Characteristics of a Radiation Beam and Field

ONCOL 355 - Tx Planning and Dosimetry. University of Alberta

Co-60 half-life and photon energies

half life = 5.27 years

photon energies: 1.17 MeV and 1.33 MeV

Why is it important to know the information about half-lives and activity for radiaiton therapy

it is important to choose the most appropriate source for the procedure and to ensure patient safety and effective treatment planning. Understanding these factors helps in predicting dose delivery and optimizing treatment schedules.

When treating with a radioactive source, we calculate a treatment ….

a treatment time

do we calculate a treatment time with LINACs?

no, because of how the x-rays are produced in the linear accelerator, we calculate a monitor unit, not a treatment time

three characteristics of radiation beams

1. beam divergence+ central axis

2. inverse square

3. penumbra

Beam Divergence and Central Axis

As radiation comes from point source, it spreads out after distance.

• This is what divercence

The very central ray is not diverging, but further out from field the angle of divergence increases

- Will diverge in all dimensions

Will the centre ray always be in the exact center of the field?

no, if the field is not symmetrical, the centre ray may not always be in the exact centre

What is the inverse square law

the intensity of the beam varies inversly as the square of the distance from the source

Inverse square law formula

Penumbra definition

the region at the edge of the radiaiton field over which the dose rate changes rapidly at as a function of distance from the beams central axis

• the amount of divergence increases further than the central axis, those rays are travelling further than the central ray, thus there is a region where the dose rate is less than what it is at the central axis

The intensity of radiation across a field isn’t ____ and then ____ at the field border

100%, 0%

Geometric penumbra

the term used when referring to the area of the edge of the field which lacks complete dose

• we don’t have a full dose, and we don’t have none

what type of photons create geometric penubra

primary photons only

• not scatted or secondary photons

  • just photons diverged from the source

What is the geometric field size

the size of the treatment field = 50% field dose line

why is penumbra not a great ting?

it gives dose beyond a treatment volume, thus it may encroach onto a critical organ

• produces a blurred effect at the edges of the field and on treatment images

• tumor should not be in this region

formula for geometric penumbra

the larger the source size, the ____ the penumbra

larger

the larger the SSD, the _____ the penumbra

larger

the larger the SDD, the ___ the penumbra

smaller

are photons direcly or indirectly ionizing?

indirectly ionizing

• Photons travel through patient before transferring energy to electrons in patient which cause the damage

electronic equilibrium definition

the point at which the number of electrons leaving a volume equals the number entering it

Where does electronic equilibrium occur

at dmax

• the depth at which 100% of dose is deposited

• when EE occurs, dmax is acheived

Dmax for 6 MV

1.5 cm

Dmax for 10 MV

2.5 cm

Dmax for 15 MV

3.0 cm

What two factors determine Dmax?

1. beam energy

2. field size

what is the advantage of a deeper dmax

the skin sparing effect

• if max dose is deposited below the skin, we can spare the skin from the higher dose that occurs at dmax

what happens after Dmax?

electrons continue to travel deeper into the tissues depositing their doses at various depths

as beam energer increases …

the penetration of the ability of the beam increases, and there is decreased absorption at shallower depths

what do we use to visualize dose depostion/attenuation

isodose charts

• a locus of points at equal dose value

three things isodose charts show

1. dose distribution on/off the central axis

2. fall off rate of a particular energy

3. shape of the isodose curves

why are the lines curved in isodose charts?

1. penumbra

2. scatter

3. beam divergence and inverse square

what effect to flattening filters have on isodose charts

Flattening filters create a smoother dose distribution across the treatment area, reducing dose variations and improving uniformity in the output of radiation beams.

• horns are also created as a side-effect

without flattending filter, isodose is more bow shaped

200 kVp orthovoltage isodose

Lots of dose going outside of field size

- Have lots of back scatter and field scatter with lower energies

- Must take OAR into consideration (such as eye for inner canthal treatment)

Co-60 Isodose chart

the larger the energy the ___ the penumbra

sharper

electron beam isodose chart

Barely gets through 4 cm of skin

Rapid fall off after 80%

Still get bulge underneath surface

- Large penumbra in electrons

Isodose curves are affected by:

1. energy

2. type of radiation

3. source size (penumbra)

4. SDD (penumbra)

5. SSD (penumbra)

6. flattening filter

7. shape of patient

8. patient inhomogeneities

9. normalization point

Geometric Field Size

Roughly passes through the centre of the geometric penumbra, and corresponds to the 50% isodose value at skin surface

what is the function of collimator jaws (secondary collimators)

they move to create various field sizes when there is no shielding used

how many sets of jaws

two sets of jaws: x and y

• Y₁ and Y₂

• X₁ and X₂

How is the field size defined with the SSD technique

it is determined at the treatment SSD (skin surface)

• used for single fields or extended fields

• isocentre is on skin surface/outside

How is the field size defined with the SAD technque

it is defined at the treatment depth (inside the patient)

• isocentre is within the patient, typically in the tumor volume

• field size on skin is irrelevant

when is the field defined as the collimator size?

• when the defined location is at the isocenter distance

• no shielding in treatment field

when is the field not defined as the collimator size?

