sbrt planning for multiple targets

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SBRT Planning For Multiple Targets

Jeremy Donaghue, MS, DABR

5/25/2021

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CE Credits• To earn CE Credits for this session, you need to view

the entire session and complete both the assessment questions and evaluation.

• These need to be completed by Thursday, July 15

[email protected]

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SBRT Planning for Multiple Targets

AAMD Annual Meeting 2021

Jeremy Donaghue, MS

Disclosures

• None

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• How to decide to use one or multiple isocenters

• How to handle dose spillage between targets

• How dose criteria can change in multi target situations

• Advanced: Creating “robust” plans

Overview

• SBRT should be taken as a very serious treatment technique. If things are done wrong we can severely hurt someone. This should not be taken lightly.

• I will not get into explicit optimization settings, but if interested, reach out, and I can share my planning/optimization strategies.

• My techniques are a combination of peoples’ opinions and my learning over the years. I cannot necessarily point to who gave me what, but I am appreciative of all planning conversations I have had and will have.

• We will not talk about intracranial treatments outside of one footnote. (no spine srs either)

Notes

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• No matter how good we think we are, it’s always good to ask for help. Sometimes it takes someone else’s different thought process to get around a problem. Hopefully, this interaction can add another tool to our toolbox for future cases.

Last thoughts before getting into this

• My planning system doesn’t care how many isocenters I use!

• Patient and those involved in treatment do.

• With single isocenter there is: - Less time in/out of the room verifying set up.

- Less imaging is required (less time again, less dose)

- Overall less time on the table means that the patient is less likely to move. Tight margins of SBRT are less tolerable to patient movement.

• We try and use the fewest isocenters that can deliver an accurate treatment for the patient.

Why use single Isocenter?

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• Considerations- Location of treatment area(s)

• Is there a lot of motion in the area? If so can you expect them to move in similar directions together?

- Distance between targets

- Treatment equipment• 6D couch allows for better alignment of multiple targets

• Breath hold

• All of the above factors affect each other, so just passing one isn’t enough.

Single vs Multiple Isocenters

- In my experience, most of our multiple targets end up being in the lung or abdomen

- Lung targets mostly rely on which lobe of the lung, but type of breather can also matter.• Breath hold treatments are significantly more reproducible, but many patients

have issues during treatment (especially if these are extended treatments)

• Treatments that use 4D scans are still acceptable, but you may end up with larger margins with targets in the lower lung by the diaphragm.

- Targets in bone tend to align well to each other (ex. Pelvic girdle)

- Targets that can be close to organs that can have a lot of change should more than likely use multiple isocenters• Bladder fill varies quite a lot (unless empty bladder is used) so anything

abutting the bladder wall I would not trust to move consistently relative to other targets

Single vs Multiple Isocenters(Location considerations)

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Disance between targets (edge to edge) Comment

4 cm and below Good

4-6 cm Normally pretty reproducible, 6D couch makes

this in the “good” range

6cm+ Highly depends on location/stability of targets. If

going to larger distances like this, consider using

techniques that allow for a fallback

treatment/planning that will be discussed later

Single vs Multiple Isocenters(Distance between targets)

This data is based on my own machine performance and experience.

CHECK WITH YOUR LOCAL PHYSICIST

• During simulation, we will normally make an educated guess here with the possibility that we will possibly change the number of isocenters used for treatment.


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• To help determine distance, I will adjust the isocenterPOI diameter to see how far apart the targets are.

• When creating a new isocenter, I like to find the center of the geometry, but adjust the coordinates to be “round” shifts ex. Multiples of 1 cm.

• When using a single isocenter, the planner may consider using a combined PTV. While acceptable, you must remember that coverage to a combined PTV does not mean that all targets are getting adequate coverage.

Target Coverage (under dosing)

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• Set up a static beam with blocking on a combined PTV structure, and find the collimators that give you the best advantage.

• Give the machine a chance to close leaves between targets to avoid dose bleed

• Consider if there is any advantage to collimator angle relative to possible critical/nearby OARs also

• Since we’re normally using arcs, we have to consider the whole arc region vs just a single gantry angle.

• This can stand true for single targets also, but normally not as important.

Geometry can be important!(single isocenter)

Geometry can be important!(single isocenter)

Better blocking available to surround targets

Possible dose bleed between targets

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• Planning/Optimization- I will use both a combined PTV, and the individual PTVs.

