beamdirection devices and simulators

1. PRINCIPLES OF BEAM DIRECTION AND USE
OF SIMULATORS
Moderator- Dr Bhavana Rai

2. What is beam direction
During treatment planning whole plan
of the treatment is worked out in
advance of the actual treatment and
certain devices are used to direct the
beam towards the tumor , so that
better tumor control is achieved with
minimum complications

4. Beam
direction
It is the
containment of
radiation by
using various
methods so that
useful beam is
directed towards
target

5. Why it is required?
• In order to reduce unnecessary irradiation to
patient
- beam size should be minimum required to cover desired
. treatment zone
-beam must be accurately aimed towards the zone

6. Better tumor control
Low normal tissue
complication
Better Therapeutic ratio

7. How is Beam Direction achieved?
Using Beam direction device
in radiotherapy machine
Planning in advance
Accurate calculations

8. Beam Direction Device
Collimators
Front and Back pointer
Pin and arc
Isocentric Mounting
Laser
Light Field
Distance Meter
Breast Cone

9. What is Collimation?
• Some radiation is emitted out in every
direction,
• Even the useful beam is far greater than is
needed for most treatments
• Methods, to produce beam of the shape and
sizes, so that desired zone is irradiated and
normal tissues are spared is k/a COLLIMATION

10. Collimators
• Radiation source is mounted, surrounded with
metal box, except for a thin ‘window’- ‘Fixed’
or ‘Master’ collimator

11. Fixed Collimators
• Radiation allowed to escape is
usually accepted for which
primary exposure rate at the edge
is 50% of that at centre
20°

12. • Protects the patient from bulk of the
radiation.
• Dictates the maximum field size for the
machine.
Maximum field size
Co-60 -> 35 35 cm
Linac -> 40 40 cm

13. Treatment or movable collimator
• Placed below master
collimator required for
- cutting down the
penumbra
- defining the required
field size
• Types
- Applicators
- Movable jaws

14. APPLICATORS
• As electron scatter is so significant, applicators
are necessary to delineate the field sharply
close to the patient

15. Applicators: Design
• Applicators are constructed
of low Z materials (e.g.
aluminium, plastic, etc.) to
minimise x-ray
contamination.
• A frame is generally
provided near to or at the
final section, into which
irregular or non-standard
size cut-outs can be
positioned for tertiary field
shaping.
Metal Plate with hole
Metal Sheet
Box
Plastic Cap

16. • They indicate
immediately the shape
and size of beam
• Give useful indication of
its direction
• Position the patient at
desired distance
• Acts as compressor- by
pressing the applicator
end into the tissues

17. Jaws/Movable Diaphragms
• Are blocks of lead which
by a suitable
mechanical linkage can
be made to move in and
out at the turn of a
handle

18. PENUMBRA
• One of the objective of
collimation is to produce
a beam of some size and
shape and sharp edge as
much as achievable i.e.
smallest possible
penumbra
• Penumbra size depends
upon-
- Size of source
- Distances of diaphragm
from plane interest and
the source
The minimum penumbra is obtained
when final diaphragm is on the skin

19. • A strip, a centimetre or
so wide, of copper-
tungsten alloy is placed
on the inner faces of
the jaws
• Also called heavy alloy
• It has more attenuation
than lead

20. • With the gantry vertical
and the head at 0
degree-
• Y2 - at the gantry end
X2 - on right side (i.e.
supine patient’s left) as
viewed facing the
gantry.

21. Front and Back Pointer method
• Based on the principle
that any line can be
defined by two points
• Entry point- A
Tumor centre- T
Exit point- B
Front pointer/SSD
indicator
Back pointer

22. Pros and cons
Pros
• Easily available
• Easy to use
• Cheap
• Accurate
Cons
• Accurate localization of
tumor required
• Requires skin marks-
unreliable
• Front and back points must
be accessible

23. Pin And Arc
• Consists of
- Ruler(R)
- Slider(S)
- Arc(T)
- Pin(V)
- Carriage(U)

24. Pin & Arc
Arc
Bubble

25. Pin & Arc: Principle
Based on principle of
Parellogram

26. Pin & Arc : Method
d
D
d D
D
This is the
isocenter

27. Isocentric Mounting
• The axis of rotation of
the three structures:
– Gantry
– Collimator
– Couch
coincide at a point known
as the Isocenter.