• when extended SSD technique is used

  • have to make 12.5×12.5 to get a 15×15 at 120 cm SSD

• shielding is used in the treatment field

What is shielding?

physical devices put into the path of the radiation to block it from reaching the patient

why do we shield out some of the treatment field

to protect tissue/organs that don’t need treating or those that are critical structures and to shape the field

what three things can we used to create shielding?

1. lead blocks

2. cerrobend blocks

   • both these essentially obsolete in LINACS, used in ortho/e-

3. Multi-leaf collimators

   • main type of shielding used today

Pros and Cons of Pb

pro: good attenuator

Cons: high melting point, toxic, not practical for customization

Pros and Cons of Cerrobend

pros: low melting point, practical for customization

cons: toxic, custom blocks are time consuming

Divergent blocks

blocks that matched the divergence of the radiation beams

• the further the block was from the central axis, the more angled it needed to be because of increased beam divergence

Non-divergent blocks

striaght sided blocks

• this is what the lead blocks were

do divergent blocks or non-divergent blocks cause more transmission penumbra

non-divergent blocks

Transmission Penumbra

the region irraddiated by photons which have traversed part of the thickness of the collimator/shielding blocks

• side A shows a non-divergent block creating transmission penumbra

physical penumbra

the sum of the transmission penumbra + geometric penumbra

• this is the full penumbra the reduces the dose at the field edge due to a decrease in side scatter

what is the physical penumbra measurement

between 90-20% isodose lines @ dmax

clinical penumbra

between 90-50% isodose lines @ dmax

HVL

the thickness of an attenuator that reduces the intensity of the intensity of the beam to half of it’s original value

what does HVL depend on

• energy of the beam

  • larger energy = larger HVL

• type of attenuating material

5 HVL reduces the intensity of the beam to …

3.125% of its original value.

• this is the standard accepted level of transmission, so shielding blocks were made with 5 HVLs

Lead HVLs for various energies of lead

why is the HVL for 18 MV less than 15 MV

we get more pair production at 18 MV

HVLS for various energies using cerrobend

not as good as an attenuator as lead, thus the blocks had to be larger

although blocks are nearly obsolete, when would we use cerrobend and lead

field shaping in electron/ortho for superficial treatments

What are MLCs

tungsten leaves/rods which move independently in and out of the field to shape it

• located in the head of gantry after the collimator jaws

if there are 120 leaves, the machine is called caled ‘60 pairs’

are MLCs divergent or non-divergent

non-divergent

Advantages of MLCs over blocks

tungsten = good attenuator

don’t need to lift heavy block

no storage issues

not toxic

Disadvantages of MLCs compared to blocks

• jagged shaping causes squared off ends

  • not necessarily a problem, but needs to be accounted for

• not beam divergent (since the ends of the MLC leaves are rounded)

  • roundal shape and penumbra effects

• can experience mechanical isues

planning considerations that need to be taken into account with MLCs

• shaping considerations

• Non-Divergen MLC leaves

• single plane MLC system

• leakage between leaves

If there is leakage between leaves, how can this be fixed

fix with collimator jaws to adjust the beam margin

Primary Radiation

photons have not interacted with anything other than air before they reach the patient

scatter radiation

primary photons have hit something and interacted

• involves a change in direction of the particle

not useful for tx purpopses

is there lots of scatter with with primary photons once inside the tissue

Yes, once primary photons enter tissue, they scatter significantly, reducing the effectiveness of the beam.

how does scatter differ with field size

Long rectangular field scatter may not get back to the calc point like the 10×10 field will

What is the equivalent square

there is a square field for every non-square field size in terms of radiation characteristics

formula for equivalent square

What is the equivalent square for a (20×5) field

Eq square = (4x(20×5)/(2(20+5)) = 8 cm x 8cm

What happens to the equivalent square when we have shielding to a field

the volume of tissue irradiated with an open beam is different than the total volume of tissue irradiated when shielding is used to block out parts of the beam

• there is also a change in scatter in the treated volume

shielded area definition

the total area of all shielding within a field size

treated area definition

the remaining unshielding area within a field size

Effective field size

the equivalent rectangular field dimension of the treated area with shielding accounted for

Blocked equivalent square

the equivalent square field with shielding accounted for at the calculate depth

• this is the size used in our MU calculations for tissue absopriton factors

• see math problem on notes

% shielded

the ratio of the total shielded area to the area of the opening field

What is the Clarkson method

A technique used in radiation therapy for calculating dose distribution and patient geometry within a non-uniform field, accounting for the impact of irregular field shapes and patient contours.

• this accounts for the location within the field not accounted for in blocked equivalent square method

What ratio does the Clarkson Method use

the Scatter air ratio (SAR) is used to figure out what the effect of in field shielding has on the calc point

Describe how the Clarkson Method was used to calculate the equivalent square

• first the treatment field is divided into segments using radii

• every 9 defrees is measured from calculation point to shielding or a field edge

• using a SAR data table, each radius is looked up and an SAR is found at the depth of interest

• all the SARs are added up and then divided by the # of segments (Radii) to find the average SAR

• the field size with the same SAR is the equivlaent square is used in treatment calculations

when shielding is used we need to account for the change in the area being treated, and the subsequent change in scattering within the field. what needs to be done then?

the effective field size is converted to an blocked equibalent square, which is the size to be used in tx dose calculations