ARC Flashcards
step and shoot
-beam is off in between movements tf less radiation leakage
-easier to deliver and plan
-more time consuming
dynamic IMRT
faster
has smooth intensity modulation
beam on throughout—-leakage ↑
more susceptible to tumor motion
another QA needed for MLC motion accuracy
what do you need for VMAT
- CBCT imaging system with soft tissue matching
- A high number of MLC leaves (122)
- Arc Planning software
- Modifications to the gantry
Control IMRT
dynamic dose delivery defined as segments/subfields
* Fixed aperture shape
* Fixed gantry angle
* Fixed number of MUs
superimposed field segments:
step and shoot
fixed gantry
dynamic MLC
sliding window
fixed gantry
tomotherapy
fan beam
non-linear
-binary leafs
dynamic gantry
VMAT
dynamic gantry
leaf motion between adjcent angles is limited by
leaf travel and gantry rotation speed
vmat collimator angles
-variable
-more freedom
- reduce leakage in stationary leaves
VMAT MLC shape
variable
-as the gantry rotates and the MLC moves ,the bealmets of the adjcent beam angles must be similar
ie smooth transition and small distances to travel
Vmat dose rate
-variable
-if the dose rate required is to high or low the gantry speed can be adjusted to an extent
gantry speed
-variable
-only tolerate small and smooth variations because of its weight
-up. to 1 rev per minute is irrelevant
interplay occurs in
any dynamic MLC tx
happens in lung
interplay is
relationship of motion of MLC and motion of lung
if these motion coincide the interplay ocurs
when is interplay likely to be a problem
smallno.of fractions
if. theres large no. , interplay artefact is averaged out
excessive modulation =
more uncertianity
can lead to differences in planned dose and delivered dose
Modulation of the hardware creates
*Dose calculation uncertainty ↑
* Mechanical uncertainty ↑
overall IMRT:
plan
simplish
single arc
fewer MLC movements tf less wear on machine
overall IMRT:
plan quality
dose conformity and tissue sparing improve
VMAT similar plan quality
overall IMRT:
tx time
reduced
-↑ comfort
-↓ external motion
-can be used w FFF to ↓ tx time more
overall IMRT:
MU
30-60% reduction
tf reduced:
secondary cancer
head scatter
MLC scatter
out of field dose
integral dose
