Motion Management

Question 1

What is interfraction motion

Answer 1

o Motion between fractions
o Soft tissue shifts relative to bones (head and neck, thorax, abdomen, pelvis)
o Tend to occur day-to-day regardless of immobilisation and localisation

Question 2

What can cause interfractional variations

Answer 2

o Due to adjacent OAR/target organ filling
o Due to changes in tissue geometry

Question 3

What is Intra-fraction motion

Answer 3

o Motion during delivery of a fraction
o Primarily a fraction of the extent of the target motion during treatment
o Voluntary (wriggling, scratching)
o Involuntary (coughing, sneezing, swallowing, cardiac motion)

Question 4

How to manage intra-fraction motion

Answer 4

o Consider patient comfort
o Reduction of patient anxiety
o Minimise length of treatment

Question 5

Patient Preparation and Setup Techniques to Minimise Motion

Answer 5

Immobilisation Devices
 Important to maintain reproducibility and accuracy
 Can be a source of anxiety – may exacerbate motion

Bowel and Bladder Prep
 Consistency of filling can improve positional reproducibility of pelvic organs

Compression techniques
 Aim to reduce abdominal motion due to respiration (abdominal compression)
 Potentially increase baseline shifts (setup errors)

Education and Compliance
 Understanding of positioning equipment, preparation protocols – shown to increase compliance

Question 6

What is the optimal arm position when treating a lung?

Answer 6

Arms above head
 Greater choice of beam angles
 Improves target coverage
 Sparing of normal tissue
 May be unsuitable for certain patients

Question 7

What can be used to stabilise the arm when treating a lung?

Answer 7

Use a stable arm support in combination with knee support to increase comfort
 Generally vac-bags – provide support through arms, neck and torso

Question 8

Which aspect of the lung is tumour motion the greatest?

Answer 8

Different aspects of lung have variable motion
 60% lung tumour – move < 1 cm
 35% lung tumour – move 1 – 2 cm
 5% lung tumour – move > 2 cm

Motion increases towards the diaphragm and is largest in the liver

Question 9

Goal of Image Guided RT

Answer 9

 Reduce uncertainties
 Maximise reproducibility
 Improve set up accuracy
 Account for organ motion
 Sparing NTT

Question 10

What are fiducial markers and their purpose?

Answer 10

o Routinely used for localising prostate irradiation

o Small gold seed (0.9-3.0 mm) implanted under transrectal US guidance

Question 11

Benefits of Fiducial Markers

Answer 11

FMs surrogate of prostate motion

Very fast method of localisation

Staff have great confidence when aligning FMs
- very low intra- and inter-observer variability

Question 12

Disadvantages of Fiducial Markers

Answer 12

o Associated expense
o Invasive procedure = risk of infection at implantation
o Some patients ineligible, e.g. Warfarin dependency
o Rely on few (three) discrete points to localise the prostate (particularly when used with planar imaging)

Question 13

In which direction is prostate motion most common

Answer 13

Dominant changes were in anterior and/or superior directions.

Question 14

What can affect the position of the uterus

Answer 14

o Bladder filling significant factor in motion – filled can push posterior and superior
o Pelvic Tilt

Question 15

What can affect the position of the prostate

Answer 15

Pelvic Tilt

Bladder Filling

Question 16

Motion Artifacts in CT simulation

Answer 16

Distortions along the axis of motion can either lengthen or shorten the target
(in a somewhat random effect)

 If CT scanning speed < tumor motion speed, → smeared tumor image
 If CT scanning speed > tumor motion speed, → tumor position and shape captured at an arbitrary breathing phase
 If CT scanning speed ~ tumor motion speed, → tumor position and
shape heavily distorted

Question 17

4DCT

Answer 17

• 10 Bins are recorded per breath cycle
• 1 CT dataset is created with the same bin from each breath cycle
  -> e.g., Bin 1 (Cycle 1) + Bin 1 (Cycle 2) + Bin 1 (Cycle 3)
  -> results in 10 CT datasets
• 10 CT datasets are fused together -> creates a single CT dataset in which ITV can be contoured
• allows more accurate estimate of tumour position

Question 18

Benefits of IGRT

Answer 18

Tightly conformed dose distributions are at
increased risk of missing the moving target

Resultantly, IGRT can provide more knowledge on the target position –> more precisely irradiate the intended target

Question 19

Benefit of Planar kV imaging

Answer 19

Relatively low imaging dose
Efficient means of isocentre verification

Question 20

Forms of Volumetric Imaging

Answer 20

Fan Beam (CT on Rails) (Tomotherapy)

Cone Beam (CBCT) (MCVT)

Question 21

Benefits of Volumetric Imaging

Answer 21

Provide visualisation of soft tissue

Great potential to improve treatment delivery

Question 22

What is CT on Rails

Answer 22

• Diagnostic CT directly opposite (or orthogonal)
• Single couch for both gantries
• Couch rotates between the Linac and CT
• CT gantry “slides” over patient
• Assumes fixed relationship between the
  isocentres of the two systems (CT and Linac)

