Chronic pain was among the first conditions treated with the Gamma Knife . In the 1990s, we developed a technique to perform medial thalamotomy using stereotactic magnetic resonance imaging (MRI) localization and the Gamma plan computer dose planning system . The target includes portions of the medial dorsal, intralaminar, center median, and parafascicular thalamic nuclei. Lesions are now made with a single isocenter using the 4mm secondary collimator helmet of the Gamma Unit and maximum radio-surgical doses of about 140 Gy. These parameters produce, on the average, a spherical lesion of about 90 mm3 in volume that develops over a period of 3 to 6 months after treatment. Coincident with development of the lesion gradual reduction in chronic pain occurs in the contralateral body and face without any loss of normal sensory function in about two thirds of patients. Nociceptive pain responds better than neuropathic pain to medial thalamotomy. Larger lesions may be made with two or three adjacent isocenters, which results in a higher success rate in terms of pain relief, but also a much higher rate of complications caused by excessively large lesions. We do not recommend multi-isocenter medial thalamotomies any longer.
Radiosurgical cingulotomy or hypophysectomy has been used in a very limited number of patients for treatment of chronic pain with a strong anxiety component or for pain resulting from metastases from prostate or breast cancers.
For cingulotomy, bilateral lesions are made in the cingulum bundle at a point 25 mm posterior to the anterior end of the frontal horns of the lateral ventricles, as determined on stereotactic MRI scans. We have tried both the 4-mm and 8-mm secondary collimator helmets of the Leksell Gamma Unit for making such lesions at various radiosurgical doses. At this point, our experience is not sufficient to make a firm recommendation about radio-surgical dose.
For hypophysectomy, multiple 4-mm isocenters are directed at the anterior pituitary gland. The maximum safe radiosurgical dose is determined by the dose to the optic apparatus, which we believe should generally not exceed 8 Gy.
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