Monthly Mechanical QA Guide

This will be a guide on how to perform the monthly mechanical QA for the Elekta Synergy machine, though the steps may apply for other machines.

Set the gantry angle, the couch rotation, and the collimator rotation to 0°. Retract the MV and kV image panels to open up space for you to move between the gantry and the head of the table. Place a bubble level on the gantry, and set the gantry such that the bubble indicates the appropriate level for 90°. Position another bubble level on the gantry to check for the collimator, and set the collimator such that the bubble indicates the appropriate level for 0°.

Leveling the collimator may unlevel the gantry, thus, adjust the gantry and collimator in combination to find the exact spot where both become appropriately leveled. Set the collimator to its mechanical angles of 270°, 180°, 90°, and 0°, and check that the digital readout is within tolerance at each angle (gantry/collimator angle indicators (@ cardinal angles) (digital only): 1.0°). With the collimator at the mechanical angle of 0°, rotate the gantry back to its mechanical 0° position as well.

Enter into Mosaiq and open the patient for mechanical QA. Go into RO treat, select QA mode, and mode up a 20 x 20 CM field. Set the couch lateral to 0, and shift the couch into a position such that the light field is visible on the table. Pull out the supports laterally from underneath the table to reduce table sag. For an older machine, having the couch more in may result in greater table sag.

Place the Iso-Align phantom on the table, and level it using the bubble leveler in the longitudinal and lateral directions. The leveling can be changed by adjusting the pegs or by adding shims. Once the Iso-Align phantom is “flat in space”, align the crosshairs of the Iso-Align phantom to the crosshairs of the light field. Attach the front pointer mount, and set the front pointer to 100 CM. Raise the couch to set the surface of the Iso-Align phantom to make contact with the tip of the front pointer. Check that the left, right, sagittal, and overhead lasers are in alignment with the crosshairs of the Iso-Align phantom (localizing lasers for Non-IMRT, IMRT, and SRS/SBRT: ±2 mm, ±1 mm, and < ±1 mm).

Record the current couch vertical position with the front pointer set 100 CM to the surface of the Iso-Align phantom. Set the front pointer at varying distances, and shift the couch accordingly. Record the difference between the expected reading and the actual couch vertical digital readout. For example, if the couch vertical position with the front pointer set at 100 CM reads -30.0, the couch vertical position with the front pointer set at 90 CM should, in theory, display -20.0 on the digital readout. However, the couch indicator may display -19.9 or -20.1 despite shifting by the mechanical amount (treatment couch position indicators for Non-IMRT, IMRT, and SRS/SBRT: 2 mm/1°, 2 mm/1°, and 1 mm/0.5°).

Return the couch to 100 CM SSD by using the front pointer and confirming the shift by using the couch vertical readout previously recorded for the couch vertical indicator checks. Remove the front pointer and its mount, and verify that the crosshairs of the light field are still in alignment with the crosshairs of the Iso-Align phantom. The Iso-Align phantom has various field sizes etched on its surface which will be used to check the jaw position indicators and the lateral/longitudinal treatment couch position.

Check various symmetric and asymmetric field sizes by visually matching the light field to the etchings of the Iso-Align phantom. For each field edge, record the difference between the intended/expected digital readout, i.e., the geometry of the light field, and the actual digital readout (jaw position indicators (symmetric): 2 mm); (jaw position indicators (asymmetric): 1 mm). For symmetric field sizes, a tolerance of 2 mm refers to the total tolerance for only the X or only the Y opening. The Elekta Synergy does not allow the user to set the field size by using the pendant in the treatment room, but instead, a field must be moded up through Mosaiq. Rotate the collimator through a full rotation to check that the crosshair intersection of the light field does not deviate/”walkout” compared with the etchings of the Iso-Align phantom surface. If a “walkout” is visibly evident, align a graph paper with the Iso-Align phantom surface and measure the deviation distance to check that it is within tolerance (cross-hair centering (walkout): 1 mm).

