Radiosurgery of epilepsy. Long-term results

Epilepsy

GKNS has been introduced as an alternative to microsurgery for the treatment of certain types of focal epilepsy. It was first noted that epilepsy associated with AVMs often improved before the obliteration of the vascular lesion. It was subsequently shown that the treatment could be effective in mesial temporal lobe epilepsy (MTLE) producing remission in around 60% of patients, provided the treatment was carried out as originally designed. GKNS has also been useful in the treatment of gelastic epilepsy associated with hypothalamic hamartomas.

Stereotactic radiosurgery for epilepsy and functional disorders

Stereotactic radiosurgery is used for many indications. In functional neurosurgery, it is used to modulate the function of axons, neurons, and related brain circuits. In this article, indications, current techniques, and outcomes are discussed.

Trends and importance of radiosurgery for the development of functional neurosurgery

Functional neurosurgery includes surgery conducted to ablate, augment, or modulate targets that lead to improvement in neurological function or behavior. Surgical approaches for this purpose include destructive lesioning with different mechanical or biologic agents or energy sources, non-destructive electrical modulation, and cellular or chemical augmentation. Our purpose was to review the role of stereotactic radiosurgery used for functional indications and to discuss future applications and potential techniques. Imaging and neurophysiological research will enable surgeons to consider new targets and circuits that may be clinically important. Radiosurgery is one minimal access approach to those targets.

Advances in the radiosurgical treatment of epilepsy

Radiosurgery is the precise application of focused radiation to a targeted volume area within the brain, which has been identified on MRI. With recent advances, radiosurgical treatment is now being evaluated as an alternative treatment to open resective surgery for intractable epilepsy. Recent prospective trials suggest that radiosurgery may be an effective and safe treatment for medically intractable epilepsy associated with mesial temporal sclerosis, cavernous malformations, and hypothalamic hamartomas.

Brain arteriovenous malformations and endovascular treatment: effect on seizures

We report our experience in treating patients with seizures associated with brain arteriovenous malformations (AVM) without a clinical history of intracranial hemorrhage. Between 2001 and 2003, the neurovascular unit at Beijing Tiantan Hospital treated 109 patients with brain AVM endovascularly. Thirty patients (27.5%) experienced seizures before treatment. We studied the following factors: sex, age, AVM size, AVM location, seizure type, duration of seizure history, endovascular treatment and AVM obliteration. Clinical follow-up was via telephone interview. Thirty patients with seizure disorders due to brain AVMs were endovascularly treated. The age of the patients ranged from eight to 55 years. There were 22 males and eight females. The AVMs were smaller than 3 cm in five patients, between 3 cm and 6 cm in 22, and larger than 6 cm in three. The most frequent location of the AVMs was in the frontal, followed by the parietal, temporal and occipital lobes. Sixty-seven embolization procedures were performed and total obliteration was achieved in four patients. Two patients developed a hemiparesis and three suffered temporary dysphasia after embolization. Two patients had visual field deficits. There were no deaths. The results of post-embolization seizure control during the average follow-up period of 80 months were excellent in 21 patients, good in four, fair in two and poor in three. Successful seizure control can be obtained with endovascular embolization.

The implementation of ablative hypofractionated radiotherapy for stereotactic treatments in the brain and body: observations on efficacy and toxicity in clinical practice

Radiosurgery has a long history dating back to the 1950s. Only in the last decade or so have advances in radiation delivery and visualization allowed export of this paradigm to extracranial sites. This review evaluates the efficacy and safety of such ablative radiation courses using dose per fraction schedules of 10 Gy or above. Retrospective published experience in functional and benign tumor radiosurgery is reviewed. Prospective controlled clinical trials in ablative cancer therapy of early-stage lung cancer and metastatic disease in the brain, liver, and spine are reviewed.

[Radiosurgery for drug-resistant epilepsies: state of the art, results and perspectives]

BACKGROUND

There is growing interest in the use of radiosurgery in epilepsy. We analyzed our experience in this field in an attempt to define the potential of radiosurgery in epileptology.

MATERIAL AND METHODS

[corrected] Our local clinical experience (134 patients), accumulated over the last 15 years, mainly includes treatment of temporal lobe epilepsy without space-occupying lesions (59 patients), including 53 with pure MTLE, 61 cases of hypothalamic hamartoma, two cases of callosotomy, and 12 other types of epilepsy.

