Stereotactic Radiotherapy for Cranial Lesions Flashcards
1
Q
stereotaxy
A
- refers to the 3D localisation of a particular point in space by a unique set of coordinates that relate to a fixed, external frame
- a Cartesian (x,y,z) co-ordinate system can then reference any point in the body
2
Q
history of CNS stereotaxy
A
- 1949 Dr. Lars Leksell, Sweden
- Inaccessible intracranial target
- larger number small stationary fields
- 1950: ‘Gamma knife’
3
Q
SRS and SRT
A
SRS:
- singe fraction treatment: doses of 12-30Gy: CNS tumours
SRT:
- fractionate: useful if the lesion is extremely close to critical structures in the CNS or for extracranial sites
4
Q
Goal of Stereotactic Irradiation
A
- conform therapeutic dose to 3D target
- deliver large doses of radiation in fewer fractions
- fusion of excellent immobilisation, imaging and precise treatment delivery techniques in order to.
5
Q
Indications for SRS
A
- malignant & benign lesions
- generally <3-4cm max diameter
- potential to respond to single, large dose
- ideally spherical in shape = one isocentre
Gliomas - grade 3 and 4
- no evidence to suggest that SRS has a benefit over 3DCRT/wbrt
- techniques in either newly-diagnosed or recurrent lesions (RTOG93-05)
6
Q
brain metastases
A
- brain mets are the most common malignant lesion treated with SRS
- brain mets are ideal targets for SRS, as they are generally small, spherical in shape and are radiographically demarcated
- SRS over WBRT considered in those patients with limited brain metastases for improved cognitive function (but remember for intracranial control)
- ISRS guidelines define this as 1-4 metastases
- dose and fractionation varies depending on the volume of the tumour (region of 16-30Gy)
Chao et al (2018)
Ballangrud, Ase et al (2018)
7
Q
benign CNS lesions
A
- Arterio-Venous Malformations (AVM)
- Vestibular Schwannoma (Acoustic Neuromas)
- Meningioma
- Trigeminal Neuralgia
8
Q
Frame Based SRS
A
Head frame attachment
- invasive
- resource intense treatment (MDT)
- time constraints between scan and treatment (1 day procedure)
- Frame is then attached to the table top
- Frame offers immobilisation not positioning
need to establish spatial relationship between ring and target
- Optical Guidance Platform (OPG) system
- single infrared camera
- infra-red emitter or reflector: localisation
9
Q
Brown Robert Wells System
A
Procedure
- attached by neurosurgeon/RO
- anchored to scalp through 4 posts
- 2 above supraorbital ridge
- 2 above occipital proturbance
- plastic & aluminium set pins
- local anaesthetic only
- planning CT
- planning
- plan checks
- treatment delivery
- frame removed
10
Q
frame-less SRT or SRT
A
- reinformed thermoplastic masks
- biteblock / mouthbite (upper dentition mould) + mask
- open face mask
11
Q
biteblock systems
A
external coordinate system attached to the mouthbite/upper teeth +/- vacuum pump
12
Q
construction and use of bite block
A
- importance of viewing bite block immobilisation as a whole system
- importance of patient position
- construction of the head rest/thermoplastic mask
-> forehead and bridge of nose well fixed
-> posterior mask conformity to neck curvature to avoid head tilts
-> extra strip of thermoplastic applied across the bridge of nose, underneath the anterior mask can reduce head rotations - selection of mouthpiece size and fit with teeth
- correct introduction of the putty compound into the mouthpiece
- vacuum / saliva stopper - not always used
- patient compliance
-> patient education and support
-> RT assessment of patient - repositioning of patient for SRS
13
Q
biteblock systems
A
- mask providing positioning & mouthbite providing localisation
- mouthbite providing positioning and localisation
14
Q
accuracy of bite block systems
A
- performance of a bite block fixation system for SRS can be defined in terms of the accuracy with which the system can be placed at the treatment unit isocentre and the accuracy with which the patient can be placed in the stereotactic space (Kumar et al., 2005)
- need to test the reliability of the attachment to the upper teeth if localising to a mouthbite
- alternatively bite block can add to positioning stability in a mask
- (Masi et al, 2008)
-> found thermoplastic masks and a bite block to result in fewer set up errors than mask alone in 57 patients with SRT for brain metastases
-> for a single fraction, the set-up correction was 2.7 +/- 1.4mm and was 3.2 +/- 1.4mm for SRT - (Van Santvoort et al., 2008)
-> adapted a mask system to incorporate a vacuum mouthpiece (VM)
-> analysed accuracy of 20 patients with mask and upper jaw support (UJS) and 20 with mask and VM
-> found the initial and verification positioning accuracy to be increased as well as the intrafraction stability (initial set-up accuracy of 2.1 +/- 1.2mm for UJS and 1.7+/- 0.7mm for VM)
15
Q
disadvantages of bite block
A
- edentulous patients (patients with no teeth)
- claustrophobic patients
- breathing difficulties
- poor construction of bite block
- misalignment: incorrect positioning of bite block. specialist team of RTs
16
Q
planning scan acquisition
A
- CT localiser attached to head ring
- localiser defines a Cartesian co-ordinate system
- can reference any point by unique co-ordinates
17
Q
SRS planning
A
- The images are transferred to the treatment planning system Separate planning system for stereotactic at the moment. The TPS software references the CT images to the frame. The radiation oncologist delineates the target volume and all OARs. The goal is to achieve conformality of the target coverage, to minimise dose to the normal tissue and to achieve dose homogeneity within the target.
