SRS Flashcards
What are the indications for SRS?
Mets Benign tumours (meningioma, pituitary) Vascular lesions (AVM) Functional treatments (trigeminal neuralgia)
What is SRS?
A single fraction of highly conformal radiotherapy delivered to precisely delineated target volumes, delivered using stereotactic localisation techniques
What is SRT? (How many fractions?)
A term used to describe treatment to a small targey given in multiple fractions. It usually imples a cranial target. (2-5 fractions.)
What does fSRT stand for?
Fractionated SRT - conventional fractionation.
What platforms are available for SRS delivery?
Gammaknife
Cyberknife
Linac
What were the principles of the early Gamma Knife?
201 Cobalt-60 sources focussed on isocentre
Interchangeable helmets with small circular collimators for each source (4, 8, 14, 18mm collimators)
Produced spherical dose distributions with diameter dictated by collimator size
Immobilisation and localisation using invasive frame
Treatment plan involves sphere packing: varying number and position of isocentres (=shots), size of collimator and treatment time
Can further optimise plan by blocking individual sources by manually plugging collimator
What are the principles of the Gamma Knife Perfexion?
192 Cobalt-60 sources focussed on isocentre
Circular collimators in-built (4, 8, 16mm)
Sources split into 8 sectors which can use different collimator sizes or be blocked to produce non-spherical shots
Option of using relocatable frame to allow fractionated treatments
Icon model comes with IGRT (CBCT)
What are the principles of the Cyberknife?
6MV Linac mounted on robotic control system with stereotactic radiographic localisation of bony anatomy or implanted fiducials.
Patient position tracked during treatment using tubes in ceilings and detectors in floor.
Synchtrony camera follows patient and adjusts robot to deliver correctly.
Multiple beams from any direction.
Older cyberknife systems had circular collimators (12 sizes), newer have MLCs.
Iris is a dodecagon shaped variable collimator.
What are the principles of stereotactic Linacs?
Cone based techniques are used.
Cones produce sharp penumbra and dose gradients.
Dose is delivered in several arcs intersecting at the isocentre.
A spherical dose distribution is produced (ideal for spherical lesions), similar to Gammaknife.
Typical cone sizes are 4-50mm (50mm cone would be used for QA as larger field sizes are desired).
What is the main disadvantage of cone based techniques?
To achieve homogeneity for larger or irregular lesions, multiple isocentres are required. This can result in high inhomogeneity and long treatment times.
What are the advantages of Cyberknife?
Track patient motion – good motion management Imaging Can treat extracranial Can fit in smaller room Non-isocentric
What are the advantages of Gammaknife?
Conformal sharpest dose gradients Simpler QA Good up time compared to Linac Doesn’t breakdown much – not much to break Immobilisation (?) – could be used on other machines No couch or gantry movements Well established
What are the advantages of stereotactic linacs?
Use for other treatment – patient throughput
Potential for patient motion tracking
Quick patient treatment times – can use FFF
Lots of imaging options
What are the disadvantages of Cyberknife?
Small volume only Can’t treat below couch Very specialised Hard QA Would need primary barrier everywhere in room
What are the disadvantages of Gammaknife?
Small volume only
Change sources every 3-5 years (?) (half life of Co-60 is 5.25 years)
Active sources result in lots of paperwork & security arrangements
Can only treat brain (now base of skull) – intracranial targets only
Multiple isocentres
What are the disadvantages of stereotactic linacs?
Limited non-coplanar treatments
Minimum treatment size limited by MLC leaf width and dosimetric accuracy
What are the advantages of using frameless immobilisation?
Less invasive
Good for SRS, SRT and fSRT
Less time pressure for planning/QA = better plan?
Accuracy now approaching frame based techniques?
What are the advantages of using framed-based immobilisation?
High accuracy (not perfect – not 0) Treatment on same day
What are the disadvantages of using frame-based immobilisation?
Invasive Local anaesthetic required Time pressure for treatment on same day Only suitable for SRS Can still get sag/slipping of patient within the frame
What is automatic segmentation?
Elastically fuses an advanced human atlas onto the patient data set considering mutual information, then transfers atlas contours and fine-tunes using local grey-value analysis.
Brainlab cranial atlas based on T1 weighted MRI (which has been fused to CT in localiser box)
Can create customised templates
Name and explain the 2 potential future developments of MRI protocols for SRS.
Fibertracking
based on diffusion tensor imaging (DTI) MRI, selects fiber bundles starting, passing and ending in defined ROIs. Converts to 3D objects for planning
BOLD
Blood Oxygen Level Dependent MRI mapping. Image processing for motor and speech functional areas. Convert to 3D structures and set as an OAR for better protection
What considerations should be made for SRS planning?
Fine dose matrix used for calculations due to steep dose gradient.
Avoid entrance/exit dose through OAR
Minimise couch angles for efficient delivery
Single isocentre technique may be more efficient than multiple for multiple mets
Small PTV margins
What is the main advantage of using multiple isocentres?
Better shaping and sparing as the smaller MLCs in the centre of the field would be utilised for each met.
What is the equation for coverage?
PTV V100% (cc) / GTV or PTV volume (cc)
AKA: the volume recieving 100% of the prescription dose over the volume of the PTV/GTV
What is the equation for selectivity?
PTV V100% (cc) / Body V100% (cc)
AKA: the volume recieving 100% of the prescription dose over the volume of the body
What is the equation for the Paddick Conformity Index?
coverage * selectivity
= (PTV V100 / PTV vol) * (PTV V100 / body vol)
Higher is better: 1 is perfect, >0.75 is good plan
What is the equation for the Gradient Index?
Body V50% (cc) / Body V100% (cc)
V50% / PTV Vol = Body V50% (cc) / PTV volume (cc)
Lower is better: want under 3.5
What are the 3 main sources of error in small field dosimetry?
Loss of lateral electronic equilibrium
Source occlusion leading to penumbra overlap and reduction in output
Strong dependence on size and construction of detector used
What is the definition of a small field?
A field size smaller than the lateral range of the charged particles.
(normally <3x3cm)
What methods can be used to avoid errors in small field dosmetry?
Multiple layers of redundancy (use more than one detector)
Peer review of data
Ask for manufacturers reference data
Independent end-to-end testing (clinical trials, national audits)
What does the Starshot QA check test for?
Rotational accuracy of the gantry, collimatory and couch.
What does the Winston Lutz QA check test for?
Accuracy of the radiation isocentre.
What are the tolerances of accuracy for GammaKnife, CyberKnife and a stereotactic Linac?
GK: <0.9
CK: <0.95
Linac: 1
Which test can be used to check geometric accuracy?
Hidden target test: Set up to an isocentre ball bearing embedded in a phantom and image using a small cone or MLC field
(Lucy phantom can be used)
What are the 4 tiers of SRS treatments according to NHS England?
Tier 1 = neuro-oncology (metastases & non-skull base meningiomas)
Tier 2 = skull-base (meningiomas, vestibular schwannomas etc)
Tier 3 = vascular (AVMs & cavernomas)
Tier 4 = other rare indications (trigeminal neuralgia, rare tumours etc)
