HyperArc - How Many Targets are Possible?
Since we are the only HyperArc department in Austria, we are often asked: "What is the maximum number of lesions that you can treat simultaneously?" This will be today's topic.
Lesions and Targets
First we have to clarify the terminology. The Treatment Planning System (TPS) for HyperArc is Eclipse. In Eclipse, each Structure that should receive a certain minimum dose is called a Target Structure, or simply Target. Clinicians on the other hand speak of Lesions. If, for instance, a patient has, say, five Brain Metastases (BMs) that should be irradiated, there are five lesions to treat.
Using the common Margin Concept, each lesion consists of three (overlapping) structures: the Gross Tumor Volume (GTV), the Clinical Target Volume (CTV), and the Planning Target Volume (PTV). In HyperArc planning, we contour the GTV based on registered MR information, and create the CTV out of the GTV by adding 1 mm margin, and the PTV out of the CTV by adding 2 mm. This means that the GTV-to-PTV margin is 3 mm. This is done for all lesions.
As described in the previous article on HyperArc, the desired dose distribution in each lesion has the prescription dose at the PTVs outer border, and increases towards center. In our department, the innermost structure, the GTV, should get the Center Dose. The dose distribution inside the GTV should be homogeneous, which means that all voxels inside the GTV should receive the same dose1. A typical Dose Prescription would be: all lesions should receive 6 Gy at the border and 7.5 Gy in the center in five fractions, which means that the Total Prescribed Dose is 30 Gy, and center dose is 37.5 Gy.
To reach the goal of the desired dose distributions, we have to convert the Lesions into Targets by taking into account that the Photon Optimizer (PO) of Eclipse works best with non-overlapping structures.
Since PTV/CTV/GTV are overlapping, we segment them by forming non-overlapping shells. This was also described here. Each shell is a separate Target during VMAT Optimization. If we use the PTV-CTV shell, the CTV-GTV shell and the GTV as Targets, we get three Targets per Lesion2. The title of the previous article is therefore somewhat click-bait, because it spoke of "45 targets", which could be falsely interpreted as "45 lesions".
Now that we have clarified the terminology, we come back to today's topic. The idea is to use a clinical case, add some more (non-clinical) lesions, create the new targets, perform PO optimization, calculate dose, and repeat the cycle until something either fails in Eclipse or the process becomes so slow that it unpractical to proceed. Since the GPU card in the Eclipse workstation has limited memory, and both PO optimization and AXB dose calculation use the GPU, there must be a practical limit.
We expect that there is no "hard limit", because other factors have an influence, such as the size of the CT dataset, the grid sizes, voxel sizes and so on, and these depend on the individual patient.
We start by describing the start conditions in the clinical (treated) plan:
Start Conditions
Today's patient has 7 clinical lesions. All lesions are to receive the same fraction dose (9 Gy at the border and 11.25 Gy at the center) in three fractions. This gives 27 Gy total prescribed dose, 33.75 Gy center dose.
The CT dataset has 299 slices (1 mm slice thickness), and the lateral pixel size is 0.6 mm x 0.6 mm. Based on CARE kV, the scanner selected 80 kV for the scan. Dose will be calculated on this 80 kV scan.
Since the seven lesions are distributed rather evenly within the brain, the HyperArc module selected a central isocenter location. All four arcs, with couch angles 0°, 45°, 315° and 270° can be used:
(Field arrangement of the clinical HyperArc case, two different views, "Normal" and "Halloween".)
The plan timing for the clinical case with seven lesions was as follows. When the Physics team received the case, GTV/CTV/PTV were already contoured, and we started the clock. We started by defining the several non-overlapping target shells and some other structures such as Brain-GTV, which are used to evaluate the clinical goals (Timmerman tables). It took 30 minutes until all structures were ready and the the actual optimization could start. The first acceptable plan was ready after another 35 minutes. This is fast, considering that all resolution settings were set to "High" or "Fine" (PO dose calculation resolution, Structure resolution) and the calculation grid size was set to 1 mm.
We have seen that a good HyperArc plan can be created within an hour or so, but of course, one tries to optimize further, creating more plan variants. In our example, the third plan was accepted for treatment3.