- generally multiple non-coplanar arcs
- either micro-MLC or tertiary collimator (cones)
- Definition of prescriptions are different ICRU 91 for stereo
- We prescribe stereo treatments in a different manner to conventionally fractionated treatments
- Conventionally fractionated: prescribed to a volumetric prescription D95% or D98% or sometimes Dmean (not good practice)
- In stereo we prescribe to a much lower isodose. Depends on location of tumour relative to stuctures
- Non co-planar = not coming in at 0deg through patient. Different floor angles
- Micro MLC system typically used
- Cones/tertiary collimators usable with brain mets
18
Q
practical planning considerations
A
- difficulties with potential couch-collimator-patient collisions
- overlapping of beam entrance and exits should be avoided
- arcs should have maximum separation to avoid overlapping beams to minimise dose to normal tissue
19
Q
SRS planning
A
- CT images transferred to planning system - may be from diagnostic scanner
- co-registration of MRI if indicated
- target +/- OAR delineation
- isocentre placement, collimator selection, arcs set, dose optimisation, plan evaluation
- conform tightly to target prescribed to 50-80% isodose (therefore high max dose, poor homogeneity)
20
Q
SRS Planning: Margins
A
- may apply GTV to CTV margin of 1mm
- may apply CTV to PTV margin of 1mm
- often no margin - GTV=CTV=PTV
21
Q
pre-treatment QA
A
- careful dosimetric QA required because of steep dose gradients
- verification of mechanical isocentre alignment - with couch off 0 degrees
- extra to therapist daily QA - Winston Lutz SRS QA (iso calibration at floor angles) - up to 90 minutes machine time
- vendor specific solutions may also have QA requirements
22
Q
non-coplanar arcs
A
- Patient stationary on couch
- gantry moves through give arc (‘start’ and ‘stop’ angles)
- move couch angle only (isocentre remains the same)
- repeat
- generally 3-6 arcs per isocentre
23
Q
treatment procedure
A
- addition of stereotactic equipment to linac e.g. frame attachment/tertiary collimator
- preparation of post SRS equipment if required e.g. saline, swabs, band aids
- patient & frame lowered to attachment
- frame secured in position
- patient (and tumour) is located into the stereotactic space i.e. bringing the patient’s planned isocentre(s) into coincidence with the linac isocentre (visually verify this)
- this can be done using infra-red tracking system/surface monitoring etc.
- 6 DoF essential
- check feasibility of arcs - collision risks assessed
- correct tertiary collimator attached (scan and verify x2)
- no field through metal frame parts
24
Q
treatment procedure - IGRT
A
- solutions include CBCT, KV orthogonal imaging +/- surface tracking/ infra red tracking/ high frequency/ high frequency imaging
- inter- and intra- fraction verification
25
Q
post-treatment procedure: frame based
A
- frame unlocked from couch attachment
- patient slowly raised to sitting position
- doctor removes screws, RT holds frame
- puncture sites swabbed and covered
- patient released into care of comparison
- sterilisation procedure for equipment
26
Q
ICRU 91
A
- 2017 report - prescribing, reporting and recording of stereotactic RT with small photon beams
- prescribing - create plan then treat prescription
- reporting - extra conformity index and gradient index
