Positioning, leveling and immobilization done in the treatment position.
Adequate internal contrast – external landmarks to be delineated too.
Preferably images to be transferred electronically to preserve electron density data.
Imaging in multiple planes without formatting.
Greater tissue contrast – essential for proper target delineation in brain and head and neck
No ionizing radiation involved.
Lower spatial resolution
Longer scan times
Inability to image calcification or bones.
Includes PET – CT imaging and Fusion MRI.
Allows “ biological modulation ” of radiation therapy.
Expensive : requires additional software
Final clinical utility – still remains to be realized
Patient positioning is the most vital and often the most NEGLECTED part of beam direction:
Good patient position is ALWAYS :
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.
Help 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.
Positioning aids… Pelvic Board Prone Support Breast Board
Possibility of skin reactions in the infra mammary folds
Access to CT scanners hampered
Prone treatment support.
Allow comfortable arm up support ► brings arms out of the way of lateral beams.
Positions patient so that the breast / sternum is horizontal ► avoiding angulation of the collimator.
Pulls breast down into a better position by the pull of gravity.
Breast boards… Modern Breast Board Indexed Arm supports Indexed wrist support Head rest Carbon fiber tilt board Wedge to prevent sliding
Mould making : Contd..
Mould making : Contd..
Thermoplastics are long polymers with few cross links.
They also possess a “plastic memory” - tendency to revert to normal flat shape when reheated
Made of polyurethane
Ability to cut treatment portals into foam.
Mark treatment fields on the foam.
Rigid and holds shape.
Chance of spillage
Environmental hazard during disposal
Consist of polystyrene beads that are locked in position with vacuum.
Can be reused.
However like former immobilization not perfect.
A simple yet elegant design to immobilize the head.
A dental impression mouthpiece used.
The impression is attached to the base plate and is indexed.
Gill Thomas Cosman System.
TALON ® Systems – NOMOS corp.
Contour is the representation of external body outline.
Plaster of Paris
Thermoplastic contouring material
Geometrical : Area DEFINED by the light beam at any given depth as projected from the point of origin of the beam.
Physical : Area encompassed by the 50% isodose curve at the isocenter. In LINACs often defined at the 80% isodose.
Criteria for acceptability:
Dose distribution to be uniform ( ±5% )
Maximum dose to tissues in beam ≤ 110%.
Critical structures don ’ t receive dose exceeding their normal tolerance.
2 Field techniques
Can be :
Less chance of geometrical miss
Dose homogeneity depends on:
Disadvantage of 2 field techniques
-large amount of normal tissue gets radiation
-if separation is more there is an arc like distribution
so, in ca cx if separation is
>16 cm four fields are used.
3 field techniques
-deep seated tumors
-to save vital structures
-ca nasopharynx with ant extention
-ca maxilla with ethmoidal extention
4 field techniques
Used in 3DCRT & IMRT
Used to obtain a “conformal” dose distribution in the modern radiotherapy techniques.
Integral dose increases
Certain beam angles are prohibited due to proximity of critical structures.
Setup accuracy better with parallel opposed arrangement.
Radical treatment of lymphoma(HD)
Whole body irradiation
Hemi body irradiation
DOSE DISTRIBUTION ANALYSIS
Done manually or in the TPS.
Manual distribution gives a hands on idea of what to expect with dose distributions.
Inefficient and time consuming.
Adequate for most clinical situations.
SSD technique (PDD method)
Dose to be delivered.
Number of fractions
Number of fractions per week
PDD is the ratio of the absorbed dose at any point at depth d to that at a reference depth d 0 .
D 0 is the position of the peak absorbed dose.
D max is the peak absorbed dose at the central axis .
Total Tumor dose Number of fields x Number of #s = T Incident dose = T x 100 PDD Time = ID Output
Uses doses normalized at isocenter for calculation.
In this technique the impact of setup variations is minimized.
Setup is easier but manual planning difficult.
Time taken for treatment reduced.
SAD calculations Total Tumor dose Number of fields x Number of #s = T Incident dose = T x 100 TMR/TAR Time = ID Output
SSD vs SAD technique
Relatively less homogenous dose distribution
Setup possible without requiring expensive aids e.g. Laser
PDD charts can be used for simple dose calculations
More skin reactions
Less number of MUs required
Time taken is less
Impact of setup inaccuracies is minimized in 2 field techniques
Ease of setup reproducibility in multi field treatments.
TAR vs. SSD
TAR = Tissue Air Ratio
TAR introduced by Jones for rotation therapy.
Allows calculation of dose at isocenter WITHOUT correcting for varying SSDs.