Adding More Targets
We now leave the clinical case by duplicating the Structure Set. To the copy, we want to add 8 more lesions which have no clinical meaning and which we distribute randomly inside the brain.4 We first draw the GTV (about 3-4 mm diameter), generate CTV and PTV by adding the margins, and create the non-overlapping shells.
This gives a total of 15 lesions:
(One of the lesions - lower left - is a double-lesion, which counts as one. Eyes, Brainstem are for orientation.)
The user interface starts to slow down during editing: we always use "upper" objectives for each target structure during HyperArc optimization, whereas the default Varian HyperArc workflow only generates "lower" objectives for each target. This means that for each of the 45 targets, an upper objective must be added, plus the desired dose level, plus the Priority for each objective. The list on the left of the PO interface (yellow frame) is actually more than 6 pages long!
Doing all this adding and edditing takes considerably longer the mor items are added. But once the edits are finished, optimization itself still performs without issues.
Going For 20 Lesions
Twenty lesions means 60+ targets, and we start being cautious and save after each major editing step. This screenshot is from Contouring workspace, where we currently generate the PTV17 by adding the marging to CTV17:
We now change our naming convention regarding the shell structures. Instead of writing t16a and t16b, we write ATV16 and BTV16. This has many advantages, from creating new structures (right-clicking ATV16 and selecting "Duplicate Structure" automatically creates ATV17) to alphabetic ordering: ATV20/BTV20/GTV20 are now the three non-overlapping targets for lesion 20. We repeat the renaming for all lesions and do not forget to update the Brain-GTVs, the Brain-PTVs and PTV total structures after adding more lesions.
The advantages of the naming scheme change become obvious when we select "Modify HyperArc Plan" from the menu. The "A" and "B" shell targets are nicely grouped together, which makes it easy to insert the correct prescribed doses:
Unfortunately, since this is Varian philosophy, only the lower (prescribed) doses can be added here, not the upper limits. This has to be done in the PO Optimizer window.
After each edit in the user interface, one has to wait a few seconds until the screen is updated. With a little experience, on can click a few fields in advance and enter the values, until the UI responds.
With 20 lesions (61 targets), it took 4 minutes 25 seconds after clicking "Start VMAT Optimization", until the first iteration appeared.
(20 lesions, 13min into GPU optimization.)
After four optimization levels (MR Level 1-4), at 27 minutes into optimization, Intermediate Dose was calculated. This took about 3 minutes. 35 minutes into optimization, we are still looking good:
Exactly 49 min 15 sec after start of optimization, we had the final 3D-calculated plan:
(Final dose for 20 lesions. The good image contrast in the brain is due to the 80kV scan.)
We can conclude that 20 lesions (60+ targets) are still no problem, if the planner is not in a hurry.
Next Step: 25 Lesions!
Adding five more lesions is straightforward. In Contouring, the editing process is still fast. Adding the "Upper" objectives to the ATVs, BTVs and GTVs in the optimizer window and editing the dose levels and priorities however takes even longer than before.
From "Start VMAT Optimization", it took 6 minutes 10 seconds until the first iteration appeared.
(14 minutes into optimization with 25 lesions and 76 targets.)
Intermediate Dose started after 35 min 40 sec. The final dose appeared after 62 minutes:
(Final dose for 25 lesions or 76 targets. Here is the calculation log.)
Discussion
By adding more targets step by step, no hard limit could be found. The system only seems to slow down a little5. Clinically thinking, the question is rather: does it make sense to irradiate 25 lesions? But it is certainly good to know that it is possible, just in case ...
Literature
1Different lesions can be optimized to receive different doses in a HyperArc plan, but for simplicity, we choose the same prescribed dose for each lesion.
2As described, we sometimes add more structures as Targets, such as PTV total. This is optional.
3Only one goal according to the Timmerman tables for three fraction was missed in the clinical plan, which says that the V23Gy of the Brain-GTVs structure should be < 5 cm3. This is practically not achievable for multi-met plans and our margin settings. The situation worsens if more lesions are added.
4Remember that the sole purpose of this study is to find out whether the TPS is able to handle a large number of structures (targets).
5Memory consumption is about 26 GB during the last optimization with 25 lesions.
