Tx planning info Flashcards
incr fs = ? Sc & Sp
incr
fs output based on
open fld equiv square
Sc (CSF)/Sp (PSF) involves
blocking
gap calc e- can abut separated only by width of
line
fld size correction factor accounts for difference in ? with diff cone sizes & field sizes
output or dose rate
incr E= ? geom penumbra
decr
TAR only for what type of tx
SAD or iso for 4MV or Co-60
TMR only for
SAD & isocentric for 6MV & up
isodose curves shows what variaion
2D
more depth= ? uniform & symmetrical
more-b/c incr scatter
smaller beam=? flatness
poor
larger fs= ? dmax
smaller
hot spot is considered ?
@ least 2cm^2 (khan)
dose @ any depth greatest @
CAX
isodose shift measures ?
air gap or extra tissue
wedge pair is 2 fields separated by less than? deg
180deg
4 fld pelvis has ? target dose
higher
4 fld diamond lowers dose where on pt
@ side of pt
arc therapy is how much of a rotation
less than full rotation
adding wedge reduces ?
hot spot
wedges do not alter CAX of what E
Co-60 or 6x
tissue inhomogeneity is inversely dependent on
photon E
equal distribution is
unequal weighting of beams to get even distrib of dose/hotspots
120% < D90 < 130% if dose is 160Gy means?
90% of ptv should receive b/n 192-208 Gy
weighting is used for asymmetric
volume
normalization relates a series of #s to
specific value
non-coplanar is used to produce ? distribution
more uniform distrib & avoid healthy tissue
conformal therapy is the usage of ? fields
MORE THAN 4 FIELDS w/a lot of blocking
cumulative DVH is the plot of volume of a structure receiving ?
certain dose or higher
differential DVH is a plot of volume receiving a dose within
a specific dose interval as a fxn of dose
obstacle in achieving optimal dose distrib is adequate knowledge of
tumor extent
optimization requires
fld apertures, beam weights, beam directions, modifiers, # flds
registration is correlating different image sets to
identify corresponding structure region
beam aperture is beam directions that create greater
separation b/n targets & critical structures
correction-based algorithm is limited for
3D heterogeneity correction in lungs & tissue interfaces
model based algorithm computes
dose distribution w/a PHYSICAL MODEL
convolution superposition is how accurate
most accurate model based algorithm
Clarkson method accounts for ? contribution determined by?
-scatter contribution, by avg SAR corresponding to each radius
ICRU stands for
-int’l commission of Rad Units & Meas
tx of superficial tumor with extended depth of 7 cm is
wedge pair technique
with adjacent flds for deep tumors need ? on skin
-separation; to enable junction or overlay @ depth
adjacent flds for superficial tumors can be ? on skin
abutted
primary reason to tattoo pts
to locate previous tx flds
advantage of universal wedge
beam widths
wedge pair assoc w/hot spot with larger ?
field size
ssd advantage over sad enables
fixed gantry location on the # of fields
pt with kyphosis beam is directed toward
head
upper extremity tx POP- arms are ? to axis of gantry
parallel to axis of gantry or rt angle to beam
hot/cold spots common with what technique
wedge pair
max & min tumor dose to surrounding tissue increase #
portals
to deliver highest dose to tumor & minimal to surrounding tissue, have Dmax dose centered over
tumor volume
isodose curves represent ? variation of absorbed dose
volumetric or planar variation; @ diff levels
increasing geometric field size makes dose build up
higher
first hvl of a heterogeneous x-ray beam is usually less than
2nd (thinner)
divergent blocks ? potential of partial transmission vs non-divergent block
decreases
cerrobend blocks are typically how thick
7 cm
field size ? if dose rate increases due to ?
increases; scatter almost entirely dependent on field size
a 10x10 field around a calc point will produce ? scatter than a 2x50 field
more
absorbed dose rate decr w/incr ?
distance
interactions more concentrated when distance is ?
increased
the size of the inhomogeneity will affect ?
dose
wedge angle is the angle through which an isodose curve is
tilted @ cax @ specific depth
the side of field where starting position of moving leaf is located will receive higher dose than side where final position is located is called
dynamic wedge
correcting isocharts for irreg surfaces with photons is
2/3 of air gap
-depth @ which max dose or equivalent occurs is ? as fs is incr
reduced
single fld tx photons have a dose range w/i target of +/-?
5%
wedge flds are used in tx of what depth tumors
small shallow tumors
in an indep jaw, higher/lower dose is delivered where jaw was first set
higher
when ssd becomes ? or depth where beam sides converge increase, size of gap will be ?
shorter, larger
-bentel fig 6.56; 6.46 pf 143/pg 153
proper gap shift is achieved by ? size of 1 field & ? size of adjacent field
incr, reducing
decr E = ? geom penumbra
incr
Sc is based on open
equivalent square
Sp is ? collimator setting
effective
Sp is based on blocked
equivalent square
PDD decr with ? depth
incr- b/c more attenuation
PDD comprises ? components
attenuation & inverse square
PDD falls less rapidly with ? E
high
TAR is ? at Dmax
backscatter
TAR is used for what kind of fields
-stationary isoctr & irreg flds- calc dose in block fld by breaking down into prim & scatter components
in TAR, calc dose in blocked fields by breaking down into components
primary & scatter components
TAR at Dmax is
1 or higher
TMR at Dmax?
always 1 @ dmax for any photon E but never >1
isodose curves are lines of
absorbed dose
isodose curves for high E appear
flatter & sharper
isodose curves display distribution in phantom by
joining points of equal absorbed dose
Clarkson Method is devised to calculate ? segments
separately calc scatter dose by dividing blocked field into 24 segments & finding SAR
Clarkson Method compares SAR to ? fields
circular
Clarkson Method corrects for
pt contour & blocks
rotational factors are used to find average ? for rotation fields
TAR
rotational factors use contour divided into ? segments
16-24 segments
isodose shift shifts curve ?
