proton Flashcards
source of protons for proton therapy
hydrogen as it only has protons (1)
mass of proton
2000 times the mass of e-
protons have high
inertia mass
which is the difficulty to accelerate an object in order to change object motion acceleration
protons require
2000 times more force to get the same kind of deviation of e-
deviation =moving the beam direction
features of protons
-lateral scatter is low as protons are not deflected by e
-some side scatter
-RBE=1.1
-proton loss per interaction is small
-secondary nuetrons and gammas are produced
range of secondary e- is..
small tf secondary dose is all absorbed locally
nuetronal contimation
occurs in very high energies
what is bragg peak
radiation instantly disappears after “BP”.
TF surrounding normal tissues dint get exposed to radiation
how does bragg peak occur
-velocity of protons ↓ as they go thru deeper in tissue,
- energy is lost from protons via non-ionsing attenutaion
the probability of …
causing ionisation ↑ as velocity↓
tf when protons go further into body, they are more likely to ionize tf less energy , tf velocity↓
native beam
-proton beam has a positive feedback effect
-peak is v high and narrow
-build up of dose is slower and lower
-tail off is virtually instantaneous
native beam is only useful for tumour @
c.25cm depth
c.2cm thick
modified beam:
any modified beam must fit 3 goals:
-maintain (spread out bragg peak) characteristics
-allow beam to treat wide area
-treat different depths in 1 field
when to use
-isolated and hard to treat surgically regions
*eye
*tumours close to spinal cord
*h&n
*paediatrics
proton source : H
-H gas obtained from electrolysis of water
-high voltage applied to h gas to strip e-, leaving protons suspended in magnetic field
the protons are magnetically guided into synchroton or cyclotron
synchroton
small p linac used to create high en p and then these are injected into synchroton (which is a circular accelerator ring)
-varaiable en
-lower beam current than cyclo
what happens in the synchroton
electromagnets accelerate p
cyclotron
-fixed energy machine
-high en gives max depth
-lower en only achieved by en degraders
proton relativity
p accel. up to2/3 speed of light
when accel. , they gain mass
tf inertia ↑ which opposes changes in motion
beam shaping
uses thin penicl beam of high en protons
2 ways of shaping beam to useful size
-passive scanner
-beam scanning
tissue denisty
p is v sensitive to tissue denisty
-BP is clser to surface of pt if path goes thru bone
-BP is deeper if path goes thru lung
tf tx spots are at different depths for some beams
disadvantages pt1
-very expensive
-↑ skin dose
-bending proton beams is v. difficult bc of high mass
disadvantages pt2
-beam delivery is challengeing bc of v large gantry
-range uncertianity tf OAR less than 2cm may not be spared
