Gamma Knife Flashcards
What treatments are done
C2 upwards
Trigeminal neuralagia
Post-surgical ed
SRS pre-operative
Safety
If one part fails, a different part takes over
Source is well shielded, not much bunker shielding needed
The patient positioning system and machine are separate so that if there is an error or malfunction in one component, it shouldn’t harm the patient
Why gamma knife
Treats with high dose to small areas
Can do frames or masks for stabilisation
Limited recovery time compared to surgery
Imaging for gamma knife
MRI
T1 MPRAGE +GD, CISS protocols
1mm met scans
Iso is created by frames
How is movement detected
Monitors nose to detect movement
When patient goes above 1.5mm movement tolerance, it will hold treatment
If patient goes over threshold for long, patient gets ejected out of machine
Trigeminal neuralagia
Treatment of nerves
GK is the least invasive treatment
75-90Gy in 1# as it is not a malignancy has a lower a/b ratio
Benfits
Integrated safety- if one part of machine stops working, the other part takes over
No hospital stay required
Limitations
Staff training
New bunker
Wound healing following surgery
Meningioma
<4cc 12-15Gy
>4cc or near optics 25Gy
Most tumours are slow growing but some can grow quickly
Brain Mets
Used to avoid complication with WBRT
Improved QOL
15-22Gy in 1#
Parkinson’s disease
Disabling tremor
If they are unfit for deep brain simulation
120-130Gy in 1#
Acoustic neuromas
Slow growing, benign usually develop from the vestibular portion of the 8th nerve
Symptom - unilateral hearing decline
<5cc dose 12-13Gy in 1#
>5cc 25Gy in 5#
Arteriovenous malformations
Goal is complete obliteration
Reduce lifetime risk of haemorrhage to 1% or less
20-22Gy 1#
Larger tumours 18Gy in 1#
Pituitary lesions
Role in functioning and non-functioning pituitary tumours
Good for recurrent or residual non-functional pituitary adenomas
High tumour control for patients with Cushing disease
Functioning - 18-24Gy in 1#
Non-functioning: 12-15Gy in 1#
Components of gamma knife
Patient positioning system
Radiation unit: collimator, 60 cobalt sources with half life of 5.26 years
Collimator system: 8 sectors, 24 sources per sector, sources arranged in 5 rings, 4/8/16mm/blocked
Patient follow up process
GP-> Neurosurgeon/specialist -> gamma knife
MDT
Initial outpatient consultation
Patient sees RO on treatment day and have follow up appt booked with referring doctor post treatment
Treatment process
Frame: frame fitted-> MRI and CT/angiograms for AVMs -> target and OAR delineated on MRI -> QA -> Treatment delivered
Mask: MRI -> thermoplastic mask and custom headrest -> CT sim -> target and OAR delineated on MRI ->QA -> treatment delivered
When to use frames
Functional cases e.g. trigeminal neuralgia, tremor
Vascular cases e.g. AVM
Long treatment times >45 mins
Only one fraction
IF CRANIOTOMIES -> masks will be used
Imaging process
Frame provides spatial references and immobilises patients head
Patient scanned with an indicator box in place - locating pins and must fit perfectly
Reference fiducials during image acquisition
Creates the stereotactic space for each image
Images sent directly to gamma plan via DICOm
Motion management
ICON Nanor mask
Reflective marker on nose tip
Infrared stereoscopic camera
What are some exclusion criteria’s for pre-op SRS
Prior SRT to treating lesion
Need for immediate surgery
Presence of leptomeningeal disease outside anticipated resection zone
Unable to undergo MRI with contrast
Surgery unable to proceed within 7 days of SRS
Received chemo 7 days prior to SRS
Why do we do pre-op SRS
Improved target delineation
Sterilisation of surgical bed
Lower dose
Less radio necrosis
Overall shorter treatment
Difference between radiosurgery and radiotherapy
Radiosurgery is a single dose of radiation (12-150 Gy), with ablative intent which requires a defined target that is less than 3-4cm.
Radiotherapy - Radiotherapy is delivered in small doses (1.8-3 Gy), with mitotic cell death in dividing cells as the goal. Often used with margins to treat regions and can be used for large volumes.
Inclusion criteria
- 18 years ECOG 0-2
- One brain met requiring surgical resection which has not had previous RT
- Gross total resection must be reasonably expected
- Between 2-5cm
DSA
Digital subtraction angiography
All AVM patients to have an angiogram
Done same day as planning to localise imaging to the frame coordinates
Thalamotomy
- identified at movement disorder clinic
- disabling tremor
- unfit for deep brain stimulation
- refractory to medical treatment
