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Physics for FRCR CO1 > 4.9 Quality assurance in radiotherapy > Flashcards

4.9 Quality assurance in radiotherapy Flashcards

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1
Q

What is the primary aim of radiotherapy?

A

To deliver radiation with accuracy and prescision

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2
Q

What is Accuracy?

A

How close the dose is given to the intended place

aka how on-target it is

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3
Q

What is dosimetric accuracy?

A

The correct dose delivered (within a set of tolerences)
Usually 3%

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4
Q

What is Geometric accuracy?

A
  1. Patient in correct position
  2. Treatment delivered to right place

Geometric inaccuracy can lead to dosimetric inaccuracy

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5
Q

What is Precision?

A

How reproducible it is - e.g. are all the doses delivered close together (even if not to the right place?)

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6
Q

What is Quality Assurance (QA)?

A

Quality assurance makes sure there is a system in place to ensure specific levels of consistensy are achieved. This shuld be regularly audited.

This is a bit like the recipe book, with the recipe being the system

Examples are IRMER 2018, IRR 2017, plan checklists, trial protocols

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7
Q

What is Quality Control (QC)?

A

Tests, measurements, and monitoring required to ensure standards are maintaned within specific tolerences

e.g. standard output measurements
flatness and symmetry of the beam
temperature checks

This is a bit like tasting the cake you baked from the recipe inside the recipe book

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8
Q

How is machine output quality controlled?

A
  • Daily - constancy check,
  • Weekly - Ion champer check (farmer or roos) this is more accurate but slower
  • Annual - water/perspec phantoms

IPEM states 5% tolerence, the christie uses 2%

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9
Q

How is beam symmetry and flatness quality controlled?

A

Measured by scanning an ion chamber across the beam profile at 10cm deep in water or using an array of ion chambers.

Beam symmetry is measured at the distance from the central axis where the difference is bigger (ears on the graph)

IPEM 81 states this should have a 3% tolerence (christie is 2%)

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10
Q

What is the formula for calculating beam symmetry?

A

Symmetry = D+80/D-80x 100%

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11
Q

What affects beam symmetry and flatness?

A
  • Electron gun
  • Radiofrequency power
  • Input bem stearign systems
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12
Q

What is the formula for calculating beam flatness?

A

Flatness = ((Dmax/Dmin)-1)x100%

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13
Q

How is beam energy quality controlled?

A

Determined by electron gun current and microwaves in waveguide

Characterised by central axis PDD in water under refernce conditions

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14
Q

How are photon beams quality controlled?

A

Farmer chamber using TPR 20/10 or constancy check TPR 15/5

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15
Q

How are electron beams quality controlled?

A

Ion chamber (R50D - dose at 50% Dmax) or constancy check

IPEM 81 2% (christie 1%)

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16
Q

How is field size quality controlled?

A
  1. Use a visible light field - measure with graph paper, 2mm tolerence
  2. Use an x-ray gfield - measure with film or an MV imager panel (EPID)
17
Q

What is an EPID (Electronic Portal Imaging Detector)?

A

Unfolds behind the patient and gives direct imaging of the treatment fields as x-rays exit the patient

Image is then compared to the DRR

18
Q

What are the mechanical tests used to ensure correct patient set-up?

A
  • Machine isocentre (point at which the gantry/couch rotate)
  • Optical crosswire - compared with the isocentre and rotational accuracy
  • Optical Distance Indicator (ODI) - projects a reticle (line) image onto the patient - must be correct at the isocentre and linearly - compare FSD
  • Lasers - straight and intersection at isocentre
19
Q

What are the steps in checking a treatment plan?

A
  1. Patient data - right patient?
  2. Prescription
  3. Isocentre postion
  4. Check the machine;
    - Gantry
    - Collimator angles
    - Couch angles
    - Field size
    - MLC shapes
    - Wedges
  5. Optimisation - does plan meet prerequisites
  6. Coverage
  7. DVH statistics - evaluate and optimise dose distribution in the patient’s anatomy to ensure OAR and PTV dosed correctly
20
Q

How do we ensure the prescription is correct?

A
  1. Use standard protocols for each treatment site, these state:
    - clinical indication
    - modality
    - localaisation
    - planning technique
    - dose and fractionation
    - timing
    - immobilisation techniques
  2. Use protocols for geometrc verification:
    - imaging equipment and techniques
    - frequency and timing of images
    - anatomical reference points
    - tolerence and action levels (caused by systemic and random errors)
21
Q

What are the sources of geometric error?

