RT techniques Flashcards
summary of 3DCRT
medium @ treatment time
worst @ dose conformity and to OARs
3DCRT
pelvic RT tend to be 3/4 fields
localised the tumour and target and how they change shape, density and location
abdominal tends to be three fields
a couch angle an help assist in avoiding organs reducing dose to healthy tissues
conventional
limited number of fields
large volumes can be treated
immobilisation
margins added to CTV
MLC
vol-delineated
positional accuracy +/- 5mm
often co-planar
what is therapeutic index
higher the dose, the higher the probability of giving a tumourcidal dose. The gap between the TCP and NTCP is the therapeutic index, we want to kill the tumour whilst minimising dose to normal tissues
stereotactic
non-coplanar
precise VOI delineation
micro MLC
right set up margin
very small treatment volumes
precise positional accuracy +/- 1mm max
more field
high degree immobilisation
SABRT
high doses to small volumes, precisely defined
limited normal tissue in the high dose region
required mm accuracy localisation and immobilisation
extreme hypo fractionation = 1-5 fractions
what early stage cancers receive SBRT
prostate
liver
adrenal gland
lung
oligometastatic disease
prostate SGRT
total margin between GTV-PTV = 3mm
verification via implanted gold seeds
7+ non coplanar beams > high conformity of high dose region
5 fractions = best toxicity profile
describe PACE-B
36.25Gy in 5 fractions (7.25Gy/ fraction) = 85Gy conventional
planning target goals for prostate SGRT
99% of PTV receives greater than or equal to 999% of prescribed dose
max = 107%
oars to contour: rectum, bladder, femoral head, small bowel
DART
pretreatment plan is changed during treatment to account for observed anatomical changes in the target localisation or OAR
results in more account plan which is patient specific, changes are only implemented if within tol
plan of the day
library of plans created, with a best fit plan being used based on online imaging
three images created at planning
daily CBCT, most appropriate is chosen
improve target coverage and lowers bowel dose
replan
for more systematic changes, plan goes back to planning
plan changed to adapt to CTV changes on set
suitable fro random changes
online adaptive therapy
CBCT based autosegmentation of contours and automated plan allows for fully adaptive treatment
potential imaging for internal margin reduction
BEV
conforms and shapes the beam to the tumour, reducing toxicity and improving QOL
summary for VMAT
best @ dose conformity, dose to OAR and treatment time
VMAT
consists of 2/3 arcs per treatment
fewer arcs = less time
gantry and MLC move
dose rate varies
standard linac has a max of 600MU
if the gantry speed varies, this reduces the time
lateral fields are slower to avoid OARs
similarities in VMAT + IMRT
dose coverage, homogeneity, conformity rates for PTV
VMAT disadvantages
increased plan optimisation dose
extended 20% isodose
VMAT advantages
reduced treatment times
50% isodose volumes, slightly better than IMRT
reduced MU
IMRT
intensity varies across the portal, with more dose distribution in some area
modifying the intensity, has a relationship with the dose distribution
more beam angles
odd number of beams
POP rare
easier to avoid OAR
sharp dose gradients, sharp dose fall off
differences in IMRT
MLC define non uniform beam intensity in each field
more gantry angles
fields planned as intensity maps
fields separated into segments
composite dose may/may not be homogeneous
inverse planning
what is dose tracking
allows for an estimation is dose to be made across the structures, to determine whether a adaption is required
tracking the dose to VOI allows for beneficial adaptation
has the potential to improve accuracy by lowering exposure to OAR, reducing margin size and increasing conformity, so the structures receive the planned dose
improves therapeutic index, improved local control
techniques for respiratory motion
breath-hold
active breathing control
real time VOI tracking
gating
4D planning
higher dose to prostate =
increase in TCP
halcyon
FFF
higher dose rate
leaves have twice the speed
gantry is x4 speed
more patient friendly
work load friendly
what is step and shoot
when MLCs are static
series of static segmented subfields at each gantry angle, MLC position only changes during beam off
extends treatment time, easier to plan can be forward or inversely planned
sliding window
MLC’s constantly moving at each angle creating dynamic boomlets
auto dynamic MLC during beam on
leaves are more variable to pre-defined positions
inversely planned
limitations for IMRT
high target conformity: movement/error
risk of carcinogenesis
unexpected toxicity
clinical indications for IMRT
complex/irregular shapes
concave targets
near critical structures
near an area previous to RT
other applications for IMRT
local advanced
multiple sites simultaneously treated
different dose prescriptions at different sites
dose boost to TV with high recurrence risk (simultaneous boost for sub volumes )
deliberate dose in homogeneity
remove unwanted in homogeneous dose distribution
what is dose is for high risk to malignancies and prostate cancer
> 76Gy
VMAT (tomotherapy, halcyon, arc therapy)
high dose conformity, homogeneity
sharp dose gradients
sparing adjacent OAR from high dose regions
enables dose escalation (higher TCP and lower NTCP)
what is a concave target (nodal volume) perfect for
IMRT
