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
