3B Equipment - Beam Divergence & Modifying Devices

What is field size always set to

Isocenter

Beam divergence formula

Direct proportion (A/B = a/b)

As you get farther from the source, the divergence & field size increases

Half-beam blocking

When either the X or Y jaw is closed all the way

Beam will diverge where the jaw is open, but at central axis has a flat edge and doesn’t diverge

What’s the max field size you can have with a half-beam block

20

What is half beam blocking good for

Good for avoiding nearby critical structure or avoiding radiating previously radiated areas

Common for breasts

Types of beam modifying devices

Field blocking & shaping devices

• Hand blocks

• Custom blocks

• MLCs

Bolus

Compensators

Wedge filters

Methods of beam blocking

Non-divergent blocks (Hand blocks)

Custom blocks (Divergent blocks)

MLCs (Divergent leaves)

Hand blocks

• Can be divergent or non-divergent, but mostly non-divergent

• Lead hand blocks are at least 5 HVLs thick and will transmit 3% of the dose

• Might not be large enough to block the entire field

• Not custom

To cut a custom blocks, what measurements do you need to cut with divergence

FFD (focus to film distance) → determines magnification

TTD (target to tray distance)

What are custom blocks composed of

Made with Lipowitz’s Metal (Cerrobend)

• 1. Bismuth

• 2. Lead

• 3. Tin

• 4. Cadmium

What does cerrobend vs lead melt at

Cerrobend melts at 158 F

Lead melts at 625.1 F

What needs to be done when pouring cerrobend and how do you check it

Must remove all air bubbles when pouring it

Check density by using a densitometer on various places on a film

Where should all blocking trays be placed

15-20 cm from patient surface to maintain skin sparing effect

What percent transmission is acceptable for custom blocks and how many HVL is needed

5% max transmission is acceptable

You need 4-5 HVL to get less than 5%

HVL equation

n = log (1/decimal transmission) / log2

To determine block thickness, what factors do you need to know

Transmission desired

Block material/density

Beam energy

What are the types of blocks

Positive and negative blocks

Positive vs negative blocks

Positive = cavities are cut in the styrofoam and filled with cerrobend

• Block is blocking out critical structure, rest of field is open

• Also called island blocks

• Looks like the shape of the critical structure

Negative = treatment target is open, peripheral area blocked

• Electron blocks are negative

• Blocks around the treatment area

What is the width range for MLC leaves

0.25 - 1 cm

How many and what kind of leaves can a linac have

52, 80, 120, 160

Rounded or flat ended leaves

What does the quality of the field definition depend on

The width of the leaves

What do the upper vs lower jaws determine

Upper jaws determine the length (y)

Lower jaws determine the width (x)

What are MLCs made of

Tungsten alloy

How thick are MLCs

6-7.5 cm

How much transmission through MLCs

Allows 2% or less transmission through leaf

3% or less intra-leaf transmission (between each individual leaf)

Advantages of MLCs

• No more lifting heavy blocks

• Field shaping is computer controlled, faster, and more accurate

• Reduce setup time and errors of block placement

• Dynamic motion of the leaves allows for advanced techniques such as dynamic wedges, IMRT, and Volumetric Modulated Arc Therapy (VMAT)

Disadvantages of MLCs

• Jagged field edges. Serious when blocking around small and critical structures located close to target volume

• Physical penumbra is larger due to lateral dose distribution near the field edge

• Blocking a part of the field within the open field or island block is difficult

• Matching of adjoining field is difficult

MLC collimation in Elekta/Varian & Siemens

Elekta & Varian - collimator moves in & out of the field along a linear path perpendicular to central axis

Siemens - collimator moves in an arc to follow the divergence of the beam edge

What is bolus

Tissue equivalent material used on skin surface to alter depth of max dose

What “tissue equivalence” qualities must bolus have

• Material with the same density as tissue (water)

• Same atomic # as tissue

• Same absorption properties as tissue

• Same scattering properties as tissue

What color / HU should bolus be on CT

HU should be around 0

Gray on CT

Does bolus increase or decrease skin sparing

Loss of skin sparing (greater skin reactions)

Where is bolus applied

To the entire field or areas of concern

Purpose of bolus

• Increase dose to skin

• Compensate for uneven skin surface or a lack of tissue

Types of bolus

.Decimal, Paraffin Wax, Vaseline gauze, wet gauze/towels, lucite, “Superstuff”, “Superflab”, Lincolnshire bolus

