SABR Flashcards
What does SABR stand for?
Stereotactic Ablative Body Radiotherapy.
What does SBRT stand for?
Stereotactic Body Radiotherapy.
What is SABR?
The precise irradiation of an image-defined extra-cranial lesion with the use of high radiation dose in a small number of fractions.
Ideally the tumour motion is less than 1cm (SABR consortium guidelines).
It is characterised by: Low number of fractions High dose per fraction Small volumes Good immobilisation Rigorous image guidance
What is the main difference between SABR and SRS?
The radiation is extra-cranial for SABR.
What is hypofractionation?
Higher doses in fewer fractions, resulting in a higher BED.
Which sites are suitable for SABR?
Oligometastases: intermediate state of cancer spread between localised disease and widespread metasteses. Lung Liver Adrenals Spine Pelvic Nodes
What advantage does using a fast CT protocol have?
Can image more of the patient without the tube overheating as couch can move faster.
What imaging is recommended for Lung SABR?
4DCT. If not available: kV flouroscopy Slow acquisition CT 3D with extremes of breathing – max inhale, exhale (hold breath), and free breathing & combine
Which manufacturers & systems are available for 4DCT?
Varian RPM system
ANZAI system
C-RAD catalyst
VISIONRT gatect
Describe the Varian RPM respiratory measuring system.
Small marker block with 6 infra-red reflective dots placed on/close to patient’s xiphisternum.
Position of marker block tracked by in-room camera sensitive to visible and infrared.
Camera and associated PC output the position of the bock over time to get waveform.
Describe the ANZAI respiratory measuring system.
Belt with internal pressure sensor placed around patient’s abdomen.
Patient’s breathing creates changes in pressure within belt to get waveform.
Describe the C-RAD catalyst respiratory measuring system.
Optical surface scanning
No markers or fiducials
Can also be used to assist patient posture correction
Useful as nothing to put on patient, can track different parts of patient – change of shape & abdomen movement.
Describe the VISIONRT gatert respiratory measuring system.
3D surface mapping
No markers or fiducials
Can also be used to assist patient posture correction
Useful as nothing to put on patient, can track different parts of patient – change of shape & abdomen movement.
Describe the VISIONRT gatect respiratory measuring system.
3D surface mapping
No markers or fiducials
Can be used to assist patient posture correction
If this is on the CT and the Linac, then there is additional confidence in the patient setup.
Describe the Calypso respiratory measuring system.
Implanted electromagnetic transponders (patient uncomfortable & already ill)
Electromagnetic array detector
Not a motion surrogate
Invasive
What is the equation for pitch of a CT table?
Pitch = travel per rotation / beam width
What are the drawbacks of retrospective gating?
Imaging dose significantly increased with 4DCT compared to comparable 3DCT (approx. 3 times larger)
Automatic dose control may not be available for 4DCT acquisitions
Limitations on scan length limitations (tube overheating)
Imaging anatomy outside the CT’s normal field of view (extended field of view) may not be possible
Larger patients – ipsilateral breast
What are the 2 methods of binning for 4DCT? What are the advantages/drawbacks of each?
Amplitude binning - fewer image artefacts, may not get full motion.
Phase binning - better for reconstruction of peaks, tends to contain image artefacts
What is prospective gating?
The CT acquisition is only gathered during a defined interval of the breathing cycle (gated acquisition).
E.g: acquire and treat at full exhale.
What can cause 4DCT artefacts?
Loss of data
Long scan period with no definitive maximum inhale/exhale
Stepping artefact/Bed moving slowly (assumes same breathing cycle)
May be significant change in breathing cycle from breath-to-breath
What is the MIP?
Maximum Intensity Projection
Usually used for outlining
What is the AvIP?
Average Intensity Projection
Usually used for planning
What is the TWAvIP?
Time Weighted Average Intensity Projection
What considerations should be made for compression for 4DCT?
Does it adequately restrict tumour motion?
Can it be used for more than one tumour site?
Does it attenuate the beam?
If the patient is large, fat is malleabe, and so will compression actaully help?
Is it restricting everything as expected? (abdominal compression may only restrict front of diaphragm and rear is still free to move).
What is passive motion management?
Accepts that motion exists but attempts to quantify the range of motion. The treatment is created to incorporate the movement PRIOR to delivery.
What is active motion management?
Actively attempts to reduce the amount of tumour motion, like with real-time tracking. The treatment is adapted during delivery.
Give an example of passive motion management.
Free breathing 4DCT acquisition.
ITV delineated over all phases of 4DCT
Free breathing treatment delivery with no motion monitoring
Give an example of active motion management.
