Radiotherapy

Question 1

Why use CT for treatment planning

Answer 1

Geometrically accurate in 3D
HU map to electron density
Good resolution
Good bony-soft tissue contrast
Low distortion

Question 2

Limitations of CT and other options to get around this

Answer 2

Poor definition of soft tissue - MR has good soft tissue contrast
No functional information - PET CT or functional MRI would give this

Question 3

Additional requirements for RT planning CT over normal CT

Answer 3

Needs to be in consistent and reproducible position, the same as for treatment
Flat top couch to mirror that of the LINAC
Scan patient in treatment position with any immobilisation that will be there
Lasers establish reference system, ball bearings define orthogonal points on patient surface

Question 4

What affects shape of TCP curve and how

Answer 4

Total dose or dose per fraction - increasing moves to left
Initial number of clonogenic cells - moves to right
Parameter alpha - moves to left (and gets steeper)
Alpha/beta ratio - curve moves to right
Spread of alpha - gradient decreases

Question 5

Properties of an ideal radiation detector in RT

Answer 5

Accurate and precise
High sensitivity
Appropriate spatial resolution
Linear response with dose
Minimal angular dependence
Dose rate independent
Energy independent
Temperature independent

Question 6

Define primary shielding

Answer 6

The primary collimator absorbs any radiation that may have gone astray, before it scatters around the room and irradiates some part of the patient that should not be treated.

Question 7

Define secondary shielding

Answer 7

The secondary collimators or moveable jaws/ MLCs define the field size.

Question 8

How and why is electron energy reduced when using electron beam instead of photon beam

Answer 8

o Electron energy is reduced by reducing the current
o Lots of energy lost as passes through the target as heat and attenuation through flattening filter etc
o (Flattening filter also hardens the beam and this does not occur for electrons-because of this need to alter the energy of the beam)

Question 9

Uses of CT in planning

Answer 9

4D reviews - lung movement
Contouring
Used during patient set up
Can be fused with other modalities
Can check dose distribution through patient

Question 10

What is on axis of cell survival curve

Answer 10

x: dose
y: surviving fraction (logorithmic)

Question 11

Larger alpha/beta ratio and smaller: types of cell

Answer 11

Larger ratio: tumours, early responding tissue
Smaller: late responding tissue

Question 12

5 R’s of radiotherapy

Answer 12

Repair (of sublethal damage) - hyperfractionation
Repopulation (following irradiation) - accelerated fractionation
[more resistent]

Redistribution (of cells within cell cycle) - chemotherapy
Reoxygenation (of surviving cells) -drugs
[more sensitive]

Radiosensitivity (intrinsic per cell)

6th? Reactivation of immune response

Question 13

Tumour control probability

Answer 13

TCP = exp(-k) = exp(-k0 SF)
= exp(-k0 exp(-alpha BED))

Question 14

BED

Answer 14

BED = D[1 + d/(alpha/beta)]

Question 15

Systematic vs random error in setup

Answer 15

Systematic: an error propagated through the process
Random: error at single event that won’t be reproduced

Question 16

Treatment prep errors

Answer 16

Systematic:
Doctors delineation
Organ position and shape at localisation
Error in TPS
Image registration

Question 17

Treatment delivery errors

Answer 17

Systematic:
Linac geometry errors
Systematic set up error

Random:
Daily set up error
Daily position and shape of organ

Question 18

Calculating random and systematic errors

Answer 18

Systematic: difference to mean positions day to day
Random: standard deviation, spread of points from mean

Question 19

Reducing systematic and random errors

Answer 19

Systematic: off line verification, acquire images on successive days and calculate and correct for mean
Systematic and random: online verification, acquire image each day, calculate displacement and correct for this

Question 20

What is 2D verifiation

Answer 20

Checking patient position or check treated and shielded position with beams eye.
Quality of match depends on type of image used
Might image for 3 days and see out of tolerance and shift

Question 21

Methods of matching during verification

Answer 21

Bony registration
Soft tissue registration
Online verification: register localisation and verification CT data, overlay 95% isodose on verification and ensure CTV still enclosed.

Question 22

QA vs QC

Answer 22

QA: ensuring standards are established and maintained
QC: undertaking of tasks to ensure defined parameters remain in tolerance

Question 23

What are two parts to QC

Answer 23

Geometric accuracy
Dosimetric accuracy

ICRU 83 recommends 3.5mm on geometric and 3.5% on dosimetric

Question 24

Pre treatment imaging QA

Answer 24

Check geometric accuracy (couch movement, laser alignment), CT number, image quality (SR and CR)

Need geometry to be correct for planning - use phantom of known geometry
Need lasers/ couch position to be correct because this is how we set the patient up
Need CT number correct because its how we end up calculating the dose

Question 25

TPS QA

Answer 25

Need patient model and linac model
Some systems use check sums - database checks, some perform routine dose calculation algorithms.
Daily - recalculate standard plans - ensures consistency of CT –> ED
Routine testing of functionality - ROI contouring, algebra, DVH statistics.

Question 26

Process/data transfer QA

Answer 26

Test process using phantom - scan on CT, data transferred to TPS, planned here, plan exported and transferred to linac, treatment delivered and check it’s correct.

Question 27

QC in patient pathway

Answer 27

Photo/DOB/name/address.
customised immobilisation
Comprehensive plan checks

Question 28

Daily vs monthly checks

Answer 28

Daily: output consistency, interlocks, laser alignment, light field and cross wire set up
Monthly: photons, electrons, mechanicals, on-board imaging

Question 29

Radical vs palliative radiotherapy

Answer 29

Palliative: symptom relief not cure. Quick, pain relief, simple fields, POP/ single field, critical structures less of a concern
Radical: aiming to treat. Immobilisation, CT, planning system, dose distribution, plan check

Question 30

What is a monitor unit

Answer 30

Ionisation chamber ‘monitors’ beam
constantly.
* 1 monitor
chamber unit=1 dose unit
(usually 1cGy) under ‘calibration
conditions’.
* Monitor chamber linked to beam control - beam is switched off at correct MUs.
* TPS calculates MUs required

Question 31

What is PTV

Answer 31

Margin added to account for geometric considerations and patient motion. Treating the PTV ensures coverage of the CTV with setup variability

Question 32

What happens below saturation voltage

Answer 32

• If voltage not high enough, measured charge proportional to voltage
• At a particular saturation voltage (Vsat) the current is saturated for a given exposure rate. i.e. nearly all ions formed are collected.
• All ionisation chambers must operate under saturation conditions if their response is to be linear with exposure.