RESULTS

The analysis of our material, as well as other clinical and experimental data, suggest that the use of radiosurgery is beneficial only to patients in whom a strict preoperative definition of the extent of the epileptogenic zone (or network) has been achieved and strict rules of dose planning have been applied. As soon as these principles are not observed, the risk of treatment failure and/or side effects increases dramatically. Long-term outcome data are now available and published for MTLE but not yet for other types of epilepsy. Long-term safety and efficacy in MTLE are comparable to surgical resection but radiosurgery has the advantage of sparing verbal memory in patients operated by Gamma Knife (GK) on the dominant side. In small hamartomas, the efficacy is comparable to microsurgery but with a dramatic reduction in risk.

CONCLUSION

The vast amount of clinical materiel and long-term evaluation now support the use of GK surgery in small hypothalamic hamartomas and MTLE when the patient is at risk of verbal memory loss.

Stereotactic radiosurgery for functional disorders

✓ Stereotactic radiosurgery (SRS) with the Gamma Knife and linear accelerator has revolutionized neurosurgery over the past 20 years. The most common indications for radiosurgery today are tumors and arteriovenous malformations of the brain. Functional indications such as treatment of movement disorders or intractable pain only contribute a small percentage of treated patients. Although SRS is the only noninvasive form of treatment for functional disorders, it also has some limitations: neurophysiological confirmation of the target structure is not possible, and one therefore must rely exclusively on anatomical targeting. Furthermore, lesion sizes may vary, and shielding adjacent radiosensitive neural structures may be difficult or impossible.

The most common indication for functional SRS is the treatment of trigeminal neuralgia. Radiosurgical treatment for epilepsy and certain psychiatric illnesses is performed in several centers as part of strict research protocols, and radiosurgical pallidotomy or medial thalamotomy is no longer recommended due to the high risk of complications. Radiosurgical ventrolateral thalamotomy for the treatment of tremor in patients with Parkinson disease or multiple sclerosis, as well as in the treatment of essential tremor, may be indicated for a select group of patients with advanced age, significant medical conditions that preclude treatment with open surgery, or patients who must receive anticoagulation therapy. A promising new application of SRS is high-dose radiosurgery delivered to the pituitary stalk. This treatment has already been successfully performed in several centers around the world to treat severe pain in patients with end-stage cancer.

Radiosurgical treatment for rolandic arteriovenous malformations

Object

The authors reviewed the radiosurgical outcomes in patients with arteriovenous malformations (AVMs) located in the rolandic area, including the primary motor and sensory gyri.

Methods

The study population consisted of 38 patients with rolandic-area AVMs who underwent linear accelerator radiosurgery at the University of Toronto between 1989 and 2000. Obliteration rate, risk of hemorrhage during the latency period, radiation-induced complications, seizure control, and functional status were evaluated. Patients were also divided into two subgroups according to AVM volume (< 3 cm3 and ≥ 3 cm3).

Patients were followed up for a median of 42.4 months (range 30–103 months), and the median age of the patients was 40 years (range 12–67 years). The median AVM volume was 8.1 cm3 (range 0.32–21, mean 8.32 cm3), and the median dose at the tumor margin was 15 Gy (range 15–22, mean 16.8 Gy). The risk of hemorrhage after radiosurgery was 5.3% for the 1st year, 2.6% for the 2nd, and 0% for the 3rd. Two patients (5.3%) sustained adverse effects related to radiation for more than 6 months. Complete nidus obliteration after a single radiosurgical treatment was achieved in 23 patients (60.5%). The obliteration rate for AVMs smaller than 3 cm3 was 83.3% (10 of 12) and that for AVMs larger than or equal to 3 cm3 was 50% (13 of 26). Among the patients who had seizures as the initial presentation, 51.8% were free of seizures after radiosurgery and the seizure pattern improved in 40.7% during the 3rd and last year of follow up. Overall, excellent results (obliteration and no new or worsening neurological deficit) can be achieved in approximately 60% of patients. This percentage varies according to the AVM size and can reach 83% in patients with AVMs smaller than 3 cm3.

Conclusions

Radiosurgery is a safe and effective treatment for people with rolandic AVMs. The low rate of morbidity associated with radiosurgery, compared with other treatments, indicates that this method may be the first choice for patients with AVMs located in this area.

