Fractionated radiotherapy of small inoperable lesions of the brain using a non-invasive stereotactic frame

Arteriovenous malformations (AVMs) and benign or low grade, small malignant tumors can be treated by stereotactic radiotherapy in a single fraction. This report describes a technique for stereotactic treatment of small lesions using conventional, fractionated, photon beam irradiation. The Laitinen's stereoadapter, non-invasive head frame was used. This device was tested for accuracy by serial mountings and found to be accurate within 1 mm. The accuracy of the dose delivered was within 2%. Adaptation of this device to the linear accelerator required the design of secondary circular collimators which decreased the penumbra from 3-4 mm to 2-3 mm. The dose fall off outside the target volume is steep enough when using two non-coplanar arcs (90 to 10% within 1 cm). Thermoluminescent dosimetry (TLD) in a humanoid phantom showed good correlation with the calculated dose. This system permits delivery of fractionated radiation therapy to small volumes, easily and accurately, under stereotactic conditions.     

Radiosurgery target point alignment errors detected with portal film verification.

Stereotactic radiosurgery with a linear accelerator requires an accurate match of the therapeutic radiation distribution to the localized target volume. Techniques for localization of the target volume using CT scans and/or angiograms have been described. Alignment of the therapeutic radiation distribution to the intended point in stereotactic space is usually accomplished using precision mechanical scales which attach to the head ring. The present work describes a technique used to verify that the stereotactic coordinates of the center of the intended radiation distribution are in agreement with the localized target point coordinates. This technique uses anterior/posterior and lateral accelerator portal verification films to localize the stereotactic coordinates of the center of the radiation distribution with the patient in the treatment position. The results of 26 cases have been analyzed. Alignment errors of the therapeutic radiation distribution in excess of 1 mm have been found using the portal film verification procedure.     

A system for stereotactic radiosurgery with a linear accelerator

A small field irradiation technique to deliver high doses of single fraction photon radiation to small, precisely located volumes (0.5 to 8 cm3) within the brain has been developed. Our method uses a modified Brown-Roberts-Wells (BRS), CT-guided, stereotactic system and a 6 MV linear accelerator equipped with a special collimator (diameters of 12.5 mm to 30.0 mm projected to isocenter) located 23 cm from isocenter. Target localization via planar angiography has been added. Treatment consists of a series of arcing beams using both gantry and couch rotations. During treatment, the patient's head is immobilized independently of the radiotherapy couch and is precisely positioned without reference to room lasers or light field. A precise verification of alignment precedes each treatment. Extensive performance tests have shown that a target, localized by CT, can be irradiated with a positional accuracy of 2.4 mm in any direction with 95% confidence. If angiography is used for localization, the results are better. The dose 1.0 cm outside the target volume is less than 20% of the prescribed dose for a medium sized collimator.     

Linac-based stereotactic radiosurgery for brain arteriovenous malformations

Aims

Most papers dealing with radiosurgery for cerebral arteriovenous malformations (AVMs) present the results of gamma-knife treatment, whereas linac radiosurgery is becoming increasingly popular. Moreover, there is still much uncertainty about the rationale of combined endovascular and radiosurgical treatment. The aims of this study were to evaluate obliteration and rebleeding rates, and to determine factors influencing obliteration and adverse effects after linac-based stereotactic radiosurgery for cerebral AVMs.

Materials and methods

Records of 62 consecutive patients were analysed. Thirty-one had partial embolisation, five surgery, 29 had no prior treatment. The mean follow-up was 28.4 months. The mean volume treated was 11.7 cm³ and included embolised portions of AVMs. Actuarial obliteration rates and annual bleeding hazard rates after radiosurgery were calculated using Kaplan-Meier survival and life table analyses.

Results

Actuarial obliteration rates after 1, 2 and 3 years of follow-up were 17, 36 and 40%, respectively. Prior embolisation did not influence the obliteration rate. In 77.3% of patients, obliteration occurred during the first 2 years of follow-up. Annual bleeding hazard rates after stereotactic radiosurgery were 3.4 and 1.1% during the first and second year of follow-up, respectively. Non-symptomatic imaging abnormalities were detected in 33.9% of patients after a median time of 8.8 months. The Spetzler-Martin grade, AVM score, radiation dose, volume and AVM nidus < 3 cm significantly influenced the probability of obliteration. A dose less than 15 Gy significantly reduced the probability of obliteration.

Conclusion

At least a 3 year follow-up is required to accurately assess the outcome. The best effects of the treatment are achieved for small (<3 cm), low-grade lesions with a low AVM score. The bleeding risk after stereotactic radiosurgery gradually decreases.     

