L 16: QA Flashcards
(25 cards)
Beam Flatness
- Field flatness for photon beams has been traditionally defined as the transverse variation of dose relative to the central axis over the central 80% of the field size (reference region)
at 10-cm depth in a plane perpendicular to the central axis - It is the flatness of the beam measured in the central 80% of the field size
- Vairation of +/- 3% acceptable
- at 10 cm depth and Dmax is where it is measured
Beam symmetry
- The cross-beam profiles obtained for flatness also can be used for symmetry.
- The mirror image from the central access.
- variation can be =/- 2%
QA main TG is
TG-142
Collimator Jaw symmetry error is ~
less than 1 mm
Acceptable ranges
- Wedges +/- 2%
- Isocenter shift with couch motion +/- 2mm
- Field size indicator +/- 2 mm
- Optical density indicator +/- 2mm
- Lasesr alignment +/- 2mm
- Table sag +/- 2mm
- Field symmtery +/- 2%
- Beam Flatness +/- 2%
- Gantry rotation / Collimator rotation 1deg
- mlc 2%
Pacemaker
- Pacemaker malfunction is increased with cumulative doses > 2Gy or neutron producing therapy (photon beams > 10 Mv)
- Surgical relocation of pacemaker if it is in the treatment field
mass absorption coefficient
- It is equal to the sum of mass absorption coefficients for PE + compton + PP
- Units cm^2/gm
- Dependent on energy
Field size increase
Sc- collimator scatter increases due to
1. Increase scatter to the measurement point azs more of the flattining filter in exposed
2. Decrease backscatter from the jaws to the monitor unit chamber
RPN
Risk priority number
RPN = O.S.D
It is used to identify and correct potentially hazardous process deficiencies
TG- 142 tolerance level for linac output
Daily = +/- 3%
Monthly & Annual = +/- 2%
TG-142 Daily checks
- Door Interlocks
- AV /monitoring
- Daily output
- Laser localization
- Field size
- DIstance indicator
TG- 142 QA for Medical accelerators
Based on intended treatment techniques
Laser accuracy
TG 142
SRS/SBRT - 1mm
IMRT 1.5mm
Non IMRT - 2 mm
Daily Lineac QA checks done by
Radiation therapist
Checking Isocenter
SHould be done daily for KV imaging and MV treatment
QA check for linac
- Mechanical
- Dosimetric
- Imaging
- Safety
Linac output
Checked Daily
acceptable tolerance in +/- 3%
Helical MVCT for tomotherapy dose
1-2 cGy
acceptable pacemaker dose
2-5 Gy
Picket fencing test
Test for MLC accuracy and positioning
X-ray and electron output calibrations
Done annually
Testing leaf accuracy
Monthly checks
- Safety checks (mechanical, switches)
- Mechanical checks (panel movement)
- Image quality (Isocenter check, Image contrast)
Radiation survey meters vs radiation detectors
Radiation survey meters have highly pressurized chambers
