SRS Flashcards
What are SRS treatments?
Techniques that are highly conformal radiotherapy treatments delivered to precisely delineated targets, delivered using stereotactic localisation techniques
What is the difference between SRS, SRT, and fSRT?
SRS - 1 fraction
SRT - 2-5 fractions
fSRT - conventional fractionation
What are the indications for SRS?
Metasteses
Benign tumours
Vascular lesions
Functional treatments - neurological conditions
What is the difference between frame based and frameless immobilisation?
Frame based - stereotactic system of external coordinates used for localisation and positioning
Frameless - Anatomy and IGRT - can’t lose accuracy, needs online monitoring
What is the difference between invasive and non-invasive immobilisation?
Invasive - patient rigidly fixed to stereotactic system using invasive techniques
Non-invasive - patient friendly immobilisation which can be used for multiple fraction treatments
What are the platforms for delivering SRS?
Linac
Gammaknife
Cyberknife
How do Gammaknife Perfexion systems work?
192 Co-60 sources focused on the isocentre
Circular collimators are inbuilt (4, 8, 16mm)
Sources split into 8 sectors that can use different collimator sizes or be blocked to produce non-spherical shots
Can use relocatable frame for multiple fractions
IGRT on Icon model
What dose algorithms are used for Gammaknife treatment planning?
Historically used factor based algorithm -TMR10 - assumes head is water - planned on MRI scan for target/OAR, head shaped using skull scaling instrument
Now use convolution algorithm to take density variations into account - use CT scan for planning
How does a cyberknife system work?
6MV linac mounted on robotic control system with stereotactic radiographic localisation of bony anatomy or implanted fiducials to track the patient and alter beams accordingly - multiple beams from any angle - except through couch
Used to use collimators - now uses 41 MLCs
How is tumour tracking achieved with cyberknife?
Fiducial tracking
Spine tracking
Lung tracking
6D skull tracking
What are the features of APEX?
Add on to head
Dynamic micro-multileaf collimator
2.5mm MLCs
Largest treatable area - 12x14cm
Why were cones used for SRS delivery?
Produces sharp penumbra and dose gradients
Dose delivered in several arcs intersecting at isocentre - produces spherical dose distribution
What are the issues with cones?
For larger or irregular leasions multiple isocentres are needed - inhomogeneous dose, long treatment times
Why are MLC techniques used?
More precise shaping for larger and/or irregular lesions - use conformal fields, dynamic arcs (forward planned), or IMRT
What are the advantages for Gammaknife?
Very conformal Sharpest dose gradient Lowest reported uncertainties No couch or gantry movements Can treat very small lesions
What are the disadvantages of Gammaknife?
Long treatment times
Radioactive sources need to be replaced
Intracranial targets only
Have to use multiple isocentres for multiple lesions
What are the advantages of Cyberknife?
Non-isocentric - very conformal treatments
Excellent motion management
Can treat extracranial lesions
What are the disadvantages of Cyberknife?
Long treatment times
Can’t treat through couch
What are the advantages of Linacs?
Used for other techniques Fast treatment times - FFF Lots of imaging options Some motion management Can treat extracranial lesions Can treat multiple lesions with 1 isocentre
What are the disadvantages of Linacs?
Limited non-coplanar treatments
Minimum treatment size limited by MLC leaf width and dosimetric accuracy
What are the features of using frame based immobilisation?
Invasive Frame placed under local anesthetic Tx on same day Only for SRS Can get sag/slip of patient in frame High accuracy
What are the features of using frameless immobilisation?
Less invasive Good for SRS, SRT, fSRT Less time pressure for planning/QA Accurate to 1mm Frameless mask system attached to carbon base plate fixed to extension of the imaging couch top
How can localisation be achieved?
Automatic CT localisation detects fiducial rods to make 3D coordinate system - can use immobilisation backplate
Scan full skull in order to use vertex fields
Angiography localisation for AVMs brings 2D and 3D data into same coordinate system
What imaging modalities can be used for SRS?
At least CT and MRI. Also use SPECT, PET
Need slice spacing < CTV-PTV margin
How is automatic segmentation used in SRS?
Elastically fused an advanced human atlas onto the patient data set
Cranial atlas based on T1 weighted MRI
What are the future advancements for automatic segmentation?
Fibertracking - based on diffusion tensor imaging MRI, selects fiber bundles starting, passing through, and ending in defined ROIs
BOLD - uses blood oxygen level dependent MRI mapping - image processing for motor and speech functional areas
What are the treatment planning characteristics for SRS?