Stereotactic proton beam therapy for intracranial arteriovenous malformations

PURPOSE: To investigate hypofractionated stereotactic proton therapy of predominantly large intracranial arteriovenous malformations (AVMs) by analyzing retrospectively the results from a cohort of patients. METHODS AND MATERIALS: Since 1993, a total of 85 patients with vascular lesions have been treated. Of those, 64 patients fulfilled the criteria of having an arteriovenous malformation and sufficient follow-up. The AVMs were grouped by volume: <14 cc (26 patients) and > or =14 cc (38 patients). Treatment was delivered with a fixed horizontal 200 MeV proton beam under stereotactic conditions, using a stereophotogrammetric positioning system. The majority of patients were hypofractionated (2 or 3 fractions), and the proton doses are presented as single-fraction equivalent cobalt Gray equivalent doses (SFEcGyE). The overall mean minimum target volume dose was 17.37 SFEcGyE, ranging from 10.38-22.05 SFEcGyE. RESULTS: Analysis by volume group showed obliteration in 67% for volumes <14 cc and 43% for volumes > or =14 cc. Grade IV acute complications were observed in 3% of patients. Transient delayed effects were seen in 15 patients (23%), becoming permanent in 3 patients. One patient also developed a cyst 8 years after therapy. CONCLUSIONS: Stereotactic proton beam therapy applied in a hypofractionated schedule allows for the safe treatment of large AVMs, with acceptable results. It is an alternative to other treatment strategies for large AVMs. AVMs are likely not static entities, but probably undergo vascular remodeling. Factors influencing angiogenesis could play a new role in a form of adjuvant therapy to improve on the radiosurgical results.     

Stereotactic irradiation for intracranial arteriovenous malformation using stereotactic radiosurgery or hypofractionated stereotactic radiotherapy

PURPOSE: To investigate the appropriateness of the treatment policy of stereotactic irradiation using both hypofractionated stereotactic radiotherapy (HSRT) and stereotactic radiosurgery (SRS) for arteriovenous malformations (AVMs) located in an eloquent region or for large AVMs and using SRS alone for the other AVMs. METHODS AND MATERIALS: Included in this study were 75 AVMs in 72 patients, with a mean follow-up of 52 months. Of the 75 AVMs, 33 were located in eloquent regions or were >2.5 cm in maximal diameter and were given 25-35 Gy (mean, 32.4 Gy) in four daily fractions at a single isocenter if the patient agreed to prolonged wearing of the stereotactic frame for 5 days. The other 42 AVMs were treated with SRS at a dose of 15-25 Gy (mean, 24.1 Gy) at the isocenter. The 75 AVMs were classified according to the Spetzler-Martin grading system; 21, 23, 28, 2, and 1 AVM were Grade I, II, III, IV, V, and VI, respectively. RESULTS: The overall actuarial rate of obliteration was 43% (95% confidence interval [CI], 30-56%) at 3 years, 72% (95% CI, 58-86%) at 5 years, and 78% (95% CI, 63-93%) at 6 years. The actuarial obliteration rate at 5 years was 79% for the 42 AVMs <2.0 cm and 66% for the 33 AVMs >2 cm. The 5- and 6-year actuarial obliteration rate was 61% (95% CI, 39-83%) and 71% (95% CI, 47-95%), respectively, after HSRT and 81% (95% CI, 66-96%) and 81% (95% CI, 66-96%), respectively, after SRS; the difference was not statistically significant. Radiation-induced necrosis was observed in 4 subjects in the SRS group and 1 subject in the HSRT group. Cyst formation occurred in 3 patients in the SRS group and no patient in the HSRT group. A permanent symptomatic complication was observed in 3 cases (4.2%), and 1 of the 3 was fatal. All 3 patients were in the SRS group. The annual intracranial hemorrhage rate was 5.5-5.6% for all patients. CONCLUSION: Our treatment policy using SRS and HSRT was as effective as the policy involving SRS alone. The HSRT schedule was suggested to have a lower frequency of radiation necrosis and cyst formation than the high-dose SRS schedule. The benefit of HSRT compared with lower dose SRS has not yet been determined.     