Hypothalamic hamartoma associated with anterior paraclinoid aneurysm of the internal carotid artery

A 15-year-old boy presented with a history of medically refractory gelastic seizures and cognitive impairments. Neuroimaging demonstrated a sessile type hypothalamic hamartoma, which was treated by gamma knife surgery. However, the gelastic seizures only partially decreased and the frequency of seizures remained unchanged. One year later, angiography before surgery detected anterior paraclinoid aneurysm of the left internal carotid artery. Blood pressure and endocrinological examinations showed no abnormality. Direct surgery was performed to treat the aneurysm and hamartoma. No sclerotic changes were noted in the arterial wall. The aneurysm was treated with clipping, and the hamartoma was partially removed. Postoperative course was uneventful and the gelastic seizures disappeared. No evidence for a causal relationship between the hamartoma and aneurysm was found.

Gamma knife radiosurgery for callosotomy in children with drug-resistant epilepsy

OBJECTIVES

Gamma knife radiosurgery as a noninvasive procedure is increasingly used as a treatment option in patients with intractable seizures. We investigated efficacy and safety of gamma knife radiosurgery (GKRS) for callosotomy in children.

MATERIALS AND METHODS

Between 2000 and 2004 three children between 4 and 14 years (mean 8 years) underwent radiosurgical callosotomy. One child presented with Lennox-Gastaut syndrome, two with hemispheric cortical dysplasia. These two children underwent functional hemispherotomy before GKRS. GKRS was performed with a marginal dose of 55-60 Gy on the 50% isodose.

RESULTS

Mean follow-up was 35 months. Radiosurgical callosotomy was ineffective in one child with the Lennox-Gastaut syndrome, whereas in the remaining two children, a 100% seizure reduction of generalized tonic-clonic seizures, 20-70% reduction of partial seizures, and a progress in mental and physical development was achieved. No postradiosurgical side effects were observed in all children.

CONCLUSION

Radiosurgical callosotomy might be offered after hemispherotomy to complete callosal resection. However, larger number of patients and longer follow-ups are needed to draw final conclusions.

Gamma Knife Surgery in Mesial Temporal Lobe Epilepsy: A Prospective Multicenter Study

PURPOSE

This article is the first prospective documentation of the efficacy and safety of gamma knife surgery (GKS) in the treatment of drug-resistant epilepsies of mesial temporal lobe origin.

METHODS

From July 1996 to March 2000, three European centers selected 21 patients with mesial temporal lobe epilepsy (MTLE) for a temporal lobectomy. The preoperative investigations included video-EEG with foramen ovale electrodes, magnetic resonance imaging, neuropsychological testing, and the ESI-55 quality-of-life questionnaire. In place of a cortectomy, radiosurgical treatment was performed by using the Leksell Gamma Knife (LGK) at a dose of 24 +/- 1 Gy at the margin. The target included the anterior parahippocampal cortex and the basal and lateral part of the amygdala and anterior hippocampus (head and body). One patient (a heavy smoker) died of a myocardial infarction. Twenty patients were available for prospective evaluation. A minimum 2-year follow-up period included clinical, neuropsychological, and radiologic evaluations.

RESULTS

At each 6-month follow-up evaluation, the frequency of seizures was significantly smaller than that at the previous visit. The median seizure frequency of 6.16 the month before treatment was reduced to 0.33 at 2 years after treatment. At 2 years, 65% of the patients (13 of 20) were seizure free. Five patients had transient side effects, including depression, headache, nausea, vomiting, and imbalance. There was no permanent neurological deficit reported except nine visual field deficits. No neuropsychological deterioration was observed 2 years after treatment. The quality of life was significantly better than that before surgery.

CONCLUSIONS

The safety and efficacy of the radiosurgical treatment of MTLEs appears good in this group of patient over short-to-middle term. Delay of the seizure cessation was the major disadvantage of GKS. A longer follow-up period is required for confirmation of these results.

Stereotactic radiosurgery for hypothalamic hamartomas

Hypothalamic hamartomas are nonneoplastic lesions often characterized by central precocious puberty and gelastic epilepsy. Due to the delicate location surgery is often unsuccessful and associated with considerable risks. In the presented series, Gamma Knife radiosurgery was applied. Four cases (aged between 5-13 years) who presented with medically intractable gelastic epilepsy and increasing secondary generalization, abnormal behaviour and precocious puberty (3 cases) are reported. Hypothalamic hamartomas sized 11-17 mm had been diagnosed by MR imaging. Radiosurgical treatment was performed in general anaesthesia with margin doses of 12-14 Gy to the 50-90% isodoses covering volumes of 600-2300 mm3. After follow-up periods of 12 to 68 months, a continuing decrease both in seizure frequency and intensity was noted (outcome according to Engel: II a (3 cases) and III a (1 case)). All patients are socially reintegrated. MR imaging did not reveal significant changes concerning the size of the lesions. Gamma Knife radiosurgery can be an effective and safe alternative treatment modality for HH capable of achieving good seizure control and improving behavioural disorders in selected cases.