TAR is the ratio of dose at a point in the phantom to the dose in free space at the same point (D q /D 0 )
D q D 0
TAR removes the influence of SSD as it is a ratio of two doses at the SAME point.
However like PDD the TAR also varies with:
EXECUTION & VERIFICATION
Can be done using:
Electronic Portal images
Cone Beam CT mounted on treatment machines (IGRT).
Seen during treatment
-infrared camera (in imrt)
But have several disadvantages.
Port film disadvantages
Factors leading to poor image contrast:
High beam energy (> 10 MV)
Large source size ( Cobalt)
Large patient thickness (> 20 cm)
Slow acquisition times.
Image enhancement not possible.
Electronic Portal Imaging
Video based EPIDS
Fiber optic systems
Matrix liquid ion chambers
Solid state detectors
Amorphous Si technology*
BEAM DIRECTION DEVICES
The main beam direction devices are:
Front pointer / SSD indicator
Pin and arc
Collimators provide beams of desired shape and size.
Fixed / Master collimator.
Movable / Treatment collimator.
Protects the patient from bulk of the radiation.
Dictates the maximum field size for the machine.
Maximum beam size is when exposure at periphery is 50% of that of the center.
In megavoltage radiotherapy beam angle used is 20° .
Master Collimator : Design
In megavoltage x ray machines beam energy is maximum in forward direction.
Beam energy is equal in telecurie sources so primary collimators are spherical.
Define the required field size and shape.
Placed below the master collimators results in trimming of the penumbra.
Jaws / Movable diaphragms
Applicators: Design Lead Sheet Box Plastic Cap Metal Plate with hole
Indicate size and shape of beam.
Plastic ends allow compression .
Compression allows immobilization .
Useful for low energy only.
Separate sizes and shapes required.
Shapes may change due frequent handling.
Handling of heavy weight not required.
Skin sparing effect retained.
Jaws moved mechanically – accurately.
Jaw border lies along the line radiating from focal spot
Jaws: Disadvantages Disadvantages Remedy Size and shape of field remain unknown Light beam shining through the jaws Patient to source distance unknown SSD indicator used. Compression not possible A Perspex box may be applied to the head
Front & back pointers
This method requires the identification & marking on the patient’s surface, of two points lying on a line passing through the tumor centre.
Entry point- A
Tumor centre- T
Exit point- B
Front Pointer/ SSD indicator
Detachable device to measure the SSD and align the beam axis.
Designed so that it may be swung out of the beam path during treatment.
The pointer can be moved in the sleeve.
A nipple is used to allow compression.
The arrow lies along the central ray.
Front pointer and back pointer used in the following situations:
Head and Neck
Requires skin marks – inherently unreliable.
Back pointer is unreliable when compression is desired.
Both front and back points must be accessible.
Accurate localization of tumor center is mandatory.
Pin & Arc Pin Arc Bubble
Based on the principle of parallelogram
How does it work?
arrangement of pin & arc is such that when pin is at it’s lowest position, it’s lower end is on the central axis of beam & on centre of curvature of arc.
Depth of the tumor is already known.
Pin is withdrawn the reqd. distance & it’s lower end is brought in contact with the surface mark.
So long as the pin is vertical the rest of equipment & applicator will rotate about the centre of tumor and central ray will always pass through it
Thus keeping the pin vertical & in contact with surface mark any particular angle can be selected
The oblique distance can be read off the scale or bar by applying principle of parallelogram.
Advantages of Pin & Arc
Allows Isocentric treatment of
Can be used with compression e.g. in treating deep seated tumors.
Can be used for manual verification of Isocentric placement of machines
Sites where used
Mostly in midline tumors situated at a depth
First used by Flanders and Newberg of Hammersmith Hospital for early linear accelerators.
The axis of rotation of the three structures:
coincide at a point known as the Isocenter .
Why Isocentric Mounting?
Allows faster setup and is more accurate than older non isocentrically mounted machines.
Makes setup transfer easy from the simulator to the treatment machine.
LASER = Light Amplification Of stimulated Emission Of Radiation
Typically 3 pairs are provided with the machine and intersect at the isocenter.
Other uses :
Checking the isocenter
Reproducing the setup on the simulator at the treatment couch.
Accurate setup depends on proper alignment of the lasers themselves
Lasers known to move frequent adjustments needed.
Beam direction devices & methods are important part of radiotherapy which aids in accurate treatment.
To neglect the extra accuracy that can be gained by beam direction is to throw away much of the value of the powerful and expensive apparatus now in use in radiotherapy.