Linear Accelerator QA

acceptance testing

•performed upon equipment installation

•performed by physicist and vendor representative

commissioning

•performed by a qualified physicist

•establishes baselines

quality assurance testing

•performed by physicist

•equipment calibrated by Accredited Dosimetry Calibration Laboratory (ADL)

spot checks

•performed by therapists

•equipment less precise

•use array detectors

•multiple tests in 1 exposure (output, symmetry, constancy checks)

daily QA dosimetry

•xray output constancy: 3%

•electron output constancy: 3% (all energies at least 2x weekly)

daily QA mechanical

•lasers: 2 mm (non IMRT), 1.5 mm (IMRT), 1mm (SBRT)

•ODI: 2mm

•collimator size: 2 mm (non IMRT/IMRT), 1 mm (SBRT)

daily QA safety

•door interlock: functional

•audiovisual monitor: functional

daily QA checks

•water level/temp/pressure

•gas pressure (SF6)

•general conditions

•tx devices

•supplies

monthly QA check dosimetry

•xray output constancy: 2%

•xray beam profile constancy: 1%

•electron output constancy: 2%

•electron beam profile constancy: 1%

•backup monitor: 2%

•dose rate constancy: 2%

monthly QA check safety

•emergency off: functional

•wedge interlocks: functional

monthly QA check mechanical

•lasers: 2mm (non IMRT), 1mm (IMRT), 1mm (SRS/SBRT)

•light vs xray: 2mm/1mm (asym)

•FS indicators: 2mm/1mm (asym)

•tx couch position: 2mm/1 degree

•gantry/collimator: 1 degree

•cross hair center: 1 mm

•light field intensity: functional

•tray/wedge latches: functional

temperature and pressure correction

•most ion chambers unsealed

•chamber sensitivity changes with temp and pressure

•increased temp, reading goes up

•increased pressure, reading goes down

=(reading x temp K x 760 mmHg x Cal factor)/(exp time x 295 K x pressure in mmHg)

monthly QA respiratory gating

•beam output constancy: 2%

•phase, amplitude beam control: functional

•in room respiratory monitoring system: functional

•gating interlock: functional

annual QA check dosimetry

•xray flatness/symmetry: 1%

•electron flatness/symmetry: 1%

annual QA check safety

•per manufacturer

annual QA check mechanical

•collimator rotation iso: 1 mm dia

•coincidence of radiation and mechanical iso: 2 mm dia

•table top sag: 2 mm

•table angle: 1 degree

•electron applicator interlocks: functional

stability of isocenter

•stable: convergence is at iso

•wobble: nonconvergence at iso

•split field test: used to QA POP fields

split field test

•used to QA POP fields

•materials: film, plastic tray on 4" feet, 2 shielding blocks

split field test technique

•gantry @ 0, film on tray

•shielding block on top of film

•shielding block under tray, opposite side/jaw

•expose film at 0 and 180

couch isocenter

•technique: gantry at 0, reference mark with table at iso height, rotate thru various angles

•not to exceed 1mm

•other sources of error: bad reference mark, collimator, light crosshair alignment

machine dose rate constancy

•constancy: machine output, cGy/MU

•spot check confirms baseline parameters

•often combined with flatness, symmetry, beam penetration

•possible reasons: magnetron current, electron gun current, change in monitor ion chamber

•instruments: ion chamber, diodes, film , TLD

flatness

•maximum difference from CA dose over 80% field dimension

•defined at a depth of 10cm

•Max= (Ymax-Yo)/(Yo)x100%

•Min= (Ymin-Yo)/(Yo)x100%

symmetry

•difference in dose rate between any 2 points within 80% of the field dimension

=(Y1-Y2)/Yo x 100%

flattening filter free

•flattening filter is removed in beam to attenuate photons

beam penetrative quality

•percentage depth dose/HVL (established upon commissioning using precision instruments)

•spot check (standardized SSD and FS)

•materials: tissue equivalent, ion chamber/diodes

•technique: 2 measurements, compare ratio to established baseline usually at dmax and some depth

imagers QA kV and MV

•daily: collision/door interlock, warning lights/sounds, laser/image/tx iso coincidence (2mm), phantom localization and couch shift

•monthly: kV/MV/laser alignement (1mm), high contrast spatial resolution, CT# accuracy

•annual: imaging dose and xray generator performance