Gamma Knife

goals of gamma knife

•precisely located target

•hold target still

•aim 192 beams at target accurately

•deliver radiation dose that conforms to tumor shape

gamma knife indication

•smaller tumors up to 4cm, sometime 6cm in mean diameter

•standalone tx/combination w/ surgery, chemo, immunotherapy or inoperable brain tumor

conditions treated with gamma knife

•brain tumors (mets, pituitary adenoma, pinealoma, craniopharyngioma, meningioma, chordoma, chondrosarcoma, glial tumor)

•acoustic neuroma (vestibular schwannoma)

•arteriovenous malformation (AVM)

•tremors (essential tremor, Parkinson's)

•trigeminal nerualgia

•some types of epilepsy

components

•radiation unit

•beam focusing technology

•patient couch

•electric bed system

•control console

•planning computer system

helmet

•frame pinned to pt head

•frame attaches to collimator helmet

•plus to block individual beam

•some systems replaced helmet with internal collimation system

gamma knife sources

•Co-60

•1/2 life: 5.26 yrs

•sources replaced every 5-7 yrs

•192 sources in machine

•beams intersect at point of interest

collimator system

•192 Co sources divided into 8 movable sectors

•24 sources in each sector

•sectors can be collimated to 4/8/16mm in diameter or be blocked

shot

•timed treatment at specific coordinates and collimation

isocenter

•has set of 3 Cartesian stereotactic coordinates (x,y,z)

•QA performed to ensure accuracy

frame surgery

•local anesthesia into scalp in pin sites (2 in front, 2 in back)

•pt gets MRI/CT/Cerebral angiogram for tx plan

•tx plan (1-few hours)

•frame attaches to bed adapter and locks pt into place

•frame can be angled 70/90/110 degrees

•pt treated in frame for 1 fx

•if 5 fx, pt tx in frame for 1 fx and rest in mask or all 5 fx in mask

treament process

•preparation

•applying frame

•imaging studies

•tx planning

•gamma knife tx

•recovery

mask setup

•thermoplast mask w/ accuform

•sensor placed on nose communicating with HDMM

•must stay within 1mm tolerance range (couch moves out of collimator cap after >30sec out of range)

•therapist can coach pt back into head position

•up to 1-5 fx

gamma knife vs linac

•tx time longer for GK (30-60 min)

•Co-60 dmax: 5mm

•1.25 MeV energy from gamma decay

treatment planning

•many prescribed to 50% isodose line (periphery of tumor will receive at least prescription dose, and dose will be higher than prescribed dose in tumor)

•dose rapidly falls off outside tumor to spare OAR

treatment prescriptions (50% isodose line)

•meningioma: 18 Gy in 3 fx

•pituitary adenoma: 20 Gy in 5 fx

•mets: 20 y in 1 fx

•cerebellar mets: 22 Gy in 1 fx

•pineal (meningioma): 12 Gy in 1 fx

benefits of gamma knife

•low risk to nearby tissue (individual beams have low intensity)

•minimal discomfort

•outpatient procedure

•can reach tumors deep in brain

•can target multiple tumors at same time

•rapid fall off

side effects

•swelling of brain

•HA

•N/V/D

•numbness

•hairloss of tx area (temp)

•seizures

•soreness and swelling of tx area

•weakness

•loss of balance

•vision problems

•radiation exposure

•pain at pin insertion sites