radiotherapy planning Flashcards
(31 cards)
what is radiotherapy
use of high energy radiation (xray, e-, protons, gamma) to kill cancer cells, shrink tumours
- delivered externally or internally
e.g dose in CT pelvis scan = 20mGy
dose for prostate radiotherapy = 60Gy x 3000
reminder of photoelectric effect and Compton scattering, which effect do diagnostic and therapy xray use
diagnostic = photoelectric effect
therapy = pair production (Compton)
what is the energy range for diagnostic xrays and therapy xrays
diagnostic = 0.1 ± MeV
therapy = 10±MeV
photoelectric interaction is proportional to ATOMIC NUMBER cubed
Compton interaction is proportional to ELECTRON DENSITY
what is a machinery used for external radiotherapy, how does it work
linear accelerator
- also known as linacs, are devices used to treat cancer. They are most commonly used for external beam radiation treatments. Linacs work by speeding up electrons to deliver therapeutic X-rays or electrons to a patient’s tumor.
a linear accelerrator can also shoot xrays of different energy levels perpendicular to each other, what is the purpose of this
usually u have a MV xray for radiotherapy in one direction
- but you can additionally use kv xrays (perpendicular to MV beam) to capture projectional images
- use both for image verification orthogonal imaging (90 degrees)
- so it works as MV treatment and Kv imaging
for treatment planning you counter the target and OAR, what does oar stand for
organ at risk
what immobilisation tool is used if radiotherapy is on ur face/neck
RT immobilisation mask
- thermoplastic (mesh) mask, helps to keep still
- mesh plastic that becomes soft and pliable when heated in warm water. The warm plastic is draped carefully over your head. It will feel like a warm face cloth placed over your face at first but it cools very quickly. The holes in the plastic will allow you to breathe easily through it
treatment steps for RT:
- PLAN CT (use other imaging modalities to see cancer)
- contour target + OAR
- create RT plan
- image verification (compare contour to actual anatomy to see if its accurate)
- image verification orthogonal imaging
what beam from the linear accelerator is used for a) image verification b) image verification orthogonal imaging
a) MV beam for image verification
b) both kv and mv beam for orthogonal imaging
what is CBCT
cone beam CT
- fan beam
- 3D reconstruction of image matrix
how long does it take CBCT for 200/360 rotation
38-60 secs
what artefacts are CBCT prone to why
motion (as it a fairly slow scan)
ARC?
why would you use CBCT for RT and not diagnostic CT
- enables radiation therapists to correct for changes of the target position prior to treatment and
- allows monitoring of complex changes of the patient and tumor anatomy, typically caused by patient’s loss of weight and tumor regression
- provides 3D viewing
- allows matching to soft tissue structures
(e.g imaging after, you may find that anatomy has moved out of the contour lines initially drawn for RT)
what is stereo static radiosurgery
a way of giving radiotherapy to a brain tumour. It is given as a single high dose of radiotherapy, to a small area.
Sometimes it may be divided into smaller doses given over a number of days.
what do you refer to each daily dose in stereo static radiotherapy as
fraction
MRI linear accelerator exists too
what is Cerenkov radiation
the blue glow emitted when electrons are moving at speeds fast than that of light in a specific medium
(captured with Cherenkov cameras)
why is proton radiotherapy better than standard radiotherapy
- protons have unique properties that allow doctors to better target radiation to the size and shape of the tumor
- precisely delivers a beam of protons to disrupt and destroy tumor cells
what are the 4 parts of the machinery that forms proton RT beam and what does each part do
- cyclotron, using magnetic fields, accelerates hydrogen protons to 2/3 speed light
- electromagnet (focus proton beam to gantry)
- gantry ( can rotate 360 around patient to position nozzle)
- nozzle ( guides beam to patient thru nozzle)
what is transit/non-transit dosimetry
transit = dosimeter used on items/ people moving thru different radiation fields
non-transit = dosimeter used in a fixed location/ on person not moving thru different radiation fields
(part of checks and QC in RT)
purpose of 2d imaging in RT?
- verify position of RT beam
- visualise surgical clips etc
compare image quality and dose b/w 2d KV/MV images
2d kv = good quality and contrast, low dose
2d MV = not as good contrast but mimic treatment field shape and skin outline
