Thalamic lesions produced by gamma thalamotomy for movement disorders

Gamma knife versus linear accelerator thalamotomy for essential tremor and Parkinson's disease: A systematic review and meta-analysis

BACKGROUND

Tremor, either in patients with Essential Tremor (ET) or Parkinson's disease (PD), constitutes the most common movement disorder. Stereotactic radiosurgery using Gamma Knife (GK) and linear accelerator (LINAC) systems, is an effective, incisionless treatment modality for ET and PD. Although these technologies have been used clinically since the 1990's, most studies have focused on GK, and efficacy, safety and time to treatment effect (latency) of GK and LINAC have not been compared.

OBJECTIVE

We therefore aimed to conduct a systematic review with network meta-analysis examining efficacy, adverse events (AEs) and latency of GK and LINAC for treating tremor in ET and PD.

METHODS

We conducted a systematic review with network meta-analysis in accordance with PRISMA guidelines, using the Embase and PubMed databases. We included all primary GK/LINAC thalamotomy studies in ET/PD patients with at least 6 months of follow-up, reporting unilateral Fahn-Tolosa-Marin Tremor Scale (FTM-TRS) or Unified Parkinson's disease rating scale (UPDRS) scores pre-treatment/post-treatment and/or AEs and/or latency. The primary efficacy outcome was FTM-TRS Scale A or UPDRS Item 16 score reduction. AEs were presented as an estimated incidence, and latency as average time to first recorded clinical improvement in tremor.

RESULTS

Six studies of 311 patients and 2 studies of 60 patients met inclusion criteria for GK/LINAC efficacy comparison, respectively. Network meta-analysis showed similar tremor reduction between modalities (standardized mean difference between pre- and post-treatment scores: GK: -2.18 (95 % CI: -2.79, -1.57); LINAC: -2.13 (95 % CI: -5.13, 0.87). GK also had a higher absolute AE rate, while LINAC was associated with a greater latency period. There was no correlation between GK efficacy and AE rate.

CONCLUSIONS

Despite the relatively small sample sizes, these results demonstrate similar efficacy between GK and LINAC for ET and PD, with a trend toward higher efficacy but greater AE incidence and slower onset of tremor improvement in GK compared to LINAC.

Stereotatic radiosurgery for the treatment of motor symptoms in Parkinson's disease: A systematic review

BACKGROUND

Stereotaxic Radiosurgery (SRS) is a non-invasive lesioning technique for movement disorders when patients cannot undergo DBS due to medical comorbidities.

OBJECTIVE

To describe and summarize the literature on SRS's application and physical parameters for Parkinson's disease (PD) motor symptoms.

METHODS

The MEDLINE/PUBMED and EMBASE databases were searched in July 2022 following the PRISMA guideline. Two independent reviewers screened data from 425 articles. The level of evidence followed the Oxford Centre for Evidence-Based Medicine. Pertinent details for each study regarding participants, physical parameters, and results were extracted.

RESULTS

Twelve studies reported that 454 PD patients underwent Gamma KnifeⓇ (GK). The mean improvement time of the treated symptoms was three months after GK. Tremor is the most common symptom investigated, with success rates ranging from 47.5% to 93.9%. Few studies were conducted for caudatotomy (GKC) and pallidotomy (GKP), which presented an improvement for dyskinesia and bradykinesia. Physical parameters were similar with doses ranging from 110 to 200 Gy, use of a 4-mm collimator with an advanced imaging locator system, and coordinates were obtained from available stereotactic atlases.

CONCLUSIONS

GK thalamotomy is a good alternative for treating tremor; however, its effects are delayed, and there are cases in which it can regress after years. The outcomes of GKC and GKP seem to be promising. The existing studies are more limited, and effects need to be better investigated.

A Systematic Review Comparing Focused Ultrasound Surgery With Radiosurgery for Essential Tremor

BACKGROUND

Focused ultrasound (FUS-T) and stereotactic radiosurgery thalamotomy (SRS-T) targeting the ventral intermediate nucleus are effective incisionless surgeries for essential tremor (ET). However, their efficacy for tremor reduction and, importantly, adverse event incidence have not been directly compared.

