sbrt planning from start to finish...1.patient selection 2.respiratory motion 3.contouring 4.3d...

SBRT Planning From Start to Finish

Thomas Costantino CMD RT(T) [email protected]

Disclosures


I would rather be racing

Objectives


1.Patient selection2.Respiratory motion3.Contouring4.3D planning5.IMRT & VMAT planning6.Evaluation 7.Conclusion

Note: I will be using the Varian Eclipse TPS but the techniques are applicable to all planning systems

Patient selection


This is a very important part of the SBRT process. The team must consider several factors when determining if a patient is a candidate.

1. Target size: As target size gets larger it will be more difficult to control the 50% isodosevolume as beams will interact before reaching the target. This will also play a role in prescribed dose. Targets larger than 110cc will usually pose a challenge to meet the 50% gradient index as well as 2 cm gradient.

2. Target location: What critical structures are in close proximity to the target and is it realistic to achieve the dose constraints for those critical structures.

3. Comorbidities: Can the patient tolerate positioning .The immobilization devices can be uncomfortable for debilitated patients.(many times this is not known until simulation).

4. Curative or not? This part is the decision of the Radiation Oncologist and their goals for the patient.

The planner on the team should have a good idea ahead of time as to what can and cannot be achieved when reviewing imaging during this step.

Respiratory motion


There are several technologies available to determine the motion of a target. We use the Varian RPM system with the GE CT sim. If this technology is unavailable in a center there are other ways to determine motion. Do 3 CT scans on the patient sequentially. A free-breathing helical, an inhale series and an exhale series. The inhale series usually overestimate the motion as patient will take a larger breath than normal.

Once the 4D scan is done it is time for some more decisions.1. How much does the target move?2. Will gating be used on certain phases? In our clinic if we use gating on inhale or

exhale phases only then VMAT is usually not used as treatment time gets very long due to machine starting and stopping.

3. Will the target be defined from all phases creating an ITV. If this method is chosen then do plan IMRT or 3D? Our policy for this scenario is if motion exceed 1 cm then 3D planning is used.

There are many published articles on this and is a presentation in itself so this just an overview of what we use.

Contouring


This is probably the most important step in the planning process if IMRT or VMAT is to be used. I will focus on structures created for optimization as I believe we all know how to contour normal structures.1. Dose control rings: These are concentric rings at different distances

from the PTV. I use 3 an inner, outer, and 2cm gradient ring.Inner ring is an expansion from the PTV of 1 cm with a 1mm gap to the PTV. This is where I want my Rx isodose line to fall. I will limit this to just under Rx Dose.

1mmgap

Contouring


2. Outer ring: A 2 cm expansion from PTV subtracted from inner ring. I will try to limit the dose to this ring to 50% but depends on target size.

Contouring


3. 2 cm gradient ring: a 3 cm expansion from PTV then subtracted from outer and inner ring. This will be used to ensure 50% isodose line stays within this ring.

Contouring


4. Dose control structures. If PTV overlaps a critical structure then create a structure out of that overlap volume and add min dose of 95% and max of RX dose. This can be used in non-SBRT planning as well.

Contouring


5. Subtraction structures: Create a structure inside a critical structure and subtract away from target. This will help control volumetric doses to critical structure. For example, Spinal cord

Dose control structureResultant forced fall off

Contouring


6. Target expansion structures: Create an expansion structure from the PTV in the superior and inferior dimension only by 1mm. This will help to achieve coverage of the Target especially if using Eclipse TPS. It will also help smooth changes in contours from slice to slice.

1mm exp sup and inf

Helps smooth variations

Contouring


7. At this point convert your target and optimization structures to high accuracy if using Eclipse. I do this at the end because once a structure is converted many contouring tools will be unavailable such as margins. It my behoove you to create a copy of the structure before making it a high accuracy structure.

3D planning


3D planning is much more difficult than IMRT or VMAT as it requires the planner to manipulate beam arrangements and modifiers to achieve not only good coverage of the target but also reduce 50% isodose volume. This was all we hade in 1990’s and I am grateful as learned a great deal about manipulating beams. Remember the best IMRT plans come from the best 3D plans. I will discuss 2 methods, fixed field and Dynamic conformal arcs.