Ways to use bolus for electrons

• Eliminate skin sparing with low-energy electron beams

• The dose depth fall off can be changed by choice of electron energy and using bolus to decrease penetration depth

• Dose may be evenly distributed in irregular surfaces or within air cavities when applying bolus

Types of wedges

• Individualized = requires a separate wedge for each beam width

• Universal = single wedge used for all beam widths (used on linacs)

• Dynamic/Virtual = defined by the movement of the collimator (either jaws or MLC that will simulate a wedge) (most common)

What are plans are physical wedges used for

3D conformal plans

Where does more dose get through, the toe or heel of the wedge?

More dose gets through the toe

Purpose of the wedge

Wedges are used to reduce the chance of a hot spot

Causes a progressive decrease in intensity across the beam, resulting in tilting the isodose curves from their normal position

Hot spot

When 2 beams intersect at an angle of less than 180 degrees, a hot spot will occur

Materials a wedge can be made from

Tungsten, brass, lead, steel

What does a wedge angle refer to

The tilt of the isodose curve, not the actual angle of the wedge filter

Do wedges decrease or increase the output of a machine

Wedges decrease machine output, requiring larger MU

2 main uses of wedges

1. Compensate for a lack of tissue

2. Alter isodose curves when two beams are intersecting an angle of less than 180 degrees (hinge angle)

What kind of tumors are wedges often used for

Commonly used on non-centrally located tumors (superficial or offset)

How should you place wedges

Place wedges with heels together

Ensure they are not reversed, that will create a greater hotspot

Should be placed 15-20 cm from skin surface

Wedge/Hinge angle formula

Wedge angle = 90 - (hinge angle/2)

Common wedge angles

15, 30, 45, 60

What moves in a virtual/dynamic wedge to get the desired wedge angle

Independent jaw moves to generate the wedge angle

How do fields open/close in virtual/dynamic wedge systems

Fields can start open and one jaw will shut

OR fields start closed and one jaw will open

Heel will always be closed the longest

How determines the angle of the sloping isodose at 10cm for virtual/dynamic wedges

Speed of the jaw movement determines the angle of the sloping isodose at 10 cm

Fast-closing = thinner wedge

Slow-closing = thicker wedge

When are custom tissue compensators used

When the body surface is irregular in length and width

What materials are tissue compensators

Copper, brass, lead, lucite

How are 3d compensators made

Now, based on CT image

Back in time, they used a pointer along patient skin which records depth measurement of missing tissue. Device then hollows styrofoam to be filled with cerrobend for tissue

How thick do electron blockers need to be

Thickness of lead in mm equal to ½ the incident energy of electrons in MeV

Ex: For 6 MV machine, lead thickness of 3mm

Typically 1-2 cm thick

Do proton blockers need a thinner or thicker blocking that electrons

Thicker

What are the purpose of electron cones

To prevent electron scatter (also why they are so close to the skin)

How big are electron cones

95 or 100 cm cones

What should cones be marked with

Type of machine

Distance of the cone

Field size of cone

Collimator opening of cone

What’s the max field size with an electron cone

25

Are electron cutouts positive or negative blocks

Negative blocks

Do proton beams have MLCs or Arcs

Neither

Where do proton beams come from and how

The proton beams come from the accelerator as a few millimeter diameter beam and enters the nozzle

What is the nozzle on a proton machine

The nozzle is after the accelerator and is what delivers the beam to the patient and monitors the beam uniformity, alignment, and dose delivered

The head of the unit

What is the snout of a proton machine

After the nozzle, where the beam exists to enter the patient and is only used with scattered beams

Two methods for delivering a protein beam through a snout

Passive scattering

Active scanning (pencil beam scanning)

Bragg Peak of a pencil beam vs passive scattering

Pencil beam has immediate fall off, is a straight peak

Passive scattering has a spread out, wide peak

How does passive scattering occur

By placing 2 filters or scatter filters to spread the beam

The aperture will shape the beam liken MLCs would

Compensators put in place to change dose distribution

How does pencil beam scanning work

Magnets used to move the beam as it paints the dose over the tumor one layer at a time

Energy of the beam is changed to paint the tumor at different depths