brief description of 3DCRT and IMRT

1. RADIATION PHYSICS – 3DCRT AND IMRT
DR KIRON G

2. CONFORMAL THERAPY

3. CONFORMAL THERAPY
 creates high-dose volumes that are shaped to closely “conform” to the
desired target volumes and prescription doses
 minimizing the dose to critical normal tissues
 maximizing tumor control probability (TCP)
 accuracy in localization of CTV is critical
 minimizing normal tissue complication probability (NTCP).

4. 3-DCRT

5. 3-DCRT
 based on 3D Anatomic information
 treatment planning system that is capable of calculating 3D dose distribution
and dose volume statistics for contoured structures

6. TREATMENT PLANNING PROCESS
Patient
positioning and
Immobilisation
CT Simulation
Image
Segmentation
Forward
planning
Plan evaluation
Treatment
Delivery

7. CT SIMULATION
 CT simulator is a CT scanner equipped with laser localizers to setup the
treatment isocenter, a flat couch
 Relationship between pixel value (CT number) and tissue density is established
and this allows pixel by pixel correction for tissue inhomogeneities in
computing dose distribution
 CT is related to electron density and atomic number
 CT provides the best geometric accuracy.
 MRI shows proton density distribution

8. IMAGE REGISTRATION
 Process of correlating different image data sets to identify corresponding
structures or regions
 Eg: MRI and CT fusion

9. IMAGE SEGMENTATION
 Slice by slice delineation of anatomic regions of interest

10. BEAM APERTURE DESIGN
 After image segmentation has been completed, next step is to select beam
direction and designing beam apertures
 Multiplicity of Fields remove the need of using ultrahigh energy beams
(>10MV)
 Multiple fields requires beam shaping blocks or MLC

11. PLAN OPTIMIZATION AND EVALUATION
 Isodose curves and surfaces
 Dose Volume Histograms

12. PLAN OPTIMIZATION AND EVALUATION – ISODOSE CURVES AND SURFACES
 Dose distribution of plans are evaluated by viewing isodose curves in
individual slices or 3D ISO dose surfaces
 It shows regions of uniform dose, high dose or low dose and their anatomic
location and extent

13. PLAN OPTIMIZATION AND EVALUATION - DOSE VOLUME HISTOGRAMS
 Provides quantitative information about how much dose is absorbed in how
much volume
 Also summarizes the entire dose distribution into a single curve for each
anatomic structure of interest
 Can be represented in two forms
 Cumulative integral DVH
 Differential DVH

14. CUMULATIVE DVH
 plot of the volume of a given structure receiving a certain dose or higher as a
function of dose
 found to be more useful
 Any point on the cumulative DVH Curve shows the volume of a given structure that
receives the indicated dose or higher
cumulativ
e

15. DIFFERENTIAL DVH
 Plot of volume receiving a dose within a specified dose interval as a function
of dose
 shows extent of dose variation within a given structure

16. DOSE COMPUTATION ALGORITHMS
 Correction Based
 Model based
 Convolution-Superposition Method
 Direct Monte Carlo

17. IMRT

18. DEFINED AS
 non uniform fluence is delivered to the patient from any given position of the
treatment beam to optimize the composite dose distribution
 Optimal fluence profiles for a given set of beam directions are determined
through inverse planning

19. IMRT
 To produce intensity modulated fluence profiles, accelerator must be
equipped with a system that can change the given beam profile to a profile of
arbitrary shape
 Computer controlled MLC is the most practical system for delivering intensity modulated
beams

20. IMRT DELIVERY
 Fixed gantry angle
 Rotating fan beams (tomo therapy)
 Rotating cone beams
IMRT
Fixed gantry angle
Segmental MLC
delivery
Dynamic MLC
delivery
Rotating fan beams
Tomotherapy
Rotating cone
beams
Intensity
Modulated Arc
therapy
Volumetric
Modulated Arc
therapy

21. IMRT WITH FIXED GANTRY ANGLES
 Segmental MLC delivery
 Dynamic MLC delivery

22. SEGMENTAL MLC DELIVERY / STEP AND SHOOT / STOP AND SHOOT
 treated by multiple fields
 Each field is subdivided into a set of subfields irradiated with uniform beam
intensity levels
 Subfields are created by MLC and delivered in a stack arrangement one at a
time in sequence
 Accelerator is turned off while the leaves move to create the next subfield
 A nine field plan could be delivered in less than 20 min

