machine QA Flashcards
describe E2E test for stereo
Use hidden targets in a phantom
Localize with CT/MR and complete a treatment plan
Treat with beams and also use film to get exposure
Displacement of image of target from radiation exposure should be < 1 mm
How do you measure dose profile for gamma knife?
plug 200 of 201 collimators and use film dosimetry. Also plug all 201 collimators to get background.
definition of nominal stereo
sphere within which gantry, couch, collimator intersect
what tests should be performed for new ion chambers
-stem effect (worse for small fields)
-radiographs for mechanical soundness
-leakage current (10 ^ -14) A
-quantify stem effect
-microphonics- place cables etc under stress and test of current measured changes
-measure radiation equilibration time – at most 2 X 200 cGy readings
-test atmospheric communication- chamber should obey ideal gas law
-ensure polarity effect < 1 %
-measure collection efficiency; recombination should be < 1.02
-check orientation dependence
pros and cons of diodes
-smaller volumes and higher sensitivity than ion chambers
-include silicion (high Z) which leads to energy dependence
-directional dependence
-lose sensitivity over time
pros and cons of diamond detectors
-almost soft-tssue equivalent, but higher density than water 93.5 g/cm3)
-very sensitive
-good mechanical stability
-sensitivy doesn’t change with radiation
-small directional dependence
-dose rate dependence
desriable properties of electrometers
accurate, linear, stable, sensitive, high impedance, low leakage
what are the options for 2D dosimetry
film, computed radiography, diode arrays, ion chamber arrays
List TG-142 tests where constraints for stereo are different than for conventional
o Daily:
Laser localization: 2, 1.5, 1 mm
Collimator size indicator: 2, 2, 1 mm
o Monthly:
Couch position indicators: 2 mm/1º, 2 mm/1º, 1 mm/0.5º
Lasers: 2, 1, <1 mm
o Annual:
Coincidence of mechanical and radiation isocenters: 2, 2, 1 mm from baseline
o Daily imaging:
Positioning/repositioning for planar kV, EPID MV: 2, 2, 1 mm
Imaging and Tx coordinate coincidence for all: 2, 2, 1 mm
o Monthly imaging:
Imaging and Tx coordinate coincidence for planar kV, EPID MV: 2, 2, 1 mm
Planar kV scaling accuracy: 2, 2, 1 mm
CBCT geometric distortion: 2, 2, 1 MM
how do you check that the ion chamber is on CAX?
take measurements by rotating collimator at different angles
-if not at CAX, readings will be different because x and y jaws are at different heights
consideration for FFF beam (additional correction)
Prp correction from TG51 addendum
/(average field intensity across chamber sensitive volume, normalized to 1 at centre of sensitive volume) Prp will be >1 to correct for doses being generally lower away from CAX for FFF beams.
how to use shielded diodes?
scan direction matters, especially with shielded diodes since the shielded is not uniform around the active material. Should scan right-left, then scan left-right, and then average the two scans (this is referred to as hysteresis)
2 things you can get from starshot test
-size of radiation isocenter
-distance from radiation isocenter to mechanical isocenter (must put bb at intersection of crosshairs to indicate where mechanical isocentre is)
2 competing effects for Pfluence in electron beams
-in-scatter effect which increases f;luence in the cavity because electrons are not scattered out by the gas in the cavity
-obliquity effect- decreases fluence in cavity because electrons go straight instead of being scattered
Pfl < 1 which means the in-scatter dominates, making the observed fluence too large. Note that the correction is
very large at low-energies or for large diameter chambers and in this case it is best to use plane-parallel chambers with large guard rings.
-dominance of in-scattering over out-scattering gives rise to increase of fluence towards back of cavity and hence an increase ovf the reading of an ion chamber where electrons can enter at different dephts (i.e. cylindrical ones)
-PP chambers reduce fluence effects
why do we monitor the pulse rate with the reference chamber?
so that we can account for any pulse drops in the flatness/symmetry scan
where do you get best measure of flatness/symmetry?
horn on beam profile (as opposed to 10 cm depth)
where should reference chamber be placed?
in corner of treatment field but not too close to the scan chamber (as it cause cause scatter into the measurement area)
how much does output change with FS at 10 cm depth?
87 % for 4x4, 110% at 20x20, and 119 % for 40x40 compared to 10x10
how is scatter from the linac arm compensatred for?
