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Physics for FRCR CO1 > 4.7 Principles of radiotherapy treatment planning > Flashcards

4.7 Principles of radiotherapy treatment planning Flashcards

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1
Q

What is involved in patient set up?

A
  1. localisation with tattoos and fixed room lazers (also light fields and cross wires)
  2. Immobilisation devices e.g. head shells, abdominal compression, intracranial frames in GK
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2
Q

What are the aims of immobilisation?

A
  1. Accurate repositioning
  2. Maintain position for long periods
  3. Comfort
  4. Reduce movement of OARs
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3
Q

What needs to be considered when immobilising and setting up patients?

A
  1. Reproducivle
  2. Comfort/compliance
  3. Reduce dose to normal tissue
  4. Treatment technique
  5. Practical - avoid distortion of imaging
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4
Q

What are the 10 steps involved in planning radiotherapy?

A
  1. Immobilisation
  2. Tumour localisation
  3. Visual representation
  4. select treatment geometry
  5. Optimise dose distribution
  6. Calcualte dose / MU
  7. Independent dose check
  8. Information documantation
  9. Set-up verification
  10. Treatment delivery
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5
Q

What types of 2D verification imaging are there?

A
  • kV film
  • EPID (Electronic Portable Imaging Device)
  • kv EPID
  • MV EPID
  • CT DRR
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6
Q

What types of 3D verification imaging are there?

A

Cone Beam CT
Single rotation of an x-ray tube - multiple kV radiographs are obtained and then back projected to give volumetric data
Done on the treatment machine
Poorer quality than planning or diagnostic CT but good for verification (good for bone, ok for soft tissue)

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7
Q

What types of planning imaging can be used?

A
  • USS
  • 2D Plain film
  • CT
  • MRI
  • PET-CT
  • 4DCT
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8
Q

When is 2D film used in radiotherapy planning?

A

kV imaging is only really used for diagnosis
Very occasionally used for simple pall RT planning
Can be used for on-treatment verification at kV or MV energies

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9
Q

How is CT used in radiotherapy planning?

A

Provide high resolution imaging for outlining target volume and OAR

Hounsfield units are then used to calculate dose within the planning system

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10
Q

What are Hounsfield units?

A

a.k.a CT number - represent electron denisty

Hounsfield units are linearly related to the linear attenuation coefficient - quantify how much tissues attenuate beams

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11
Q

How are Hounsfield units used to calculate radiotherapy doses?

A
  1. The CT measures the electron density of tissues and represents them as HUs
  2. The treatment planning system uses CT-to-electron density calibration curves to convert HUs into relative electron density (RED)
  3. RED is the ratio of tissue’s electron density to that of water - this is necessary because dose deposition depends on electron density of the tissue
  4. Algorithims model Photon interactions (PE effect, compton scattering and pair production) and electron interactions (energy deposited into tissue). The algorithms account for variations in tissue density to predict how radiation is absorbed and distributed.
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12
Q

How is MRI used in radiotherapy planning?

A

MRI has better soft tissue contrast than CT but no electron density information without conversion/fusion with CT

Prone to geometric uncertainty - needs to have registration distortion correction and special QA

Occasionally used as primary data set

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13
Q

How is PET-CT used in radiotherapy planning?

A

Tracer demonstrates physiology
F18-FDG is a positron emitter - pair of 511 keV photon pairs produced. PET image is cerated from annihilation photons.
PET - functional data
CT - anatomical data

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14
Q

How is 4D CT used in radiotherapy planning?

A

MIP - GTV contouring on composite image
AIP - Planning and OAR contouring

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15
Q

Why is CT usually used as the primary data set?

A
  • Quick
  • Anatomically correct
  • Reproducible
  • HU (CT numbers) correlated to electron density so can model attenuation for planning
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16
Q

How to minimise breathing motion?

A
  • Abdo compression
  • Deep inspiration breath hold - Active Breathing Coordinator (ABC) or monitoring on patient surface
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17
Q

How are simulators used?

