Changing Practice with MapRT

Dave Parsons (00:03):

Okay, so good morning everybody. I’m Dave. Today, I’m going to talk about how we really changed our practice using MapRT, which sort of echoes that again.

Dave Parsons (00:17):

I’m going to structure the talk is I really want to set the stage to why we wanted something like MapRT. So I’m going to go through what UT Southwestern is as a department why non coplanar treatments were important to us, how we made them safe in the past, how we now do that with MapRT and how we implement that into our clinic. And then go through a couple examples of where it’s useful and then summarize.

Dave Parsons (00:46):

SoUT Southwestern is in Dallas, Texas. If you’re questioning where my accent is or disappointed that it’s not a Texas accent, I’m actually from Nova Scotia, Canada originally the nice gentleman at the front lobby when I asked for directions thought I was Scottish. And I was like, I don’t think it’s that strange of an accent. But yeah. So we’re actually celebrating our 20th year as a cancer center, well not cancer center, but a radiation department at UT Southwestern. And you could see we started with just treating over 1200 patients a year. But now in, well, last year in 2024, we’re treating just over 4,600 patients. And even though we have added linear accelerators to our department, the main driver of that growth has been going to more hypofractionated. So back when we started, we were doing about 22 fractions per course of treatment. Now we’re doing about 12 fractions per course of treatment on average. And you can really look at why that is. So being in the US, we do have a healthy conventional population with tangent, for example, or whole brain treatments. But a lot of that other stuff being IMRT and SBRT that drives the department where SBRT or SABR is the largest chunk of our patient population that gets treatment.

Dave Parsons (02:10):

Now, if you ask my boss which is Robert Timmerman, how you should plan those patients, he would say the following, when you’re doing a SABR treatment, you need to respect the tumor coverage and the dose compactness and how that’s, that really defines SABR. So when you’re thinking about analyzing those plans, you want to be a very conformal plan. So pad, conformity, index approach, stream one low d2cm, a low grade index, I’ll go over those on the next slide if you’re not familiar with them. And what you’re doing to achieve those is to have really many beams coming from many different directions to spread out that entrance and exit dose effectively trying to create an isotropic fall off where possible.

Dave Parsons (02:55):

So just to go over those d2cm. So if we have our PTV there in red, a two cm expansion around it in any direction is that green structure. And we’re just trying to limit the max dose going into that. And I am a physicist, so I need my one equation per talk, and it’s just going over the grad index, which is the volume receiving 50% of the RX divided by the volume receiving a 100% of the rx. And if you’re familiar with lung planning, you would see something like this for from the RTOG, which says, oh, for your PTV volumes, you expect to have a gradient index somewhere here. And then the max d2cm in this column. And which usually you can achieve if you’re doing non coplanar treatments.

Dave Parsons (03:38):

Mu-Han Lin, one of my colleagues, had this nice slide that sort of shows that process. So here she has a lung lesion with two coplanar arcs which, you know, met the constraints with a grad index of about 4.6 a d2cm of 50% of the rx. But you can make that better by taking one of that, those complainant arcs and changing it to be non coplanar, where the gradient index is the same, but you’re getting that d2cm to be lower. And then if you go completely non coplanar, in which case she had 10 3D conformal arcs. So not even modulating the fluence, she’s able to get that gradient index to be lower and the dose compactness to be lower as well. And you can really see that there in the cyan. We’ll try to put it on that screen too, where that’s really becoming more and more compact as you increase the number of non coplanar beams.

Dave Parsons (04:30):

So hopefully that’s set up the stage to why we want to do non coplanar treatments overall, almost 40% of our patients are SABR and from the top down we’re being directed to do non coplanar plans to give the best quality we can. So how do we make that safe?

Dave Parsons (04:50):

So we did what you would normally do for us, which was you set up the immobilization on the couch and you check the clearance. The therapist here are showing that. So we use the elect body frame still, and you can see they set it up to the lasers in the room put the isocenter on the frame coordinates, and then actually ran through every combination of coaching gantry and collimator and says, does it pass or does it fail? And if it fails, could we recommend something that would clear that would then go back to dosimetry to say, yes, you’re good, or, oh no, you need to revise your plan.

