TG179

Question 1

What is TG179 on?

Answer 1

Quality assurance for image-guided radiation therapy utilizing CT-based technologies

Question 2

difference between fan beam and cone beam kV CT

Answer 2

fan beam: acquires one or more thin slices per tube/detector rotation
cone beam: acquires entire volume in single rotation

Question 3

kV CBCT imaging dose

Answer 3

0.2-2 cGy

Question 4

energy of beam for MV CBCT

Answer 4

3.5 meV

Question 5

pro of MV FBCT vs kV CBCT

Answer 5

-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

Question 6

MV FBCT imaging dose and MV CBCT dose

Answer 6

1-3 cGy for FB
3-10 cGy for CB

Question 7

localization precision of MV FBCT and MV CBCT

Answer 7

2 mm

Question 8

benefit of CT on rails

Answer 8

-diagnostic quality
-adaptive planning
-same CT used for planning and imaging- facilitates image registration

Question 9

advanages of kV CBCT over projection imaging

Answer 9

-soift tissue structures can be directly imaged and targeted

Question 10

advantage of kV CBCT in SBRT

Answer 10

low dose- can image in between arcs and reduce impact of motion

Question 11

is it easy to modify subsequent SBRT fractions if an error is made?

Answer 11

difficult b/c of small number of fractions

Question 12

test for geometric accuracy of imaging system

Answer 12

-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.

Question 13

when do you do flexmap?

Answer 13

-at commissioning
-verified on monthly basis
-after system upgrades or service

Question 14

where can errors be introduced in MVCT?

Answer 14

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.

Question 15

why is CBCT lower quality than FBCT?

Answer 15

-CB usually require lrge area detectors that are inferior in dyanmic range and DQE
-xray scatter is issue
-blurring from internal structure motion

Question 16

does spatial resolution depend on dose or location of phantom wrt isocenter plane?

Answer 16

No

Question 17

spatial resolution for kV CBCT and MV CBCT

Answer 17

-6-9 lp/cm for kV
-4 lp/cm for MV

-can see small objects 1-2.5 mm in size

Question 18

what is more important in IGRT, spatial res or low contrast detectability

Answer 18

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.

Question 19

HU values from kV CBCT?

Answer 19

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

Question 20

if you you use fan beam MVCT for dose calculations, what monthly test should you do?

Answer 20

-monthly test of HU calibration

Question 21

calibration of MV-CBCT images

Answer 21

-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

Question 22

calibration of MV-CBCT images

Answer 22

-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

Question 23

when are image contrast gains offered by kV over portal imaging immediately obvious?

Answer 23

-lung
-pediatrics
-brain

Question 24

As PTV margins get smaller, what happens to importance of geometric accuracy of the equipment?

Answer 24

-it becoms more important
-just as important as dosimetric accuracy

Question 25

where should you pay a lot of attention to for CT QA?

Answer 25

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