A

kV x-ray - can do planar x-ray for targets/OARs or fluoroscopy for motion
Set up like the linac for treatment simualtion and verification

Used for simple plans - now not really used

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18
Q

What is the GTV?

A

Gross Tumour Volume - the visible disease

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19
Q

What is the CTV?

A

Clinical Target Volume

CTV + microscopic disease (anatomical + clinical)

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20
Q

What is the ITV?

A

Internal Target Volume

CTV + internal margin = ITV

Essentially due to CTV motion relative to the patient’s anatomy e.g. breathing/bowel movement/ bladder filling

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21
Q

What is the Internal Margin?

A

Expected motion of the CTV within the patient due to anatomy

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22
Q

What is an OAR?

A

Organ at Risk

Tissue whose radiosensitivity may influence the plan

These can be serial, parallel, serial-parallel, or combination

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23
Q

What is a serial OAR?

A

An organ at risk where max doses in one part of it can impair function of the whole organ e.g. spinal cord

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24
Q

What is a Parallel OAR?

A

An organ where the overall function depends on individual subunits working in parallel. Damage to some of the subunits doesn’t mean it completely fails e.g. Lungs

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25
Q

What is serial-parallel OARs?

A

Where one organ has elements of both serial and parallel OAR

e.g. the Heart where coronary arteries are serial but the myocardium is parallel

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26
Q

What are the planning volumes within the patient?

A
  • GTV
  • CTV
  • ITV OAR
27
Q

What are the planning volumes with relation to the LINAC?

A
  • Set-up margin
  • PTV
  • Treated volume
  • Irradiated volume
  • Planning OAR volume (PRV)
28
Q

What is the Set-up margin (SM)?

A

CTV motion relative to the LINAC

e.g. inaccuracies in beam alignment, cough sag, errors when transferring from treatment planning system to Linac, machine stability

29
Q

What is the PTV?

A

Planning Target Volume

makes up for geometric uncertainties e.g. in patient set up or internal motion, in planning and ensures the CTV receibes the planned dose within defined statistical accuracy

Aim for 90% of patients receiving 95% dose

30
Q

What is the Treated Volume?

A

Volume encompassed by the 95% isodose - the dose required to achieve treatment (pall or cure)

31
Q

What is the Irradiated Volume?

A

Volume of tissue receiving a dose relevant to normal tissue tolerence - 20% isodose

32
Q

What is the Planning OAR volume?

A

PRV

Ensures the OAR dose does not exceed the planned max dose, even with set up variation, spares the oAR

A.K.A the PTV but for the OARs not the TVs

33
Q

What are the Treatment Parameters that can be adjusted?

A
  • Source of the radiation e.g. photons vs electrons
  • Energy
  • Number of fields
  • Beam angles (number of fields will influence beam angles)
  • Size of the field
  • Shielding e.g. MLCs, lead
  • Wedge filters
  • Relative beam waiting
  • Source to Skin Distance

These are all restricted by the machines available and will vary with treatment intent

34
Q

What does an increase in Energy result in?

A
  • Increased skin sparring - skin dose falls
  • Increased penetratin - Dmax gets deeper with increased energy. Increased dose @Depth
35
Q

What are the options for beam angles and how are beam angles useful?

A

Ant, post, lateral, oblique

Contribute to ease of set-up/verification

Can adjust beam angles to avoid critical OARs and shape treated volume around PTV

36
Q

How can Field Size affect RT treatment?

A
  • Too small = under treated
  • Too large = normal tissue complications - may require a reduction in prescribed dose

Need additional margin to account for field-edge penumbra

37
Q

How is Shielding used in RT treatment?

A

Shielding can be arranged to spare clinical structures and shape the treated volume to PTV.

Dose in shielded areas is ~5-50% of dose to unshielded areas

Still need to take into account penumbra

38
Q

How does Irregular Blocking affect dose?