Dave Parsons (05:30):

Of course this adds time. So this plot here is showing the number of angle checks and then how long it took from when the dosimetrist requested it to when they got it back from the therapy team at the machine to say, yes, you’re good to go. If your plan, you can continue your planning. And most of the time we’re averaging that to be within one working day for a dosimetrist. So a mean time about 6.2 hours. However, in some cases, say the plan was submitted at the end of the day, the morning, those symmetry or therapists didn’t get to it because they had patients to treat and they didn’t really get to it until the following end of day. So now you’ve added that whole day of just waiting to know if that planner can proceed with their treatment plan they wanted to use.

Dave Parsons (06:16):

And in some cases for these 60 patients we looked at, it was almost 24 hours. So, and that would be roughly three clinical days of working time.

Dave Parsons (06:26):

Of course, you can always get the scenario where you have drastic failures in this, where the plan failed on multiple reasons. So this is the feedback one of the dosimetrists got. If you can’t read it, we’ll just make it really bold for you and say your ISO two posterior, your cone beam won’t clear the panel, hits the couch or hits the immobilization, a their plan, send it back and hopefully get the all clear. However, in some scenarios, even though they aren’t having any issues with the immobilization or the couch being an issue and two of your couch gantry combinations also collide with the immobilization. So you have massive changes to do in that you’re doing for your changing your actual isocenter in the plan. And if you’re that the dosimetrists you’re probably starting to panic now.

Dave Parsons (07:02):

And then they would have revise axis with clearance, they might say something like, you’re all clear, but you should really watch the elbow fraction one. And you could see that for this patient, the elbow actually goes quite above the immobilization and they know that it’s there, but they don’t know where it really is. They’re just guessing that it could be an issue. And we’ll actually come back to this patient later in the talk to show it actually was an issue fraction one.

Dave Parsons (07:39):

So of course is what we believe to be a solution for this. I know some of you are familiar with it, but for those in the audience, I’m just going to give a brief explanation of what it is.

Dave Parsons (07:53):

So it’s a surface guided technique that captures the patient’s surface at the time of simulation. So that’s this guy here. And then it combines it with a LIDAR scan of linear accelerators. So here’s one of our, the true beams and they combine those two to look at how that would, how the clearance would be for a prospective plan.

Dave Parsons (08:13):

So if you don’t have this in your clinic, you essentially get two cameras on either side of your CT and then that is used to acquire the patient’s surface for us. We also have SimRT in our vault, so that’s the third camera there. We really wanted the monitor to be in the room. So we really proposed that initially when they installed it. So we would have an extra monitor for SimRT and MapRT in our vault and then also in the console two. But you can use the same keyboard and most it’s to between the two systems.

Dave Parsons (08:47):

After you acquire a surface you would get this interface on the software. You say, I want to use this machine model with this surface capture. In this case it’s a head and neck patient with laryngeal cancer.

Dave Parsons (08:59):

And then once you select that, you can proceed to this interface, which shows you your imported DICOM parameters here. So we have our imaging fields, our arcs for treatment, and I don’t know if I can do that over there, but yes, your imaging fields and arc for treatment. And then here you can see what that looks like where you have gantry on the Y and couch on the X axis, in which case we’d have two non coplanar partial arcs and then one full coplanar arc for this patient. And then of course your collision zones, which would be areas that you’d ideally want to avoid. You also have this interactive window which you could call like a room’s eye view where you’re going through the treatment. Which you’re going through the treatment to see what that looks like as if it were to happen. And in which case you can see for these two partial arcs, everything is fine. We have no issues there at the clearance. This is using a d2cm safety buffer around the whole treatment. However, if you were to navigate into one of those collision zones, you can see what that would look like, where we would definitely be hitting that patient’s pelvis if we continued to keep going, which we ideally want to avoid. So that’s really why we thought found MapRT attractive. Cause it shows us what you don’t know, which is where the patient is relative to their mobilization.

