Attendee Question from 9th International Novalis Circle Congress.
Section: Physics Elements - Benefits of 4pi Optimization w/ Cranial SRS
Speaker: Christina Skourou
Why did your VS plan have 0mm margin? How do you take into account your machine mechanical and localization errors?
When schwannoma is abutting brainstem how do you ensure brain stem remains within tolerance with no margin?
Comments
C
Dear Ethan,
Thank you for your question. The decision for 0mm margin for benign targets was made after long discussions with our radiation oncologists and neurosurgeons at the onset of our SRS service. The argument to remove any margin is that for these patients our priority is to reduce dose to normal tissue (and a PTV margin is by definition comprised of normal tissue). They have full understanding that the target may be undercovered, and their clinical justification for that is that the aim when treating benign tumours is not to eradicate every cell but to reduce the bulk of the disease and stop them from growing further. In scenarios where there is risk of harm to normal structures neurosurgeons often leave behind tumour tissue at surgery without concern for rapid regrowth or metastasis. We therefore adopt that same mentality when treating with radiosurgery.
If the target is abiding the brainstem, we used to plan with a 1mm PRV to it and accept undercover of the target. The Cranial Element has the "Most Important OAR" tool which essentially does that for us without us needing to physically create a PRV structure to the brainstem.
We also use ExacTrac to correct the setup of our patients with 0.5mm/0.5-deg action levels.
I hope this answers your question. Happy to discuss further.
Regards,
Christina