TG179 Flashcards
What is TG179 on?
Quality assurance for image-guided radiation therapy utilizing CT-based technologies
difference between fan beam and cone beam kV CT
fan beam: acquires one or more thin slices per tube/detector rotation
cone beam: acquires entire volume in single rotation
kV CBCT imaging dose
0.2-2 cGy
energy of beam for MV CBCT
3.5 meV
pro of MV FBCT vs kV CBCT
-fewer scatter and beam hardening artifacts compared to kVcBCT
-gets rid of artifacts from high Z materials
-however, MV has poor subject contrast and are noisier
MV FBCT imaging dose and MV CBCT dose
1-3 cGy for FB
3-10 cGy for CB
localization precision of MV FBCT and MV CBCT
2 mm
benefit of CT on rails
-diagnostic quality
-adaptive planning
-same CT used for planning and imaging- facilitates image registration
advanages of kV CBCT over projection imaging
-soift tissue structures can be directly imaged and targeted
advantage of kV CBCT in SBRT
low dose- can image in between arcs and reduce impact of motion
is it easy to modify subsequent SBRT fractions if an error is made?
difficult b/c of small number of fractions
test for geometric accuracy of imaging system
-like WL, but actively corrects for misalignment
-ball bearing is moved iteratively toward radiation isocenter using portal images at the 4 angles
-acquire images with imaging system
Ana-lyzing the apparent travel of the ball bearing on the projec-tion images used for reconstruction of volumetric datasets provides a measurement of the components’ flexing as a function of gantry angle. Once this relationship is known,the pixel coincident with the isocenter can be determined and the projection image pixel locations referenced to the isocenter pixel. A plot of the distance between the measured isocenter pixel and the pixel that would nominally intersect isocenter is termed a flex map. The shifts identified in the measured flex map are performed automatically by the image-guidance software.
when do you do flexmap?
-at commissioning
-verified on monthly basis
-after system upgrades or service
where can errors be introduced in MVCT?
For fan-beam MVCT units, the imaging beam is gener-ated by the same source that generates the treatment beam and the two beams share a common geometry. This leads to an inherent robustness of the MVCT imaging system geome-try. However, the image acquisition, reconstruction, and registration process uses hardware and software components that have the potential to introduce geometric errors in the fan-beam MVCT IGRT process.
why is CBCT lower quality than FBCT?
-CB usually require lrge area detectors that are inferior in dyanmic range and DQE
-xray scatter is issue
-blurring from internal structure motion
does spatial resolution depend on dose or location of phantom wrt isocenter plane?
No
spatial resolution for kV CBCT and MV CBCT
-6-9 lp/cm for kV
-4 lp/cm for MV
-can see small objects 1-2.5 mm in size
what is more important in IGRT, spatial res or low contrast detectability
low contrast detectability
Routine QA of the spatial resolution is, nevertheless,useful because a reduction in spatial resolution may indicate changes in scanner geometry and=or gantry angle readout calibration.
HU values from kV CBCT?
No, b/c small deviations from acceptance phantom conditions significantly affect CT numbers
-suffers from sensitivity of scatter-to-primary xray fluence to object and/or field size
-not the case for fan beam MVCT or CT on rails
if you you use fan beam MVCT for dose calculations, what monthly test should you do?
-monthly test of HU calibration
calibration of MV-CBCT images
-more elabroate than fan beam due3 to cupping artifact and beam hardening
-however, in MR range, have smaller amt of scatter and reduced energy dependence of photon interactions
-therefore have cupping artifact that is predictable and easy to correct for
calibration of MV-CBCT images
-more elabroate than fan beam due3 to cupping artifact and beam hardening
-however, in MR range, have smaller amt of scatter and reduced energy dependence of photon interactions
-therefore have cupping artifact that is predictable and easy to correct for
-this makes MV CBCT suitable for dose recalculation
when are image contrast gains offered by kV over portal imaging immediately obvious?
-lung
-pediatrics
-brain
As PTV margins get smaller, what happens to importance of geometric accuracy of the equipment?
-it becoms more important
-just as important as dosimetric accuracy
where should you pay a lot of attention to for CT QA?
Since the primary aim of image-guidance is to detect and correct positional uncertainties,particular attention should be paid to their geometric accu-racy assessment
