Gy
should be planning goal.
• Eisbruch et al (2007): Substantial parotid flow recovery
(upto 86% of pretreatment levels) at 2 years if mean
doses are between 25-30Gy.
• Eisbruch et al (2010): Severe xerostomia (<25% of
baseline) avoided if mean parotid dose kept to <20Gy
(if one parotid is to be spared) or <25 Gy (if both are to
be spared)
Parotid dose and xerostomia

48. CONCLUSION
• Avoidance of circumferential irradiation of rectum to 55Gy with
minimal compromise of PTV coverage is achievable with IMRT ƒ.
• use of IMRT reduces acute GI/GU toxicity rate when compared with
the 4FB technique.
IN HEAD AND NECK CANCER:-
• IMRT plan show improve dose distribution especially sparing of
parotids.
• Plans with more beams is better than with less beams, however
improvement saturates after beyond certain number of beams.
plan quality deteriorates when beam number is <=5.
IN PROSTATE:-

49. Thank you.