SRS Question

[Reaganite]

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How do you guys handle SRS planning with closely adjacent lesions...say separated by a few mm? What kind of hot spot are you willing to accept in normal brain tissue in the "bridge" between the lesions?

 

[Neuronix]

There shouldn't be any hot spot in the area between them. It's just a question of how much dose falloff you can get between the two lesions. That's technique dependent.

 

[RickyScott]

Obviously it depends on size and distance, but we try not have 50% bridging when lesions are a few cm apart.

 

[Ray D. Ayshun]

We go with v10 < 10 ml and v12 < 8 ml for single fraction, in this case just considering it a single lesion. If that won't work, then fractionate.

 

[scarbrtj]

How do you guys handle SRS planning with closely adjacent lesions...say separated by a few mm? What kind of hot spot are you willing to accept in normal brain tissue in the "bridge" between the lesions?

The situation is kind of like black hole mergers; the closer the two things get, the "hotter"* the intervening space gets. Can't defy the physics in either situation. You make it be as good as it can is the cop-out answer. The highest dose(s) in the plan should still be the centers of targets... not out in the invisible bridges between (keeping in mind what a "hot spot" truly is). Inverse optimization allowing heterogeneity (ie Dmax's that are up to 50% hotter than the Rx dose, aka Rx'ing up to the ~67% line) plus a single isocenter approach are your friends, if they're not already. Another help is to use beams that are in a plane, or close to a plane, that is perpendicular to a plane that holds the isocenters of the two targets; only applies in two target problems cases really though.

* I know there aren't "heat maps" in the video

 

[evilbooyaa]

Try to avoid 50% bridging. If they are really close (like within 2-3mm of each other) then we just contour it all as one PTV, and focus on V12 constraint. Consider IMRT planning as it won't be a nice clean circle rather than DCA.

 

[Neuronix]

In addition to what's all been reasonably suggested, depending on size and risk some people would come back and treat the other one at a later date to give the normal brain in between some time to repair.

I try not to let V12s touch in general. But if the total area is small I let it go. GK has the tightest fall-off followed by cone based linac, especially if you favor the cone or shot edges to try to separate the two lesions.

Greater than 8 cc with 18 Gy prescription dose is when I'd strongly consider fractionating the entire volume.