Test 2 Flashcards by Courtney W

Family of isodose curves usually drawn at equal increments of percent depth dose (PDD), depth dose values are usually normalized in reference to the prescription dose
Ex: 100%, 90%, 80%, etc.

Isodose chart

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Isodose lines are usually normalized in reference to the prescription dose

Absolute dose

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Isodose lines are given in percentages relative to the prescription dose; 105%, 100%, 90%, etc.

Relative dose

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4 isodose line properties

Dose at any depth is greatest on central axis (CA) and decreases laterally away from the CA
Near beam edges the penumbra region exists
Near beam edges, the dose reduction is not only due to geometric penumbra but also from reduced side scatter
Outside the geometric limits of the beam and penumbra, dose is due to side scatter as well as leakage

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Lateral distance between 90-20% isodose lines at a depth of Dmax
Rapid falloff region of dose
Scatter only coming from light side

Physical penumbra

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Dose variation across the field while staying at a specified depth

Beam profile

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Coincidence of the light field and the 50% isodose line of the radiation field
Verified with QA test: marking the light field on radiochromic film, then exposing the film

Beam alignment

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Another way of depicting dose variation across a field is to plot isodose curves in a plane __________ to CA

Perpendicular

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Most common tool to measure isodose curves

Ion chamber

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6 parameters of isodose curves

Beam quality/energy
Source size
Beam collimation
Field size
SSD
SDD

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Higher energy carries dose deeper in a medium and is more ________ peaked
Lower energy has wider penumbra regions so isodose lines ________ out on the side

Forward, bulge

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Source size, SDD, and SSD affect penumbra by virtue of __________ penumbra

Geometric

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Increase source size = ________ geometric penumbra

Increase

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Increase SSD = ________ geometric penumbra

Increase

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Increase SDD = ________ geometric penumbra

Decrease

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A smaller field size(FS)/collimation eliminates more scatter, so dose at depth ________

Decreases

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Makes a more forward peaked beam and has a hardening effect

Flattening filter

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Beam at a depth of 10 cm with flattening filter; beam is within 3% across 80% of the field or 1 cm from the field edge

Flat

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3 accelerators that don’t need a flattening filter

Radiosurgery machines: very small field sizes
Tomotherapy
Modulated fields

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3 advantages of flattening filter free (FFF)

Higher dose rate
Less side scatter outside the field
Shorter treatment times

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Field size selection must always be made __________ rather than geometrically; a certain isodose should be selected to cover a field, rather than a predetermined __________

Dosimetrically, field border

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Caution should be used with small field sizes as a large portion of the field will lie within the __________ region; isodose curves tend to be _______-shaped
Ex: if there is 1 cm of penumbra on a given field, this is much more pronounced in a 5x5 cm field compared to a 20x20 cm field

Penumbra, bell

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2 types of wedge filters

Physical

Nonphysical

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Wedge shaped absorber that causes a progressive decrease in beam intensity, resulting in a tilted isodose line
Has more scatter to patient because it’s mounted outside treatment head; forgetting to place this leads to over-treating a patient

Physical wedge

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Single wedge serves for each beam width | 60 degree wedge used with relative open field

Universal wedge

Electronic filter that generates a tilder isodose line by moving a collimator jaw Superseded by IMRT technology (MLC) movement Varian: Enhanced Dynamic Wedge (EDW); Siemens': Virtual Wedge

Nonphysical wedge

3 advantages of nonphysical wedges

Automation of treatment delivery Less chance of user error Less scatter to patient: 15 cm minimum distance away from patient

1 disadvantage of nonphysical wedges

More effort for commissioning

Angle of isodose lines at CA at a reference depth of 10 cm Isodose curve angle at the central axis at a specified depth, ICRU recommends this depth to be 10 cm Dosimetrically a 45 degree angle Angle of isodose lines

Wedge angle

Ratio of dose with and without wedge, always less than 1

Wedge factor (WF)

WF _________ MUs in proportion

Increases

Require a separate wedge for each beam width Designed to minimize loss of beam output Physics labor intensive; must measure beam data for every small change

Individualized wedge system

_______ of wedge should be at border; if center of wedge oriented at CA, MUs __________

Toe, increase

3 criteria for using a single field

Target uniformity is within 5% Max dose to tissues in the beam is not excessive: over 110% Normal critical structures don't exceed tolerance dose

Simplest combination of two fields

Parallel opposed fields

3 advantages of parallel opposed fields

Simplicity and reproducibility of setup Homogeneous target dose Less chance of geometric miss

1 disadvantage of parallel opposed fields

Excessive dose to normal tissue above and below tumor

All doses close to prescription; depends on patient thickness and beam energy and flatness

Dose uniformity

Increase patient thickness/diameter = _________ uniformity

Decreased

Increase beam energy = _________ uniformity because higher energy pushes dose further Lower energy has more entry and exit dose; higher energy carries dose through

Increase

Increase beam flatness in profile = _________ uniformity

Increase

Dose closer to the surface relatively compared to the dose at the midpoint/isocenter

Peripheral dose

The lower the peripheral dose/midpoint dose ratio (closest to 1), the ______ uniform dose distribution is Higher energies have _________ peripheral dose/midpoint dose ratios

More, better

Data shows that there is _________ biologic damage with using higher daily dose from one field, even though the total dose is the same