Question 23

Disadvantages of CT on Rails

Answer 23

Couch sag that occurs at the CT on rails gantry could not be totally corrected for

Time lag between image acquisition

Question 24

What is an MVCT

Answer 24

Uses the treatment beam and EPID to acquire
cone beam CTs

Question 25

Benefits of MVCT

Answer 25

• images high-density images without artefact
  (e.g. prosthetic hips)
• Same isocentre of imaging and treatment
• Standard hardware – no additional arms

Question 26

Disadvantages of MVCT

Answer 26

• Low contrast (high energy)

Dose several times greater than kV

Question 27

Benefit of CBCT (kVCT)

Answer 27

Able to monitor patient throughout treatment

Volumetric rather than few discrete points

Question 28

Prostate Interfraction Motion Management

Answer 28

Recommend complementary use of implanted FMs and volumetric imaging with CBCT for daily localisation

“Aligning FMs on kVCT provides the best IGRT solution; offering minimal intra-observer error and prostate and OAR deformation information.”

Question 29

Bladder ‘Plan-of-the-day’

Answer 29

Adaptive RT for Bladder Cancer

Initial plan has generous CTV-PTV margins (based on single CT, bladder empty)

Initial fractions treated with this conventional plan
* CBCT guided soft-tissue match
* CBCTs from first 5 fractions subsequently used to adapt plan

Develop a ‘small’, ‘average’, ‘large’ plan -> chosen to best suit the patient bladder on the day

Can reduce amount of tissue irradiated

Question 30

How is surface imaging utilised

Answer 30

Utilises two or three ceiling mounted 3D camera units, designed to image the patient at simulation or treatment.

Reference surface model is produced from either: * importing contours from CT data, or by
* acquiring a 3D surface with AlignRT at simulation.

At each treatment fraction, the system images the current patient position

This image is registered to the reference surface

Couch shifts then calculated to correct inconsistencies between actual and planned positions.

Question 31

What is the difference between gating and tracking

Answer 31

Gating
- beam ON only when target is at desired position or phase

Tracking
- the treatment beam follows the target during the whole cycle

Question 32

Clarity 3D Ultrasound Guidance - use in simulation

Answer 32

Can be employed at CT simulation to aide
soft-tissue visualization.

Ultrasound probe is calibrated to the same
isocentre as the CT system

Question 33

Clarity 3D Ultrasound Guidance - use in treatment

Answer 33

Used for patient alignment in the treatment room
* requiring just 90 seconds for scanning and
repositioning
* The treatment couch position is tracked using the same system as the ultrasound probe, providing highly accurate alignment.

Verify target position without ionizing radiation

Provides continuous real time imaging

Question 34

Deformation and rotations

Answer 34

Most IGRT corrections only account for linear displacements
-> other sources of geometric uncertainty
1. Deformation of organs (adjacent organ position, filling)
2. Patient Weight loss/gain
3. Rotation of tumours and OARs

Question 35

Suggested bladder volume

Answer 35

150-300cm3

Question 36

When would you use 4DCT vs gating

Answer 36

4DCT - 8mm and greater motion
Gating - 13mm motion and above

Question 37

Interplay effect

Answer 37

• observes as the dose deposition occurs when the target is moving
• results in a sub-optimal dose distribution contributed by a lack of synchronisation between the beam delivery and the desired target position (can cause regions of over-dose or and under-dose)
• much less likely with static treatment

Question 38

Electromagnetic tracking of lung

Answer 38

Anchored transponder
- contains beacon transponder core and has 5mm anchors and legs to secure positioning in small airways

• Calypso monitoring system provides 3D target position -> predicts lung motion during treatment -> provides adjustment to 3D leaf position if necessary (in accordance with breathing cycle)
• Allows for significant reduction in the volume of ITV with MLC tracking delivery

Question 39

Benefits of Markerless Tracking in Lung Cases

Answer 39

• Markerless 3D monitoring of the proximal bronchial tree (PBT) could help manage this risk by tracking the position and motion of airways relative to the
tumour, while offering a more reliable tracking option than reflective or implanted markers.

• Accurate real-time position monitoring could alert users to interrupt radiation treatment if the PBT position deviates beyond a pre-determined threshold.

Question 40

Markerless tracking application and benefit in Rt

Answer 40

Example - Lung - MRL
* Automatic tumour tracking triggers a beam-on when target volume is within the boundary, and triggers a beam-hold when the target volume exceeds the boundary

• This allows patients to hold their breath at their own pace by using an adjustable mirror to see the real-time sagittal
  projection on the in-room monitor.

Other Benefits
-> real time tracking
-> reduced dose to NTT
-> real time adaptation to organ motion

Question 41

4DCT disadvantages

Answer 41

Huge dataset
Irregular breathing pattern
Movement