3D planning


Fixed field: It is very important to use as many non-coplanar fields when using this technique. The reason is we do not want entrance dose overlapping with exit dose from another beam. The idea is the same as cone based intracranial SRS. I usually end up using 13 fields. Note the field entry shapes on the body to ensure non overlapping fields. This becomes more difficult with larger target volumes.

3D planning


Fixed field: If you have a FFF beam use it as the pronounced dose profile will help with dose fall off.

3D planning


Fixed field: Posterior fields should be kept coplanar as the risk of couch and gantry collision goes up.

3D planning


Fixed field: Set MLC margin to 0 or 1mm. This will force a very sharp fall off. Calculate the dose and re-normalize to get coverage. I calculate to a point in the center of the PTV. Be prepared to normalize around 80%. Proceed to adjust field weights to shrink 50% isodose line. I always have my plans in absolute dose.

3D planning


Fixed field: If 100% isodose line is a little loose find a field where you can crop the jaw into target .

Pull jaw into targetloose

result

3D planning


Fixed field: Continue this process and adjust collimator angles to get the desired result. Yes, this is a time consuming process. Check for streaking of 50%. The goal with 3D is target coverage as well as 50% fall off. The conformality ratio will suffer as there is no modulation to adjust for changes in shape. Notice that the 50% isodose line is within the 2cm gradient ring. Also the 50% volume constraint is also met at 4.9<5.2

3D planning


Fixed field: Target coverage at 95%. This is a good time to mention Hot spots. In order to shrink the 50% isodose line you have to let dose escalate within the target. Do not try to limit this. You want a heterogeneous dose. I have seen hot spots as high as 150%. Do try to keep the hot spot inside ITV.

3D planning


Fixed field. Directly from TG101

1 A limited volume of tissue, containing the gross tumor and its close vicinity, is targeted for treatment through exposure to a very high dose per fraction, and hotspots within the target are often deemed to be acceptable.2 The volume of normal tissue receiving high

doses outside the target should be minimized to limit the risk of treatment toxicity. Thus, the gradient describing the dose fall-off outside the target should be sharp.

3D planning


Dynamic conformal arcs: Arc planning for SBRT is great and we can use the principles from intracranial SRS to apply to the body. As we know the arcs will focus their energy at the isocenter and spread out the dose to the periphery. Arcs by nature will shrink the 50% isodose line. Justin Hayes, MS, DABR gave a great webinar on this technique a couple months ago which was very informative and well presented using Pinnacle software. I encourage all to watch it. I start with similar arc geometry. 2 non-coplanar partial arcs

3D planning


Dynamic conformal arcs: result with the 2 arcs. 50% too big and 100% loose on target. “How to fix?” I will add a fixed posterior oblique field and crop jaw into target. I will also split each existing arc into 2. an anterior half and post half. This will allow me to weight them separately.

3D planning


Result:

IMRT & VMAT planning


OK now for the fun part. We have already determined high quality field arrangements which produce good plans. If we optimize those we will get superior results. This section will deal with optimization and does not matter whether you use fixed fields or VMAT. You will be able to use less fixed fields as the modulation can make up for interactions of the beams. Usually 8 to 10 beams yield great results just choose the angles wisely. With VMAT I stick with the 2 non-coplanar partial arcs. If I have a mid line lesion I will probably go with full arcs but less couch kick maybe 10 degrees. Know your couch and where iso is in relation to the table top. If 15 cm or less I can achieve 12 degree kicks for full arcs. Also if you have an HDMLC that will help with conformation.



Change calculation grid size for more accurate evaluation



Ensure enough DVO points on structure by changing resolution.This is more important for small targets and avoidance structures. Lower resolution to increase points.



Using NTO adjust settings for realistic expectations then adjust priority during optimization. Do not use defaults.



If using fixed field IMRT, smoothing levels can be reduced to achieve a better DVH, but be careful not to over-modulate. Lowering the smoothing levels creates high dose gradients which may cause the plan to fail QA and lead to patient dose inaccuracies.



OK remember those dose control rings from contouring well now we are going to use them. Note objective value for inner ring is just under Rx Dose and value for outer ring is just around the 50% dose. I am trying to force fall off by 1 cm which is the distance to the outer ring. The gradient ring is 2 cm away where less than 50% is required.