Rotate the collimator back to the mechanical 0° by aligning the crosshairs of the light field with the etchings of the Iso-Align phantom. Return to a mechanical 20 x 20 CM field size, and shift the couch laterally, starting in either direction, to align the field border to the longitudinal crosshair of the Iso-Align phantom. Record the difference between the mechanical shift and the shift displayed by the digital readout (treatment couch position indicators for Non-IMRT, IMRT, and SRS/SBRT: 2 mm/1°, 2 mm/1°, and 1 mm/0.5°). Perform the same steps for the opposite lateral direction and the longitudinal direction.


The following check for distance check can be done in various ways. Certain machines like the Varian 21iX Silhouette will have tray which slides into the gantry head which will mount the front pointer. The tip of the front pointer, when set to 100 CM, will display the mechanical isocenter. Whereas using a front pointer for the Elekta Synergy requires a mount to be attached onto the gantry head first and will only represent the mechanical 100 CM SSD. For the first scenario, turn on the room lasers, and check that the room lasers intersect at the very tip of the front pointer. Use a bubble leveler to rotate the gantry to varying angles and checking that the room lasers still intersect at the tip of the front pointer (distance check device for lasers compared with front pointer: 1 mm). For the Elekta Synergy, attach a pointer onto the side of the Iso-Align phantom, and position the couch such that the room lasers intersect by the tip of the pointer. Again, use a bubble leveler to rotate the gantry to varying angles and checking that the room lasers still intersect at the tip of the front pointer. For both of these scenarios, though it is optional, check that the ODI reads 100 CM SSD at the tip of the pointer.

The FC-2 phantom will be used for the following step to check that a light field and its respective radiation field have the same geometry. On the FC-2 phantom, there is an outline for 10 X 10 CM field, and at every corner of the field outline, there is a tungsten ball by which a software will analyze to check for the light/radiation field coincidence. The center marker phantom has a crosshair with a tungsten ball at the intersection of the crosshair. This phantom is placed directly on top of the FC-2 phantom, and will be used to check for the collimator indicators.

Rotate the gantry back to its mechanical 0° position. Place the FC-2 phantom on top of the Iso-Align phantom, and align the FC-2 phantom to the crosshairs of the light field, and use the front pointer to set the surface to 100 CM SSD. The FC-2 phantom can be placed directly on the couch, but it will not be properly leveled which is why it is placed on top of the Iso-Align phantom. Mode up a 10 x 10 CM field and align the light field to the FC-2 phantom. Taking a MV image and running it through a program will tell us how the light field and radiation field match up (light/radiation field coincidence: 2 mm or 1% on a side); (light/radiation field coincidence (asymmetric): 1 mm or 1% on a side). Gently place the center marker phantom on top of the FC-2 phantom, and align its crosshairs to that of the light field. The FC-2 phantom will tell us how well the light field and radiation field match up, and the center marker phantom will tell us if the X1, X2, Y1, and Y2 jaws are actually of 5 CM each, or are of 4.7 CM and 5.3 CM to make up 10 CM (jaw position indicators (symmetric): 2 mm); (jaw position indicators (asymmetric): 1 mm). Take MV images for varying symmetric and asymmetric field sizes and run the information through a software to check that the variables are within tolerance.

Remove the FC-2 and center marker phantom, and set the surface of the Iso-Align phantom to 100 CM SSD. Mode up a field with a wedge and/or a compensator, and open the jaws wider than the dimensions of the wedge. The light field will begin to remain at the same size when the jaws are open beyond the wedge. Measure the distance from the mechanical isocenter to the edge of the shadow, and check that the distance is within tolerance (wedge placement accuracy: 2 mm); (compensator placement accuracy: 1 mm). If the machine uses physical wedges or block trays, insert them into the accessory tray and verify that they latch correctly (latching of wedges, blocking tray: functional). Insert the graticule tray into the accessory mount, and visually check that the crosshairs of the light field and the crosshairs of the graticule tray are in alignment with one another. With the graticule tray mounted, take a MV image using the EPID, and compare physical graticule with the electronic graticule of the image software to see that its within tolerance (accessory trays (i.e., port film graticule tray): 2 mm).

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