Stereotactic Radiosurgery and Radiotherapy Flashcards
1
Q
what is stereotaxy
A
stereotaxy refers to the 3D superposition of a fixed coordinate system upon a given organ
2
Q
differences between stereo and conventional
A
Conventional (3DCRT)
- coplanar setup
- larger volumes
- small(ish) number of fields
- target volume delineation
- positional accuracy +/- 5mm
Stereotactic
- non coplanar
- small volumes
- larger number of fields
- positional accuracy +/- 1mm
3
Q
what do you need
A
- the accurate definition of the treatment area and normal anatomy in terms of a coordinate system
- the development of a treatment plan that delivers the dose delivered and protects critical sites including immobilisation
- the accurate delivery of the treatment plan
4
Q
small field dosimetry
A
- most normal field dosimetry is referenced back to a 10x10 field
- 0.6cm x 0.6cm beam data (with identical setup) gave discrepancies of up to 15%
- this applied to SABR, SRS, SRT and VMAT
5
Q
conditions for small field dosimetry
A
- lateral electron disequilibrium (beam dependent)
- if the beam half width (radius) is less than the practical range of secondary electrons - partial occlusion of the primary beam by the collimating device (beam dependent)
- jaws, MLCs, cones, or tertiary collimator
- linacs have a focal spot of generally a few mm (2-4) therefore electron disequilibrium starts at slightly larger field sizes
* both 1 and 2 get worse (because electron range increases) with increasing energy of beam and decreasing tissue density - the size of the detector is similar or larger than the beam being measured (detector dependent)
- the signal for the detector is averaged over the volume
- this is often the biggest problem as the presence of the detector affects the fluence and very small uncertainties in the detector build have large effects on the reading
* any of these conditions will create an overlap of the penumbra with the detector
* in general the beam half width should be at least the electron range + half the external size of the detector
* increased effect of the mid portion of the flattening filter
-> shift in the energy spectrum due to hardening
-> reduction in intensity
6
Q
Modified Linac with added tertiary collimator
A
- advantage
-> familiarity with technology
-> much cheaper - disadvantage
-> slightly less accurate cf gamma knife
-> often longer treatment times than conventional RT
-> tumour site depends solely on the frame type - Gamma Knife is a dedicated machine
-> disadvantage: cannot be used for other sites, specific training / education needed
-> advantage: increased accuracy, patient throughput. Only likely to be effective in large centres with large amounts of patients
7
Q
Micro MLC
A
- smaller leaves gives better field shaping
-> 1cm leaves are not generally used
-> 2.5mm or 0.5mm are more common
-> historically add-on MLCs were used
-> there would appear to be little point reducing the leaf width below the beam penumbra size (c. 2mm for 6MV)
The lateral electron range is about 2.5mm for 6MV which is a major contributor to physical penumbra
-> smaller fields mean that the leaf positioning is a bigger issue
-> leaf transmission from add-on MLCS is generally smaller because of the use of the back up jaws
-> interleaf leakage will be greater with larger numbers of MLCs
-> microMLCs have smaller maximum field sizes
limited overtravel - should be half the max field size in order to create narrow intensity peaks at the field boundaries
-> clearance to isocentre. only an issue with add-on MLC devices and then only on extra-cranial treatments (30cm clearance generally enough)
8
Q
targets
A
Spherical target
- circular field - single isocentre
- Gamma knife and Linac
Irregular target
- circular fields and multiple isocentres
-> gamma knife and linac
- irregular fields (uniform or modulated fields)
-> linac with microMLC
Micro MLC
- improved target homogeneity
- sharper dose fall off
- shorter treatment time
- possible less scatter
9
Q
Preparation
A
- the entire radiosurgey process should be tested for geometric accuracy before treatment
- test phantom
- image the phantom in treatment position (and in imaging position)
- and determine the measured coordinated of the structures within the phantom
- add in quadrature for all three directions to give a measure of the total error
10
Q
Main components of the Gamma Knife
A
- source core
- 201 cobalt-60 sources
- shutter mechanism
- helmet and secondary collimators
- treatment couch
11
Q
what do you need for gamma knife
A
- radiation unit (couch and collimator helmet)
-> 201 cobalt-60 sources
-> central beam is tilted at 55deg to door
-> each source contains 15-20 pellets
-> total activity = 209 TBq
-> half life 5.26 years
-> energy 1.17MeV and 1.33MeV - stereotactic frame
- planning system
12
Q
collimation
A
- primary
tungsten and lead in the tube holding the cobalt source - secondary
in the collimator helmet (interchangeable)
choice of 4 aperture sizes - 4, 8, 14 and 18mm (at focus) - each individual collimator can be plugged with lead to prevent exposure
- mechanical beam intersection accuracy (+/- 0.4mm)
- helmet to body alignment (+/- 0.1mm)
13
Q
procedure
A
- collimator helmet is attached to the couch of the gamma knife
- then the patient is positioned into the collimator helmet (prone or supine)
- helmet contains microswitches sensitive to 0.1mm displacement for continuous verification
- when the staff leave the room and start the procedure the couch moves into the housing
- the couch locks in place when the channels of the housing are aligned with the channels of the collimator unit
14
Q
outcome
A
- treatment time 10-20 minutes
- total time including imaging, planning, etc = 5 hours
- whole body secondary irradiation can be low
for a 50Gy target dose
-> scalp in region of beam entry = 0.5Gy
-> scalp not in region of beam entry = 0.1Gy
-> thyroid = 0.13Gy
-> sternum = 0.1Gy
-> knees = 0.01Gy
15
Q
Gamma Knife
A
Advantages
- <1mm precision (0.1mm motion tracking)
- multiple lesion in one session
- long history and evidence base
Disadvantages
- limited to the brain
- placement of the frame can be painful if used
- peripheral brain can be difficult to treat
