Test 3 Flashcards by Courtney W

4 ways electrons interact as they travel through matter

Inelastic collisions with atomic electrons
Inelastic collisions with atomic nuclei (bremsstrahlung)
Elastic collisions with atomic electrons (electron-electron scattering)
Elastic collisions with atomic nuclei

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Some of kinetic energy (KE) is lost producing ionization and excitation or converted to other forms such as Bremsstrahlung
More common in low Z mediums like water or tissue

Inelastic collisions

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KE is not lost, but it may be redistributed among particles emerging from collision
More common in higher Z mediums such as lead

Elastic collisions

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Rate of energy loss depends on electron density of the medium

Collisional losses (ionization and excitation)

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Rate of energy loss per gram per cm^2 is greater for low atomic (Z) number materials compared to high Z materials due to high Z materials having fewer electrons per gram compared to low Z materials
Also due to high Z materials having tighter bound electrons/higher BE

Mass stopping power

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Rate of energy loss of electrons of 1MeV and above water is about ___MeV/cm

2Mev/cm

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Probability of radiation loss relative to collisional loss _______ with electron energy and Z

Increases

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Equation for 90%, 80%, 50%, and the practical range (Rp) electron isodose lines

90% = E/4
80% = E/3
50% = E/2.5
Rp = E/2

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Increased field size (FS) leads to _________ scatter from collimator as well as the phantom

Increased

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Increased FS = _______ PDD

Increase

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Increase FS = depth of Dmax shifts toward the _________

Surface

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After passing through vacuum window, bend magnet, scattering foil, monitor chamber and air column, the electron beam appears to diverge from a point
Point where electrons start to diverge
3 cm when they go through accelerator, point after scattering foil closer to patient
Close to patient and further from head of machine than photon source

Virtual source (VS)

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3 things electron beam energy selection is dictated by

Depth of target volume
Minimum target dose required
Dose to normal tissue

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Beam obliquity = ________ side scatter at Dmax depth

Increased

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Beam obliquity = shift of Dmax towards the __________

Surface

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Beam obliquity = ________ depth of penetration

Decreased

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Electron correction factor/effective thickness for tissue inhomogeneities related to stopping power and depends on energy and depth

Coefficient equivalent thickness (CET)

Electron density

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Effective dose (Deff) formula

Deff = d1(CET) + d2(CET) d3(CET)

d = measured depth

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CET of spongy and compact bone and lung

Compact = 1.65
Spongy = 1
Lung = 0.2-0.33

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3 purposes of bolus

Flatten out irregular surfaces
Reduce penetration
Increase surface dose

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When an electron field is abutted to a photon field, a hot spot develops on the side of the _______ field and a cold spot develops on the side of the _______ field

Photon, electron

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Rule of thumb for electron lead cutout field shaping devices

1/2 the energy + 1mm

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3 situations the require internal shielding during electron treatments

Lip
Buccal mucosa
Eyelids

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While lead can be a good stopping medium, it can cause backscatter; to eliminate the effect backscatter, a ____-Z absorber is placed between the lead and preceding tissue

Low-Z

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3 total skin irradiation (TSI) techniques

Transitional Large field/Stanford Modified Stanford

Patient lies on a motor driven couch and is moved in a downward motion or the patient is stationary and the radiation source is translated horizontally

Transitional technique

Large electron fields can be produced by scattering electrons through wide angles and using large treatment distances Patient is treated in a standing position with 4-6 fields equally spaced around the patient X-ray contamination becomes a limiting factor

Large field technique

Uses 6 fields spaced 60 degrees apart (AP/PA and four obliques) AP and two obliques, PA and two obliques

Stanford technique

2 treatment planning algorithms

Pencil beam | Monte Carlo

Algorithm assumes a collimated photon beam striking a patient is a collection of many smaller, narrow pencil beams These pencil beams have a central axis where it deposits dose which varies with intensity and spectrum of beam

Pencil beam

Algorithm takes into account millions of interactions (Co, Pho, and Com) which lead to more electron interactions Large statistical probability calculation More accurate dose calculation algorithm, but very time consuming due to number of statistics it must consider

Monte Carlo

Gap calculation formula

(1/2)(L1)(d/SSD) + (1/2)(L2)(d/SSD)

Distance that's equivalent to that measured in water | Distance x equivalent thickness

Equivalent thickness/path

Same tissue density

Homogeniety

Different tissue density

Heterogeneity

Maximum range obtained by electrons incident on the surface

Practical range (Rp)

Electrons have _________ block margins than photons because of scatter and penumbra

Wider

Increase electron energy = _________ skin dose and dose at depth

Increase

Electron Dmax is a __________

Range

4 electron PDD curve characteristics

Buildup Range Fall-off Photon contamination tail

2 causes of photon contamination

Head of machine (majority from high Z material) | Patient

For head and neck (H&N) treatments; treat with photons until cord tolerance is reached, then treat with electrons of cord so they fall off before reaching cord depth and still get dose to LNs

Posterior triangles

What is a treatment that commonly uses a bolus?

