Patient contouring and beam modifying devices

1

PATIENT CONTOURING AND BEAM MODIFYING

DEVICESMayur Mayank

15.05.2012

2

PATIENT CONTOURING

3

PATIENT CONTOURING◦ Introduction

A. Manual contouring- Plaster of Paris strips- Contour tubes- Contour wire- Flexible curve- Manual Contouring device- Mobile Contour Plotter- Clarke’s Model of Contouring Device

ADVANTAGES AND DISADVANTAGES

B. Image based contouring

OVERVIEW

4

Contour : It is a cross sectional outline of the patient’s external surface.

◦ Required for outlining the Planning target Volume (PTV)

◦ Also helps in dose limitation to the normal organs

PATIENT CONTOURING

5

•The patient contour must be obtained with the patient in the treatment position. •A line representing the tabletop must be indicated in the contour so that this can be used as a reference for beam angles.

•Important bony landmarks as well as beam entry points must be indicated on the contour.

•Checks of body contour should be done during the treatment course as the contour is expected to change due to a reduction of tumor volume or a change in patient weight.

•If body thickness varies significantly within the treatment field, contours should be taken in more than one plane.

RULES FOR CONTOURING

6

Plaster of Paris strips

I. Extra fast setting 4 inch wide POP strips can be used for contouring.

II. The strip should be long enough to from the couch top from one side, over the patient, and down to the couch top on the opposite side.

III. The strips are dipped in hot water and then placed along the transverse axis of the patient.

MANUAL CONTOURING

7

Contour Tubes

I. Hollow, low-temperature thermoplastic tubes. II. Heat and cool quickly to form rigid contours in

minutes.III. Easy application and removal without altering

the shape of the finished contour. IV. The shape is retained until reheated.

8

9

Contour Wire

I. Thin Lead Contour Wire 2mm (0.080") Diameter used for head and neck areas

II. Thick Lead Contour Wire 3mm (0.125”) Diameter used for body sections

The wire is formed around the patient in the treatment position.

10

11

Flexible Curve

I. Flexible Contour Device is covered with tough vinyl.

12

Manual Contouring Device

I. This device is an economical and convenient unit for making contours.

II. Apparatus consists of a hinged frame assembly with two feet.

III. The plastic frame guides twenty-seven rods. Small replaceable washers provide friction on the rods to prevent movement.

13

14

-The patient is placed in a prone position and the unit is placed over the area to be contoured.

- The aluminium rods are positioned so that the tips just touch the skin.

- After all the rods are positioned, the hinge locking knob is removed. The end unit is opened and the unit is removed from the patient.

15

Mobile Contour Plotter

- Pantograph-type apparatus - A rod can be moved laterally as well as up

and down. When the rod is moved over the patient contour, its motion is followed by a pen that records the outline on paper..- This contour drawing can then be used on treatment planning computers and in conjunction with CT information for the treatment plan.

16

- The contour plotter has a mechanical mechanism which links a drawing pen to a stylus arm.

- Upon contact with the body, it translates body contours to an overhead drawing board.

- When the finger plunger is depressed, engaging the pen, a continuous plot is drawn as the operator follows the physical contour of the patient.

- Marks can be made along the contour to indicate beam entry and laser light locations.

17

STYLUS

DRAWING PEN

DRAWING BOARD

18

By far the most accurate of the mechanical devices used for manual patient contouring in

Radiotherapy.

19

Clarke’s Model of contouring device

Electromechanical device in which motion of the rod over the patient contour is read by a sensing device and

transferred to an X-Y recorder.

◦ Consists of a master unit with a probe which is made to follow the contour of the skin, and a recording unit which produces a life size drawing of the contour on paper.

◦ The recording unit is made to reproduce the motion of the master unit by the use of Selsyn motors.

◦ The device was readily adaptable to computerized planning system.

*A contouring device for use in radiation treatment planningBy Hector C. Clarke, M.Sc.

Department of Radiology, The General Hospital, St. John's, Newfoundland

1969, Br. J. Radiol, 42, 858-860

20

*A contouring device for use in radiation treatment planningBy Hector C. Clarke, M.Sc.

Department of Radiology, The General Hospital, St. John's, Newfoundland

1969, Br. J. Radiol, 42, 858-860

21

ADVANTAGES◦ Cheap◦ Easy to use◦ Readily available

DISADVANTAGES◦ Inaccuracy◦ High chances of

errors due to manual translation on the contouring paper

ADVANTAGES AND DISADVANTAGES OF MANUAL CONTOURING

22

Used for 3D treatment planning mostly CT/MRI/PET/Ultrasound images are used for

contouring the patients anatomical landmarks, target volumes and the organs at risk

Much more accurate than the manual system of contouring

Expensive and requires trained professionals for the image acquisition and contouring.

IMAGE BASED CONTOURING

23

BEAM MODIFYING DEVICES

24

INTRODUCTION TYPES OF BEAM MODIFICATION TYPES OF BEAM MODIFYING DEVICES CONCLUSION REFERENCES

OVERVIEW

25

Beam ModificationDefined as desirable modification in the spatial distribution of radiation - within the patient - by insertion of any material in the beam path.