Failure of low-dose radiosurgery to control temporal lobe epilepsy

Radiosurgical treatment of intractable epilepsy has emerged as a noninvasive alternative to resection. Although gamma knife surgery (GKS) reportedly is effective when the radiation dose is sufficient to cause a destructive reaction in the targeted medial temporal lobe, the optimal target area and dose distribution are largely unknown. Some investigators have suggested that focused irradiation from a nondestructive dose is also effective. In this article the authors report two cases of medial temporal lobe epilepsy in which the patients underwent GKS performed using a 50% marginal dose of 18 Gy covering the amygdala. hippocampal head and body, and parahippocampal gyrus. In both cases this procedure failed to control seizures. Both patients became seizure free after undergoing anterior temporal lobectomy 30 and 16 months, respectively, after radiosurgery.

Subnecrotic stereotactic radiosurgery controlling epilepsy produced by kainic acid injection in rats

OBJECT

Any analysis of the potential role of stereotactic radiosurgery for epilepsy requires the experimental study of its potential antiepileptogenic, behavioral, and histological effects. The authors hypothesized that radiosurgery performed using subnecrotic tissue doses would reduce or abolish epilepsy without causing demonstrable behavioral side effects. The kainic acid model in rats was chosen to test this hypothesis.

METHODS

Chronic epilepsy was successfully created by stereotactic injection of kainic acid (8 microg) into the rat hippocampus. Epileptic rats were divided into three groups: high-dose radiosurgery (60 Gy, 16 animals), low-dose (30 Gy, 15 animals), and controls. After chronic epilepsy was confirmed by observation of the seizure pattern and by using electroencephalography (EEG), radiosurgery was performed on Day 10 postinjection. Serial seizure and behavior observation was supplemented by weekly EEG sessions performed for the next 11 weeks. To detect behavioral deficits, the Morris water maze test was performed during Week 12 to study spatial learning and memory. Tasks involved a hidden platform, a visible platform, and a probe trial. After radiosurgery, the incidence of observed and EEG-defined seizures was markedly reduced in rats from either radiosurgically treated group. A significant reduction was noted after high-dose (60 Gy) radiosurgery in Weeks 5 to 9 (p < 0.003). After low-dose (30 Gy) radiosurgery, a significant reduction was found after 7 to 9 weeks (p < 0.04). During the task involving the hidden platform, kainic acid-injected rats displayed significantly prolonged latencies compared with those of control animals (p < 0.05). Hippocampal radiosurgery did not worsen this performance. The probe trial showed that kainic acid-injected rats that did not undergo radiosurgery spent significantly less time than control rats in the target quadrant (p = 0.03). Rats that had undergone radiosurgery displayed no difference compared with control rats and demonstrated better performance than rats that received kainic acid alone (p = 0.04). Radiosurgery caused no adverse histological effects.

CONCLUSIONS

In a rat model, radiosurgery performed with subnecrotic tissue doses controlled epilepsy without causing subsequent behavioral impairment.

Gamma knife radiosurgery for hypothalamic hamartomas in patients with medically intractable epilepsy and precocious puberty. Report of two cases

Hamartoma of the hypothalamus represents a well-known but rare cause of central precocious puberty and gelastic epilepsy. Due to the delicate site in which the tumor is located, surgery is often unsuccessful and associated with considerable risks. In the two cases presented, gamma knife radiosurgery was applied as a safe and noninvasive alternative to obtain seizure control. Two patients, a 13-year-old boy and a 6-year-old girl, presented with medically intractable gelastic epilepsy and increasing episodes of secondary generalized seizures. Abnormal behavior and precocious puberty were also evident. Magnetic resonance (MR) imaging revealed hypothalamic hamartomas measuring 13 and 11 mm, respectively. After general anesthesia had been induced in the patients, radiosurgical treatment was performed with margin doses of 12 Gy to 90% and 60% of isodose areas, covering volumes of 700 and 500 mm3, respectively. After follow-up periods of 54 months in the boy and 36 months in the girl, progressive decrease in both seizure frequency and intensity was noted (Engel outcome scores IIa and IIIa, respectively). Both patients are currently able to attend public school. Follow-up MR imaging has not revealed significant changes in the sizes of the lesions. Gamma knife radiosurgery can be an effective and safe treatment modality for achieving good seizure control in patients with hypothalamic hamartomas.