Greater

6 ways dose uniformity and normal tissue sparing can be achieved; treatment planning seeks to deliver maximum target dose while preserving the function of normal tissues

``` Appropriate FS Increasing the number of fields Beam directions Beam weighting Beam energy Beam modifiers ```

Distance from source to axis always remains the same

Isocentric techniques

2 types of isocentric techniques

Static beams | Rotational arcs

With static beams (IMRT uses computer-modulated MLCs), SAD of 100 remains constant but SSD varies with what?

SSD = SAD - depth

Beam moves continuously around patient, best suited for deep seated tumors Can be faster Go all the way around patient, use computer-modulated MLCs

Rotational arcs

3 contraindications for rotational arcs

Irradiated volume is too large External surface differs too much from a cylinder Tumor is far off center

Partial arcs have hotspots displaced toward the surface, so they should be aimed at a distance just beyond the tumor

Past pointing

Hot spot of up to ___% in the treatment volume is usually acceptable

10%

Hot spots often occur under the ______ edge of the wedge; however _____________ can occur with large hotspots under the toe

Thin, over-wedging

Wedges generally suitable when a tumor is ___-___ cm deep in tissue

0-7 cm

Purpose is to modify the shape of the isodose curves by changing the beam intensity across the field Most desirable feature is rapid dose falloff beyond overlap region Do not always have to match Used in breast treatments, larynx, etc. Heels go together

Wedge (filters)

Gross demonstrable extent and location of a tumor, delineation possible with imaging

Gross tumor volume (GTV)

GTV plus presumed tumor/microscopic disease

Clinical target volume (CTV)

Compensates for physiologic movements and CTV size, shape, and position variation

Internal margin (IM)

CTV and IM

Internal target volume (ITV)

Compensates for movement and setup uncertainties

Planning target volume (PTV)

Includes organs at risk plus a margin for movement

Planning organs at risk (PRV)

Represents the volume enclosed by the isodose line that covers the PTV adequately

Treated volume (TV)

Corresponds to the 50% isodose volume

Irradiated volume (IV)

Highest dose in the target volume that covers 2 cm^3

Maximum target dose

Lowest dose in the target volume

Minimum target dose

Value between minimum and maximum dose values in the target

Mean target dose

Most frequent dose that occurs in the target volume

Modal target dose

Area outside target that covers a volume of 2 cm^3

Hot spots

4 criteria for reference point that target dose should be specified and recorded at

Clinically relevant and representative of dose throughout the PTV Easy to define in a clear way Selected where dose can be accurately calculated Not in penumbra region or within a steep gradient

Lines passing through points of equal dose

Isodose curves

2 things physical penumbra is a function of

Geometric penumbra | Lateral scatter

Distance between the 50% isodose lines at Dmax

Field size (FS)

Field defining light should coincide with 50% isodose lines within 2 mm (+/- 2 cm)

Alignment

Hinge angle (HA) formula

HA = 180 - 2WA WA = wedge angle

2 components 2 beams have

Entry | Exit

The calculation point is at the _________ part because you want it to get the whole prescription

Thickest

3 beam modifiers

Wedges Dynamic wedges by jaws MLCs

Angle between two beams

Hinge angle (HA)

Increase HA = ________ WA; when beams spread out, overlap is not as bad

Decrease

Wedge over attenuates and now apex is cold

Over-wedge

Wedge ________ treatment time/MUs

Increases

SAD MU formula with WF

MU = TD / (Dfs x INV^2 x TAR x WF)

SAD POI formula with WF

POI = MU x Dfs x INV^2 x TAR x WF

Modulated fields (IMRT, blocking field in segments, etc.) __________ MUs

Increase

Increase number of fields = ________ uniformity in target

Increase

Uniform dose formula

Peripheral dose/midpoint dose = 1

WF affects _____, not doses relative to each other

MUs

Dose is hotter on ________ weighted side

Higher

Object has a very irregular shape, not square or circle, and amount of scatter is unknown; ex: mantle field

Irregular field size

Calculates irregular field sizes

Clarkson algorithm

Ratio of the scattered dose at a given point to the dose in free space at the same point

Scatter air ratio (SAR)

Total radiation formula used to find scatter

TARd,fs = TARd + SARd,fs ``` TARd,fs = total TARd = primary, no FS just Dfs SARd,fs = scatter ```

4 benefits of low MUs

Decreased treatment time Patient doesn't have to lay on table as long (mets) Uncertainty in dose plans with more MUs because of more leakage and scatter More economical

Peripheral versus midpoint dose (max/Rx) usually _______ 1 with one and two fields (uniformity)

Greater than

Increase energy and number of fields = _______ (max/Rx) = ________ uniformity

Decrease, increase

Increase patient thickness = _______ (max/Rx) = _______ uniformity

Increase, decrease

Sum of primary and scatter radiation, can measure total and primary radiation with ion chamber

Total radiation

Cumulative histogram, plot of target or normal structure volume as a function of dose Percent of volume dose, or percent of prescribed dose, or above Tells if organ is failing Want 100% of target to get all of dose without exposing other organs

Dose volume histogram (DVH)

Increased number of fields = _________________ dose

Spread-out

Wedge factor (WF) formula

Dose with wedge/dose without wedge