Dave Parsons (10:28):

So how do we implement that in the clinic?

Dave Parsons (10:31):

So we actually got this back in August, 2022 which was one of the research versions installed at our center. And then there was a second one in Raigmore in Scotland. And then over that next year we validated it, gave feedback, had iterative updates, and then despite it not being approved by FDA in the us, we as a department said, we’ll take the liability. We want this to be clinical now. We don’t want to wait for more updates to go live. So we went live in October 23. We then installed our second system on January, 2024. This also added more models in that update that happened then. So you can see we have a Halcyon here, a TrueBeam with its imaging arms out a TrueBeam with a electron applicator and a versa. But you notice the pendants down in this Versa model. Originally that wasn’t, which was a source of constant, is it going to clear or not? So now they’ve actually put that pendant down, which hopefully elect that someday we’ll just move it to the end of the couch. And then just last month we installed our third system, which is on a PET CT that we just installed. So we, we definitely think it’s the, the future and we’re, we’re pretty happy to have it on our three simulation machines. Hopefully, someday we’ll go on an MR simulator, but we’ll see.

Dave Parsons (11:55):

So our first question was what is the accuracy? And this has work done by Siqiu Wang, who was a resident in our department at the time, but has since been joined us as faculty. And she said, well, let me map that surface. The interface of that collision space where she took one of the cube phantoms scanned it, went through the process with capturing it and said every point of that surface, I want to see how close that is to reality. So she then went to the vault, set it up and said, okay, how close can I get without, you know, causing the sweat to pour down my head as I get closer and closer to the machine and see if I have a constant couch, what would be the gantry angle to cause a collision? Or if I have constant gantry, how much would I have to move the couch to cause a collision? We didn’t actually want to collide, I didn’t want to explain to the department leadership why their linac was broken. So we got reasonably close, probably a millimeter in reality.

Dave Parsons (12:47):

And what she saw was if we have gantry angle here and coach angle here where red is the predicted MapRT collision and blue is the measured band, either how much you move gantry or how much you move coach for a given angle what that measured result is. And then that green band says, well, what if you expand the MapRT result by a degree? In which case we saw that all the measurements that she did was were within a degree of the prediction. So for us, we say that the accuracy, at least our install at our center was within a degree either for gantry or couch, which we’re pretty happy with for being roughly a millimeter from colliding the machine.

Dave Parsons (13:30):

How does that compare to what we were doing though? So that same 60 patients that we looked at earlier for seeing how long did it add to your planning timeline, we also went back and saw what was the success ratio. So 55 of those patients, no issues. They, the therapist did the angle check fraction one treatment went fine, but for five of those patients, fraction one didn’t go so fine, in which case either the mobilization had to be changed such as it was a prostate, you could move an arm to get out of the collision. Or in the case of a lung you aborted the fraction completely and said, sorry patient, we need to send you home and redo your plan because we’ve, we have a collision that wasn’t identified. And when we went back and looked at those same patients with the MapRT surface that we had, you could predict every collision that they saw and then when they did their plan update, that was also still true in MapRT as well. So we’re saying at least for our patient population that we treated and can compare to real data, it’s a hundred percent successful in identifying collision risk.

Dave Parsons (14:35):

So with that of course we said we went live. So for our true beams, which we have four in our clinic, this was the number of angle checks we were doing for essentially the last three to four years, going back to November, 2021. And you can see it goes up and down but the average is somewhere around 38 angle checks per month. Since going live with MapRT we saw that start to decrease, cause we’re getting more and more confidence. Of course we still have another CT scanner that was installed which was here and we saw it go back up cause when we installed that second system, we also updated the software. So it took a two week pause and said is the new update as good as the previous version, which it was. And then after having both systems live, you can see we sort of flatlined here with roughly three angle checks per month that we still are doing. Now you might be saying why still three?

