Adaptive Radiotherapy Flashcards
Overall benefits of ART
- Accounts for temporal changes in anatomy and changes in tumour biology/function
- Accounts for organ motion
- Accounts for variation in patient position
What is ART?
- Adaptive radiotherapy
- Adaptive radiotherapy is defined as changing
the radiation treatment plan delivered to a
patient during a course of radiotherapy to
account for:
u Temporal changes in anatomy (e.g. tumor
shrinkage, weight loss or internal motion)
u Changes in tumor biology/function (e.g. hypoxia)
Advantages of IGRT
- Can lead to reduced target margin
- May decrease dose to surrounding OAR
- Provides rich 3D information which can be used as the basis for adaptive planning intervention - modification of the initial plan
IGRT Role in ART
- Images taken just prior to treatment delivery
- Assess changes in patient position relative to treatment plan
- Adapting positioning to account for variation —> increased treatment precision
Clinical Examples of IGRT
- 4DCT capabilities (Elekta Symmetry)
- Patient Motion Detection (iGuide, SGRT)
- Detection of Correct Floor Rotation (Exactrac)
- Video-based systems (Varian RPM)
- Ultrasound (Clarity Autoscan)
- RF Tracking (Calypso)
- MRI
Limitations of IGRT
Image guidance in isolation can not correct for non-rigid changes
Timepoints of ART Application
Offline (between fractions)
* Most common form of AR
* Continue treating the patient while the plan is being re-planned and assessed
Online (immediately before a fraction)
* Plan is adapted prior to treatment - XRT phase is not continued until plan is assessed and complete
Real-time (during a fraction)
* Dose calculation during real time
* Some technology can locate the target and treat simultaneously
It can also be a combination of online and offline
Real-time workflow
Technology to locate the target during real-time
RF waves: Calypso, RayPilot
Dose calculation during real-time
Technology to hit the target in real-time
Limitations with Current Bladder XRT
Organ Motion
* Size, shape and position can significantly change both intra and interfractionally
* Leads to generous margin (2-3cm) patients experience side effects which potentially can be avoided
* Need to decrease prescriptive dose to account for this (dose escalation is often related to poor treatment outcomes)
Current Dosing for Bladder ART
- General prescribed dose: 50-66Gy (relatively low in comparison to other sites)
- Could be related to poor outcomes with bladder cancer
- Retrospective studies suggest significant improvement in outcome with dose escalation (may be possible with ART)
Bladder: Online ART Workflow
Step 1: Daily pre-treatment CBCT
Step 2: Plan of the Day (most popular method currently used clinically)
Bladder: Offline ART Workflow
- Adaptive PTV delineated based on information from the first 5# CBCT’s.
- Utilisation of patient specific margins
Limitations with Current Prostate XRT
- Size, shape and position is dependent on bordering organs (rectum, bladder)
- Can lead to under/overdosing of prostate, bladder, rectum → side effects
Prostate: Offline ART
- Delivered dose and variation of OARS is accounted for in the planning objective function.
- Dose distribution accomplished in the adaptive plan is used for the remaining treatment.
- Optimiser increases/reduces dose in sections of target volume, dependent of dose already delivered in treatment.
Prostate: Online ART
- kV intrafraction monitoring
- Real time localisation method
- Can be used to track gold seeds image is taken approx. every 30 degrees and couch can be shifted to account for intrafraction motion.
Prosate: Real-Time ART
- Together, KIM and MLC enable IGART using a standard LA without additional equipment.
- Current onboard CBCT imaging is suboptimal for online guidance of prostate
Limitations with Current Lung XRT
- Prognosis for NSCLC is poor (<50% 5 year survival)
- Dose escalation studies show promising results – increased loco-regional control and improved survival.
- Routine dose escalation restricted by surrounding dose limiting structures/side effects.
- Respiratory motion - very patient dependent and can vary from 1cm to 2cm.
Benefit of ART in Lung XRT
- Improved tumour delineation and margin selection
- Adaption for treatment plan due to biological and function response
- Allows for dose escalation, dose maintenance and normal tissue sparing.
Briefly explain ‘Gating’
Imaging and treatment devices are periodically turned on and off, in phase with the patient’s breathing pattern, in order to restrict the range of positions of the tumour and internal anatomy.
What are two primary methods of ‘Gated’ XRT
- Breath Hold
- Free Breath (with real-time monitoring)
What is the benefit of ‘breath control’ techniques?
Can be used to suspend breathing at any predetermined position along the normal breathing cycle
Provide the steps involved in the application of ABC in treatment
- Patient breathes normally
- Operator activates system →machine will be shut down at specified volume
- Patient proceeds with specified lung volume
- Valve is inflated with an air compressor for a predefined duration of time.
- Breath hold duration is patient dependent, typically 15-30 seconds.
- Radiation delivered during breath hold.
Briefly explain ‘tumour tracking’ in Lung XRT
Dynamically shifting dose in space as to follow the tumours changing position during free breathing.