BEAM MODIFYING DEVICES

26

There are four main types of beam modification:◦Shielding: To eliminate radiation dose to some

special parts of the zone at which the beam is directed.

◦Compensation: To allow normal dose distribution data to be applied to the treated zone, when the beam enters a or obliquely through the body or where different types of tissues are present.

◦Wedge filtration: Where a special tilt in isodose curves is obtained.

◦Flattening: Where the spatial distribution of the natural beam is altered by reducing the central exposure rate relative to the peripheral.

Types of beam modification

27

Field blocking and shaping devices:◦Shielding

blocks.◦Custom blocks. ◦Asymmetrical

jaws.◦Multileaf

collimators. Compensators. Beam spoilers

Wedge filters. Beam flattening

filters. Bolus

Types of beam modification devices

28

SHEILDING BLOCKS

An ideal shielding material should have the following characteristics:◦ High atomic number.◦ High-density.◦ Easily available.◦ Inexpensive.

The most commonly used shielding material for photons is Lead (Pb).

FIELD BLOCKING AND SHAPING DEVICES

29

The thickness of shielding block used depends upon the energy of the radiation.

The shielding material which reduces beam transmission to 5% of its original is considered acceptable.

The term half value-layer is an expression for the attenuation produced by any material. Half-value layer is defined as the thickness of material, which will reduce the intensity of the primary beam by 50%.

Practically thickness of lead between 4.5 - 5 half-value layers results in 5% or less of primary beam transmission.

30

Beam energy Required lead thickness

Co60(1.25 MeV) 5.0 cm

4 MV 6.0 cm

6 MV 6.5 cm

10 MV 7.0 cm

31

CUSTOM BLOCKS

Material used for custom locking is known as the Lipowitz metal or Cerrobend.

Melting point 70°C. Density 9.4 g /cm3 at 20°C (83% of lead). 1.21 times thicker blocks necessary to produce

the same attenuation. Most commonly thickness of 7.5 cms used.

32

Lead, 26.70%

Bismuth, 50.00%Cadmium, 10.00%

Tin, 13.30%

Bismuth Lead Tin Cadmium

33

Custom blocks

Outline of the treatment field being traced on

radiograph using a

Styrofoam cutting device.

Electrically heated wire pivoting around a point (simulating the

source) cutting the styrofoam block

Cavities in the styrofoam block

being used to cast the

Cerrobend blocks.

34

INDEPENDENT JAWS

Used when we want to block of the part of the field without changing the position of the isocenter.

Independently movable jaws, allows us to shield a part of the field, and this can be used for “beam splitting”.

Here beam is blocked off at the central axis to remove the divergence.

Use of independent jaws and other beam blocking devices results in the shift of the isodose curves.

This is due to the elimination of photon and electrons scatter from the blocked part of the field.

35Ref : The Physics of Radiation Therapy by Faiz M Khan

36

MULTILEAF COLLIMATORS

Multileaf collimators are a bank of large number of collimating blocks or leaves

Can be moved automatically independent of each other to generate a field of any shape.

40 pairs of leaves or more having a width of 1 cm on less (projected at the isocenter).

Thickness = 6 – 7.5 cm Made of a tungsten alloy. Density of 17 - 18.5 g/cm3. Primary x-ray transmission:

◦ Through the leaves < 2%.◦ Interleaf transmission < 3%.◦ For jaws 1% ◦ Cerrobend blocks 3.5% .


38

ADVANTAGES◦ Time for shaping and

inserting of custom blocks is not required.

◦ The hardening of beam, scattered radiation, and increase in skin doses and doses outside the field, as seen with physical compensators is avoided.

◦ Automation of reshaping and modulation of beam intensity in IMRT.

◦ MLCs can also be used to as dynamic wedges and electronic compensators (2D).

DISADVANTAGES◦ Island blocking is not

possible.◦ As the physical penumbra

is larger than that produced by Cerrobend blocks, treatment of smaller fields is difficult, as is the shielding of critical structures, near the field.

◦ The jagged boundary of the field makes matching difficult.

MULTILEAF COLLIMATORS

39

A beam modifying device which evens out the skin surface contours, while retaining the skin-sparing advantage.

It allows normal depth dose data to be used for such irregular surfaces.

Compensators can also be used for ◦ To compensate for tissue heterogeneity. This was

first used by Ellis, and is primarily used in total body irradiation.

◦ To compensate for dose irregularities arising due to reduced scatter near the field edges (example mantle fields), and horns in the beam profile.

COMPENSATORS


41

Two-dimensional compensators

Used when proper mould room facilities are not available.

Thickness varies, along a single dimension only. Can be constructed using thin sheets of lead,

Lucite or aluminum. This results in production of a laminated filter.

Constructed by gluing together sheets of lead or other material in a stepwise fashion to form a laminated filter.

The total thickness of the filter at any point is calculated to compensate for the air gap at the point below it.