Radiosurgery: where we were, are, and may be in the third millennium

Radiosurgery will celebrate its Golden Jubilee in the year 2001. More than 100,000 patients throughout the world have undergone radiosurgery since Lars Leksell first described the technique in 1951. Rapid developments in neuroimaging and even robotic technology in the past decade have contributed to improved outcomes and wider applications for radiosurgery. A variety of different radiosurgical techniques have been developed in the past two decades. Numerous studies have examined the benefits and risks of radiosurgery performed with various devices. The long-term results of radiosurgery are now available, and these results have established radiosurgery as an effective noninvasive treatment method for intracranial vascular malformations and many tumors. Additional applications of radiosurgery for the treatment of malignant tumors and functional disorders are being assessed. Radiosurgery is an impressive combination of minimally invasive technologies administered by a multidisciplinary team of surgeons, oncologists, medical physicists, and engineers.

Corpus callosotomy with radiosurgery

OBJECTIVE

Corpus callosotomy is a surgical option for medically uncontrolled generalized epilepsy in appropriate patients. Because numerous complications related to open callosotomy are still reported, we performed radiosurgical corpus callosotomy with the gamma knife.

METHODS

Between October 1992 and June 1995, three patients underwent stereotactic radiosurgery to ablate the anterior third of the corpus callosum. The patients had intractable epilepsy: two had Lennox-Gastaut syndrome, and one had multifocal epilepsy with atonic, tonicoclonic, and atypical absence seizures. The history of seizures ranged from 20 to 37 years' duration. Stereotactic radiosurgery was performed with a cobalt-60 gamma knife using a 4-mm collimator, targeted to the rostrum, genu, and anterior third of body of the corpus callosum. Two patients were treated once with 150 and 160 Gy at maximum, respectively, and one patient was treated in two stages with 50 Gy and then 170 Gy at maximum.

RESULTS

The severity and frequency of seizures were significantly reduced in all three patients. The types of seizures associated with the most improved outcome were atonic and generalized tonicoclonic seizures. The mean follow-up period was 38 months. Hospitalization required for this procedure was 3 days. No complications related to irradiation were recorded except transient headache in one patient.

CONCLUSION

The outcomes suggest that radiosurgical corpus callosotomy may be a promising alternative treatment to open callosotomy.

Functional radiosurgery

Although the application of stereotactic radiosurgery for the management of functional brain disorders began in 1951, almost 50 years elapsed before it received appropriate attention. Radiosurgical techniques are used to create image-guided, physiological inactivity or focally destructive brain lesions without neurophysiological guidance. The lack of neurophysiological guidance remains the greatest argument against the use of radiosurgery for selected disorders. Current anatomic targets include the trigeminal nerve (for trigeminal neuralgia), the thalamus (for tremor or pain), the cingulate gyrus or anterior internal capsule (for pain or psychiatric illness), the globus pallidus (for symptoms of Parkinson's disease), and the hippocampus (for epilepsy). The use of radiosurgery as a "lesion generator" is based on extensive animal studies that defined the dose, volume, and temporal response of the irradiated tissue. The usefulness of radiosurgery has been compared with that of microsurgical, percutaneous, and electrode-based techniques used for functional neurological disorders. At present, the long-term results after functional radiosurgery procedures remain to be documented. The current indications and expected outcomes after radiosurgery are discussed.

Control of epilepsy associated with cerebral arteriovenous malformations after radiosurgery

OBJECTIVE

To investigate the effect of radiosurgery for symptomatic epilepsy associated with cerebral arteriovenous malformations (AVMs).

METHODS

Thirty five patients with unruptured epileptogenic AVMs were studied with a mean follow up of 43.0 months. The duration of epilepsy before radiosurgery ranged from 2 months to 21 years (mean 2.8 years). Fifteen patients showed partial seizures; eight of these had associated secondary generalisation. The remaining 20 patients showed only generalised seizures without preceding focal seizures.

RESULTS

At the final follow up examination, 28 patients remained seizure free, whereas seizures continued in seven. Variables significantly associated with continuity of seizures after radiosurgery were the number of seizures before therapy (p<0.01) and duration of epilepsy (p<0.05). According to Engel's classification, the 10 patients with intractable seizures before treatment included five with grade I, four with grade III, and one with grade IV. The frequency of seizures began to decrease several months after radiosurgery; much shorter than the time required for morphological change in the AVMs.