28. Why Isocentric Mounting
• Better accuracy
• Faster setup
• Easier transfer from
simulator to machine

29. LASERS
• Lasers used to identify isocentre
• Laser lines pass through isocentre
perpendicular to each other
• A typical set of lasers comprises
-Cross lasers (coronal and lateral
lines) placed on both walls lateral
to isocentre
- Line laser (sagittal) placed at foot
of bed passing through isocentre
-Cross laser (lateral and sagittal)
placed on ceiling above isocentre

30. LIGHT FIELD
• In order to enable the
treatment staff to see where
the radiation beam will be
located, a light beam is
generated that coincides with
the useful limits of the beam.
• by using a high intensity light
bulb containing a small
filament that must be optically
coincident with the source
position
• Cross on field indicates
central axis (and wedge
direction)

31. DISTANCE METER
• Also called optical distance
indicator(ODI)
• Uses the light field and a
second light source to indicate
distance from radiation source
to patient surface
• Second light source marked
with lines at set distances
• Intersection of the two light
beams varies with distance to
surface – indicating distance

32. Breast Cone
• A beam direction and
beam modification device
• Directs beam to central
axis of beam, where
tangential beam is
applied to curved surface
• Helps position the patient
with accurate SSD
• End plate provides
compensation
• Effective shielding of
lungs

33. Steps of advance treatment planning
Positioning Immobilisation Localisation Field selection
Dose
distribution
Calculations
Execution &
verification

34. RADIOTHERAPY SIMULATORS

35. RADIOTHERAPY SIMULATORS
• Is an apparatus that uses a
diagnostic x-ray but
duplicates a radiation
treatment unit in terms of
its geometrical, mechanical,
and optical properties
• Has diagnostic X ray instead
of treatment beam
• Called simulation because
the treatment is being
"simulated," or not really
given.

36. Need Of Simulators
(a) Manual marking not accurate due to anatomical
uncertainities.
(b) Geometrical relationship between the radiation beam and
the external and internal anatomy of the patient cannot be
duplicated by an ordinary diagnostic x-ray unit.
(c) Field localization directly with a therapy machine by port film
quality is poor.
(d) Field localization is time-consuming, if carried out in the
treatment room.
(e) Unforeseen problems with a patient set-up or treatment
technique can be solved.

37. Function
• It displays the treatment fields so that the target volume may
be accurately encompassed without delivering excessive
irradiation to surrounding normal tissues
• By radiographic visualization of internal organs, correct
positioning of fields and shielding blocks can be obtained in
relation to external landmarks.

38. Types
X-ray simulators
CT simulators
4D CT
MRI Simulator

39. X-Ray(conventional) Simulators
• Major sub-systems are-gantry,
collimator, x-ray tube, imaging unit,
patient support system(couch), remote
control console
• Similar to MV machine except instead
of MV beam it has diagnostic quality X
ray
• X-ray passes through one side of the
patient and attenuated beam is
converted to digital image by image
intensifier
• Image is either radiographic mode or
flouroscopic
• The variable focus to axis distance
makes it suitable for therapy simulation
for number of teletherapy machines

40. Pros and Cons of X Ray simulator
Pros
• Cheapest among all
simulators
• Less radiation exposure
than CT
• Fast and efficient
• Generate verification
radiograph
• No limitation patient
positioning
Cons
• Only 2 D planning possible
• Inability to accurately
distinguish the different
densities of areas- making it
difficult to visualize
anatomy
• Radiation exposure
Best for checking radiotherapy plans and planning
palliative treatments very quickly and efficiently

41. Simulator CT
• X Ray simulator with CT
capability
• Uses X ray cone beam to
acquire image volume in
single rotation
• With the large variable
aperture size, patients
can be imaged in the true
treatment position and
without set-up
restrictions often found
with regular CT scanners.

42. CT Simulator
• A dedicated radiation therapy
CT scanner with simulation
accessories such as flat table,
lasers, immobilization and
image registration device, and
software for virtual simulation
• Uses CT scanner to localize the
treatment fields
• Positioning, levelling and
imobilization is done in
treatment position
• Images are sent electronically
to preserve electron density
data
GE LightSpeed VFX
70 cm bore
48 cm scan FOV

43. Design
Simulator Table Top
- Similar to treatment
machine for better
reproducibility
Wall Laser
- horizontal,
vertical
Saggital laser
- Can move
sideways
Scanner laser
- Internally
mounted vertical,
horizontal and
sagittal laser
Gantry
Console room

44. What is Virtual Simulation
• A software(exclusively written
for simulation) provides
outlining of contours, target
volumes and critical
structures, interactive portal
displays and placement,
review of multiple plans, and a
display of isodose distribution-
k/a virtual simulation
• It is so called as patient is
represented by CT images and
radiation machine by beam
geometry and expected dose
distirbution

45. Pros and Cons of CT Sim
Pros
• Gives volumetric
distribution of dose
• DRR, 3DCRT, IMRT, VMAT
techniques can be done
• Treatment planning
algorithms are developed
specifically for CT-
attenuation coefficient can
be identified precisely
Cons
• More radiation to Patient
• Suboptimal tissue contrast
• Lack of functional
information

46. 4D-CT
Breathing/organ motion
-problems with accurate target definition
(moving targets may appear with distorted
shapes and in wrong locations on CT)
- increased irradiation of normal tissues (larger
fields are often used to ensure that the tumor
is not missed).
4D CT
• Takes respiratory motion into account that
can be used for planning, delivery and
verification
• Takes phase images of deep inspiration->
mid inspiration-> mid expiration->deep
expiration
• Other images such as MIP, miniMIP, AvgMIP
can be generated
Phillips Brilliance CT big Bore
85 cm bore
Scan FOV- 70cm

