Adaptive Radiotherapy

Question 1

What is 4D-Adaptive Radiotherapy?

Answer 1

Using IGRT information to establish the dose to the tumour and OARs on a regular basis during the treatment course, to enable the continuous adaption and re-optimisation of the plan.

Question 2

Which documents are applicable to adaptive radiotherapy?

Answer 2

• On Target: Ensuring Geometric Accuracy in RT

* National Radiotherapy Group Implementation Report: IGRT: Guidance for implementation and use

Question 3

Name and explain the 3 IGRT categories.

Answer 3

• Simple: Techniques that do not require serial imaging. Usually a gross error check on the first fraction.
• Complex: Techniques that involve significant serial imaging for systematic error correction. Usually an off-line correction: 2D or 3D image acquisition on a sequence of fraction.
• Adaptive: Techniques using 3D image acquisition to enable a positional change of the treatment field at the time of treatment. Usually on-line correction: 3D image acquisition with immediate intervention.

Question 4

What are the levels of adaptive radiotherapy?

Answer 4

• 4a: Imaged once-weekly to reduce uncertainties from shape change. Scehduled repeat planning imaging during course with offline dosimetric assessment. Corrected by replanning when dosimetric action level exceeded.
• 4b: Imaged as seen (triggered by a change) to reduce uncertainties from shape change. Treatment unit imaging & online/offline dosimetric analysis. Replan to assess for dosimetric changes & implement changes.
• 4c: Image each fraction to reduce uncertainties from shape change. Treatment unit imaging & online/offline geometric analysis with a comparison to best fit plan. Corrected by delivering ‘plan of the day’ for that fraction. (for example the bladder trial: RAIDER)
• 4d: Image each fraction to reduce uncertainties from shape change. Treatment unit imaging & online dosimetric analysis. Corrected by real-time ART.

Question 5

What are the disadvantages of using a plan-of-the-day technique?

Answer 5

• Increased workload - eg planning 3 VMAT plans.
• Risk of delivery of wrong plan as more plans will be available to treat during a single fraction.
• How can you be sure you are using the correct plan for delivery?
• How do you account for bladder filling during treatment?

Question 6

What are the drawbacks of adaptive radiotherapy?

Answer 6

• Need quick integration between imaging, replan and delivery.
• Predictive aspects of tracking are complicated as things are always a split second behind.
• The match is only within 3 mm
• We are not changing target/OAR shape currently (cannot see the sub-clinical disease to adapt the GTV/CTV - so even if the tumour appears to shrink, it is unknown as to whether the current standard margins account for microscopic spread during treatment).

Question 7

What are the challenges of Biological Image Guided Adaptive Radiotherapy?

Answer 7

• Thresholding of metabolic images for accurate contouring
• Multi-modality registration: RTS registration
• Inter-disciplinary integration
• Inter-disciplinary training
• Inter-disciplinary collaboration

Question 8

How can F-18 FDG be used in Biological Image Guided Adaptive Radiotherapy?

Answer 8

As a cellular metabolism marker.

Question 9

How can F-18 FLT be used in Biological Image Guided Adaptive Radiotherapy?

Answer 9

As a cell proliferation marker.

Question 10

What is the name of the current bladder adaptive radiotherapy trial called?

Answer 10

RAIDER.

Question 11

What is the pathway for the RAIDER trial?

Answer 11

Bladder prep: Patient voids then drinks 350 ml of water.
CT scans are acquired at 30 and 60 minutes without voiding between scans.
Plans are created for small and large PTVs.
Plan of the day is chosen for which suits anatomy at time of fraction delivery.