Treatment planning TV, OAR, Margins 07/02 Flashcards
What does TPS stand for?
Treatment planning system
What does the TPS do?
Generates beams and angles
Shapes the beam
Attributes dose
(it is directly superimposed and based on patients CT anatomy)
What is the aim of the TPS?
To maximise tumour control and minimise normal tissue complications (being irradiated) > Side effects occur not only as a result of the tumour being treated but also as a result of normal tissue being ‘treated’ (giving dose to normal structures) e.g., erythema
‘Localise a tumour’
Identify a target volume and use PET/CT/MRI data to look at the relationship between the anatomy
‘Localisation’
Target tumour = what we want to treat
What is 3D conformal planning
Treatment volume of radiation ‘conforms’ to the Target Volume
Several treatment fields can be added to ‘conform’
MLC’s increase the conformity (shape the beam)
We can manipulate our treatment fields and beams to make sure only the target volume is treated rather than healthy tissue.
What can aid target delineation (structures outlined by clinician) to increase conformity?
Multi-leaf collimators (MLCs) – shape beam
CT scan – provides us with a set of data > we can make the outlining by the clinician better by using multi-modal imaging so by using an MRI and a PET in addition to the CT gives us the best picture that allows us only the area (tumour) that we need to treat and save the soft tissue around.
What does GTV stand for and what colour is it?
Gross tumour volume - red
What is the GTV?
The demonstrable extent and location of the malignant growth
Can be identified by palpation / visualisation or through imaging techniques e.g., CT slice
What does CTV stand for and what colour is it?
Clinical target volume - blue
What is the CTV?
Contains the demonstrable tumour (GTV) and microscopic invisible tumour
CTV contains cancer cells (potentially following surgical removal) and MUST be treated with the prescribed radiation dose adequately to achieve a cure
Depending on where the tumour is radiation does will all be different
Can have additional volumes for presumed subclinical spread e.g. lymph nodes
From research – to treat a tumour, the CTV must be treated with the right dose. If the CTV is missed, tumour control and cure is compromised
Can have two different CTV’s (e.g., CTV1, CTV2 etc) with 2 different prescribed doses. Often seen in head and neck patients e.g., tonsil tumour and neck nodes being treated simultaneously. Often referred to as ‘CTV node’ and ‘CTV tumour’.
What does PTV stand for and what colour is it?
Planning target volume - light blue
What is the PTV?
Allows for intra and inter fractional motion when the beam is on and between each fraction (each day).
Includes a margin around the CTV to allow for variations (intra- and inter-fractional) due to patient position, patient set-up, physiological changes (respiration), machine variations and human factors.
What is intra-fractional motion?
Motion of the target volume during the radiation treatment
What is inter-fractional motion?
Measurement of day-to-day difference in the target position
When could margins be modified?
Margins can be modified to be larger on one side than the other if close to critical structures e.g., spinal cord
What does a yellow line represent?
Organ at risk (OAR)
What is an organ at risk?
Any tissue NOT in the CTV = normal tissue
Radiation dose needs to be kept as low as possible
Tissues are not equally sensitive to radiation
Organs that are sensitive to radiation / are at risk are outlined in the TPS
Tolerances to that tissue need to be considered
What is the isocentre?
A single point within the treatment room (in space) towards which the radiation beam always points.
The central axis passes through this point & the three principal rotational movements of gantry, collimator and floor are all around axis which intersect at this point (on a linac).
What occurs in isocentric treatments?
The axis of gantry rotation is fixed within the target volume The SSD (skin to source) to the patient will vary as the gantry rotates
What is the isodose line/curve?
A line joining points receiving the same absorbed dose (to water)
Gives a visual representation of the dose distribution
What factors can affect distribution?
Field size
Source size
Flattening filter
SSD
Radiation beam OR the dose distribution inside the patient can be shown via what 3 things?
Isodose Lines
Percentage Depth Dose
Beam Profile
What is dose delivered to tissues is dependant upon
The energy of the photon
- It needs enough energy to release an electron
- And then it depends upon the binding energy of the electron
- More photons = more scattered electrons = more dose
More dose
Photon beam out of head of linac - releases energy – thousands of collisions – as they collide, they lose energy – until collision produces an electron – dose
More photons = more interactions = more electrons = more dose
What is isodose distribution normalised to at Dmax on central axis?
100%
What is the normalisation point?
Where the prescription point is calculated (usually 100% isodose line)
What is Dmax?
Depth where dose is at its greatest
Dmax is a depth (cm rather than a percentage)
Normalisation point - Dmax relationship
When the normalisation point is prescribed to Dmax then you have 100% @ Dmax
Dose is usually normalised to the prescription point
What is percentage depth dose (PDD or %DD) ?
Representation of dose in an irradiated volume as a spatial position along a single line (central axis)
What does Dmax increase with?
Increasing beam energy
What is the build-up effect?
Surface dose (skin sparing) decreases with increasing beam energy
What happens at 15MV?
Skin sparing occurs. The higher the energy we treat, the larger the skin sparing is so the deeper you have to go to get the Dmax.
Entrance and exit doses
For 6MV beam, entrance dose is around 55% and at 15MV it is around 30%. Exit dose is a lot higher with a higher energy beam than with a lower energy beam. Most tumours require 6MV.
What is the prescription point?
When prescribing, at that point, generally there will be 100% of the dose
Generally at midplane or the Isocentre
@depth for simple single fields
Prescribing 20Gy @ 2cm will have different effects to prescribing 20Gy @ Isocentre
What is a beam profile?
A plot of dose across the beam in a direction perpendicular (90*) to the central axis, passing through the central axis, normalised to the dose at the central axis.
Profile is FLAT due to the flattening filter
Variation of %DD
Varies with:
Field Size
Energy
FSD
What information does a treatment plan contain?
Total Dose / # Prescription Point Machine information – treatment machine, beam energy, field size, beam modifications, technique Moves to isocentre Bolus Target Volumes Coverage of volumes & OAR information MU required to deliver each beam DRR’s / Verification images In-vivo dose measurements
