Adaptive Radiotherapy

Question 1

What are changes that can occur during treatment?

Answer 1

Patient and isocentre positioning
Organ motion
Anatomical changes (tumour shrinkage/growth)
Biological variation (how the tumour is responding to different treatments biologically).

Question 2

Define Adaptive Radiotherapy (ART)

Answer 2

changing the radiation treatment plan delivered to a patient during a course of radiotherapy to account for random variations like: temporal changes in anatomy (tumour shrinkage , weightloss, internal motion) Changes in tumour biology/function (e.g. hypoxia)

Question 3

Provide examples of ART in practice?

Answer 3

Patient position correction
Image Guided RT - in room treatment imaging (i.e. Catalyst, Sentinel). Complimentary imaging during treatment - & subsequent couch shifts
Modification of treatment pla.

Question 4

What are ways patient positioning can be modified?

Answer 4

Re-setting up the patient .
Change in specific set-up instructions
Modifications to patient specific immobilisation devices
Patient positioning - is a simple example of adaptive radiotherapy

Question 5

What is IGRT and how can it be utilised for adaptive radiotherapy?

Answer 5

Images taken just prior to treatment delivery.
Assess changes in patient position relative to
treatment plan.
Adaptation via couch shifts to account for variation —> increases treatment precision.
Allows us to correct for positioning variation in more than just 3 planes.

Question 6

What are ways IGRT can be used for patient/organ motion detection?

Answer 6

• 4DCT capabilities (e.g. Elekta Symmetry)
• Patient motion detection (e.g. iGuide, Catalyst)
• Detection of correct floor rotation (e.g. Exactrac)
• Video-based systems (e.g. Varian RPM)

Question 7

What are the advantages and limitations of IGRT?

Answer 7

Advantages: Provides 3D information which can be used as the basis for adaptive planning intervention – modification of the initial plan.
Limitations: IGRT by itself cannot correct for non-rigid changes. E.g.i if matching to bone for a specific patient It cant correct for changes in the relationship between tumour and mediastinal node (if one match is lining up and the other is off). - see slide 23

Question 8

What are examples of IGRT techniques?

Answer 8

Ct based
   - Cone beam kv CT
   - Cone beam mV CT 
MRI 
Video-based 
  - AlignRT - surface guided 
  - Real time video guided IMRT
MV xray 
  - EPI

Question 9

What is deformable image registration and why is it important in ART

Answer 9

Finds geometrical correspondences between imaging datasets that differ in time, space and modality (e.g. MRI to CT)
It adapts contours required for re-planning. Used in ART because it is efficient and saves time (don’t need to spend as much time re-contouring, speeds up planning process). Also reduces workflow.

Question 10

What are the 2 main reasons deformable registration is used in ART?

Answer 10

1. Propagation of planning contours to daily images (educe time required for re-contouring).
2. Dose deformation and accumulation (can overlay the planned dose onto to the daily image to see how much the patient has received) - use of deformable vector field to propagate dose distribution.

Question 11

Why might Bladder ART be necessary?

Answer 11

Organ motion - bladder is a mobile, hollow organ that can change
significantly in size, shape and position during treatment.
This motion leads to the use of large margins (2-3cm) resulting in irradiation of healthy tissue (small bowel and bladder itself).
IGRT and ART could be used as a tool to reduce margins (from 2cm to 1.2cm) and facilitate dose escalation.

Question 12

Provide examples of bladder ART.

Answer 12

Online adaptive radiotherapy:
•Daily pre-treatment CBCT
•Treatment staff select ‘plan of the day’ (based on a conventional [same as planning] small, medium or large bladder size - these three plans are created from the first 5 treatment fractions).
•Currently most popular method particularly in research studies.
Offline adaptive radiotherapy:
• Adaptive PTV delineated based on information from first 5 fractions CBCT
• Utilisation of patient-specific margins

Question 13

Why might Prostate ART be necessary?

Answer 13

Size, shape and position of prostate is highly dependent
upon state of bordering organs (i.e. gas in rectum, bladder too small).
Can lead to under or over dosing of prostate and/or
overdosing of bladder and rectum —> increased
side effects.

Question 14

Describe Offline prostate ART methods.

Answer 14

(used if the patients rectum/bladder or prostate is continuously larger/ smaller or in a different location from planned). Can use offline ART: use the CBCT to determine how much dose has been delivered so far, then replan to deliver the required remaining dose. (can result in improvements in the therapeutic ratio ~10% and decreased side effects.

Question 15

Describe online prostate ART methods.

Answer 15

Real time adaption where the beam aperture and MLC segments are modified and adjusted to account for changes in the prostate (from gas movement during treatment). This can gain better PTV coverage but also help to spare the amount of rectum or bladder receiving dose.

Question 16

How can IGRT improve prostate ART?

Answer 16

Will improve and promote online image guidance techniques.
Current onboard CBCT imaging system is sub-optimal due to poor soft tissue contrast.
Onboard MRI can potentially enhance online IGRT and improve treatment accuracy.

Question 17

Why might Lung ART be necessary?

Answer 17

Respiratory motion and associated tumour movement a major limitation of treatment.
• Motion is very patient dependent and can vary from <1cm to >2cm
• Affects accuracy of tumour delineation
• Potential of increased side effects
• Potential of tumour moving in and out of treatment field
•Bones good surrogates for nodal disease but may lead to misalignment of primary tumour.
Changes in lung (see other card)

ART could improve:
• imaging techniques for tumour delineation and margin selection
• treatment delivery techniques
• reponse to tumour and surrounding tissue changes.

Question 18

What changes could occur in lung patients?

