Introduction to Treatment Planning Module Flashcards
The Planning Process
Pre-planning considerations
Image Registration
target volume delineation
contouring
additional contouring
field set up
Optimisation
Dose Calculation
Plan evaluation
Pre Planning Phase
immobilisation (internal and external considerations)
use of contrast
Pre Planning Phase: Bolus
-tissue equivalent material that increases dose to skin
-counteracts w skin sparing effect from MV beams
also used for tissue irregularities
Image Registration
fusion of multimodality images with the planning CT scan to assits in target and OAR definition
OAR Contouring :
impact on
function
-dose calculation and plan quality in dosimetry
-creates dose volume histogram (DVH) info and predicts toxicity
OAR Contouring :
requirement for inverse planning optimisation process
IMRT
VMAT
driven by volumes
– Target and OAR relationship
-Competition of planning goals
– Sensitivity in optimisation
accurate imaging assists the contouring process
–Decrease in interobserver variability
– DVH calculation
– Greater confidence in predicting toxicity
Additional Contouring:
The PTV Eval concept
caused by expansions of CTV
breast
parotid
anus
post-void residual measurement PVR
-margin around a OAR to account for positional uncertainties
forward planning
place beams in RT treatment planning system
required decisions include :
angles of beams
wedges used or not
MLC configuration will be used to shape the radiation from each beam
when is forward planning used
tumour simple shape and not near any critical organs.
inversed planning
RO defines critical organs and tumour,
planner then gives target dose and important factors for each
Serial Organs
Parallel Organs
loses functionality if one of the sub volumes are damaged eg spinal cord , brain stem
loses functionality if only all sub volumes are damaged eg liver kidney
components of field set up
isocentre location
bolus
beam energy
beam aperture
beam orientation
technique
weighting
wedge
field setup: isocentre location
geometric centre of PTV
exceptions= half beam block techniques ,mono-isocentric technique
field set up
technique
single feild
Parallel opposed
junctioning and mono-isocentric
weighting
the relative contribution of beam to overall plan
beam Orientation
entry and exit dose has a huge impact on sparing critical structures for 3DCT
less relevant for IMRT
Beam Aperture
penumbra
sheilding
beam energy
linked depth of penetration ((effective path length)
skin sparing effect and % depth dose curves
wedges
tilts isodoses relative to their normal position at central axis
tissue compensator
achieves a uniform distribution in multi field technqiues
types of wedges
physical , dynamic and universal
what does eclipse TPS use
dynamic wedges:
may need to spin collimator thru 90 degreesin planning to make sure wedge direction is correct
Normalisation
point where dose is forced at 100% and dose everywhere is changed by the same ratio
also known as prescription point
plans are generally normalised at geometric centre of PTV
The isocentre and norm pt location will differ if
the isocentre is:
at the beam or block edge
in inhomogeneous tissue
Plan evaluation
What needs to be considered
target coverage,conformity and homogenity
OAR doses
integral dose
field arrangement
monitour units
treatment time
integral dose meaning
the total energy absorbed by the body , the product of mass of tissue irridicated and absorbed dose.
monitor unit
measure of radiation beam on time
one monitor units = 1cGy of absorbed dose in water under specific conditions
CB-CHOP : plan evaluation
-contours- review target volumes and OARs
-Beam arrangements / Fields - appriopate and reasonable
-coverage-evaluate on graphic plan and DVH
-Heterogenity/hot spots -value and location
-OARs- review specific constraints ,DVH, isolines on plan
Prescription- total dose, dose per fraction and image guidance
What is a DVH
graphical representation of the dose-volume relationship inside a pre-determinded volume such as ptv or oar
computer calculates volume of structures that recieves at least the given dose and ploys the volume (or percentage volume) vs dose