47. How does it works
Infrared marker- moves
with breathing motion
Infrared camera -
records movement
Converted to digital
data
Encorporated to CT
imaging

48. Pros and Cons of 4D CT Simulator
Pros
• Takes respiratory/organ
motion into account
• Highly precise techniques
such as gating/tracking can
be done
Cons
• Respiratory motion is not
constant
• Needs higher expertise to
operate

49. MRI Simulators
• Due to major drawback
- Inherent geometric distortion in
the images
- Low signal intensity of bone causes difficulty in obtaining
DRRs
- lack of inherent electron density
information
• But it’s demand is rising due to
- Better soft tissue contrast
- No radiation exposure
- By using MR gel dosimetry that utilises radiosensitive material
that can be irradiated and then ‘read out’ by virtue of the
changes in magnetic properties thus,a true 3D dosimetric analysis
may be performed with good accuracy, if properly callibrated
Treatment
Plan
Two
volumes
overlayed
MRI
dosimetry
data

50. Current Practice
Is still to fuse to CT for this purpose
Proper fusion is not possible until and unless MRI is
taken in treatment position
Treatment position not possible every time in radio
diagnosis MRI machine due to narrow bore of gantry
and curved couch
Newer MRI machines have come up with 70 cm wide
bore and flat top and also lasers installed to facilitate CT
fusion

51. Future Aspects of MRI simulators
• Work is going on to develop planning
algorithms for MRI based planning
• Pseudo DRRs are being researched where CT
data is used as a surrogate for MR signal
intensity
• MR gel dosimetry is being further researched
and calibration mapping is going on, which
can provide better dosimetric calculations

52. PET Scanner
• Earlier fusion imaging
was practiced
• Now combined PET/ CT
simulator are present
• First came in 2000
• Can also be used as CT
simulator also
• What is advantage of
PET?

53. PET = Functional Imaging
• Entails imaging of biodistirbution of a
radiolabeled compound selected based
on its biochemical behaviour
• FDG-PET-> demonstrate metabolically
active disease
• 18F-FLT(flouro deoxy thymidine)-a
marker for cellular proliferation can be
used to assess response to radiation
therapy( FDG is inferior as radiation
causes inflammation which is picked up
by FDG)
• Cu-ATSM demonstrateshypoxia
---> radiation resistance
• 18F-FMISO
• Choline PET- recurrent prostate cancer
• 11C-metomdate-adrenocortical tumors

54. Simulation Process
• Is a team approach which needs physician,
physicists, therapist, nurses etc
• Team needs to understand individual
components of the process and their specific
technical requirements
• Well informed and knowledgeable personnel are
necessary to exploit the benefits of modern
treatment simulation equipment
• Better to write steps of simulation procedure so
that reproducibility is better achieved during
treatment

55. Steps of Simulation
3D Simulation
Patient positioning
and immobilization
Patient marking
CT scanning
Verification of scan
at console room
Transfer to virtual
workstation
Conventional Simulation
Patient positioning and
immobilization
Verification of patient
positioning using
fluoroscopy
Determination of
isocenter location
Beam placement design
Marking of patient
Acquisition of X ray films
Outlining of treatment
portal on x ray films
Transferring or acquiring
of patient setup data for
record and verify system

56. PATIENT POSITIONING
• Defn-
• It is one of the weakest link in treatment
planning process
• Good patient position is ALWAYS:
– Stable.
– Comfortable.
– Minimizes movements.
– Reproducible.

57. Standard Positions
• MC used body position.
• Also most comfortable.
• Best and quickest for setup.
• Minimizes errors due to
miscommunication.
• Best for treating posterior
structures like spine
• In some obese patients setup
improved as the back is flat
and less mobile.
Supine
Prone

58. Positioning aids/ device
• Helps to maintain patients in non standard
positions.
• These positions necessary to maximize
therapeutic ratio.
• Accessories allow manipulation of the non rigid
human body to allow a comfortable,
reproducible and stable position.

59. Other Positioning Devices

60. IMMOBILIZATION
• Patient’ concerned part is immobilized using
POP cast or thermocast or various devices
• More reproduciblity

61. Thermocast
• Thermoplastics are long
polymers with few cross
links.
• They also possess a
“plastic memory” -
tendency to revert to
normal flat shape when
reheated

62. Other Immobilization devices

63. CONCLUSION
• Accuracy is vital for treatment
• Better to use kilovoltage beam with careful beam
direction than megavoltage beams without it
• To neglect extra accuracy that can be gained by beam
direction is to throw away much of the value of
powerful and expensive apparatus now in use in
radiotherapy
• Simulators is an important part of advance planning
as they deal with target location, treatment planning,
and spatial accuracy in dose delivery