Provide examples of Biological and Functional Imaging used for Lung Patients
- PET Imaging
- SPECT Imaging
- Ventilation Imaging
- Perfusion Imaging
Application of PET Imaging in Lung XRT
- FDG-PET is increasingly used to monitor tumour response in NSCLC patients.
- May allow prediction of tumour response and or radiation induced lung toxicity.
- Early identification has potential to reduce side effects and costs of ineffective or toxic treatment
Application of SPECT Imaging in Lung XRT
- Single Photon Emission Computed Tomography
- Assess lung function.
- Can be used to design and plan in accordance with tissue variation.
What is Ventilation Imaging?
- Assessing 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.
- Can be used before, during and after treatment
What is Perfusion Imaging?
- The movement of blood through the pulmonary capillaries
- Can be used before, during and after treatment
- May be able to adapt the plan and ‘boost’ areas of active tumour
Limitations with Current H/N XRT
- Can undergo considerable anatomic and tumour change.
- Weight change
- Change in size and shape of tumour and nodal disease.
- Change in OAR size and shape.
- Post op changes (oedema)
Effect of Changes in Anatomy in H/N XRT
Dose distribution:
* Altered dosimetry
* Altered dose gradient
* Inadequate coverage
Clinical impact:
* Loco-regional recurrence
* OAR’s exceeding tolerance dose → increased severity of side effects
Potential methods of Application of ART to account for Positioning Variation
Method One:
* ART aims to customise treatment plan to patient specific variation by evaluating and characterising systematic and random patient specific variations through feedback
Method Two:
* Quantify errors in setup error → re-optimise plan to account for varied dose distribution to the target across the delivered fractions → alter dose distribution for following fractions.
What is Deformable Image Registration?
- Finding geometric correspondences between imaging data sets (2D/3D/4D) that differ in time, space, modality
- Propagates planning contours to daily images (reduced time for re-contouring)
- Dose deformation and accumulation (use of deformable vector field to propagate dose distribution)
Benefits of MRI Linac in ART
- Hybrid linac combined with MRI
- 1.5T linac can be used for image guidance in multiple sites providing diagnostic quality images during treatment delivery
- Allows for accurate image-guided daily ART
- Allows for daily fast adaptive replanning and gated or tracked treatment
- Functional imaging may allow for adaptive focal boosting and personalised inhomogenous target dosage based on response
MR Linac - Adapt to Position
- Allows for plan adaptation based on the online patient position
- Based on rigid registration → isocenter position in the reference data is updated
- Re-treatment plan recalculated or re optimised to reproduce or improve the target coverage
- No contours need to be edited as original contours will be used for adapted plan
MR Linac - Adapt to Shape
- Allows for plan adaptation based on new patient anatomy and plan is optimised on daily MRI and adapted contours
- Pre-treatment contours automatically propagated by deformable registration onto the online planning MRI (if necessary, contours can be edited by RO)
- Plan is recalculated or re optimised on online planning MRI and adjusted contours
Adapt to Position vs Adapt to Shape
- ATP plan adaptations are faster than ATS
- ATP can lack target coverage
- Total treatment time ranges from 30 to 85 minutes (most completed within 60 minutes)
Brief explanation of Radixact Synchrony
- Updated form of Tomotherapy
- Integrated intrafraction motion management based on the Synchrony
- Used software to predict motion based on implanted fiducials or tumour itself (using imaging information to actively track and correct for movement)
Overall Limitations of ART
- Time and resource intensive (major barrier to routine clinical adoption)
- Financial burden on radiation therapy department
- Determination of appropriate patient selection
- Determination of optimal number of replans
- Lack of consensus guidelines across locations
- Extensive re-contouring may be required → more automation may be required
- Determination of appropriate POD creation
- Accounting for intra-fraction motion
- Imaging distortions in CT and MRI modalities
- Optimal method of image registration not yet decided - rigid vs deformable
Why do we need Lung ART
Tumour changes:
- decreases in size
- increases in size
- can be difficult to determine due to atelectasis or unhealthy lung
- variation in size and shape
- surrounding tissue change
What happens to parotid glands during RT
Shifts medically
Significant decrease in size
What does the magnitude of dosimetric impact depend on in ART
Location of high dose target volumes
Proximity of OARs to high dose region
Dose gradient
Head and neck trials
ARTFORCE
MiADAPT
MRADAPTOR
Deescalation of dose based on functional imaging assessments
Biological adaption for H+N
Would allow modification of treatment based on
changes in tumor resistance factors and normal tissue
function
u Allow adaptation based on assessment of early
response
u Define volumes that would benefit from dose
escalation
u Utilisation of MRI Linac
Use of ART in H+N
Positioning errors
Anatomical change
Biological response (only in research)
Why do we do functional imaging for lung patients
- early identification of non-responding patients
- patients at high risk for radiation induced lung toxicity -> reduce side effects
- FDG uptake highlights this
- highlights functioning lung: can reduce dose to this region
What should tumour tracking be able to do to be integrated clinically?
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
