Treatment planning TV, OAR, Margins 07/02

Question 1

What does TPS stand for?

Answer 1

Treatment planning system

Question 2

What does the TPS do?

Answer 2

Generates beams and angles
Shapes the beam
Attributes dose
(it is directly superimposed and based on patients CT anatomy)

Question 3

What is the aim of the TPS?

Answer 3

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

Question 4

‘Localise a tumour’

Answer 4

Identify a target volume and use PET/CT/MRI data to look at the relationship between the anatomy

Question 5

‘Localisation’

Answer 5

Target tumour = what we want to treat

Question 6

What is 3D conformal planning

Answer 6

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.

Question 7

What can aid target delineation (structures outlined by clinician) to increase conformity?

Answer 7

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.

Question 8

What does GTV stand for and what colour is it?

Answer 8

Gross tumour volume - red

Question 9

What is the GTV?

Answer 9

The demonstrable extent and location of the malignant growth
Can be identified by palpation / visualisation or through imaging techniques e.g., CT slice

Question 10

What does CTV stand for and what colour is it?

Answer 10

Clinical target volume - blue

Question 11

What is the CTV?

Answer 11

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’.

Question 12

What does PTV stand for and what colour is it?

Answer 12

Planning target volume - light blue

Question 13

What is the PTV?

Answer 13

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.

Question 14

What is intra-fractional motion?

Answer 14

Motion of the target volume during the radiation treatment

Question 15

What is inter-fractional motion?

Answer 15

Measurement of day-to-day difference in the target position

Question 16

When could margins be modified?

Answer 16

Margins can be modified to be larger on one side than the other if close to critical structures e.g., spinal cord

Question 17

What does a yellow line represent?

Answer 17

Organ at risk (OAR)

Question 18

What is an organ at risk?

Answer 18

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

Question 19

What is the isocentre?

Answer 19

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).

Question 20

What occurs in isocentric treatments?

Answer 20

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

Question 21

What is the isodose line/curve?

Answer 21

A line joining points receiving the same absorbed dose (to water)
Gives a visual representation of the dose distribution

Question 22

What factors can affect distribution?

Answer 22

Field size
Source size
Flattening filter
SSD

Question 23

Radiation beam OR the dose distribution inside the patient can be shown via what 3 things?

Answer 23

Isodose Lines
Percentage Depth Dose
Beam Profile

Question 24

What is dose delivered to tissues is dependant upon

Answer 24

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

Question 25

More dose

Answer 25

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

Question 26

What is isodose distribution normalised to at Dmax on central axis?

Answer 26

100%

Question 27

What is the normalisation point?

Answer 27

Where the prescription point is calculated (usually 100% isodose line)

Question 28

What is Dmax?

Answer 28

Depth where dose is at its greatest

Dmax is a depth (cm rather than a percentage)

Question 29

Normalisation point - Dmax relationship

Answer 29

When the normalisation point is prescribed to Dmax then you have 100% @ Dmax

Dose is usually normalised to the prescription point

Question 30

What is percentage depth dose (PDD or %DD) ?

Answer 30

Representation of dose in an irradiated volume as a spatial position along a single line (central axis)

Question 31

What does Dmax increase with?

Answer 31

Increasing beam energy

Question 32

What is the build-up effect?

Answer 32

Surface dose (skin sparing) decreases with increasing beam energy

Question 33

What happens at 15MV?

Answer 33

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.

Question 34

Entrance and exit doses

Answer 34

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.

Question 35

What is the prescription point?

Answer 35

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

Question 36

What is a beam profile?

Answer 36

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

Question 37

Variation of %DD

Answer 37

Varies with:
Field Size
Energy
FSD

Question 38

What information does a treatment plan contain?

Answer 38

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