OBJECTIVE

To present a comprehensive systematic review with network meta-analysis examining both efficacy and adverse events (AEs) of FUS-T vs SRS-T for treating medically refractory ET.

METHODS

We conducted a systematic review and network meta-analysis according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, using the PubMed and Embase databases. We included all primary FUS-T/SRS-T studies with approximately 1-year follow-up, with unilateral Fahn-Tolosa-Marin Tremor Rating Scale or Clinical Rating Scale for Tremor scores prethalamotomy/post-thalamotomy and/or AEs. The primary efficacy outcome was Fahn-Tolosa-Marin Tremor Rating Scale A+B score reduction. AEs were reported as an estimated incidence.

RESULTS

Fifteen studies of 464 patients and 3 studies of 62 patients met inclusion criteria for FUS-T/SRS-T efficacy comparison, respectively. Network meta-analysis demonstrated similar tremor reduction between modalities (absolute tremor reduction: FUS-T: -11.6 (95% CI: -13.3, -9.9); SRS-T: -10.3 (95% CI: -14.2, -6.0). FUS-T had a greater 1-year adverse event rate, particularly imbalance and gait disturbances (10.5%) and sensory disturbances (8.3%). Contralateral hemiparesis (2.7%) often accompanied by speech impairment (2.4%) were most common after SRS-T. There was no correlation between efficacy and lesion volume.

CONCLUSION

Our systematic review found similar efficacy between FUS-T and SRS-T for ET, with trend toward higher efficacy yet greater adverse event incidence with FUS-T. Smaller lesion volumes could mitigate FUS-T off-target effects for greater safety.

Deep Brain Stimulation for Tremor: Update on Long-Term Outcomes, Target Considerations and Future Directions

Deep brain stimulation (DBS) of the thalamic ventral intermediate nucleus is one of the main advanced neurosurgical treatments for drug-resistant tremor. However, not every patient may be eligible for this procedure. Nowadays, various other functional neurosurgical procedures are available. In particular cases, radiofrequency thalamotomy, focused ultrasound and radiosurgery are proven alternatives to DBS. Besides, other DBS targets, such as the posterior subthalamic area (PSA) or the dentato-rubro-thalamic tract (DRT), may be appraised as well. In this review, the clinical characteristics and pathophysiology of tremor syndromes, as well as long-term outcomes of DBS in different targets, will be summarized. The effectiveness and safety of lesioning procedures will be discussed, and an evidence-based clinical treatment approach for patients with drug-resistant tremor will be presented. Lastly, the future directions in the treatment of severe tremor syndromes will be elaborated.

Microelectrode Recording and Radiofrequency Thalamotomy following Focused Ultrasound Thalamotomy

Magnetic resonance imaging-guided focused ultrasound (MRgFUS) is a novel method for stereotactic brain lesioning and has primarily been applied for thalamotomies to treat essential tremor (ET). The electrophysiological properties of previously MRgFUS-sonicated thalamic neurons have not yet been described. We report on an ET patient who underwent an MRgFUS thalamotomy but experienced tremor recurrence. We expanded the MRgFUS-induced thalamic cavity using radiofrequency (RF), with good effect on the tremor but transient sensorimotor deficits and permanent ataxia. This is the first report of a patient undergoing RF thalamotomy after an unsuccessful MRgFUS thalamotomy. As we used microelectrode recording to guide the RF thalamotomy, we could also study for the first time the electrophysiological properties of previously sonicated thalamic neurons bordering the MRgFUS-induced cavity. These neurons displayed electrophysiological characteristics identical to those recorded from nonsonicated thalamic cells in ET patients. Hence, our findings support the widespread assumption that sonication below the necrotic threshold does not permanently alter neuronal function.

[Historical view on the pathogenesis and surgical treatment of cervical dystonia]

In the past few decades, approaches to surgical treatment of dystonia passed through paradigmatic shift. Intradural upper cervical anterior rhizotomy was replaced by selective peripheral denervation with lesser spectrum of side-effects. Such techniques as microvascular decompression of accessory nerve or spinal cord stimulation for cervical dystonia were abandoned due to lack of proven efficacy. Introducing globus pallidus interna (GPi) DBS in 1990's to treat all types of dystonia, including cervical dystonia, was a fundamental factor. With the growing body of knowledge on the pathophysiology of dystonia, GPi DBS appears to be the most expedient, effective and safe method with limited indications to peripheral destructive procedures.

