Advancing Motion Control: An Evaluation of VRT In-Bore Guided Breath-Hold Using a CBCT-Guided Adaptive System

Xinran Zhong (00:04):

So my name is Xinran, and I’m currently a physicist working at the UT Southwestern Medical Center. And today I’m excited to share our experience using the AlignRT InBore system to guide the breath hold and its application on a CBCT guided adaptive system. I have no conflict of interest to disclose.

Xinran Zhong (00:34):

And here is the outline of today’s presentation. So, in the beginning, I’d like to briefly talk about the CBCT guided online adaptive treatment for whoever is less familiar with it. And then we’ll move on to the breath-hold application in the online adaptive radiation therapy and its benefits. And then I will briefly introduce how we use the all AlignRT InBore system in our clinic Ethos, and I will go into the detail of the implementation of the breath hold from the simulation to treatment and share some preliminary results using on the inter-fraction motion analysis and time cost. And in the end, I’ll summarize some key takeaways. By the way, I think it’s still automatic move advanced slides.

Xinran Zhong (01:31):

So the CBCT guided online adaptive system is to use the daily CBCT and for the counter planning and dose calculation rather than the simulation CT. So in this way it can effectively address the infection motion and with the physician counter the daily target and OAR counter and with a potential with a reduced margin, we could achieve better target coverage and the lower OAR dose. And on the right, I show we’re using the variant Ethos system in our clinic. We have two Ethos systems equipped with a hyper set imaging technology. And as we can see from the picture, the Ethos have a ring based gantry. And the hyper set image technology enable us to acquire the city image with six seconds image acquisition time and with an improved image quality.

Xinran Zhong (02:36):

And with the adaptive therapy, help address the intersection motion. How about the intra-fraction motion? We know that the breath hold is an effective way to minimize the intra-fraction motion and in the adaptive word if we can minimize the motion within the image acquisition, it has another benefit of to reduce the motion artefact, which could improve our image quality. This is especially useful for the patient with a lot of gas in the abdominal area. And with the improved image quality the physician could have more confidence on countering the target and OAR, and it can also potentially help with the AI-based auto-countering that could help accelerate the workflow. So on the right we could see an example. The top row shows the free breathing CBCT and the bottom row shows the DIBH hyper site imaging and we could see the improvement in the image quality. And of course, the breath hold has other conventional benefits of potentially increasing the distance between the target and OAR, and also the potential for further margin reduction.

Xinran Zhong (03:58):

And to achieve a more consistent breath hold we know that surface guidance is a good option to guide the breath hold. And in our clinic, we equip the AlignRT import system on both of our Ethos systems. So here is just a very general introduction. So the AlignRT board is designed to be compatible with the Ethos system. So there would be one set of camera that’s a ceiling-mounted camera that’s used to track the surface at the set up ISO outside of the bore. And when we send the patient inside the bore, there will be a ring-based camera that’s used to check the surface in the middle of the treatment. And here we summarize some key numbers that we got from the technical specifications. So as we can see between the setup cameras and the InBore cameras, the accuracy as well as the FOV are pretty comparable.

Xinran Zhong (05:01):

So with the AlignRT InBore system installed, we not only use it for the breath hole guidance we also use it for the free breathing setup. So the application includes to achieve a more consistent setup across the machines in our situation is between the health and Ethos for our adapt on demand treatments, and it would also help with the setup for breast patients for the same omitted treatments and can be used to remake the immobilization device as well if we have any setup modification or if I have a deflated backpack and so on. And on the right, I included a pie chart showing the case distribution in our clinic from 2021 to 2024, like the site distribution. As we can see, we have a relatively even distribution across different sites. And try to design and test out our breast hole workflow, we pick the breast and abdominal lymphoma case as a starting point.

Xinran Zhong (06:15):

So, before the implementation, we do have some questions in mind when we design the clinical workflow. First, what’s the residual infection motion of the target when we do the SGRT-guided breath hold? Second, what’s the cause of time to introduce the surface guided breath hold to the adaptive workflow? And third, how does a longer treatment workflow affect the consistency across different breath holds? So after some discussion, we implemented the following workflow. So a successful motion management usually starts from the simulation and planning. So in our situation with a difference for the breast and the abdominal cases, we have a slightly different reason why we want to use the breath hold and also the target to surface distance are also different. So we try to design the simulation evaluation slightly differently based on the application.

Xinran Zhong (07:20):

So for all of our breath-hold patients our therapist would have a very thorough breath-hold coaching and evaluation using the SimRT. And so to ensure that the patient can have a long enough and consistent breath hold before we proceed with the CT. And when we are done we will acquire some CT images, and usually a physicist will be there to evaluate the CT image quality before we release the patient from the clinic. So for our breast patients, we usually would acquire one free breathing and one breath hold scans. And here the evaluation would mainly focus on the separation between the free breathing and breath-hold surface. This is to ensure that there is a symmetric value to use the breath hold for the breast planning. And we would also evaluate the shift of the spine between the free-breathing and breath-hold images. We found that this is a good indicator for a more consistent breath hold during treatment. And for abdominal cases, we would acquire more breath-hold scans. So we would usually acquire three breath-hold scans and we would compare the surface consistency as well as the target consistency. If we see significant motion in between different breast hole scans, we will definitely communicate this to our physician to make sure the final PTV margin could include this motion into consideration.