2/3 or 1/2 depending on beam E & machine
if air gap, isodose shift shifts lines ? in tissue
deeper
isodose shift factor ? as beam E incr
decr; b/c ISL & tissue attenuation
isodose shift direction of shift ? from tissue deficit
away
isodose shift direction of shift ? from tissue surplus
toward
parallel opposed fields shape
hour glass shape
wedge pair commonly used for
brain tumors, sinus tumors
wedge pair greatest depth is ? cm
9-10cm
wedge pair acceptable hot spot ?
10%
wedge pair disadvantage
hot/cold spots common
4 field pelvis reduces ? dose
entrance
arc therapy iso is placed
distal to tumor (past pointing-don’t do anymore)
wedges incr ? of xray beams
TAR
wedges can alter ? especially at large depths for high E photons
depth dose distribution
tissue inhomogeneity is directly dependent on ? of inhomogeneity
dimensions
tissue inhomogeneity is dependent on ? of inhomogeneity
depth
shrinking fields is used because dose must be ? to sensitive norm structures
decreased
weighting is used if the volume is not ?
midplane
weighting is used to remove unwanted
hotspots
normalization relates to a ?
standard
normalization uses ? approaches
4
an obstacle in achieving optimal dose distrib is pt ?
motion & organ motion & pt/organ displacement
an obstacle in achieving optimal dose distrib is adequate knowledge of PTV within ? isodose lines to Rx dose
95-105%
an obstacle in achieving optimal dose distrib is ? response of ?
biological response of tumors & norm tissue
beam aperture requires a ? cm margin b/n PTV & field edge because?
2 cm margin to assume better than 95% isodose
correction based algorithm consists of ? correction for contour irregularities, scatter corrections
attenuation
Clarkson Method corrects ? component
scatter; for pt contour & blocks
ICRU recommends regarding ?
-recommendations regarding dose uniformity, prescribing, recording, reporting photon therapy
IM is added to ? for what?
CTV for physiologic movements & variation in size, shape, & position of CTV
SM is added to account for
uncertainties in pt positioning & alignment of therapeutic beams
SM is added to what volume and not part of what volume?
added to IM or ITV & NOT part of CTV
neg blocks commonly used for what tx
h&n
pos blocks commonly used for what tx
lung
wedge pair assoc w/hot spot with larger
wedge angle
kyphosis pt- gantry & table kicked
90 deg rotation of couch & gantry perpendicular to torso
max & min tumor dose to surrounding tissue limit
field size
divergence reduces beam ?
intensity
does transmission reduce beam intensity?
no
divergent blocks ? penumbra vs non-divergent blocks
decrease
cerrobend blocks have ~ ? HVL
5
dose rate ? more rapidly in smaller fields
incr
as field size becomes ? dose rate stabilizes
larger
as field size ? absorbed dose is greater b/c of ? scatter
increases, absorbed dose is greater b/c of incr scatter
the density of the inhomogeneity will affect
dose
the E of the beam will affect ? of the inhomogeneity
dose
wedge angle degree of tilt changes with
depth
- tilt decr @ greater depth b/c of incr scatter
- refers to tilt of iso curve- NOT angle of actual wedge
wedge angle degree referes to tilt of ? not actual wedge angle
iso curve
in correcting isodose charts for irregular surface with low E
shift increases
in correcting isodose charts for irregular surface with high E
shift decreases
as dmax shifts ? to surface-dose on surface also incr
closer
obl incident beams also ? skin sparing
reduce
shift in dmax toward surface also caused by ? arising in coll & devices
secondary e-
dmax shifted toward ? when beam intercepts tx couch, immob device, clothing, etc
surface
unlike photons, actual dose for e- on skin ? w/higher E beams
increases
lower E beams in e-, dose falls off ? w/depth after it has reached 100%
rapidly
higher E beams in e- peak is ? sharp & dose remains close to 100%
less
in POP, when thickness of tissue is ?, ratio b/n mid-depth & dmax remain acceptable
small
in POP, as thickness becomes ? or beam E decreases, ratio increases
larger
in POP, when thickness is ? & extends beyond linear portion of %DD curve, dose near entrance/exit is ? than @ mid-depth
larger, higher
in POP, highest intensity line
pinches in
wedge angle needed depends on the ?
hinge angle
dose gradient ? as coll moves across the fld
decreases
in rotational therapy, most situations using ? E results in sup dose distrib
high
in rotational therapy, the rate @ which dose ? @ the POI moves toward the periphery depends on fld width & beam E
decr
in rotational therapy wide fld, fall off in periphery is less/more dramatic than w/smaller fld
less
in arc tx, when sector skipped, ? dose area is shifted away from skipped sector
high
in arc tx, it is necessary to ? in order to bring the high dose region back to target
past point
in arc tx, when 2 asym sectors are skipped, iso must be shifted ? larger sector
toward
in arc tx, when target off ctr, adv to tx only thru region when depth of iso is ? & to skip a sector where depth is larger
smaller
in adjacent fields, if abutting @ surface, there is ? @ depth
overlap
in adjacent fields, if there is a gap b/n flds, there is a (hot/cold) area in shallow region
cold
in adjacent fields, a wider penumbra makes dose (less/more) sensitive to small errors in fld sep b/c dose fall off is more gradual
less
in adjacent fields, the gap moved (dose?) to prevent overlap in same tissue
every 10 Gy
in adjacent fields, it is preferable to make ? in location where no tumor
separation
in adjacent fields, the field margin b/n ?
vertebral bodies
single fld tx photons have a max dose of < ?%
110
Absorbed dose depends on
- E of radiation
- material of mass