Dave Parsons (15:30):

Well we have a pretty diverse clinic and some patients that either are sim for our MR-Linac, which is unity from elect or CyberKnife have a mobilization that interferes with the coach markers that are placed in the CT couch for acquiring a surface. So if you’ve not seen an LEC frame or elect Unity simulation, they have this annoying couch top that runs the whole length of the couch and just prohibits you from capturing the surface. Whereas CyberKnife they just had this black thick pad about three inches thick, that’s for patient comfort and the therapists weren’t moving that prior to treatment or during their sim to capture surface.

Dave Parsons (16:10):

We’ve tried to mitigate that going forward. So we’ve actually moved the coach markers down further so we can at least capture some of these patients. But ultimately for the elective coach top for the MR-Linac you’re just limited to the field of view of the camera not being sufficient. Even though it is quite large, it’s a very long couch top that elective side to use. For CyberKnife though, it’s just simply reeducating the therapist say you could fold that pad, do your capture and then if they do go from CyberKnife to another machine like TrueBeam, you can have that surface available. So we hopefully mitigate that going forward. But we’ll see that in the future data.

Dave Parsons (16:46):

So how has that changed our clinic? So prior to MapRT we were doing about 38 angle checks per month taking roughly about nine and a half hours of vault time per month just to do that. So effectively you’re, that’s saying one day a month that machine was just being used, nothing but to do angle checks. Ideally that machine should have been treating patients but over the month it’s being used to do something that we can now do at MapRT. In terms of planning that added just under 10 days of just waiting to know if you could use that plan that you want to use. After going live with MaRT we’re now down to three manual angle checks. Those are usually, like I said, the Unity couch top or CyberKnife, those patients already usually have a delay in their planning timeline because they’re going from an attempt being an MR-Linac or CyberKnife to a C-arm Linac. So RA going long but even still those are only adding about 40 minutes of vault time per month, and only increasing the planning time by a month or not a month. That’d be way too long. Just under a day. And waiting to know if that plan is clear to go.

Dave Parsons (17:54):

So we’re quite pleased there. I just wanna go through some examples now of how you could use this.

Dave Parsons (18:00):

So that patient I showed earlier with the elbow in their mobilization, this is her plan and you can see it is quite a posterior lesion in the lung and it was in the testing phase of MapRT. So we weren’t using it clinically yet, but the angle check everybody said you should watch the elbow but we don’t know. Well sure enough fraction of that definitely collided with the elbow. It’s fairly close to the target site so you couldn’t readjust the patient. So we had to cancel the patient, the dosimetrists re-plan it by just doing a simple drop of the couch by seven centimeter and then that shift the collision zone over. Of course this was not with MapRT but you can see how that changed the clearance space. But you could see how it would be readily identifiable in MapRT as this is definitely an issue that you don’t want to have occur on your treatment.

Dave Parsons (18:56):

How could we have done better? Well, since you can see this clearance space, you have this lovely little section with no collision when your isocenter is in the target and you can really make a much better plan using non coplanar planning for map with MapRT. So realistically, this plan should have been something like this where you have this quarter arc here at roughly negative five and then this partial arc here at positive 20 degree coach rotation.

Dave Parsons (19:25):

And you can say, well how much time would that add? That adds essentially no time, cause if you start with two partial arcs, like here, you can go in the MapRT and just say, okay, how much do I have to change these arcs to be able to use them? And you can see this one, it’s still relatively close to the elbow, so maybe we want to go a little higher. Whereas this one, it’s pretty much in the collision space, we’re still avoiding it, but maybe we want to adjust that one a little bit as well. But you’re talking on the order of seconds, so you’re probably taking more time to transfer the plan to MapRT than actually figuring out how much you want to adjust your plan with MapRT to know what your clearance base is. In the end, this would be what that plan would look like.