Radiosurgery for arteriovenous malformations of the brain using a standard linear accelerator: rationale and technique

We have recently initiated a program for irradiating small, unresectable arteriovenous malformations (AVM's) in the brain. The treatments are delivered using a modified and carefully calibrated 6 MV linac. We are using high, single doses (15 to 25 Gy) with a goal of sclerosing the vessels and preventing hemorrhages. This technique, radiosurgery, is somewhat controversial in the radiotherapy community. Since the treatment is given in a single sitting, rather than in the more conventional pattern of multiple small daily fractions, there is some concern about late radiation damage to the normal brain tissue. However an extensive review of the literature leads us to the conclusion that if a technique is used that keeps the volume irradiated to high dose small, radiosurgery is a safe and efficacious treatment for small (less than 2.5 cm) AVM's. To decrease the risk of necrosis of normal brain tissue, it is important to confine the high dose region as tightly as possible to the target volume. Precise target localization and patient immobilization is achieved using a stereotactic head frame which is used during angiography, CT scanning, and during the radiation treatment. This minimizes the margin of safety that must be added to the target volume for errors in localization and set-up. The treatment is delivered using multiple noncoplanar arcs, with small, sharp edged X ray beams, and with the center of the AVM at isocenter. This produces a rapid dropoff of dose beyond the target volume. Early results in our first few patients are encouraging.     

Ultrasound-guided extracranial radiosurgery: technique and application

PURPOSE: Stereotactic radiosurgery is an effective treatment modality for many intracranial lesions, but target mobility limits its utility for extracranial applications. We have developed a new technique for extracranial radiosurgery based on optically guided three-dimensional ultrasound (3DUS). The 3DUS system provides the ability to image the target volume and critical structures in real time and determine any misregistration of the target volume with the linear accelerator. In this paper, we describe the system and its initial clinical application in the treatment of localized metastatic disease. METHODS AND MATERIALS: The extracranial stereotactic system consists of an ultrasound unit that is optically tracked and registered with the linear accelerator coordinate system. After an initial patient positioning based on computed tomographic (CT) simulation, stereotactic ultrasound images are acquired and correlated with the CT-based treatment plan to determine any soft-tissue shifts between the time of the planning CT and the actual treatment. Optical tracking is used to correct any patient offsets that are revealed by the real-time imaging. RESULTS: Preclinical testing revealed that the ultrasound-based stereotactic navigation system is accurate to within 1.5 mm in comparison with an absolute coordinate phantom. Between March 2001 and March 2002, the system was used to deliver extracranial radiosurgery to 17 metastatic lesions in 16 patients. Treatments were delivered in 1 or 2 fractions, with an average fractional dose of 16 Gy (range 12.5-24 Gy) delivered to the 80% isodose surface. Before each fraction, the target misalignment from isocenter was determined using the 3DUS system and the misalignments averaged over all patients were anteroposterior = 4.8 mm, lateral = 3.6 mm, axial = 2.1 mm, and average total 3D displacement = 7.4 mm (range = 0-21.0 mm). After correcting patient misalignment, each plan was delivered as planned using 6-11 noncoplanar fields. No acute complications were reported. CONCLUSIONS: A system for high-precision radiosurgical treatment of metastatic tumors has been developed, tested, and applied clinically. Optical tracking of the ultrasound probe provides real-time tracking of the patient anatomy and allows computation of the target displacement before treatment delivery. The patient treatments reported here suggest the feasibility and safety of the technique.     

Focal 3D conformal high-dose hypofractionated radiotherapy for brain metastases

The optimal management of patients with few brain metastases is complex. On one hand, stereotactic radiation therapy is a keystone of treatment but is only applicable to highly selected patients fulfilling specific criteria who have access to an adequate radiation unit. On the other, whole-brain radiation therapy may improve survival, but deleterious effects on neurocognitive functions are well known. It has, however, been reported that selected subgroups of patients may benefit from focal dose escalation to brain metastases to prolong survival and the time to intracranial disease progression. Here, we discuss a clinical case to consider the interest of a focal high-dose hypofractionated radiation delivered through a conventional linear accelerator on a large brain metastasis for a patient with metastatic melanoma excluded for stereotactic radiotherapy.     

Stereotactic Radiation Therapy of Intracranial Lesions Methodologic Aspects

A technique for stereotactic radiation therapy of cerebral tumours and arteriovenous malformations using a linear accelerator (6 MV photons) is proposed. Treatment relies on a fixation system that permits a precise use of the coordinates estimated at stereotactic angiography or stereotactic computed tomography. The field of treatment can be exactly outlined in the CT images during repeat examinations, thus facilitating the recognition of changes induced by radiation. The system also allows the extent of the arteriovenous malformation, as seen at angiography, to be accurately traced in the CT sections thus enabling evaluation of possible radiation damage to surrounding brain structures. The precision of the method as well as its hypothetical merits and disadvantages are discussed. The number of patients treated is still small and the follow-up time is too short in the majority of cases to allow definite conclusions. Examples of preliminary results are given.     