A
  • Dose from primary radiation unaffected
  • Dose from secondary radation is reduced
  • Dose in centre off field depends on scatter from within the filed so depends on shape as well as area
39
Q

How are wedges used?

A
  1. Minimse dose gradient in the target volume
  2. Compensate for patient shape, inhomogeneity within the patient, and dose gradients resulting from beam arrangement
40
Q

Why are wedges used?

A

Minimise the dose gradient across the PTV
Compensate for ‘missing tissue’ due to patient shape
Remove hotspots where beams overlap

41
Q

What is fixed SSD planning?

A

SSD is constant, depth within tissue changes
Patient has to keep being moved as the beam moves

42
Q

What is Isocentric planning?

A

The SSD varies
Plan to a reference point in the tumour (isocentre) always at 100cm from source
Simplified and quicker setup but different beam data required

43
Q

What does a shorter SSD result in?

A
  • Higher dose rates due to the inverse square law (pro)
  • Greater divergence (con)
  • Steeper PDDs - more treatment variation across treatment volume (con)
44
Q

What does a longer SSD result in?

A
  • Lower dose rates (con)
  • Bigger field sizes (pro)
  • Shallower PDDs - less dose variation (pro)
45
Q

What are the different beam arrangments?

A
  • Single field
  • Parallel Opposed Pair
  • Multi-field
46
Q

Where is a single field prescribed to?

A

Dmax (usually)
Need to consider field size and postion

47
Q

Where is a parallel opposed pair prescribed to?

A

Inter-field distance - prescribe to the midplane

48
Q

Where is the multi-field prescribed to?

A

Centre of target volume

Use 3D anatomy and density info

49
Q

What is Coplanar treatment?

A

Coplanar treatment has beams all on the same plane - usually transverse

50
Q

How do you deliver non-transverse RT with Coplanar beams?

A

Move the Gantry, couch, or collimator

51
Q

What is the irradiated volume like in coplanar radiotherpay?

A

All in same slab of tissue with well defined sup and inf borders

52
Q

What is non-coplanar treatment?

A

No principle plane
Can have the gantry, couch, and collimator anywhere

Can irradiate irregular shapes
Gradual dose drop off

e.g. SRS

53
Q

What is arc and rotational therapy?

A

Moving gantry
- Beam continuously moves around patient
- Isodoses are roughly circular, modified by shape of patient
- Best for small, deep seated tumours e.g. oesophageal, baldder, prostate
- Relatively large low dose bath
- Small arc e.g. 100 - non-conformal, inhomogenous distribution
- Full arc e.g. 359’ - circular distribution in transverse plane, but square in coronal, so needs MLCs

54
Q

What is meant by weighting?

A

Adjusting the contribution from each beam to optimise uniformity in the PTV

55
Q

What are the two types of planning?

A

Forward planning
Inverse planning

56
Q

What is Inverse planning?

A

Computer creates optimal solution e.g. IMRT and VMAT
- Based on optimisation criteria from planner/dosimetrist
- Computer finds best compromise between criteria based on ‘weighting’

56
Q

What is Forward planning?

A

Creating a manual plan

57
Q

What is field matching?

A

Used when two fields are abutting
Couch/gantry rotation to match beam edges

57
Q

What happens if 2 beams next to eachother have parallel CAX?

A
  • Match the 50% dose at a chosen depth
  • Beam divergence leads to underdose at shallower depth and overdose greater at depth
58
Q

How can you ensure the two fields line up?

A
  • Use two beams with tilted CAX
  • gantry angle chosen so that the 50% dose lines are parallel
  • dose matched at depth
  • half beam block - make the Axes the same
  • dose matched at depth
59
Q

What did ICRU 50 introduce?

A

GTV, CTV, PTV, OAR, TV, IrV

60
Q

What did ICRU 62 introduce?

A

Conformity index
PRV

61
Q
A

Physics for FRCR CO1 (8 decks)

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