A single case of MRI-guided focused ultrasound ventro-oral thalamotomy for musician's dystonia

Musician's dystonia (MD) is a type of focal hand dystonia that develops only while playing musical instruments and interferes with skilled and fine movements. Lesioning of the ventro-oral (Vo) nucleus of the thalamus (Vo-thalamotomy) using radiofrequency can cause dramatic improvement in MD symptoms. Focused ultrasound (FUS) can make intracranial focal lesions without an incision. The authors used MRI-guided FUS (MRgFUS) to create a lesion on the Vo nucleus to treat a patient with MD. Tubiana's MD scale (TMDS) was used to evaluate the condition of musical play ranging from 1 to 5 (1: worst, 5: best). The patient was a 35-year-old right-handed man with involuntary flexion of the right second, third, and fourth fingers, which occurred while playing a classical guitar. Immediately after therapeutic sonications of FUS Vo-thalamotomy, there was dramatic improvement in the MD symptoms. The TMDS scores before; at 0 and 1 week after; and at 1, 3, 6, and 12 months after MRgFUS Vo-thalamotomy were 1, 4, 4, 5, 5, 5, and 5, respectively. No complications were observed. Focused ultrasound Vo-thalamotomy can be an effective treatment for MD.

Stereotactic radiosurgery for tremor: systematic review

OBJECTIVE

The aim of this systematic review is to offer an objective summary of the published literature relating to stereotactic radiosurgery (SRS) for tremor and consensus guideline recommendations.

METHODS

This systematic review was performed up to December 2016. Article selection was performed by searching the MEDLINE (PubMed) and EMBASE electronic bibliographic databases. The following key words were used: "radiosurgery" and "tremor" or "Parkinson's disease" or "multiple sclerosis" or "essential tremor" or "thalamotomy" or "pallidotomy." The search strategy was not limited by study design but only included key words in the English language, so at least the abstract had to be in English.

RESULTS

A total of 34 full-text articles were included in the analysis. Three studies were prospective studies, 1 was a retrospective comparative study, and the remaining 30 were retrospective studies. The one retrospective comparative study evaluating deep brain stimulation (DBS), radiofrequency thermocoagulation (RFT), and SRS reported similar tremor control rates, more permanent complications after DBS and RFT, more recurrence after RFT, and a longer latency period to clinical response with SRS. Similar tremor reduction rates in most of the reports were observed with SRS thalamotomy (mean 88%). Clinical complications were rare and usually not permanent (range 0%-100%, mean 17%, median 2%). Follow-up in general was too short to confirm long-term results.

CONCLUSIONS

SRS to the unilateral thalamic ventral intermediate nucleus, with a dose of 130-150 Gy, is a well-tolerated and effective treatment for reducing medically refractory tremor, and one that is recommended by the International Stereotactic Radiosurgery Society.

Surgical treatment of dystonia

INTRODUCTION

Treatment of dystonia should be individualized and tailored to the specific needs of patients. Surgical treatment is an important option in medically refractory cases. Several issues regarding type of the surgical intervention, targets, and predict factors of benefit are still under debate. Areas covered: To date, several clinical trials have proven the benefit and safety of deep brain stimulation (DBS) for inherited and idiopathic isolated dystonia, whereas there is still insufficient evidence in combined and acquired dystonia. The globus pallidus internus (GPi) is the target with the best evidence, but data on the subthalamic nucleus seems also to be promising. Evidence suggests that younger patients with shorter disease duration experience greater benefit following DBS. Pallidotomy and thalamotomy are currently used in subset of carefully selected patients. The development of MRI-guided focused ultrasound might bring new options to ablation approach in dystonia. Expert commentary: GPi-DBS is effective and safe in isolated dystonia and should not be delayed when symptoms compromise quality of life and functionality. Identifying the best candidates to surgery on acquired and combined dystonias is still necessary. New insights about pathophysiology of dystonia and new technological advances will undoubtedly help to tailor surgery and optimize clinical effects.