CONCLUSIONS

Radiosurgery seems to be beneficial for seizure control in patients with unruptured epileptogenic AVM.

Subsidence of seizure induced by stereotactic radiation in a patient with hypothalamic hamartoma. Case report

The authors report on a patient who exhibited intractable epilepsy due to an inaccessible hypothalamic hamartoma and subsequently underwent stereotactic radiosurgery. This 25-year-old man had a 24-year history of intractable gelastic and tonic-clonic seizures. Magnetic resonance (MR) imaging performed at examination as well as that performed 30 months earlier demonstrated a nonenhancing and nonprogressive spherical mass, approximately 10 mm in diameter, located on the patient's right side at the floor of the third ventricle. Focal radiation treatment performed with a gamma knife unit administered 36 Gy to the center and 18 Gy to the periphery of the lesion. This treatment resulted in an improvement in seizure control. Before the patient underwent radiosurgery, he suffered from three to six generalized seizures per month in spite of attentive compliance with an anticonvulsant medication regimen. After irradiation of the harmatoma, the frequency of the seizures transiently increased and then subsided 3 months posttreatment. The patient has been free of seizures for the last 21 months, with no neurological or endocrinological complications. Magnetic resonance imaging performed 12 months posttreatment demonstrated complete disappearance of the lesion.

Monte Carlo treatment planning for stereotactic radiosurgery

OBJECTIVES

In radiosurgery treatment planning there is general acceptance that the target volume can be assumed to be homogeneous and that corrections for variations in contour are unnecessary. Thus, dose algorithms employed in radiosurgery treatment planning are quite unsophisticated; in almost every case the algorithms are the simple product of tissue-maximum and off-axis ratios and an output factor. In small photon beams, however, the lack of side scatter equilibrium compromises these assumptions.

METHODS

In this work we have employed Monte Carlo techniques in an attempt to obtain a more accurate representation of radiosurgical dose distributions. Specifically, the Monte Carlo system which we have devised traces the paths of primary and secondary radiation through a patient-specific anatomical representation defined by computed tomography data. In this manner the perturbation effects from external contour changes and internal tissue heterogeneities are accounted for completely. The ability to precisely mimic multi-beam multi-arc stereotactic delivery has been incorporated into our Monte Carlo treatment planning interface.

RESULTS

Subsequent calculations show that substantial differences can exist when homogeneity is not assumed. Tissue heterogeneities produce a lateral broadening of the beam, resulting in a smaller volume contained within the higher isodose levels (80-90%) with a corresponding increase in the volume treated at the lower isodose levels (<50%).

CONCLUSIONS

These results suggest that further investigation and refinement of radiosurgery dose algorithms is in order.

Accuracy of an experimental stereotactic system for MRI-based gamma knife irradiation in the rat

Stereotactic devices for experimental Gamma Knife irradiation and magnetic resonance imaging (MRI) have recently been developed for experimental studies using rats [6,7]. The present study examined the accuracy of these devices using the following two approaches. In the first approach, Gamma Knife irradiation was performed using the stereotactic device with targets based on a standard stereotactic atlas. Gadolinium-enhanced T1-weighted magnetic resonance imaging was performed using the MRI stereotactic device. Animals were then sacrificed after Evans blue injection, and the rat brain was sliced using an attachment to the stereotactic device. The center coordinates of the gadolinium-enhanced area from the MRI and Evans blue-stained area from the tissue sections were obtained using a computer-assisted image analysis system. These coordinates were compared with the target coordinates planned from the stereotactic atlas. In the second approach, a thermoluminescence dosimeter was implanted in the rat brain. Stereotactic MRI was performed using the stereotactic MRI device, and the coordinates of the implant were obtained. Gamma Knife irradiation was then performed at this target using the stereotactic device. The absorbed dose was measured and compared with the planning dose. These experiments demonstrated a spatial error of 0.6 mm (standard error +/- 0.07) between Gamma Knife irradiation based on a comparison of the atlas coordinates and the lesion, and a spatial error of 1.0 mm (standard error +/- 0.13) based on a comparison of the stereotactic MR images and the lesion. Gamma Knife irradiation based on MR images using the stereotactic device demonstrated a maximum error of 10% in absorbed dose at the target center. Together, the stereotactic devices for Gamma Knife irradiation and magnetic resonance imaging provide useful tools for Gamma Knife research in an animal model.