Chest wall

Increasing or decreasing the dose at a given percentage because electrons are prescribed to certain isodose lines, usually 90%

Normalization/scaling

98% scaling means a ______ increase

More oblique beam ________ skin dose

Increases

5 electron applicator/cone sizes

``` 6x6 10x10 15x15 20x20 25x25 ```

What are the different components in the linac in photon (2) versus electron (1) mode?

Photon: target, flattening filter Electron: scattering foil

2 factors electron output factor varies with

SSD | Cone/applicator size

Electron MU formula

TD/output

What is the typical electron SSD and blocking tray distance, and why?

SSD: 105 cm Blocking tray: 95 cm Since there's only 5 cm between patient and cone, extend to 105 cm so patient doesn't get hit

Do electrons follow the inverse square law (ISL) and why?

They don't follow the ISL because they repel each other

Most useful electron energies are between ___ and ___ MeV

6 and 20 MeV

The short, well-defined range of electrons makes them advantageous for treating superficial tumors at a depth of _____ cm or less and if we tried to treat past this, we'd burn the skin to get that deep

5 cm

Are electrons mono- or polyenergetic?

Monoenergetic (MeV)

Are electrons treated SSD or SAD?

SSD

Setup by looking at skin surface/scar wire; don't use imaging (IGRT) because electrons are superfiecial

Clinical setup

Small blocks put into end of applicator that shapes electron field ports

Electron cutouts

Increase cone size = _______ output factor

Increase

Relationship necessary block thickness formula; lead sufficient to completely stop electrons but some x-ray contamination may penetrate the cutout

tPb(mm) = 0.5E0(MeV) + 1

For the same transmission as lead, cerrobend cutouts needs to be a little bit thicker; thickness of cerrobend in millimeters (tC[mm]) formula

tC(mm) = 1.2tPb(mm)

What is the purpose of internal shielding?

Protect internal structures with lead and wax

Electron beams bow _______ because they're negatively charged and scatter more

Outward

Provides communication standards for sharing image information

Digital imaging and communications in medicine (DICOM)

Describe formats for the exchange of image or textual information

Information object definitions

6 information object definitions

``` Radiation therapy (RT) image RT dose RT structure set RT plan RT beams and brachytherapy RT treatment summary ```

Conventional and virtual simulation images, DRRs, and ports

RT image

Dose distributions, isodose lines, and DVHs

RT dose

Volumetric contours drawn from CT images

RT structure set

Text information that describes treatment plans, including prescriptions and fractionation, beam definitions, etc.

RT plan

Treatment session reports for EBRT or brachytherapy, may be used as part of a record and verify (V&R) system

RT beams and brachytherapy

Cumulative summary information, may be used after treatment to send information to hospital EMR

RT treatment summary

Match divergence from PA spine field (SSD)

Collimator angle

Accounts for divergence from lateral cranial fields (SAD)

Couch kick

Inverse tangent (tan^-1) formula

tan^-1 = opposite (o)/adjacent (a)

Measured depth

Physical depth

Effective depth formula

(d1)(Pe1) + (d2)(Pe2) + (d3)(Pe3)

TAR method correction factor (CF)

``` CF = TAR(effD,FS)/TAR(physical D,FS) CF = hetero dose/homo dose ```

Therapy that delivers non-uniform exposure across the radiation field using a variety of techniques and equipment; CT and tell computer treatment goals with DVH

Intensity modulated RT (IMRT)

IMRT has _____ MUs than 3D treatment planning because it modulates the whole time while 3D field is open the whole time but IMRT is more ________

More, conformal

Five or less beams per fraction

Stereotactic

Cranial treatment has less fractions, delivers a large dose of radiation on a single day

Stereotactic radiosurgery (SRS)

Body treatment from cranium down

Stereotactic body radiotherapy (SBRT)

Delivers a large dose of radiation on a fractionated treatment schedule

Stereotactic radiotherapy (SRT)

Sequence of leaves moving for repositioning, then coming to rest while beam's delivered in multiple segments at each gantry angle

Step and shoot/segmental MLC (SMLC)

MLC moves from one side of field to another within a narrow opening while beam's on, more MUs because beam's staying on the whole time

Sliding window (IMRT)

Rule of thumb for wedge placement

15-20 cm away from patient or they'll get too much scatter

Scatter comes off wedge/compensator, closer to patient ______ skin dose

Increases

2 dose at tissue interfaces

Re-dmaxing | Bone and tissue

When going through lung, why would you rather use a 6X than 18X?

``` Redmaxing effect (scatter in = scatter out) 6X used for lungs because their Dmax is shallower, builds up in tumor since there's not scatter equilibrium in lung/air The smaller the lung tumor, the more important it is to use a lower energy ```

Until you get to water, don't have backscatter to build-up to Dmax; nothing to build-up against in air Goes through air without interacting and has to build back up

Re-dmaxing

Why is 18X not used for IMRT?

Neutron contamination begins to occur at 10 MV and IMRT uses more MUs, which increases the chance of neutron contamination Neutrons want to combine with patient/hydrogenous material; weighting factor = 10, more biologically damaging

Point through tissue/water before bone would have higher dose because backscatter increases; point in tissue/water after bone would have less dose because of shielding effect

Bone and tissue interface

Beam goes through bone and has to re-dmax in water/tissue so it has less dose

Shielding effect

CT number/Hounsfield Unit (HU) formula

HU = 1000(ut-uw/uw) ``` ut = linear attenuation coefficient (LAC) of tissue under analysis uw = LAC of water = 1 ```

Intensity after half value layer (HVL) formula

Ix = Ioe^-ux ``` Ix = intensity after filtration Io = original intensity u = LAC per unit length x = filter thickness ```

Mass attenuation coefficient formula

u/P P = density

Percent transmitted formula

Ix/o = e^-ux

HVL as a thickness formula

0.693/u

Number of HVLs formula

Ix/Io = (1/2)^n