Doc, both CK and GK have slightly less than a 1mm tolerance (around 0.85 mm, if I remember correctly). The treatment plan is developed from a CT scan, not an MRI. The CT scan is more accurate for developing the treatment plan. MRIs are more easy to interpret by the human eye, which is why they are used for diagnosis (small tumors can be overlooked by the human eye when viewing a CT scan).
A "radiation cloud," which conforms to the shape of the tumor, is delivered to the tumor by way of roughly 100 (up to 140) separate beams arriving at the tumor from many different angles, resulting in healthy surrounding tissue only receiving about 1/100 or 1% of the dose that the tumor receives. The caveat is that healthy cranial nerves that are surrounded by the tumor (such as is often the case inside the very narrow internal auditory canal) receive the full dose of radiation that the tumor receives; no effort is made to avoid irradiating the cranial nerves.
This is where the differences between CK and GK become significant, at least in theory. GK delivers twice the dose of radiation at the center of the tumor with respect to the dose at the periphery, whereas CK only delivers 15% higher dose at the center. So cranial nerves at or close to the center of the radiation cloud receive much lower dose of radiation with CK than with GK. Besides the relative homogeneity of dosage delivery, the fractionated doses of CK give the cranial nerves 24 hours between each treatment to recover, which they do more expediently than the tumor does. Hence, the radiation is more damaging to the tumor than it is to the cranial nerves that are also irradiated.
Best wishes,
TW
