RGSC 1.0 - Varian's New Gating Solution for CT Scanners

Types of Gating we currently use

On the Clinac, we did not use any Gating (RPM was not installed).

We started with Gating on the TrueBeam in November 2013 (using RPM on the CT scanner).

Backup Gating

The type of Gating we started with is sometimes called Backup Gating: it contains all elements of "real" gating, but lower and upper thresholds are enclosing the whole respiratory curve.

A 4D scan is acquired first. After retrospective reconstruction (10 bins) and transfer to Eclipse 11, the phase bins which shall be used for treatment (all in the case of Backup Gating) are identified. The CTVs are delineated on the respective structure sets. From the CTVs, the ITV is generated.

A 3D scan (free breathing), acquired in the same imaging session, is registered to the 4D scan. The ITV is copied from the 4D to the 3D scan. The structure set on the 3D scan is used for optimization, dose calculation and 3D-3D matching on the TrueBeam. Eclipse 13 will give the possibility to create average CT from several phases, which can succesively be used for dose planning. This will eliminate some of the above steps.

The first two cases were lung SBRT cases. The following screenshot shows the first clinically delivered, Backup Gated SBRT plan:

Prescription was six times 7.5 Gy. Two half arcs were delivered with 10FFF and a dose rate of 2400 MU/min.

Dose calculation was done with AAA and Acuros XB (grid size 1.5 mm).

For the first patient, the PTV (ITV plus margin) was used for 3D matching on the TrueBeam.

This technique is called Backup Gating because it provides some extra security should the patient start to cough excessively during treatment. In such a case, beam would be held off automatically.

Session timing (always starting with CBCT imaging) improved from 7:57 minutes for the first session,

to 03:59 minutes for the last session (including 10FFF integrated MV-imaging):

Deep Inspiration Breath Hold

To maximize the distance between the left breast and the heart, the patient is instructed to breath in and hold her breath as long as the field delivery takes. This is called Deep Inspiration Breath Hold (DIBH).

Delivery is fast (not interrupted), and the breast-heart distance that can be achieved is larger than with gated delivery and free breathing.

Here are some screenshots during DIBH on the Truebeam. It is session 2 of 27.

First the patient is set up under DIBH.

For imaging, the patient is coached and triggered kV-kV-images are acquired, again with DIBH². This gives the necessary couch shifts, which are then applied.

After the patient has been breathing normally for several seconds, coaching starts again for the treatment under DIBH.

The first screenshot shows delivery of the medial breast field, roughly half into the treatment (click all screenshots to enlarge):

and a little later (jaw moving = EDW):

Then the patient exhales and breathes normally again for several cycles. Gantry is rotated to the second angle.

Then the audio coaching starts again, and the patient is requested to hold her breath. As soon as the lower threshold is reached, treatment of the lateral field starts:

and a little later:

After treatment is complete, patient may exhale, and breathe normally again:

The yellow shaded bars in the screenshots display the time the beam was enabled.

With TrueBeam's integrated imaging and Gating capabilities, the whole process is extremely smooth.

(Vienna, March 7, 2014): After removal of the RPM system on March 5, we installed Varian's revolutionary new Gating solution RGSC (Revolutionary Gating for SCanners¹) on our Toshiba AquilionLB. Installation, testing and acceptance took 1.5 days, without any stress. On March 7, we tested the system clinically for the first time, using both integrated coaching possibilities: audio and video coaching.

Differences between RPM and RGSC

While the gating principle is still the same (reflective marker block on patient + camera), hard- and software are completely new.

The major components of the well designed RGSC cabinet are two blue "shoe-boxes": the workstation (upper slot) and a real-time node (lower slot). The real-time node is always on, see the instruction labels on photo ("drehen" = turn, "abdrehen OK" = shut-down OK, "nix abdrehen" = do not turn off):

The cabinet also contains a 16 port switch, a Juniper firewall, and a power injector for a WLAN acces point. The latter is positioned in the CT room, has Power over Ethernet (PoE), and streams the visual coaching animation to the VCD (Visual Coaching Device), a cable-free LCD monitor. The VCD can be conveniently positioned in front of the patient's face:

This VCD comes with battery packs and a Varian charger. The VCD is on a telescopic arm, which is connected to the CT overlay with a couch mount that is specific for the carbon overlay.

The VCD can display various animations such as simple trail, a 1-D slider which should be kept within the gated range (the smiley is then displayed as "reward"), or a dog which has to be kept on its path (thereby collecting "targets" = bones):

On the next photo, the VCD has a USB mouse attached. This is only used for diagnostics and to start the demo mode. Clinically, no cable is running to the VCD. The position of the reflector block is transferred via WLAN to the VCD, which then generates the animation (It's actually a little computer).

The Camera uses the same principle as the RPM camera, but is a complete redesign. A red guiding laser is hidden behind the front cover of the camera (similar to the laser on the TrueBeam NDI gating cameras). This laser can only be operated by removing the front cover. It should point to isocenter.

We have designed an aluminum frame and mounted the camera to the same roof mount as the RPM camera:

RGSC uses the same marker block (irregular shape, four reflective lenses) as the TrueBeam:

Tracking is always 3D. A special camera calibration plate facilitates recalibration, should it be necessary. Here are software screenshots for positions 1, 2, and after the last step. Reflections of the carbon couchtop can fool the software. This can simply be avoided, by placing a sheet of paper on the couchtop, in front of the block.

Calibration check is not mandatory any more. Recommeded check interval is 1 day.

Workstation software is visually designed like the TrueBeam software. Users log in via OSP. Several user rights can be set. Messages pop up. In this sense, RGSC is a "little TrueBeam".

Whereas RPM (last version was 1.7.5 Deluxe) was a stand-alone solution, RGSC (current version 1.0) is fully integrated into the ARIA system, if TrueBeam and ARIA 13 are used. But RGSC can also be used with C-Series Clinacs (where RPM is installed on the Clinac), or with TrueBeam and ARIA 10 or 11.

The only part which is still the same (except for the new on/off switch) is the breathing phantom provided by Varian. This device simply cannot be improved!

For real phantom scans, we use our Quasar respiratory platform. One simple phantom which can be placed on the platform is the IGRT cube from CIRS. The platform can be moved around using sinusses or real patient waveforms.

First Experiences

Although it is version 1.0, the whole RGSC system appears to be very mature and stable. No problem of any kind was encountered during installation and during the first day of clinical use.

Update, March 2015: Last month, Toshiba installed a new serial interface between RGSC and the Aquilion LB, which is currently under testing in our institute. The benefit: after a 4D scan (and before the retrospective reconstruction is started on the CT console), it is not necessary any more to compare the breathing patterns peak-by-peak on the gating system and the scanner. This greatly improved our workflow!

Notes

¹Just joking. R stands for Respiratory, of course.

²For breast, a kV-kV image pair is usually only acquired during the first session. But because the MV-before-image showed a large shift, the kV-kV-image pair was also acquired in the second session.