Linac QA Procedures

Morning QA procedures

Turn machine key switch “On”

12-minute time delay (TDLY)

Rotate gantry to 0 degrees

Turn on MLCs – using HyperTerminal to initialize MLCs (3-5 minutes)

Check water (temp, pressure, and level) and gas pressure

Perform daily safety checks (door interlock, door closing safety, beam on/off, beam on indicator/sign, audiovisual monitors, safety collision interlocks, table motion function)

Check gantry, collimator, and couch position indicators (digital readouts vs. manual protractor*)

Laser and ODI alignment/radiation field light coincidence/imaging checks

Beam output checks

Laser alignment and ODI accuracy steps

Set QA device on couch – use ODI to set surface to 100 SSD

Verify that side lasers align vertically with the top of the QA device

If a discrepancy is found, it may not be immediately obvious whether the problem lies with the ODI or lasers (further tests might be necessary)

Check laser coincidence with crosshairs

Laser alignment and ODI accuracy purpose

Ensure readings are accurate

Incorrect SSD/SAD readings result in the delivered dose differing from the prescribed dose

ODI daily tolerance

2mm

Laser alignment daily tolerance non-IMRT

non-IMRT 2mm

IMRT 1.5mm

SRS/SBRT 1mm

Collimator size indicator steps

Also known as: Field size digital accuracy/light field check

With QA device, align center of device with cross hairs and set a 20x20 cm field. Verify that measured light field is within 1 or 2mm, depending on institution’s acceptable tolerance

Check to determine if a given field size setting, using the collimator size indicator, results in the production of light and radiation fields of that size at the specified treatment distance

Collimator size indicator purpose

Determines if the light and radiation fields are congruent and on the CAX

MLC checks

Perform picket fence image weekly at a minimum

Two types of QA concerns: attenuation properties and positional precision

Test deviations in interleaf transmission

Verify that MLCs are aligned as expected

Collimator angle check purpose

Mechanical or digital readouts indicate the rotational position of the collimator and the beam orientation

Accurate readouts are necessary for the translation of the set-up from treatment planning to the treatment unit to be reproducible and follow the treatment prescription

Collimator angle check steps

Use of a spirit level

Rotate the gantry so that the collimator axis of the rotation is horizontal

Open the jaws so that there is space to rest the small spirit level on the lower jaw (be

careful not to disturb the cross wires)

Rotate collimator until the bubble is exactly centered

If this is meant to be the 180-degree position, the pointer should be at 180 degrees on the scale as well as the digital display

Gantry angle check purpose

For daily reproducibility of setup and the angular position of the various beams used in treatment plans

Gantry angle check steps

Gantry angle may be indicated by means of a protractor and pointer, which move relative to one another with the gantry rotation, by a digital display on the treatment machine, and by a monitor in treatment room and on the treatment console. Can test gantry rotation readout with the use of a spirit level as well

Who performs daily dosimetric checks?

Therapists

Who performs monthly dosimetric checks?

Physicists

How often are output constancy checks and dynamic wedge checks?

Daily basis

How often are checks of flatness, symmetry, and energy?

Monthly basis

Output constancy check steps

Place QA device on treatment couch. Set SSD to 100 and align to field light.

Deliver 100MU at clinically used dose rate (600MU/min) and compare the measured dose with the expected dose of 100cGy for each photon energy and electron beam

The software compares the results with the baseline values (determined during physics calibration)

Output constancy daily tolerance

Must be within 3% (between 97-103cGy if 100cGy is delivered)

Output constancy monthly tolerance

Within 2% (between 98-102cGy if 100cGy is delivered)

Treatment couch position indicators

Therapist again should check manual protractor (if applicable) and/or digital readouts daily

Treatment couch position non-IMRT and IMRT tolerance

2mm/1 degree

Treatment couch position SRS and SBRT tolerance

1mm/0.5 degree

Treatment couch annual tolerances

Tabletop sag – 2mm from baseline

Table angle – 1 degree

Table travels max range movement in all directions 2mm

Light/radiation field coincidence monthly tolerance

2mm for symmetric fields, 1mm for each asymmetric field edge

Collimator, gantry, and couch digital accuracy monthly tolerance

1 degree

Laser positions monthly tolerance IMRT/SRS/SBRT

1mm

Laser positions monthly tolerance non-IMRT

2mm

Annual output check

Uses ionization chamber with an actual liquid water phantom

This large tank water phantom is used to take beam profile scans, depth dose scans, and make other point dose measurements that are related to MU calc parameters

Annual output check tolerance

Within 1%

EDW checks

Produce wedged isodose distributions by varying the amount of time that the beam is on in different portions of the field

The y jaw closes (IN or OUT) so that more intensity is delivered to the part of the field that remains open and less to the part that has been closed TG-142 requires a daily check that EDW are functional

EDW tolerance

CAX output must be within 2% of the commissioning value