High fraction doses
CTV=GTV for mets
Small PTV margins - GK,CK = 0mm, linacs < 1mm
Steep dose fall off
Avoid overlap of beams, careful of entrance/exit beams through OARs
Minimise number couch angles for efficient delivery
Dose homogeneity requirements relaxed for increased conformality
For multiple mets use single isocentre as more efficient
What are the dose prescriptions for the RTOG trial for brain mets
Diameter <2cm - 24Gy
Diameter 2.1-3cm - 18Gy
Diameter 3.1-4cm - 15Gy
Mets in brainstem - 25Gy/5#
Dose can vary from 12Gy-90Gy depending on area
Wide variation in practice
Where is the dose generally prescribed to?
Isodose surface (40-90%) which encompasses the target volume (GK usually to 50%, linac to 80%)
What is the radiobiology of SRS?
LQ model suggests possible new cell death mechanisms - stem cells, vasular damage, bystander effects, and immune-mediated effects
Intra-fraction time is longer so could get more repair of sublethal damage so lose therapeutic effect
What are the equations for coverage and selectivity?
Coverage = PTV V100%(cc)/GTV or PTV volume(cc) Selectivity = PTV V100%(cc)/Body V100%(cc)
What are the equations for the Paddick conformity index and gradient index?
CI = TVpiv/TV x TVpiv/PIV = coverage.selectivity GI = Body V50%(cc)/Body V100%(cc)
Why is small field dosimetry tricky?
Loss of lateral electronic equilibrium
Source occlusion leads to penumbra overlap and reduction in output
Strong dependence on size and construction of detector used
Need to select correct dosimetry equipment
How can errors be avoided in small field dosimetry?
Use multiple layers of redundancy - use more than one type of detector
Peer review of data
Ask for manufacturer’s reference data
Independent end-to-end review ie clinical trials, national audit
What does the starshot test?
Rotational accuracy of gantry, collimator, and couch
What does the Winston-Lutz test test?
Accuracy of radiation isocentre - verifies accuracy of setup, mechanical isocentre shift over time, cone mounts or MLCs
How is the Winston-Lutz test performed?
Set up ball bearing to lasers or light field, irradiated using small field - should get projection of ball bearing at centre of field on all GA/couch angles
What are the typical tolerances of GK, CK, linacs?
GK - 0.6mm, tolerance <0.9mm
CK - 0.35mm, tolerance <0.95
Linac - <1.0mm, tolerance 1.0mm
Why should centres always use margins? What can it mean if results are still achieved without margins?
Need to be certain that the prescribed dose is accurately delivered
Can mean doses don’t need to be that high
Uncertainties are incorporated elsewhere in the system
Don’t need to irradiate the entire lesion to get the desired clinical result
How can the geometric accuracy of system be tested?
Using the hidden target technique - embed a BB at isocentre in a phantom and image using a small cone or MLC field
What is the purpose of end-to-end testing?
Estimates the overall combined accuracy for a given treatment process - follow patient pathway
Why is it important to know the uncertainties in your treatment process?
SRS is unforgiving due to small margins
Set-up uncertainties~1mm - can overdose/underdose OARs/targets
Dose calibration errors are significant
What are 3 examples of errors in SRS treatments?
Incorrect linac calibration
Backup jaws set incorrectly for cones
Crainial localisation accessory error
What factors contribute to SRS errors?
Lack of training, competence, experience
Inadequate staffing and/or skill
Fatigue and stress - staffing and skill levels
Poor design and documentation of procedures
Complexity and sophistication of new tech
Over-reliance on automated procedures
Poor communication and lack of team work
Inadequate infrastructure and work environment
Changes in processes
What are the tiers of SRS treatments?
1 - neuro-oncology
2 - skull base - menigiomas
3 - vascular - AVMs & cavernomas
4 - other - trigeminal neuralgia
What is the SRS process for brain mets?
18-24Gy/1# or 24-27Gy/3#
Treat multiple mets at once, total volume < 20cc
Prolongs life, less toxic than WBRT
Can be used extra-cranially
What is the SRS process for meningiomas?
14-15Gy/1#
Used when surgery is contra-indicated
Benign tumour near spinal cord
What is the SRS process for acoustic neuroma?
Benign tumour - wraps itself around brainstem, crainial nerve
Good control, hearing preserved
12-13Gy/1#
What is the SRS process for arteriovenous malformation?
Bundle of entangled blood vessels - need destroying
15-19Gy/1#
Guided by digital subtraction angiography for target definition
What is the SRS process for trigeminal neuralgia?
Cause of facial pain
Used if medication fails
70-90Gy/1# - 45Gy isodose line touches the pons (brainstem)
Treat with GK or conical collimators 4-7.5mm diameter
