Random Flashcards
How is linear attenuation coefficient defined?
The fraction of an incident beam of photons that is absorbed or scattered per unit thickness of the target absorber
Why is the 1-g term in the relationship between mass energy absorption and mass energy transfer coefficient
Energy transferred to KE of charged particles not all absorbed by irradiated material, fraction g lost to photons.
Brem, but also includes annihilation and characteristic x-rays
What are aims of Paris system?
Rules to predict dose coverage as fn of active length and catheter spacing
Acceptable uniformity of dose distribution without hot or cold spots
Designation of dose calculation points
Designation of prescription isodose that gives desired clinical result
How does output factor vary with FFF beam?
Reduction in scatter means variation of total output factor is much less than for flattened field as FF major source of scatter.
What do mu, mu_tr and mu_en represent?
Linear attenuation coefficient - energy removed from beam
Energy transfer coefficient - energy transferred to medium (kinetic energy of electrons)
Energy absorption coefficient - energy deposited locally in medium
What is definition of kerma?
Sum of initial kinetic energy of all charged particles produced as a resullt of radation interactions in mass dm.
J/kg. Gy.
What is definition of absorbed dose?
Total energy imparted by ionising radiation to mass dm. (total energy absorbed per unit mass)
J/kg. Gy.
Direct vs grazing hit CS
Direct: amount of energy transferred depends on initial photon energy
Grazing: no energy transferred
What is knowledge based planning?
An approach using prior knowledge and experience to predict an achievable dose in a new patient or to deduce a better starting point for further optimisation
Control points in VMAT
Control points have cumulative MU and gantry angle, motion of leaves between control points is limited by gantry speed and leaf speed
What is logic behind MCO?
In MCO there is usually no feasible solution minimising all objective functions simultaneously so we look at Pareto optimal solutions: which cannot be improved without degrading one other objective
What detectors use for measuring:
Small field PDD
SF Scp
SF Sc
PDD: mini ionisation chamber (no steel electrode due to response with changing E), film?
Scp: liquid ionisation chamber or film. (could daisy chain diode and correct)
Sc: small air ionisation chamber in cap
How can US be used for in-room imaging?
Need to know position wrt isocentre
Could have probe on arm, computer attached to arm knows the position of the arm wrt isocentre
Or probe with infrared reflectors - IR cameras in room can compare position of probe to isocentre
How does exactrac IR tracking work?
IR camera opposite gantry
Reflectors either on patient or on positioning array
IR cameras are calibrated to isocentre and therefore know position of patient wrt isocentre
How does Calypso work?
Using passive RF transponders as fiducial markers
Position detected using EM array
Array position related to isocentre with IR cameras
Transponders emit RF when excited, array tracks position.