Xinran Zhong (09:05):

So after the simulation and planning, we will move to the treatment and we will use the RTC device to help the patient to achieve a more consistent breath hold. And here the setup procedure would be we set up the free breathing set up the patient with free breathing as a setup iso, and then we will move the patient into the treatment ISO and try to evaluate the breath hole consistency. And if we see a new consistent breath hole that’s different from the simulation, since it’s an adaptive machine, we’ll just recapture the surface before the Cone Beam CT. And in the whole treatment process and the imaging, we would use three millimetre motion for the vertical, lateral and longitudinal.

Xinran Zhong (09:56):

So here is a example accelerated workflow for our setup. So the therapist will use a Posture Video at the setup ISO and the free breathing surface to set up the patient, and then they will shift the patient into the treatment iso and they will also switch the surface to the breath hold surface and see if the patient’s breath hold can match the simulation. And this patient is a pretty great example. So after this verification, they will just step outside of the treatment room and then start the six seconds Cone Beam CT acquisition. So as you can see over here the patient only need to hold his breath for six seconds before the Cone Beam CT is done.

Xinran Zhong (10:46):

And after all the design, the simulation and treatment, we also want to make sure we have mechanism to evaluate the in-act motion and try to correct any motion if we find any. So in total in our workflow, we would acquire three CBCTs for the motion evaluation. So the first CBCT would be used for the planning, which is a reference CBCT. And after the whole adaptive workflow and before the treatment starts, we would have the second CBCT and the motion between the first and second CBCT we name it the pre-treatment shift. And then in the middle of the treatment beam delivery, we would acquire another CBCT. That’s just to verify the intro fraction motion and the motion between the first and third CT, we call it intact shift. And this shift is more critical related to the patient’s safety and the beam delivery accuracy. And one thing to notice is that after the second CBCT shift, we usually need have to do a recapture. So we build a new surface with this CBCT verification.

Xinran Zhong (12:06):

So in our current data analysis we collected around 50 to 60 fractions for each group of patient. So we got around 50 fractions for breast patient, free breathing and breath hold. And for this patient group, the target would be the whole breast and the prescription would be 26 Gy in five fractions. So it’s very homogeneous. And for the lymphoma patients the treatment sites and the prescription dose would vary a little bit but we manage to collect enough fractions. So on the right is just an example of the target and the dose for each group of patient respectively. So on the top is the whole breast, and on the bottom is alymphoma case.

Xinran Zhong (12:59):

So then let’s take a look at our data analysis results. So first we analyze the motion for our breath-holding patients. And on the left is a histogram of the pretreatment shift. On the right is a histogram of the inter fraction shift. So as we can see in general, the intersection shift is shorter compared to the pretreatment shift. And this we believe is related to the time from the latest surface capture to the CBCT. And as we can see, like I put the time statistics, the pretreatment shift between the previous surface to the current CBCT, it’s an average 40 minutes, but before the inter-fraction shift, since we build a new surface in the second CBCT, like the time interval is shorter, so it’s on the order of six minutes. So since the inter shift is what we care more like, that’s more related to the patient safety and we found that only one fraction of the shift that’s more than our margin, that’s five millimeter and the motion is 5.2 millimeter.

Xinran Zhong (14:12):

And this corresponds well with our SGRT tolerance. So it shows that the motion that SGRT guidance is very effective for our whole breast treatment. And how about the time cost? So here I did a box plot comparing the free breathing patient group and the breath-hold patient group. So the setup time, which is includes a lot of other steps, but it’s a fair comparison. As we can see, the time are pretty comparable between the free breathing and breath hold patients. And the session time, which is from open the patient to close the patient, which corresponds to the machine time. It’s pretty also pretty similar, like the breath hole patients take slightly longer, but on average it’s only four minutes longer, which is pretty short compared to the whole session time.

Xinran Zhong (15:08):

So similarly, we did the comparison for the lymphoma patients. As we can seethe PTV margin is slightly larger due to our motion evaluation from the simulation. But we see similar trend, like we see much smaller intra-fraction shift compared to the pretreatment shift. And among all 65 fractions, we only see one fraction that’s with a shift that’s more than seven millimeters and the shift is eight millimeter. We also notice that the time bet between the latest surface capture and this CBCT is 60 minutes, which is much longer than our average. So this fraction could be due to some very special situation that patients just couldn’t do a very consistent breath hold. And we also correct the motion with the CBCT. So similarly, we also compare the time cost. So here we can see the beam delivery time with the breath hold is slightly like higher around about five minutes. And but we can see the session time at least the average are pretty close to each other. So which shows that the breath hold although it gives a longer beam delivery time, but you won’t necessarily add the machine time for our abdominal lymphoma patients.

Xinran Zhong (16:35):

So in summary, we demonstrate that patient education and motion evaluation during simulation are essential for our successful breath hold motion management workflow, which shows that the AlignRT InBore guided breath hold can provide reliable inter-fracture motion control for our whole breast and abdominal lymphoma patients. And we also show that although the breath hold could add some time cost for some steps of the workflow, but in general, the added time is pretty minimal compared to the overall session time. And in the end, we found that it’s, if we observe a significant time passed since the latest surface capture, it may be beneficial to verify the breath hole consistency using the Cone Beam CT in the middle of the beam delivery for safety purposes. So that’s it for my presentation, and I’d like to thank our great physicist team, physician team and therapy team. Without them, we can’t implement this. Thank you.