A

**Random errors **- random errors are different for every fraction:
1. Changes in patient anatomy
- weight change
- relaxation
- tumour growth/shrinkahe
- movement of internal anatomy relative to skin markers

  1. Patient motion
    - breathing
    - bladder and bowel filling
    - voluntary movements e.g. wriggling
  2. human error

Systematic errors - errors same for every fraction
3. Imperfect mobilisation e.g. mask is made wrong
4. Equipment issues e.g. wrong laser used in set up

22
Q

What are the processes to ensure correct implementation of an RT prescription?

A
  1. Computer verifications
  2. Manual checks
  3. Online and Offline Image Guided RT (IGRT)
  4. Imaging protocols
  5. Position accuracy checks
  6. In-vivo dosimetry
23
Q

What is involved in computer verification?

A

For complex plans e.g. IMRT or VMAT an indepdent computer calculator is used. the computer can include heterogeneity calculations

24
Q

What is manual checking?

A

For simple plans manual calculations can be done although these assume homogeneity

25
Q

What is Online IGRT?

A

Verification image is taken before treatment usually a cone beam CT which then allows for the patient to be moved into the correct position
This aims to reduce systematic and random errors
Tolerences 1-3mm

Negatives are that it is time consuming and adds radiation does to patient, especially if multiple re-images occur

26
Q

What is Offline IGRT?

A

Verification image after treatment
Reduces systematic error risk

The average systematic error can be calculated and if over specified tolerence (e.g. 3mm) a shift applied for the remaining fractions

27
Q

What are the different imaging protocols that can be used in ensuring correct implication of prescribed radiotherapy?

A
  1. Cone beam CT (aka 3D kV)
  2. 2D kV imaging
  3. 2D MV imaging
28
Q

What is cone beam imaging and what are its pros and cons?

A

kV beam is rotated around the patient to create a cone beam
at kV energies the photoelectric effect dominates so good quality image

Pro:
- Good contrast and resolution
- Gives 3D information
- Large field of view - whole volume at once
- Low dose
Cons:
- Cannot verify what the treatment beam will actually be like (only patient position)
- Takes 2 minutes to aquire each iage
- HUs are variable compared to diagnostic

29
Q

How is 2D kV imaging used in verification? What are its pros and cons?

A

Some linacs have a an extra kV source on them
This creates 2D images at 2 orthoganal angles (90’) which are then used to create a Digital Reconstructed Radiograph

Pros:
- Good contrast and resolution
- Good boney nformation
- Fast to acquire
- Large field
- Low dose

Cons:
- Does not verify treatment beam
- Limited soft tissue info

30
Q

How is 2D MV imaging used for image verifcation and what are the pros and cons?

A

The beam itself is used to image - e.g. with an EPID
Compton effect fominates so not as good imaging

**Pros: **
* Shows the exact beam target
* Shows shape of treatment beam
* No extra imaging dose needed

Cons:
* Low contrast and resolution due to compton scatter domination
* No soft tissue detail
* 2D image with limited field of view (only where beam is going)

31
Q

How is in-vivo dosimetry used in RT verification?

A

Used to detect errors from:
* Incorrect MU calculations
* Incorrect transfer of treatment field parameters
* Incorrect patient set-up

Diodes and TLDs

32
Q

How are Diodes used in in-vitro dosimetry?

A

Positioned on patient skin to measure a point dose for each field. These five an instant reading and can be compared to the expected dose.

Shadowing is an issue due to dose build up within the diode, so can only really be used for 1-2#

Accuracy 3-5% and tolerence 10%

33
Q

How are LINACs qualitry controlled, and what is risk rating?

A

Series of geometric and dosimetric tests which are compared to comissioning data, to ensure the treatment machine is operating within safe tolerences (1-2mm or 1-2%)

Risk rating = liklihood of error (scale of 1-5) x severity of error (1-5)
Risk rating is then used to determine how frequently a test should be done (IPEM 81)

If risk rating >16 should top immediately
If >15 urgent action
If >8 action
>4 monitor
>1 no action

34
Q

Radiotherapy error reporting

A

Legal requirement to report any cases where an error resulted in a significant increase in dose received by patient (to CQC)

Signficiant Accidental/Unintended Exposure

**Overdose: **10% in whol treatment or 20% in a fraction

Underdose: <0.9x the intended dose

Any total geometric miss

Physics for FRCR CO1 (9 decks)

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