Single Isocenter Optimization Note

• Try not to have beams directly enter through other targets

• Be aware of where arcs are possibly overlapping

• Table kicks when necessary.- Normally, avoid overlapping entrance of target

- Always check table kicks ahead of time/early in the planning process. Normally, 5-7 degree table kicks are always good, but do not give a lot of advantage. Getting to 15-30 degrees is very dependent on isocenter location/patient setup.

Geometry can be important!(multiple isocenter)

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• Coverage- Can both plans stand alone if for some reason the treatment gets aborted between

isocenters?

- Are both isocenters being treated on the same day?

- Depending, you may accept inferior individual coverage (ex 90-94%) due to other planning factors. I would avoid this, but this is dependent on clinical needs, and physicians desires.

Geometry can be important!(multiple isocenter)

Sequential and Parallel OptimizationSequential

1. Fully optimize first target to satisfaction

2. Turn off first prescription, switch to second prescription and optimize to satisfaction

3. Turn on both prescriptions for final dose computation

4. In general, I will make the non planned GTV/ITV max dose 4% of RX, and the PTV 6-8% of Rx.

Parallel1. Optimize both targets simultaneously

Optimization Strategies

Slide courtesy of: Sherman, J SBRT Lung Treatment Planning Strategies for Multiple Targets, 12/15/2020

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Multiple Isocenter Optimization


Use Current Jaws as Max: YES Use Current Jaws as Max: NO

Multiple Isocenter Optimization


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• Depending on the proximity of the targets to each other, you may need to create some structures between targets to reduce dose bleed (if desired*)

• I normally just draw lines across the slices.

Extra planning structures

Extra Planning Structures

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Scaling to coverage

Scaling to coverage

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Scaling to coverage

Scaling to coverage

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Scaling to coverage

Patient setup/reproducibility

• I was concerned about this patient adjacent to a relatively empty bladder, and full rectum, but patient did setup well. The superior (smaller target) was more of a “we’re in the PTV” situation

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• Note: This is a bit of a different thought process so discuss with staff before going down this path.

• Depending on the area that you are treating, you can increase the dose bleed between targets to give yourself dosimetric advantages in other places.

• We primarily do this in lung. Any lung that goes over 20 Gy is considered non-functional.- “no great reason to need to spare the space between the two lesions as they are so

close it is not likely spareable to within any functional limit. Would focus on sparing the lung that is not between the two lesions. “

• Taking this into consideration, if you have 2+ targets in the lung, you can push the dose into the area between targets into this dead part of the tissue and achieve better dose sparing to surrounding tissue

Leveraging Dose Bleed

Leveraging Dose Bleed

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Leveraging Dose Bleed(Warning/comment)

• When leveraging dose bleed between targets, it’s important to take into consideration the area in the body you are at.

• In the lung, swelling in these areas between targets is acceptable because the area is spongy, and has no real hard borders (obviously respect oar restrictions)

• Using this technique in the brain can lead to severe complications since there is no way for the swelling to be relieved (ex. the skull does not move)

• I am not actually talking about the robustness optimization algorithm.

• I’ve used this technique when I was concerned that there might be a chance the patient setup would not be as reproducible as I had wanted.

Creating a “robust” plan

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• This patient had 5 separate bone metsacross the pelvis. I had wanted to be cavalier and do them in 1 iso, but decided to go with 2.

• Will only focus on the 1 area since the same concept was applied.



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• Plan was split up into sides (left/right)

• Isocenter was placed between the two sides

• Each side was treated with 2 half arcs. (left 0<->178, right 182<->0)

• By chaining the CW and CCW arcs in order and using automation, it gives the appearance of 2 continuous arcs with a brief pause at gantry 0.


• The advantage of planning this way was if we felt from the CBCT that both sides of the patient didn’t align well relative to each other, we could break the treatment into 2 separate parts with a CBCT that would align the appropriate side with the appropriate target(s).

• This also would afford us the ability to avoid having to replan the patient in the event set up was not acceptable.


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• We have PTVs for a reason. These patients (just like most other patients) will not align perfectly.

• When aligning to multiple targets try to split the difference to get all targets as centered as possible.

• I do not try and push to the very edge of the PTV, because some of that margin does include imaging innaccuracies and patient motion.

Quick note on alignment for treatment

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• [email protected]

• Twitter: @JeremyDonaghue

Contact

• Entire CCF Staff- Justin Sherman

Thank you

mailto:[email protected]

sbrt planning for multiple targets

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