Principles of beam direction and use of simulators

PRINCIPLES OF BEAM DIRECTION AND USE OF SIMULATORS

Moderator- Dr Bhavana Rai

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

Co-60Tumor

Beam direction

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

Better tumor control Low normal tissue complication

Better Therapeutic ratio

How is Beam Direction achieved?

Using Beam direction device in radiotherapy machine

Planning in advance

Accurate calculations

Beam Direction Device

Collimators

Front and Back pointer

Pin and arc

Isocentric Mounting

Laser

Light Field

Distance Meter

Breast Cone

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

Collimators

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

Fixed Collimators

• Radiation allowed to escape is usually accepted for which primary exposure rate at the edge is 50% of that at centre

20°

• Protects the patient from bulk of the radiation.

• Dictates the maximum field size for the machine.

Maximum field sizeCo-60 -> 35 35 cm Linac -> 40 40 cm

Treatment or movable collimator

• Placed below master collimator required for

- cutting down the penumbra

- defining the required field size

• Types - Applicators - Movable jaws

APPLICATORS

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

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

• 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

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

• 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.

• 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

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

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 acessible

Pin And Arc

• Consists of - Ruler(R) - Slider(S) - Arc(T) - Pin(V) - Carriage(U)• Based on principle of

Parellogram

Isocentric Mounting

• The axis of rotation of the three structures:– Gantry– Collimator– Couch

coincide at a point known as the Isocenter.

Why Isocentric Mounting

• Better accuracy• Faster setup• Easier transfer from

simulator to machine

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

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)

DISTANCE METER• 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

Steps of advance treatment planning

Positioning Immobilisation Localisation Field

selectionDose

distribution Calculations Execution & verification

PATIENT POSITIONING

• Defn-• It is one of the weakest link in treatment

planning process• Good patient position is ALWAYS:

– Stable.– Comfortable.– Minimizes movements.– Reproducible.

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

Positioning aids/ device• 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.

Breast Board

Other Positioning Devices

IMMOBILIZATION

Mould

Thermocast

• Thermoplastics are long polymers with few cross links.

• They also possess a “plastic memory” - tendency to revert to normal flat shape when reheated

Other Immobilization devices

Localization

• The target volume and critical normal tissues are delineated with respect to patient’s external surface contour.

• What to localize?– Tumor– Organ

• Methods?– Clinical examination– Imaging

Clinical localization

Advantages;

– Available everywhere. – Cheapest and quickest.– Needs little additional

equipment.

Disadvantages;– Error prone in the wrong

hands.– Accessible areas

required.– Volumetric data not

easily obtained.

Clinical localization is mandatory despite advanced imaging – need to know what to image!

Estimation of depth

• From data gained by localization studies:– CT / MRI – Accurate data– Lateral height method– Tube shift method

• Depth estimation necessary for:– Calculations– Selection of beam energy

Lateral height method

d

d

H1 + H2

2d =

H1H2

Tube shift method

• Image shift and tube shift are interrelated WHEN the tube to target distance remains constant.

• Goal: To obtain a graph of different object heights against the tube shift.

• Serial measurements of image shift measured (for same tube to film distance) while varying the height of the markers above the table.

Tube shift principles

Marker

d2

yf

S

Tumor

x1x2

d1

Calculation

d1

fy

d2

x1x2

x1

S= d1

f – d1

S

x2

S= d2

f – d2

y = d2 – d1

= fx2 + S

x2 -x1 + S

x1

Tumor

Marker

Imaging Localization

• Imaging:– X-rays– USG scans– CT scans– MRI scans – PET scan– Fusion imaging

• Type of study selected depends on:– Precision desired.– Cost considerations– Time considerations– Labour considerations

X-Ray localization • X-rays made to pass through

concerned site, which is captured by photographic film or digital detector

• Pros - cheap - takes less time - easily available• Cons - radiation exposure - 2D view

Follow up

USG scan• Uses ultrasound waves• USG generated by transducer

focussed on concerned part echoes received transformed in digital image

• Pros - cheap - no raditation exposure

• Cons - needs expertise to interpret

CT Localization• An X-ray procedure that

combines many X-ray images with the aid of a computer to generate three-dimensional images of the internal organs and structures of the body

MRI Localization

• Uses a magnetic field and pulses of radio wave energy to make pictures of organs and structures inside the body

• Soft tissue structures are better visualised

PET Localization• 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

Fusion Imaging

• Includes MRI-CT fusion and MRI-PET fusion, PET-CT fusion

• Bony landmarks visualized better in CT

• Soft tissues better visualized in MRI,

• Metabolically active disease better in PET

Field Selection

•Single field•2 Field•Multiple field

Tumor can be

irradiated by one or

more radiation

beam

Single Field

• Criteria for acceptability:1. Dose distribution to

be uniform (±5%)2. Maximum dose to

tissues in beam ≤ 107%.