Stereotactic radiosurgery for essential tremor: Retrospective analysis of a 19-year experience

BACKGROUND

Essential Tremor (ET) is a common movement disorder that can be disabling. Initial treatment is in the form of medical therapies. Patients with medically refractory ET seek surgical intervention which include radiofrequency thalamotomy, deep brain stimulation, and radiosurgical thalamotomy. Radiosurgical thalamotomy is a minimally invasive surgical option which is especially valuable for elderly and high surgical risk patients.

OBJECTIVE

The purpose of this study was to retrospectively analyze the outcomes of stereotactic radiosurgery for patients suffering from medically refractory essential tremor.

METHODS

During a 19-year period (1996-2015), 73 patients underwent gamma knife thalamotomy for intractable essential tremor. A median central dose of 140 Gy (range, 130-150) was delivered to the nucleus ventralis intermedius through a single 4-mm isocenter. We used the Fahn-Tolosa-Marin clinical tremor rating scale to score tremor, handwriting, drawing, and ability to drink fluids. The median time to last follow-up was 28 months (range, 6-152).

RESULTS

After gamma knife thalamotomy, 93.2% improved in tremor. Forty-four patients (60.3%) experienced tremor arrest or barely perceptible tremor. Eighteen patients (24.7%) noted tremor arrest and complete restoration of motor function. Tremor improvement was sustained at last follow-up in 96% of patients who experience tremor relief. Mean tremor score improved from 3.19 before to 1.27 after gamma knife thalamotomy (P < 0.0001). Mean handwriting score improved from 2.97 to 1.25 (P < 0.0001). Mean drawing score improved from 3.16 to 1.26 (P < 0.0001). Mean drinking score improved from 3.14 to 1.56 (P < 0.0001). Imaging follow-up showed three types of lesions: enhancing lesion, streaking along internal capsule on fluid-attenuated inversion recovery, and significant reactive changes. Three patients (4%) experienced temporary adverse radiation effects.

CONCLUSION

Radiosurgery is a safe and valuable treatment option for medically refractory essential tremor, especially for the elderly or those with high surgical risk for DBS or radiofrequency thalamotomy. © 2017 International Parkinson and Movement Disorder Society.

Gamma Knife Thalamotomy for Parkinson Disease and Essential Tremor: A Prospective Multicenter Study

BACKGROUND:

No prospective study of gamma knife thalamotomy for intractable tremor has previously been reported.

Objective:

To clarify the safety and optimally effective conditions for performing unilateral gamma knife (GK) thalamotomy for tremors of Parkinson disease (PD) and essential tremor (ET), a systematic postirradiation 24-month follow-up study was conducted at 6 institutions. We present the results of this multicenter collaborative trial.

Methods:

In total, 72 patients (PD characterized by tremor, n = 59; ET, n = 13) were registered at 6 Japanese institutions. Following our selective thalamotomy procedure, the lateral part of the ventralis intermedius nucleus, 45% of the thalamic length from the anterior tip, was selected as the GK isocenter. A single 130-Gy shot was applied using a 4-mm collimator. Evaluation included neurological examination, magnetic resonance imaging and/or computerized tomography, the unified Parkinson's disease rating scale (UPDRS), electromyography, medication change, and video observations.

Results:

Final clinical effects were favorable. Of 53 patients who completed 24 months of follow-up, 43 were evaluated as having excellent or good results (81.1%). UPDRS scores showed tremor improvement (parts II and III). Thalamic lesion size fluctuated but converged to either an almost spherical shape (65.6%), a sphere with streaking (23.4%), or an extended high-signal zone (10.9%). No permanent clinical complications were observed.

Conclusion:

GK thalamotomy is an alternative treatment for intractable tremors of PD as well as for ET. Less invasive intervention may be beneficial to patients.

Radiosurgery for movement disorders

Stereotactic radiosurgery (SRS) has been proposed as an alternative treatment modality to pharmaceutical administration and deep brain stimulation (DBS) for patients suffering from movement disorders. Advanced neuroimaging is required for the identification of the functional structures and the accurate placement of the SRS lesion within the brain. Atlas-based techniques have also been used to aid delineation of the target during treatment planning. Maximum doses greater than 120 Gy have been suggested for controlling movement disorders. These high delivered doses and the irreversible character of SRS require accurate placement of the created lesions. In this article, achievements in the field of stereotactic radiosurgery, neuroimaging, and radiosurgical dose planning are reviewed, and an overview is provided of the clinical experience obtained to date in the radiosurgical treatment of movement disorders.