Answer 18

Biological changes - increase or decrease in metabolic activity.
Tumour changes - decrease in size, increase in size, variation in shape, can be difficult to determine due to atelectasis or unhealthy lung (atelectasis - complete or partial collapse of lung).
Surrounding tissue changes- re-inflation or deflation of lung

Question 19

What are examples of pre-treatment lung ART?

Answer 19

Acquiring a 4DCT dataset at planning. This means CTV position, size, shape and trajectory can be more accurately determined. Also allows for the delineation of a patient specific ITV (internal target volume).

Question 20

What are examples of treatment lung ART (general)?

Answer 20

Active motion compensation techniques such as:
gating (RPM, turning on the treatment when the tumour is in the correct location),
breath control (ABC-active breathing control - through breath hold) and
tumour tracking (dynamically tracks the tumour and moves the MLCs to follow the tumour’s changing position during free breathing.

These techniques dapt the treatment to maintain constant
target position in the beam’s eye view when the beam is
on.

Question 21

Lung ART - Describe gating:

Answer 21

Imaging and treatment devices are periodically turned
on and off, in phase with the patient breathing pattern, in order to restrict the range of positions of the tumour and internal anatomy during imaging and radiation delivery.
2 Primary methods:
- Real- time monitoring of free breathing (RPM)
- Managing a long and reproducible breath-hold.

Question 22

How does RPM work?

Answer 22

RPM system tracks the respiratory cycles of the patient through a reflective plastic box placed on the patient’s abdominal surface,
Radiation is then delivered in the gated window - this is selected where the residual tumour motion is expected to be low.

Question 23

How does ABC work?

Answer 23

Also used in DIBH for Lt sided breast patients.
ABC can be used to suspend breathing at any pre-determined position along the normal breathing cycle.
ABC procedure:
• Patient breathes normally through the apparatus
• When operator “activates” the system, the lung volume and the phase (i.e., inhalation or exhalation) at which the balloon value will be closed are specified
• Patient instructed to proceed to reach the specified lung volume
• Valve is inflated with air compressor for a predefined duration of
time -“holding” patient’s breath
• Breath-hold duration is patient dependent, typically 15-30 seconds
• Radiation delivered during breath hold

Question 24

Lung ART - Describe Tumour tracking:

Answer 24

Dynamically shifting dose in space so as to follow the tumour’s changing position during free breathing. The method should be able to do 4 things:
• Identify tumour position in real time
• Anticipate tumour motion to allow for time delays in beam response
• Reposition the beam
• Adapt dosimetry to allow for changing lung volume and critical structure locations during the breathing cycle.

Question 25

What are different types of functional imaging?

Answer 25

PET - used to moitor tumour response in lung cancer patients. FDG uptake may allow prediction of tumour response or toxicity.
SPECT - single photon emission computed tomography. Assesses lung function. Can be used to design and adjust treatment plan to limit dose to functional, healthy lung tissue.
Ventilation and perfusion imaging.

Question 26

What are ventilation/perfusion scans?

Answer 26

Ventilation: The movement of air between the atmosphere and alveoli and the distribution of air within the lungs to maintain appropriate concentrations of oxygen and carbon dioxide in the blood.
Perfusion: The movement of blood through though the pulmonary capillaries.

Can be used before, during and after treatment to design, adapt and assess treatment.

Question 27

Why might H&N ART be necessary?

Answer 27

H&N cancer patients can undergo anatomic and tumour changes during treatment. This includes: weight change, change in tumour shape and size, change in OAR shape and size, post-operative changes (e.g. oedema).
These changes can have detrimental impacts on dose distribution (inadequate target coverage and increased OAR dose). Can also have a detrimental clinical impact (potential LOCO-REGIONAL RECURRENCE and increased severity of side effects).
Planning CT is only representative of patient at that one moment in time.

Question 28

How can H&N ART be used for positional errors?

Answer 28

Positioning variation can occur as a result of anatomical changes. IGRT provides a solution for daily dose re-calculation.
2 methods to do this:
Method 1: Customise treatment plan based on patient specific systematic and random variations through image feedback.
Method 2: Adaptation of each treatment fraction by taking into account dose distribution accumulated over entire course. Adjust the originally prescribed dose by decreasing or increasing dose goals for regions which were overdosed or underdosed.

Question 29

What are example of H&N anatomic variation?

Answer 29

Tumour shrinkage/ growth
Weight changes
Parotid gland changes (shift medially, significant decrease in size).
The magnitude of dosimetric impact of these changes depends on: location of high dose target volume, proximity of OAR to high dose region and dose gradient

Question 30

What has to be considered for ART - in relation to H&N planning?

Answer 30

Need to decide when is the best time during a patients treatment to adapt the plan. Some studies have found that a mean overdose of 4Gy will affect salivary output, therefore, probably not worth replanning for a patient is the dose is exceeded by ~2Gy.

Question 31

What are the limitations of ART?

Answer 31

Time and Resource intensive
- (Major barrier to routine clinical adoption & Financial burden)
More clinical outcome studies needed
- Is the outlay worth the benefit?
- What is the threshold where clinical benefits outweigh resource outlay?
Appropriate patient selection
- What patients will benefit most
- Appropriate resource allocation
What is the optimal number of replans?
- Clinical benefit vs time and resources spent
- When to intervene
Consensus guidelines lacking
- Online vs offline vs hybrid (taking into account patient benefit and also want’s practical in a clinical setting)
- Technical and QA considerations
Patient-specific margins
- How many fractions (initial observations) do you look at to determine that margin
Plan of the day (e.g. multiple bladder plans) - time/ resource intensive
- Staff training and education intensive
Extensive re-contouring required
- Increase efficiency with more automation
- Need reliable deformable image registration tools