3. Critical structures don’t receive dose exceeding their normal tolerance.

• Situations used:– Skin tumors– CSI– Supraclavicular

region– Palliative treatments

2 Field techniques• Can be :

– Parallel opposed– Angled

• Perpendicular• Oblique

– Wedged pair

• Advantages:– Simplicity– Reproducibility– Less chance of

geometrical miss– Homogenous dose• Dose homogeneity depends on:

– Patient thickness– Beam energy– Beam “flatness”

Multiple fields• Used in 3DCRT & IMRT• Used to obtain a “conformal” dose

distribution in the modern radiotherapy techniques.

• Disadvantages:– Integral dose increases– Certain beam angles are prohibited due to

proximity of critical structures.– Setup accuracy better with parallel opposed

arrangement.

Field Shaping

Custom blocking

Applicators

Independent jaws

Multi leaf collimators

Breast cone

Custom blocking

Multi leaf collimators• Developed in Japan in 1980s• In Europe first used by Scanditronix f/b Philips(Elekta) and

Varian and rest• Instead of a single block of metal, these have up to 80 pairs of

leaves that can move independently, allowing any beam shape to be produced subject to the width of the leaves.

Features • Used to shape radiation field• Can interdigitate• Fields set automatically• 52 – 160 leaves• Moves independently• Typically 60mm tall• 2.5% leaf transmission; 4% interleaf leakage• Tongue and groove offset• Focused in 1 direction• 7mm penumbra

Types of MLCsThere are three main types of MLC:a) Type A (e.g. Scanditronix and Siemens): The MLC provides

all the collimation (except for the primary collimator) and completely replaces the standard collimation system

b) Type B (e.g. Elekta): The MLC provides field shaping but additional shielding is provided by backup collimators. The MLC (including the backup collimators) replaces the standard collimation system

c) Type C (e.g. Varian): The MLC provides field shaping in addition to the standard collimation system. Although part of the head assembly, it is externally mounted and complements the standard collimation system

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.• Pros:

– Cheap– Universally available– Adequate for most clinical situations.

Calculations

SSD technique (PDD method)

SAD technique

Clarkson’s technique

Computerized

Verification Of Treatment

• The primary purpose of treatment verification is to verify the treatment volume under actual conditions

• Done when the patient is positioned on couch in treatment room just before giving radiation

Various Methods

Port films

• Films are of poor quality• Viewing is delayed as time required for processing is long

Electronic portal imaging

• Possible to view images instantaneously• Although images are of poor quality

Cone beam CT

• Can be kilovoltage or megavoltage type• Acquires images in therapy room coordinates• kV CBCT has far superior quality

Execution

• When everything is verified and treating doctors feel that beam direction is adequately achieved then execution of plan is done by giving prescribed radiation dose.

RADIOTHERAPY SIMULATORS

RADIOTHERAPY SIMULATORS

• Is an apparatus that uses a diagnostic x-ray tube but duplicates a radiation treatment unit in terms of its geometrical, mechanical, and optical properties

• Called simulation because the treatment is being "simulated," or not really given.

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.

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.

Types

X-ray simulators

CT simulators

4D CT

MRI Simulator

X-Ray(conventional) Simulators• Major sub-systems are-gantry,

collimator, x-ray tube, imaging unit, patient support system(couch), remote control console

• The variable focus to axis distance makes it suitable for therapy simulation for number of teletherapy machines

• X-ray passes through one side of the patient and attenuated beam is converted to digital image by image intensifier

Pros and Cons of X Ray simulator

Pros • Cheapest among all

simulators• Less radiation exposure

than CT• Fast and efficient

Cons • Only 2 D planning possible• Inability to accurately

distinguish the different densities of areas- making it difficult to visualize anatomy

Best for checking radiotherapy plans and planning palliative treatments very quickly and efficiently

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 snd imobilization is done in treatment position

• Image are sent electronically to preserve electron density data

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

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• Difficulty in visualize small

group of cancer cells that are separated from gross tumor

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, minMIP can be generated

How does it worksInfrared marker-

moves with breathing motion

Infrared camera -records movement

Converted to digital data

Encorporated to CT imaging

Pros and Cons of 4D CT Simulator

Pros • Takes respiratory/organ

motion into account• Highly precise techniques

such as gating can be done

Cons • Respiratory motion is not

constant

MRI Simulators• No MRI only simulator at present

• 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

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

Future Aspects of MRI simulators

• Work is going 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

IT IS THE FUTURE OF SIMULATION

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 where treatment planning can be made and reviewed

thankyou