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