Test 2 Flashcards
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
Isodose lines are usually normalized in reference to the prescription dose
Absolute dose
Isodose lines are given in percentages relative to the prescription dose; 105%, 100%, 90%, etc.
Relative dose
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
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
Dose variation across the field while staying at a specified depth
Beam profile
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
Another way of depicting dose variation across a field is to plot isodose curves in a plane __________ to CA
Perpendicular
Most common tool to measure isodose curves
Ion chamber
6 parameters of isodose curves
Beam quality/energy Source size Beam collimation Field size SSD SDD
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
Source size, SDD, and SSD affect penumbra by virtue of __________ penumbra
Geometric
Increase source size = ________ geometric penumbra
Increase
Increase SSD = ________ geometric penumbra
Increase
Increase SDD = ________ geometric penumbra
Decrease
A smaller field size(FS)/collimation eliminates more scatter, so dose at depth ________
Decreases
Makes a more forward peaked beam and has a hardening effect
Flattening filter
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
3 accelerators that don’t need a flattening filter
Radiosurgery machines: very small field sizes
Tomotherapy
Modulated fields
3 advantages of flattening filter free (FFF)
Higher dose rate
Less side scatter outside the field
Shorter treatment times
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
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
2 types of wedge filters
Physical
Nonphysical
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
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
