Protons Flashcards
Clinical applications
Paediatric tumours
Cranial/head and neck cancers
Hypo-fractionated
Re-irradiaiton
Spare side effects caused by X-ray properties
Minimise wasted dose to healthy tissue
Complex sites
Clinical advantages of protons
No dose past the target
Highly conformal
Dose distributions superior to photon plans
Clinical disadvantages of protons
No fudge factor in treatment setup
Any uncertainty can be unforgiving
Cost and lack of randomised data
Pencil beam scanning steps
- Protons are accelerated until they reach a required energy
- Protons are transported to the treatment machine
- Protons are directed toward the patient with magnets
- Proton spot deposits dose within patient
- Process is repeated for different energy protons
Snout
Jaws of machine
Can be circular/rectangular
Fixed or telescopic accessory that dictates the maximum field size
Range shifter
Tissue equivalent energy absorber of varying thickness
Used when energies of machine cannot get low enough to treat
Used for shallow tumours
SFUD
Single field uniform dose
3DCRT equivalent
Each field achieves a uniform dose over target:
- fields are decoupled - movement causes less issues /variability as both beams providing uniform dose
- more robust plans
- less opportunity for complex dosimetry
Prostate
Lung
Tumours far from OAR
IMPT
Intensity modulated proton therapy
Each field delivers a heterogenous dose to target
- can better spare OARs
- fields are strongly coupled
- more modulated
- movement causes more issues
- less robust
- more opporutnity for complex dosimetry
Used for head and neck
Multiple dose levels
Complex cases
What is the air gap and how does it effect protons
Distance of range shifter to patient
Protons can grow in size
Beam arrangement considerations
Homogenous path to target
Entry through well immobilised anatomy
No OARs following target
Clinical uncertainties
Setup uncertainties: patient shift
Range uncertainties: depth in tissue where proton dose will be deposited due to HU uncertainty, also due to range straggling
Robust optimisation
Reduces burden of mitigating changes due to setup changes
How to mitigate uncertainty with protons
Uncertainty must be mitigated at beam level
Account for individual patient anatomy and beam directions
Robust optimisation and evaluation: optimisation and analysis of plan with uncertainties applied
Spot spacing
4mm is ideal
5mm = decrease treatment time but less dose (cold spots)
3mm = closer together, more dose but more treatment time
Prostate beam arrangement
Laterals
