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PHYS852: Radiation Physics > Introduction to Treatment Planning > Flashcards

Introduction to Treatment Planning Flashcards

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1
Q

Outline the stages involved in the radiotherapy treatment planning process

A
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2
Q

What are the 2 ways that tumours are visualised in treatment planning?

A
  • Diagnostic images (CT, MRI, PET)
  • Planning images (CT)
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3
Q

What is the difference between a planning CT scanner and a traditional scanner?

A

It is adapted to position patients as if they’re on a linac couch by including:
- A flat top couch
- A wide bore
- Adaptations to position immobilisation aids
- Ceiling and wall-mounted lasers

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4
Q

Why do planning CT scanners need to have a calibration curve?

A

Because CT output is measured in Hounsfield Units (HU), but treatment planning uses relative electron density.

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5
Q

How are CT scanners calibrated?

A

Using a calibration phantom

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6
Q

Which part of the treatment planning process has the greatest uncertainty?

A

Delineating volumes (contouring)

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7
Q

Why is contouring the most uncertain part of treatment planning?

A

Because not all tumours are visible in a CT scan, meaning that multiple scans of different imaging modalities are used.

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8
Q

Who defines the standards for outlining tumours?

A

The international commission for radiation units (ICRU)

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9
Q

What does GTV stand for?

A

Gross Tumour Volume

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10
Q

What is a GTV?

A

The gross palpable, visible and demonstrable extent and location of the malignant growth.

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11
Q

How is the GTV determined?

A

Via a physical examination by an oncologist and the results of relevant radiobiological investigations.

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12
Q

What does CTV stand for?

A

Clinical Target Volume

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13
Q

What is a CTV?

A

A tissue volume encompasing the GTV but with an additional margin accounting for any microscopic extension of the primary tumour or regional lymph node spread.

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14
Q

Why is a CTV defined by an even extension of the GTV?

A

Because it is impossible to ascertain the exact degree of microscopic spread of a tumour without it being completely removed.

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15
Q

What does PTV stand for?

A

Planning Target Volume

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16
Q

What is a PTV?

A

A geometrical extension of the GTV and CTV to account for uncertainties in the planning and treatment process. This includes all organ motion, uncertainties in set-up, and uncertainties in treatment delivery.

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17
Q

Define isodose

A

A contour/line that represents a region receiving a specific, uniform dose of radiation.

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18
Q

How are isodose lines determined?

A

By combining the effect of percentage depth dose and beam profile shape.

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19
Q

Treatment planning involves administering ________ beams from ________ directions.

A

Multiple
Different

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20
Q

Why does treatment planning require multiple beams to be used?

A

Because a single photon beam cannot treat a deep tumour without giving an unacceptable dose to the patient surface.

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21
Q

Describe the two types of opposing linac beams used in radiotherapy

A

2:1 weighted: the beam is biased so that more dose is given from one beam
Equally weighted: both beams deliver equal dose

In both cases, the dose distribution is normalised to be midpoint between the 2 beams

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22
Q

Describe the result of adding four beams together in radiotherapy.

A

Adding four fields together produces an approximately cuboid-shaped high-dose region.

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23
Q

How many beams are required to produce a hexagonal high-dose region?

24
Q

What is an isocentric radiotherapy plan?

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A

A treatment approach where multiple radiation beams are directed toward a single fixed point in space, known as the isocentre, which is typically located within the target. This point remains constant while the linac rotates around the patient on a gantry, allowing for precise and efficient delivery of radiation from various angles.