The 2 objectives that will be fighting each other are PTV coverage and outer ring. You can visualize this by watching your penalty function graph. When both objectives are superimposed or very close then the optimizer is a t a standstill until you decide which one is more important and adjust your priorities accordingly.

ptv Outer ring



Also include any critical structures in the optimization and adjust priority accordingly to achieve constraints. Here you can use those subtraction and overlap structures.

Intermediate dose: If your version of Eclipse has this then use it. This is very helpful when optimizing targets that are more dense than their surrounding tissue such as lung. The PRO and DVO will come up with solution but after calculation the solution is not optimal. The optimizer will then account for this discrepancy to better the solution. The calculation algorithm being used will have a large impact on this as some are better than others. I am not going to discuss that but I warn against using pencil beam as it considerably underestimates dose in heterogeneous tissue.After the plan is calculated evaluate it. If you need to tweak some structures I do not start from the beginning but continue the optimization using the existing calculation as a base dose. During this step do not run another intermediate dose as you are already using your calculated plan. This is an iterative process where you make small changes each time. Kind of like Cyberknife planning. Evaluate after each iteration until you achieve the result you want.



Results! Note these are 2 different patients using same arc geometry as 3D with a superior result.



No intermediate dose. No problem. Copy the PTV then subtract out the ITV. This is a structure that is solely in low density tissue. You will need to put a lower objective on this structure with about 115% of the prescribed dose. This forces the optimizer to put more intensity on this structure. After calculation the dose should be close to prescribed dose. Basically tricking the optimizer.

Evaluation


There are several things to calculate in the evaluation process which will determine the quality of the plan. 1. Critical structure dose constraints: Does the plan meet them? These are

available through RTOG and other sources. We use the most stringent. If a structure exceeds a constraint it must be discussed with the Physician as it may be unrealistic to achieve it. For example rib dose. If a target is adjacent to a rib and the PTV overlaps it you will NOT be able to meet that constraint. What I try to do is avoid any hot areas in that region. The physician must make the choice as to target coverage or critical structure sparing.

Evaluation


Dose constraint document. Note ,values are different for 1,2,3,4, or 5 fractions.

Evaluation


2. Conformity: we use the Ian Paddick conformity index. This shows how well the target is covered by the 100% isodose volume.

TV x PIV• TV = Target Volume PTV• PIV = 100% Isodose Volume Inside PTV• TVPIV= Volume of the target that is covered by the 100% isodose line.A result of 1.0 is perfect conformation of 100% isodose line to target. This is unrealistic. We try to achieve >0.95 for IMRT and VMAT and >0.85 for 3D. The smaller the targets the more difficult this is to achieve.

Another conformity index isThis is just comparing 2 volumes which may not show actual coverage of target.

Evaluation


2. Dose gradients:We use the 50% gradient index for evaluation.We use the tables from RTOG.We also try to achieve less than 50% at 2 cm. As you can see this gets more difficult with larger targets.

Evaluation


OK, lets see differences in the plans. All 4 achieve critical dose constraints except the proximal rib which we accept. The differences will be in the conformity and gradient indices.

CI Paddick: GI 50%Fixed field 3D 0.88 4.9<5.5Conformal arc 3D 0.87 4.6<5.5Fixed field IMRT 0.95 4.6<5.5VMAT 0.94 4.4<5.5

Evaluation


OK, lets see differences in the plans. Comparison dvh

Note in this scenario fixed field IMRT has most homogeneous coverage. This will degrade as less beams are used. Just like Cyberknife.

I believe the best plan for this patient is the VMAT as it will have shortest treatment time.

Conclusion


High quality SBRT plans can be achieved using 3D or modulated beams. The trick is using a beam arrangement that will facilitate this. Using good optimization contours in IMRT or VMAT is critical to success. 3D planning takes much longer then IMRT or VMAT as the planner is doing the optimizing. following these techniques high quality SBRT plans can be produced in an efficient time frame without many iterations using any of the available planning systems. Usually the 1st run plan is acceptable and requires very little tweaking. This depends on many factors such as target size, location, and shape, which goes back to patient selection.

Conclusion


The future is just around the bend! Lets meet it with confidence, ability and understanding! That’s what makes us great!

Thank you for your attention


sbrt planning from start to finish...1.patient selection 2.respiratory motion 3.contouring 4.3d...

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