Compression or breath hold techniques:
- Treatment delivery is not interrupted
- Reproducibility may be a problem
- Patient compliance may be an issue
Gated delivery:
- In-efficient use of the duty cycle
- Extended beam on time
- Minimising breathing motion, but increasing the possibility of postural changes
Tracking:
- Beam on throughout
- Relies on accurate prediction model
- Relies on verification during delivery
How is verifcation of gated delivery achieved?
kV CBCT to verify setup to bony anatomy/posture
Compare reference breathing trace to measurement on the day
Acquire fluoro to assess tumour movement
What are the parameters for a 4D CBCT Thorax verification imaging protocol?
kV: 125 mA: 40 Pulse (ms): 20 Frames/sec: 7 Deg/sec: 3 mGy/mA: 1.32 Projections: 840 Time(sec): 120 mAs: 672 CTDI (cGy): 0.89 Slice (mm): 1.5 Reconstruction: Advanced 4D
What are the parameters for a 3D Abdo verification imaging protocol?
kV: 125 mA: 60 Pulse (ms): 20 Frames/sec: 15 Deg/sec: 6 mGy/mA: 1.32 Projections: 900 Time(sec): 60 mAs: 1080 CTDI (cGy): 1.43 Slice (mm): 1.5 Reconstruction: Default 3D
What are the parameters for a 3D Abdo Slow verification imaging protocol?
kV: 125 mA: 64 Pulse (ms): 20 Frames/sec: 7 Deg/sec: 3 mGy/mA: 1.32 Projections: 840 Time(sec): 120 mAs: 1075 CTDI (cGy): 1.42 Slice (mm): 1.5 Reconstruction: Default 3D
What are the parameters for a 4D CBCT Abdo verification imaging protocol?
kV: 125 mA: 64 Pulse (ms): 20 Frames/sec: 7 Deg/sec: 3 mGy/mA: 1.32 Projections: 840 Time(sec): 120 mAs: 1075 CTDI (cGy): 1.42 Slice (mm): 1.5 Reconstruction: Advanced 4D
For 4D reconstruction, what standard parameters are used?
10 bins
10% width
What does ablative mean?
Ablation: a procedure in which a tissue or body part is removed or destroyed by surgery, radiation or photocoagulation.
Which treatment techniques are encompassed by the ICRU91 definition of SRT (stereotactic radiotherapy)
SRS
SABR (a more common UK term for SBRT)
SBRT
What do SABR margins depend on?
Immobilisation and verification at the centre.
How can inter-observer variations be improved?
Consistent windowing Standard contouring guidelines Additional imaging info Training Peer review/independent checks Audit
What is the standard fractionation regime for SABR, and what BED does it give?
54 Gy 3#
BED = 154 Gy
What is the conservative fractionation regime for SABR, and what BED does it have?
55 Gy in 5#
BED = 115 Gy
What is the very conservative fractionation regime for SABR, and what BED does it have?
60 Gy in 8#
BED = 108 Gy
When would the conservative fractionation schedule be used for SABR?
If the PTV contacts the chest wall
When would the very conservative fractionation schedule be used for SABR?
When the OAR dose constraints can’t be met
What is the inter-fraction interval for SABR?
Minimum: 40 hours
Maximum: 4 days
What are the dose constraints for 5 fraction SABR: Normal Liver?
V10: Optimal: <70%
Mean Liver Dose: Optimal: < 13 Gy, Mandatory: <15.2 Gy
What are the dose constraints for 3 fraction SABR: Normal Liver?
D50%: Optimal: <15 Gy
Dose to >=700cc: Optimal: <15 Gy, Mandatory <19.2 Gy
What are the dose contraints for skin in 3 fraction SABR?
Optimal:
Dmax (0.5cc) < 33 Gy
D10cc < 30 Gy
What are the dose constraints for skin in 5 fraction SABR?
Optimal:
Dmax (0.5cc) < 39.5 Gy
D10cc < 36.5 Gy
What are SABR dose distributions characterised by?
Highly conformal dose distribution
Peaked dose distribution with high max doses
Sharp fall-off of dose to maximise sparing of surroundig OARs
What are the UK recommendations for SABR prescription dose?
95% of PTV volume gets 100% of prescription dose
99% of PTV volume gets 90% of prescription dose
Dmax is between 110-140%
Which algorithm should be used for calculating SABR plans?
Type B or MC to corectly model changes in lateral electron transport.
What are the allowed fractionation regimes?
18 x 3#
12 x 5#
7.5 x 8#
(20 x 3# is not allowed as this was used in early trials - calculated by PB)
How are SABR doses prescribed?
To the outside of the PTV (i.e. min dose to PTV)
What is CHART treatment?
Continuous Hypofractionated Accelerated Radiotherapy.
No weekend breaks, multiple fractions per day = 3 per day (still need 6 hours between fraction), finish quicker