-older true beams and clinac have a correction factor for scatter from the linac arm; newer true beams have a lead coating to minimize the effects of this scatter
define hysteresis
the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past
if you drift 10% on unservoed dose rate, how much would you drift on servoed dose rate
1%
acceptable electrometer leakage
0.1 pA without beam on
dose rate is significant if electrometer is leaky- a slower dose rate can give a larger reading
how to tune machine if it is running hot?
increase gain- machine collects more charge- machine runs less hot
why does linac output almost always trend up?
ion chambers leak air over time
-chamber doesn’t collect as much charge with less air so the machine pushes out more
linac output is trending down- what could it be?
hole in target
what QA is required if you switch ion chamber on linac
annual QA
how does dose rate affect recombination?
-faster dose rate= short time interval of pulse vs ion collection time – more time for recombination to occur
-Pion = 1.006 on 6FFF TB4
-Pion = 1.003 on 6x TB4
-Pion ~ 1.01 for electron beams
-different because dose/pulse of low E beams is 0.03 cGy/pulse
0.06 cGy/pulse for FF beams
0.08 cGy/pulse for electron beams
Dose per pulse in the unit of monitor units per pulse (MU/pulse) is dose rate (MU/min) divided by pulse rate (pulse/min). Since pulse rate of the linear accelerator for the same nominal energy does not change, Pion becomes a function of the dose rate of the photon beams.
2 types of reombination
initial and general recombination. The former process occurs most likely in densely ionized track such as α‐particles and is independent of dose rate. The latter process occurs when positive and negative ions from different tracks re‐combine on their way to the collecting electrode and is dose‐rate dependent.
dosimetric advantages of FFF
lower head scatter and lower out‐of‐field radiation
-FFF has similar dmax to FF because less electron contamination pushes dmax in but softer beam pushes it more shallow- combined effects
Also, variation in output factor is reduced when the filter is removed. Sc can change ~ 8 % when the field is changed from 3x3 to 40x40. This is reduced to 1-3% for FFF. Also, energy spectrum is essentially the same across the profile of FFF beams. Leaf transmission also reduced because of softer beam spectrum, which can improve calcs for some plans.
-With IMRT, filter not required since MLCs deliberately modulate intensity of beam- can get higher dose rate by using FFF
-may increase skin dose since beam isn’t being hardened
daily electron and x-ray output constancy
3 %, TG 142
daily laser localization
2 mm non IMRT
1.5 mm 1MRT
1 mm SRS/SBRT
TG-142
Daily ODI treshold
2 mm non IMRT
2 mm IMRT
2 mm SRS/SBRT
TG-142
daily colli size indicator
2 mm non IMRT
2 mm IMRT
1 mm SRS/SBRT
TG142
daily safety checks
door interlock
beam off
LPO interlock
key turns beam off
LPO times out
indicator lights
audiovisual monitors
monthly electron and x-ray output treshold,
backup monitor chamber constancy
2%
TG142
monthly dose rate constancy
2%
TG142
monthly photon and electron beam profile constancy
1%
TG142
monthly energy constancy
1%
TG142
Monthly light/rad coincidence
2 mm or 1 % on a side for symmetric
1 mm or 1 % on a side for asymmetric
TG142
monthly distance check for lasers
1 mm for stereo and IMRT, 2 mm for 3D
TG142
monthly gantry/colli indicators
1 degree
TG142
Monthly jaw position indicators
2 mm symmetric
1 mm asymmetric
TG142
Monthly cross-hair walkout
1 mm
TG142
Monthly treatment couch position indicators
2 mm/1 degree
1 mm/0.5 degrees for stereo
TG142
monthly respiratory gating tests
beam outpout constancy within 2 %
-phase amplitude, beam control work
respiratory monitoring works
gating interlock works
annual x-ray and electron flatness and symmetry change from basline
1%
TG142
Annual SRS arc rotation mode
monitor units set vs delivered, 1 MU or 2% (whichever is greater)
gatry arc sec vs delivered 1 degree of 2 % (whichver is greater)
TG142
annual output calibration
1%
TG142
Spot check of field size dependent output factors for photons (annual)
2% for FS < 4X4
1 % for FS > 4X4
TG142
Ouput factors for electron applicators
(1 FS/energy)
annual
2 % from baseline
TG142
annual x-ray beam quality
1 % from baseline
PDD10 or TMR20/TMR10
TG142
