Shielding (plugging) beam channels in gamma knife radiosurgery can optimize dosing for posterior third ventricular lesions by increasing the marginal dose to the tumor and decreasing the dose to nearby critical structures like the brainstem. The technique involves placing plugs in the path of beams to block radiation to certain areas. In a study of 14 patients, shielding increased the marginal tumor dose by an average of 6.2 Gy while decreasing the brainstem dose by an average of 2.3 Gy, though in one case it paradoxically increased the brainstem dose. Shielding requires skill and experience to effectively plan overall dosing.

1. Shielding in Posterior III ventricular lesions Techniques & pitfalls Deepak Agrawal Department of Neurosurgery & Gamma Knife All India Institute of Medical Sciences, New Delhi &

12. FU PET at 1 year showed recurrence with increase in size of the tumor O/E alert, No sensory/motor deficits

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