Treatment planning, software and algorithms 07/02

Question 1

What does hand planning involve?

Answer 1

Manual manipulation of isodose charts

Question 2

What does computerised treatment planning involve?

Answer 2

Plan optimisation

Question 3

Why do we use CT for planning but not just MR?

Answer 3

For MR, there are no Hounsfield units so there is no electrical density in MR which means the dose is all based upon density and how the radiation interacts based on the structure it is going through. Due to this you cannot plan on MR.

Question 4

Treatment Plans are produced – how the linac will deliver the RT - 3 main things in mind

Answer 4

Treats the TVs (Target Volume)
Reduces dose to OAR (Organs at risk)
Minimises dose to normal tissues

Question 5

What do plans demonstrate?

Answer 5

The expected dose distribution in a patients tissue

Question 6

Overall dose of radiation delivered will achieve the aim of the treatment, which is?

Answer 6

Radical – with intent to cure

Palliative - to relieve symptoms

Question 7

Radical or palliative?

Answer 7

Consider dose per fraction (daily dose)
Total dose
Number of fractions (treatments)

Question 8

Radical treatment features

Answer 8

Larger total doses given over a number of treatments (fractions #)

Daily Dose = Total Dose / No #

E.g., Typical Radical H&N 70Gy / 35# = 2Gy per #

Healthy cells have opportunity to recover during the course

Same dose over all fractions. Side effects not due to more intense dose over fractions but due to differences in DNA composition > intensity remains the same.

Question 9

Palliative treatment features

Answer 9

Overall smaller total doses over fewer fractions

Daily dose = Total dose / No #

E.g., 20Gy / 5# = 4Gy per #

Individual dose greater than radical individual dose

Often role is to improve symptoms

Question 10

Palliative Treatment Planning pathway

Answer 10

Simple plan arrangement
Lower doses
Emergency Oncology
Little risk to healthy tissues
GTV, CTV, & PTV	generally not outlined
Quick – treatment same day

Often called Virtual Simulation or V-S

Question 11

Radical Treatment Planning pathway

Answer 11

Complex plans
Higher doses
Risk to healthy tissues can be calculated accurately
Target volumes outlined
Requires geometric verification – imaging e.g., KV, Cone beam CT
May require dosimetric verification
Time consuming – days-weeks

Question 12

Treatment Planning Pathway

Answer 12

Patient has a RTP-CT in suitable & reproducible position
CT Data is imported into the TPS (Eclipse/Pinnacle/RayStation)
Contours of target volumes & OAR are delineated – by the clinician
+/- radiographer
A individualised treatment plan is produced & MU calculated using the treatment planning algorithm
Plans are reviewed by clinician
Plans are independently checked by ‘plan checker’
Plans are exported to the treatment machine

Question 13

4 common UK RTP systems

Answer 13

Philips - pinnacle
Varian - eclipse
Nucletron
Research - ray station

Question 14

What information does a treatment plan contain?

Answer 14

Total Dose / #
Prescription Point
Beam type – x-Ray, electrons, protons
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

Question 15

What do algorithms allow us to do?

Answer 15

Algorithms allows us to demonstrate how we visualise dose in a medium. This allows use to predict with as much accuracy as possible the dose delivered to any point in the patient.

Uses CT data, beam direction and beam characteristics to calculate the dose at any point within the patient

Responsible for correct representation of dose in the patient

Clinical decisions can/may be taken from the resulted dose distribution

Dose calculations are linked to monitor unit (MU)

Question 16

Dose calculation algorithms

Answer 16

Central axis models
Semi-empirical models
Pencil beam algorithms 
Convolution algorithms
Monte Carlo simulation
(increasing accuracy and decreasing speed)

Question 17

Models are based upon

Answer 17

Models of electrons striking the target in the linac
Model propagation through the head of Linac
Models of interactions with the patient

Question 18

Dose is computed by

Answer 18

Modelling the beam

Modelling the interactions in the patient model

Question 19

Pencil beam algorithm

Answer 19

Assumes lots of forward facing ‘pencil’ beams called Kernals

Does not account for inhomogeneity’s well

Assumption that each pencil beam is the same energy

Fast and cheap

Widely used for optimisation of IMRT when using inverse planning or calculation of electrons

Question 20

AAA algorithm

Answer 20

A convolution superposition algorithm

Based upon the Monte-Carlo simulated dose distribution

Correctly models the beam passing through the linac head and the patient

Calculates dose at bone/air/soft tissue interfaces better than P.B.

Fairly quick and mid-cost

Question 21

Monte Carlo algorithm

Answer 21

Based upon probabilities

Random & ‘game-like’ behaviours of beams is similar to that of gambling

Used for quality assurance

Accurate ++ (better understanding of dose inhomogeneity’s)

Cost ++

Calculation time – (long)