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