Gamma Knife thalamotomy for treatment of essential tremor: long-term results

Object

The goal of this report was to describe the safety and effectiveness of nucleus ventralis intermedius (VIM) thalamotomy performed with the Leksell Gamma Knife (GK) for the treatment of essential tremor (ET).

Methods

One hundred seventy-two patients underwent a total of 214 VIM thalamotomy procedures with the Leksell GK between February 1994 and March 2007 for treatment of disabling ET. Eleven patients were lost to follow-up less than 1 year after the procedures, so that in this report the authors describe the results in 161 patients who underwent a total of 203 thalamotomies (119 unilateral and 42 bilateral).

Results

There were statistically significant decreases (p < 0.0001) in tremor scores for both writing and drawing. The mean postoperative follow-up duration for all patients was 44 ± 33 months. Fifty-four patients have been followed for more than 60 months posttreatment. There were 14 patients who suffered neurological side effects that were temporary (6) or permanent (8), which accounted for 6.9% of the 203 treatments. All complications were related to lesions that grew larger than expected.

Conclusions

A VIM thalamotomy with the Leksell GK offers a safe and effective alternative for surgical treatment of ET. It is particularly applicable to patients who are not ideal candidates for deep brain stimulation but can be offered to all patients who are considering surgical intervention for ET.

The history and future of deep brain stimulation

The advancement of electrical stimulation of the central nervous system has been a story of fits and bursts with numerous setbacks. In many ways, this history has paralleled the history of medicine and physics. We have moved from anecdotal observation to double-blinded, prospective randomized trials. We have moved from faradic stimulation to systems that lie completely under the skin and can deliver complex electrical currents to discrete areas of the brain while controlled through a device that is not much bigger than a PDA. This review will discuss how deep brain stimulation has developed into its current form, where we see the field going and the potential pitfalls along the way.

Gamma Knife thalamotomy for essential tremor

Object

The purpose of this study was to evaluate the results following Gamma Knife thalamotomy (GKT) for medically refractory essential tremor in a series of patients in whom open surgical techniques were not desirable.

Methods

Thirty-one patients underwent GKT for disabling essential tremor after medical therapy had failed. Their mean age was 77 years. Most patients were elderly or had concomitant medical illnesses. A single 4-mm isocenter was used to target a maximum dose of 130 or 140 Gy to the nucleus ventralis intermedius. Items from the Fahn-Tolosa-Marin clinical tremor rating scale were used to grade tremor and handwriting before and after radiosurgery.

Results

The median follow-up was 36 months. In the group of 26 evaluable patients, the mean tremor score (± standard deviation) was 3.7 ± 0.1 preoperatively and 1.7 ± 0.3 after radiosurgery (p < 0.000015). The mean handwriting score was 2.8 ± 0.2 before GKT and 1.7 ± 0.2 afterward (p < 0.0002). After radiosurgery, 18 patients (69%) showed improvement in both action tremor and writing scores, 6 (23%) only in action tremor scores, and 3 (12%) in neither tremor nor writing. Permanent mild right hemiparesis and speech impairment developed in 1 patient 6 months after radiosurgery. Another patient had transient mild right hemiparesis and dysphagia.

Conclusions

Gamma Knife thalamotomy is a safe and effective therapy for medically refractory essential tremor. Its use is especially valuable for patients ineligible for radiofrequency thalamotomy or deep brain stimulation. Patients must be counseled on potential complications, including the low probability of a delayed neurological deficit.

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.

From Selective Thalamotomy with Microrecording to Gamma Thalamotomy for Movement Disorders

A theoretical and practical process from microrecording-guided thalamotomy to gamma knife thalamotomy was briefly reviewed. Based on our own experiences of selective thalamotomy with microrecording, we are trying to apply gamma knife to the treatment of movement disorders. An important technical problem is how to determine the exact thalamic target. At first we refer to the posterior commissure and coordinate of the standard atlas for approximately determining the lateral part of the ventral intermediate nucleus. Then the point is further corrected by anatomical landmark (45% of the thalamic length) to compensate the individual difference. A final lesion is made by gamma knife using a 4-mm collimator, 130 Gy in 1 shot. The average delay of clinical improvement is about 6 months after irradiation. Thus far the results are satisfactory, being 80-85% successful without any noticeable complications. Only 3 days of hospitalization with minimal invasion could be a big advantage for the patient. Further technical progress may improve the clinical results in the future.