Dave Parsons (20:09):

And then here’s the pick comparison between the two plans. So this is what was treated. You could see paddock and formulate index is pretty good 0.9, ideal would like perfect would be one gradient index5.4. If you look at the 25% grad index, 28 and a lot of MU. But you can see with that non coplanar patients essentially the same conformity. The gradient index for 50% starting to go down, but the 25% is cut in half and that 25% is this purple line here. You can see where it’s really quite different when you’re treating it non coplanar compared to just as two x or coplanar arcs. And actually it’s a more efficient plan too, where you’re cutting that mu as well.

Dave Parsons (20:52):

If we repeat that over 20 more patients, you sort of see the same thing. It is statistically different in that the conformity is usually a little bettern on coplanar arcs, but clinically is that significant? Probably not. What is probably this low dose bath, it almost is always better with non coplanar arcs. Some cases no. But that’s usually when you’re starting to get to larger size lesions.

Dave Parsons (21:19):

And you can also look at that here where this is showing the percent of the RX dose as you move away from the PTV where at the high dose it’s roughly the same for coplanar and non coplanar arcs. But after about eight millimeters from the PTV surface, you start to see that diverge where your coplanar arcs are in blue and your non coplanar arcs are in this orange color where it almost always will give you a better plan if you do non coplanar treatments.

Dave Parsons (21:46):

Another interesting site is partial breasts. So we have a healthy partial breast program at UT Southwestern where you’re just treating the tumor bed, not the whole breast. And I think from, well pretty much any breast patient, the number one thing you’re trying to spare spares the heart. As we know from Darby paper for every mean gray increase in the D mean to the heart, you’re seeing about a 7% increased risk of major Corona events in that patient’s lifetime. And a more recent paper from that same group for lymphoma patients show that there’s potentially no threshold to where you would see an impact on their long-term health of the heart. So ideally for breast patients, we’re pushing that as low as we can possibly go.

Dave Parsons (22:29):

But some patients that’s really hard to do. So this is a partial breast patient. You can see our CTV and PTV there in orange and red around the tumor bed and it’s really close to the heart. It’s about a centimeter at closest distance from the heart to the tumor bed. And if you treat this, this with coplanar arcs, you can see most of that exit dose is going into our structure we’re trying to avoid.

Dave Parsons (22:55):

But it is a nice open collision space where you can really use that to your advantage, but it doesn’t pay respect to the geometry of the patient. So you can see if you instead use one of these vertex arcs that it will easily clear here, you can effectively if the video will play, avoid the heart at every possible angle of that arc, giving you a much better heart dose what we’re trying to get to, but we know from lung patients, LAD’s probably a good correlate to heart effects and you can decrease that mean dose as well by using non than the coplanar arc would.

Dave Parsons (23:22):

How that looks like when you compare the two plans, you can see this low-dose bath going into the heart, whereas the non coplanar plan really is avoiding the heart both on the, the axial plane. And you can see what those dose differences look like. So the mean dose is decreased by just about half, whereas then the V 1.5 gray, if you plan partial breast plans, you know, that’s usually the metric we look at is also decreased by about a third. LAD we don’t really have a number for coplanar plans. And interestingly, lung also decreases by using the non coplanar treatments. If you repeat this over 18 other patients, you can see that plan quality is sort of these guys here. Pad conform index is about the same ratio of 50% dose to a hundred percent dose in the breast is the same lung dose, approximately the same contralateral max dose of the breast about the same. But all of these heart and LAD metrics for the D mean here, the V 1.5 gray and interestingly the V seven gray are all decreased. And similarly for the LADD mean is also to decreased, but what isn’t decreased is the max dose of the heart and the LED that’s about the same in either scenario.

Dave Parsons (24:46):

So with that I’d like to summarize the talk and say MapRT is a novel SGRT solution for clearance mapping. It’s more accurate than our manual angle checks, as you saw by the patients that we did in that example. It greatly reduces the planning time going from 10 days of wasted clinical time to just under an hour. It can give you better plans when you use for non coplanar treatments, which allows it to be effective and efficient in terms of your choosing those, those angles for coplanar treatments or non coplanar treatments.