23. PROS AND CONS
 Advantage
 ease of implementation
 Disadvantage
 instability of some accelerators when beam is switched off and on within a
fraction of a second

24. DYNAMIC MLC DELIVERY / SLIDING WINDOW
 accelerator beam is on while the leaves are moving
 period that the aperture between leaves remains open : dwell time
 Dwell time allows the delivery of variable intensity to different points in the
field

25. IMRT WITH ROTATING FAN BEAMS : TOMOTHERAPY
 treated slice by slice by Intensity modulated Beams
 similar to CT imaging
 special collimator designed to generate the beams as the gantry rotates
around the longitudinal axis of the patient
 2 devices
 In one device, couch moves one or two slices at a time
 In second device, couch moves continuously as in helical CT

26. IMRT WITH ROTATING CONE BEAMS
 Intensity Modulated Arc therapy
 Volumetric Modulated Arc therapy

27. INTENSITY MODULATED ARC THERAPY(IMAT)
 uses the MLC dynamically to shape the fields & rotate the gantry
 Similar to step & shoot in that each field is positioned along the arc and
subdivided into subfields of uniform intensity
 But the
 MLC moves dynamically to shape each subfield
 while the gantry is rotating
 And Beam is on all the time

28. INTENSITY MODULATED ARC THERAPY(IMAT)
 Multiple overlapping arcs are delivered with the leaves moving to new
position at regular angular interval say 5 degree
 One dimensional intensity profile generated by stacking of fields defined by
one leaf pair,
 Two dimensional profiles created by repeating the whole process for all the
leaf pairs of the MLC

29. DISADVANTAGES OF IMAT
 inefficient due to necessity of treating several arcs to deliver a single IMAT
treatment field
 only a little improvement in isodose distribution from other forms of IMRT

30. VMAT
 Delivery of a rotational cone beam with variable shape and intensity
 gantry moves continuously while the MLC leaves and dose rate varying
throughout the arc

31. CLINICAL APPLICATION – IMRT
 extra degree of freedom that is intensity modulation in achieving dose
conformity
 not limited by target size or its location
 Superficial disease sites (e.g., parotid, neck nodes, chest wall), often treated
with electrons, can also be treated with IMRT as effectively
 dose conformity is a “double-edged sword,” with more normal tissue sparing
on the one hand and greater possibility of target miss on the other
 Large integral dose

32. CAVEATS - CONFORMAL THERAPY
 Highly Susceptible to motion and setup related errors
 Extensive physics manpower
 Planning time is increased
 Increased cost

33. COMPARISON
3DCRT
 Less conformal
 Forward planning
 Less strict QA
 Less expensive
 Less Time consuming
 Less reduction of normal tissue dose
 No dose intensity modulation
 Uniform dose
IMRT
 More conformal
 Inverse planning
 More accurate QA
 More expensive
 More time consuming
 More reduction of normal tissue dose
 Dose intensity can be modulated within the
target
 Gradient dose

34. INVERSE PLANNING OR OPTIMIZATION
 Introduced by Brahme
 process by which the intensity distribution of each beam employed in a plan is
determined such that the resultant dose distribution can best meet the criteria
specified by the planner
 Criteria are typically specified in terms of
 dose and dose-volume requirements, or
 biological indices such as tumor control probability (TCP) and normal tissue
complication probability (NTCP).

35. INVERSE PLANNING OR OPTIMIZATION
 the desired dose distribution was first defined, and
 then an integral equation was solved to find an appropriate beam intensity to
provide it

36. COMPARISON
Conventional forward planning
 depends on geometric relationship
between the tumor and nearby sensitive
structures.
Inverse planning
 is less dependent on the geometric
parameters
 more on specification of volumes of tumor
targets & sensitive structures, as well as
their dose constraints

37. COMPARISON
Forward planning - Sequence
 Decide dose to PTV
 Beam arrangement
 Field shaping
 Beam modification
 Dose calculation
Inverse planning – sequence
 Dose constraints
 Beam arrangement
 Plan generation
 Technique selected

38. THANK YOU