25
What is a non-coplanar beam of radiation?
A set of radiation beams that do not all lie within the same plane relative to the patient.
26
What is non-coplanar isocentric planning?
An approach to radiotherapy planning where multiple radiation beams are delivered to a single isocentre, but the beams are directed from different planes, not just within the same geometric plane. This is achieved by rotating both the couch and the gantry.
27
Do isocentric plans use a fixed SSD?
No, the SSD of beams entering the patient at different angles varies.
28
How do modern treatment planning systems calculate the dose distribution of radiotherapy plans?
Treatment planning systems are capable of rapidly and accurately calculating the dose distribution of beams at any SSD using the measured data from treatment.
29
How was the dose distribution of radiotherapy plans calculated before modern treatment planning systems?
Hand calculations were used before advanced treatment planning systems were invented. However, as this was slow and less accurate for isocentric plans, treatments were delivered 'fixed SSD' (which involved resetting the gantry angle and the couch position for each beam).
30
Define beam weighting
A method of specifying the extent to which different beams contribute to the overall dose distribution.
31
What is 'isocentric' beam weighting?
A form of beam weighting in which the weight of each beam is defined as the fractional contribution to the total dose at the isocentre.
32
What is the typical isocentric beam weighting of a 3-field bladder plan?
Anterior beam = 50% Right lateral beam = 25% Left lateral beam = 25%
33
What is plan normalisation?
The process of normalising isodoses to a reference point, enabling plans to be correctly evaluated.
34
How is a reference point chosen for plan normalisation?
It is typically located in the centre of the PTV and must be in a region where the dose is not changing and can be reliably calculated.
35
What are wedges used for in radiotherapy planning?
For beam shaping
36
State 2 ways that beams shaping can occur in radiotherapy
- Wedges - Multileaves
37
Linacs are calibrated so that ___ MU delivers 1Gy for a 10x10cm field at a _____ of d_max, with the patient positioned ___cm SSD.
100 dose 100
38
Which calibration factors are applied when calculating the Monitor Units of a radiotherapy beam?
- Field size - Attenuation (PDD) - Distance from source - Filters (e.g. wedges)
39
Give the equation used to calculate Monitor Units (MU)
MU = monitor units MUcal = MU required to give 1Gy at calibration conditions C_fs = correction for field size C_pdd = correction for percentage depth dose C_ssd = correction for source to skin distance WF = correction for filter attenuation
40
Describe the dose distribution of a good conformal plan
- 95% isodose encompases the PTV - Maximum dose in the PTV is not more than 107%
41
Give the equation for the dose delivered at a given depth in tissue in terms of percentage depth dose
Dm = maximum dose pdd = percentage depth dose
41
Why does the field size of an irregular field have to be corrected to an equivalent square?
Because irregular fields have the same scatter effects as a square field of an equivalent size, for which there are predetermined percentage depth doses, output factors, and tissue maximum ratios for the machine.
42
The output of a linac _______ with increasing field size because of _______.
Increases Scatter
42
Describe the difference between percentage depth dose and tissue maximum ratio
PDD is the ratio of the dose at a given depth in tissue to the dose at a reference depth (usually the depth of maximum dose) along the central axis of the beam. TMR is the ratio of the dose at a given depth to the dose at the depth of maximum dose, but under conditions of constant source-to-axis distance (SAD) and no change in field size at depth. Because of this, PDD is impacted by both attenuation and the inverse square law but TMR is only impacted by attenuation.
43
What does IMRT stand for?
Intensity modulated radiotherapy
44
Give 3 reasons why IMRT improves patient care
1) It shapes isodoses around a concave target 2) It avoids normal tissue or Organs at Risk 3) It deliberately gives a higher dose to parts of the target
45
How are IMRT plans created?
Using inverse planning techniques
46
Describe the process of creating an IMRT radiotherapy plan
1) Outline the PTV and the OARs 2) Set objectives (minimum dose to PTV and maximum dose to OARs) 3) Set beam sizes, collimator angles, and gantry angles 4) Run the programme, allowing the computer to construct the ideal fluence to achieve these objectives 5) The computer constructs the MLC segments for each beam
47
What does VMAT stand for?
Volumetric Modulated Arc Treatment
48
What is a VMAT treatment?
An IMRT treatment in which the field arcs around the patient while the Dose Rate changes and the MLC's move, resulting in a plan with improved isodose coverage.
49
State 2 benefits of VMAT treatments
- Improved isodose coverage - Shorter treatment times
50
What is a Dose Volume Histogram (DVH)?
A plot of the volume receiving a dose greater than or equal to a given dose, against that dose (cumulative DVH).
51
State 6 factors that influence the accuracy of a DVH
1) Volume 2) Volume resolution 3) Dose 4) Grid spacing and size 5) Dose matrix resolution 6) Bin size
52
What is the final stage of the treatment planning process?
Transferring the plan to the treatment machine
53
What is the standard file format that linacs use to interpret a treatment plan?
DicomRTplan

PHYS852: Radiation Physics (13 decks)

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