Clinical uses of radiosurgery

Radiosurgery uses stereotactic targeting methods to precisely deliver highly focused, large doses of radiation to small intracranial tumors and arteriovenous malformations (AVMs). This article reviews the most common clinical applications of radiosurgery and the clinical results reported from a number of series using either a cobalt-60 gamma knife or linear accelerator as radiation sources. Radiosurgery is used to treat malignant tumors, such as selected cases of brain metastases and malignant gliomas (for which stereotactic radiosurgical boosts are utilized in conjunction with fractionated radiation therapy), as well as benign tumors, such as meningiomas, acoustic neuromas, and pituitary adenomas. Treatment of small AVMs is also highly effective. Although radiosurgery has the potential to produce complications, the majority of patients experience clinical improvement with less morbidity than occurs with surgical resection.     

Treatment outcome of single or hypofractionated single-isocentric stereotactic irradiation (STI) using a linear accelerator for intracranial arteriovenous malformation.

BACKGROUND AND PURPOSE: We investigated the use of hypofractionated stereotactic radiotherapy (HFSR) to reduce adverse radiation effects in comparison to single-fraction stereotactic radiosurgery (SRS) for intracranial arteriovenous malformations (AVMs). MATERIALS AND METHODS: This study includes 53 intracranial AVMs treated between 1991-1998. HFSR was selected for 26 AVMs with a maximum diameter > or 2.5 cm or at eloquent area. Twenty-seven patients were treated with SRS (18 AVMs < 2.5 cm at non-eloquent area, nine patients who were unfit for prolonged ring-wearing). The most frequent minimum dose (Dmin) was 20 Gy for SRS and 28 Gy for HFSR in four fractions. The mean follow-up duration was 34.6 months for SRS and 35.4 months for HFSR. RESULTS: As a whole, the 3 and 5-year actuarial obliteration rates were 64 and 92%. Age <20 years old (P=0.02) and a maximum diameter <2 cm were favorable factors (P=0.05). A difference in the distribution of patients was observed in size (> or =2.5 cm or not) (P<0.001) and location (eloquent or not) (P<0.001) between SRS and HFSR due to the treatment selection. However, no significant differences were observed in the actuarial rates of obliteration and transient increased signals with T2-weighted MR images between SRS and HFSR. Radiation necrosis occurred in two patients treated with SRS and in none with HFSR. Intracranial hemorrhage after treatment happened in two treated with SRS and three with HFSR. CONCLUSIONS: HFSR appears to be at least as effective as SRS in achieving complete obliteration of intracranial AVM, although its definitive role remains to be investigated.     

Impact of target point deviations on control and complication probabilities in stereotactic radiosurgery of AVMs and metastases.

OBJECTIVE: Determination of the impact of inaccuracies in the determination and setup of the target point in stereotactic radiosurgery (SRS) on the expectable complication and control probabilities. METHODS: Two randomized samples of patients with arteriovenous malformation (AVM) (n=20) and with brain metastases (n=20) treated with SRS were formed, and the probability for complete obliteration (COP) or complete remission (CRP), the size of the 10 Gy-volume in the brain tissue (VOI10), and the probability for radiation necrosis (NTCP) were calculated. The dose-effect relations for COP and CRP were fitted to clinical data. Target point deviations were simulated through random vectors and the resulting probabilities and volumes were calculated and compared with the values of the treatment plan. RESULTS: The decrease of the relative value of the control probabilities at 1mm target point deviation was up to 4% for AVMs and up to 10% for metastases. At 2 mm the median decrease was 5% for AVMs and 9% for metastases. The value for the target point deviation, at which COP and CRP decreased about 0.05 in 90% of the cases, was 1.3 mm. The increase of NTCP was maximally 0.0025 per mm target point deviation for AVMs and 0.0035/mm for metastases. The maximal increase of VOI10 was 0.7 cm(3)/mm target point deviation in both patient groups. CONCLUSIONS: The upper limit for tolerable target point deviations is at 1.3mm. If this value cannot be achieved during the system test, a supplementary safety margin should be applied for the definition of the target volume. A better accuracy level is desirable, in order to ensure optimal chances for the success of the treatment. The target point precision is less important for the minimization of the probability of radiation necroses.     

