motion managment Flashcards
how to measure motion of tumour and OARs
4DCT data
-fan beam
-3DCT recorded over multiple seconds
how is it done
CT sim by either
prospective acquisition or retrospective acquisition
prospective acquisition
-images collected at only one phase of respiratory cycle
-decided in advanced
-taking multiple breathes and each phases are added together
-only time we use linear CT anymore
retrospective acquistion
scan data acquired at all phases of respiration(continous scanning)
correlation or registration of image w/ the phase is conducted after data has been acquired
most common
uses rpm signal
features of 4DCT
-longer than conventional by 5fold
-tf more dose
-requires a signal eg rpm block
possible to use CBCT instead of CT
-still free breathing
-instead of axial slices being rebinned , the volume projections are being rebinned
motion of :
top of lung
diaphragm
-smallest motion
-largest motion
motion direction
-90% sup/inf
-10% ant/post
more time spent on exhale than inhale
when is tumour most stable
max expiration
slow CT v free breathing CT
-slow CT has blurred images → large errors in delineation
-free breathing CT creates MIP (maximum image projection) +. blurring↓
MIP
adding different phase intensities and used for target volume
MIP pros
-1 dataset to contour dataset , not 10
-captures 3D range of motion, not just ex and in
-good for lung tumours against background lung
MIP Con
-hard defining ITV for tumours in chestwall and diaphragm
-artifically ↑ all CT values
-provides data for only one structure
Avip
-takes the average of different intensities
-used for everything but the target volume(egOAR)
-reduces the am of contouring and delineation
RPM
-infra-red camera on wall
-only need one camera as rpm block moves in 2 directions
phase gating
allows automatic gating acq. and tx delivery based on same phase of patients respitatory cycle
aka motion of block and pattern its showing
amplitude based gating
allows auto gating based on position of marker block on patient , regardless of respiratory cycle
aka when block is at particular location … gating is triggered
amplitude based gating features
-better at suppressing motion artefacts
-shorter tx time
duty cycle
beam on time ÷ total tx time
ie how much of breathing cycle is the beam on for
if the duty cycle is 100%
beam always on and tumour always in field
duty cycle for:
non-gated treatments
step and shoot IMRT
dynamic IMRT
-100%
-LESS THAN 30%
-50%
if theres an increase of beam on time
this ↑duty cycle which then ↑ residual motion
if duty cycle is small
tumour wont move alot
there is a compromise between
duty cycle (effeciency) and residual motion (normal tissue irradiation)
real time-tracking method:electromagnetic field
=calypso system
surgically implanted EM transducer (gold seeds)
cyberknife
-600 or 800 cGY per min
-secondary collimnator =radiation cone (between 4 to 60mm)
-6MV