42

Three-dimensional compensators

3-D compensators are designed to measure tissue deficits in both transverse and longitudinal cross sections.

Various devices are used to drive a pantographic cutting unit.

Cavity produced in the Styrofoam block is used to cast compensator filters.

Medium density materials are preferred to reduce errors.

Various systems in use for design of these compensators are:◦Moiré Camera.◦Magnetic Digitizers.◦CT based compensator designing systems.


44

BEAM SPOILERS Special beam modification device where

shadow trays made from Lucite are kept at a certain distance from the skin.

Based on the principle that relative surface dose increases when the surface to tray distance is reduced.

First used by Doppke to increase dose to superficial neck nodes in head and neck cancers using 10 MV photon beams.

Also used in TBI to bring the surface dose to at least 90% of the prescribed TBI dose.

45

BEAM SPOILER

46

WEDGE FILTERS A beam modifying device, which causes a

progressive decrease in intensity across the beam, resulting in tilting the isodose curves from their normal positions.

Degree of the tilt depends upon the slope of the wedge filter.

Material: tungsten, brass. Lead or steel. Usually wedges are mounted at a distance of 15

centimeters from the skin surface. The sloping surface is made either straight or

sigmoid in shade. A sigmoid shape produces a straighter isodose

curve. Mounted on trays which are mounted on to the

head of the gantry.

47

Photograph of a 45* wedge for a 4MV linear accelerator

Isodose shift due to the wedge

Wedge angles used are: 60°, 45°, 30° & 15°.


49

Wedges can be of two types :- Individualized wedge- Universal wedge

Individualized wedge

Universal wedge

50

Wedged fields are generally used for relatively superficial tumors.

Beams are usually directed from the same side of the patient.

The broad edges of the wedges should be aligned together.

The wedge angle chosen depends on the angle between the central rays of the two beams also called the “hinge angle”(φ).

Wedges:◦ Reduce the hot spots at the surface ◦ Rapid dose falloff beyond the region of overlap.

The overlap region is also called the “plateau region”. Thus the 2 factors on which the wedge angle is chosen are:

◦ The hinge angle.◦ The wedge separation.

The wedge angle that will make the isodose curves parallel to each other and the hinge angle bisector is obtained using the equation.

Hinge angle = 90-Wedge angle/2


52

WEDGE FILTERS

Problem Solution

A different value of wedge angle is required for different beam angles

A small range of hinge angles can be covered by a given wedge angle without producing significant variation(±5%).

The body contour can be more curved as a result, the isodose curves are not obtained in the manner desired.

Compensators may be used to overcome the deficit or a different wedge angle can be used, so that part acts as a compensator.

FLATTENING FILTERS A beam flattening filter reduces

the central exposure rate relative to that near the edge of the beam.

Used for Linear accelerators. Due to the lower scatter the

isodose curves are exhibit “forward peaking”.

The filter is designed so that the thickest part is in the centre.

Material: copper or brass.

54

A tissue equivalent material used to reduce the depth of the maximum dose (Dmax).

Better called a “build-up bolus”. A bolus can be used in place of a compensator for

kilovoltage radiation to even out the skin surface contours.

In megavoltage radiation bolus is primarily used to bring up the buildup zone near the skin in treating superficial lesions.

BOLUS

55

The thickness of the bolus used varies according to the energy of the radiation.

In megavoltage radiation:◦Co60 : 2 - 3 mm◦6 MV : 7- 8 mm◦10 MV : 12 - 14 mm◦25 MV: 18 - 20 mm

Properties of an ideal bolus:◦ Same electron density and atomic number.◦ Pliable to conform to surface.◦ Usual specific gravity is 1.02 -1.03

56

Commonly used materials are:◦ Cotton soaked with water.◦ Paraffin wax.

Other materials that have been used:◦ Mix- D (wax, polyethylene, magnesium oxide)◦ Temex rubber (rubber)◦ Lincolnshire bolus (sugar and magnesium carbonate in

form of spheres)◦ Spiers Bolus (rice flour and soda bicarbonate)

Commercial materials:◦ Superflab: Thick and doesn't undergo elastic

deformation. Made of synthetic oil gel.◦ Superstuff: Add water to powder to get a pliable gelatin

like material.◦ Bolx Sheets: Gel enclosed in plastic sheet.

57

Superflab bolus Material

58

CONCLUSION Beam modification increases conformity allowing

a higher dose delivery to the target, while sparing more of normal tissue simultaneously.

Megavoltage radiotherapy is better suited for most forms of beam modification due to it’s favorable scatter profile.

59

The Physics of Radiation therapy by Faiz M Khan Radiotherapy Planning by Gunilla C Bentel Practical Radiotherapy Planning by Dobbs, Barrett, Ash www.rpdinc.com A contouring device for use in radiation treatment planning , Hector C.

Clarke, M.Sc. Department of Radiology, The General Hospital, St. John's, Newfoundland, 1969, Br. J. Radiol, 42, 858-860

REFERENCES

http://www.rpdinc.com/

60

THANK YOU !!!!

Education

Patient contouring and beam modifying devices