Glass rod detectors for small field, stereotactic radiosurgery dosimetric audit

This paper demonstrates the feasibility of using glass rod detectors for quality assurance audit of radiosurgery units. Five radiosurgery units (3 Gamma Knife model C, 1 Gamma Knife model U and 1 Cyberknife) located in California participated in the study. At each center glass rod detectors were used to measure a number of dosimetric parameters including relative collimator output factor and absolute dose rate. The Gamma Knife data obtained is in excellent agreement with the commissioning data generated by the manufacturer for each unit and the Cyberknife data is in general agreement with the data published by other centers. In particular the output factor of the 4 mm Gamma Knife helmet factor, a subject of abundant debate, was measured in the range 0.863-0.872 with an accuracy of better than 1% across the four participating centers. It is hoped that this pilot study will facilitate a nationwide postal audit of stereotactic radiosurgery units.

Gamma knife thalamotomy for movement disorders: evaluation of the thalamic lesion and clinical results

Object. The authors studied the effects of gamma knife thalamotomy (GKT) on Parkinson disease-related tremor and essential tremor before and after reloading of radioactive cobalt.

Methods. Based on experience in stereotactic thalamotomy aided by depth microrecording, the target was located at the lateral border of the thalamic ventralis intermedius nucleus (VIM). For more precise targeting, the percentage representation of the thalamic VIM in relation to the entire thalamic length is useful. The location of the target was determined on magnetic resonance (MR) imaging and computerized tomography scanning. A maximum dose of 130 Gy was delivered to the target by using a single isocenter with the 4-mm collimator. In more recent cases, a systematic follow-up examination was performed at 3, 6, 12, 18, and 24 months after GKT.

Since 1993, the authors have treated 70 patients with PD. Throughout the series the same dosimetric technique has been used. The course after GKT was compared between the 25 cases with PD treated before reloading and the 35 cases treated after reloading. In the majority (80–85%) treated after reloading, tremor and rigidity were reduced around 6 months after GKT. In the cases treated before reloading this effect took approximately 1 year. The thalamic reaction on MR imaging showed the same two lesion types in both series: a restricted and a diffuse. After reloading the restricted lesion was more frequent and the lesion volume was smaller.

Conclusions. The shorter delay in clinical improvement and smaller lesion size may be related to an increased radiation dose.

Gamma knife thalamotomy for movement disorders: evaluation of the thalamic lesion and clinical results

Object.The authors studied the effects of gamma knife thalamotomy (GKT) on Parkinson disease-related tremor and essential tremor before and after reloading of radioactive cobalt.

Methods.Based on experience in stereotactic thalamotomy aided by depth microrecording, the target was located at the lateral border of the thalamic ventralis intermedius nucleus (VIM). For more precise targeting, the percentage representation of the thalamic VIM in relation to the entire thalamic length is useful. The location of the target was determined on magnetic resonance (MR) imaging and computerized tomography scanning. A maximum dose of 130 Gy was delivered to the target by using a single isocenter with the 4-mm collimator. In more recent cases, a systematic follow-up examination was performed at 3, 6, 12, 18, and 24 months after GKT.

Since 1993, the authors have treated 70 patients with PD. Throughout the series the same dosimetric technique has been used. The course after GKT was compared between the 25 cases with PD treated before reloading and the 35 cases treated after reloading. In the majority (80–85%) treated after reloading, tremor and rigidity were reduced around 6 months after GKT. In the cases treated before reloading this effect took approximately 1 year. The thalamic reaction on MR imaging showed the same two lesion types in both series: a restricted and a diffuse. After reloading the restricted lesion was more frequent and the lesion volume was smaller.

Conclusions.The shorter delay in clinical improvement and smaller lesion size may be related to an increased radiation dose.