Treatment of recurrent malignant gliomas with stereotactic intensity modulated radiation therapy

Malignant gliomas are usually refractory to aggressive combined-modality therapy, and the incidence of recurrence and death after treatment is very high. State-of-the-art techniques such as stereotactic intensity-modulated radiation therapy (IMRT) are now available to deliver a high dose of radiation to the tumor with relative preservation of surrounding tissues to achieve optimal tumor coverage with minimal toxicity. We report 10 patients (median age 48 years) with recurrent malignant gliomas that were treated with stereotactic directed IMRT. Initial tumor histologies included one low grade glioma (upgraded to anaplastic astrocytoma at recurrence), four anaplastic astrocytomas, and four glioblastomas multiforme. One patient was originally presumed to have a brain metastasis secondary to renal cell carcinoma but was pathologically confirmed as having glioblastoma multiforme at the time of recurrence. Before recurrence, all patients had been treated with external beam radiation therapy (median 59.7 Gy). All recurrences were confirmed by a subtotal resection (5/10) or by imaging (5/10). The median Karnofsky performance score at the time of IMRT was 80. The median tumor volume was 34.69 cm. Treatment was delivered on a 10-MV linear accelerator with a mini-multileaf collimator, MIMiC, and planned with Peacock/Corvus software. Radiation was delivered in daily fractions of 5 Gy, to a total median dose of 30 Gy at the 71% to 93% median isodose line. Median overall survival time was 10.1 months from the date of stereotactic treatment, with 1- and 2-year survival rates of 50% and 33.3%, respectively. Fractionated stereotactic intensity modulated radiation therapy is a novel technique used in the treatment of recurrent malignant gliomas, which produces results comparable to other currently used stereotactic techniques.     

A comparison of techniques for stereotactic radiotherapy by linear accelerator based on 3-dimensional dose distributions

In order to establish the appropriate beam arrangement for use in stereotactic radiotherapy using a linear accelerator, dose volume distributions were calculated for a number of spherical targets in a head phantom and assessment was made by dose sparing of normal tissue outside the target volume. Using a single isocentre, fixed beam arrangements were compared with single and multiple non-coplanar isocentric arc rotations at target sizes from 10 to 55 mm diameter on a 6 MV Philips linear accelerator. From the dose-volume histograms produced, an arrangement of 3 or 4 arcs of rotation proved most suitable, in terms of sparing of normal tissue outside the target volume to high dose irradiation, across the range of target sizes studied. There was little further benefit with increasing the number of arcs beyond this. At target sizes greater than 20 mm diameter an arrangement of 6 static non-coplanar beams achieved sparing equivalent to multiple arc rotations and may have considerable advantages in the treatment of irregular volumes where customised beam shaping could be employed.     

The treatment of recurrent brain metastases with stereotactic radiosurgery

Between May 1986 and August 1989, we treated 18 patients with 21 recurrent or persistent brain metastases with stereotactic radiosurgery using a modified linear accelerator. To be eligible for radiosurgery, patients had to have a performance status of greater than or equal to 70% and have no evidence of (or stable) systemic disease. All but one patient had received prior radiotherapy, and were treated with stereotactic radiosurgery at the time of recurrence. Polar lesions were treated only if the patient had undergone and failed previous complete surgical resection (10 patients). Single doses of radiation (900 to 2,500 cGy) were delivered to limited volumes (less than 27 cm3) using a modified 6MV linear accelerator. The most common histology of the metastatic lesion was carcinoma of the lung (seven patients), followed by carcinoma of the breast (four patients), and melanoma (four patients). With median follow-up of 9 months (range, 1 to 39), all tumors have been controlled in the radiosurgery field. Two patients failed in the immediate margin of the treated volume and were subsequently treated with surgery and implantation of 125I to control the disease. Radiographic response was dramatic and rapid in the patients with adenocarcinoma, while slight reduction and stabilization occurred in those patients with melanoma, renal cell carcinoma, and sarcoma. The majority of patients improved neurologically following treatment, and were able to be withdrawn from corticosteroid therapy. Complications were limited and transient in nature and no cases of symptomatic radiation necrosis occurred in any patient despite previous exposure to radiotherapy. Stereotactic radiosurgery is an effective and relatively safe treatment for recurrent solitary metastases and is an appealing technique for the initial management of deep-seated lesions as a boost to whole brain radiotherapy.     

Radiosurgery of cerebral arteriovenous malformations with the dynamic stereotactic irradiation

From December 1986 through December 1988, 33 patients with inoperable arteriovenous malformation (AVM) were treated in our center with the dynamic stereotactic radiosurgery, which uses a standard 10 MV isocentric linear accelerator. There were 18 females and 15 males with a median age of 26 years (range: 9-69) and a median follow-up time of 16 months (range: 7-32). The arteriovenous malformation volumes treated ranged from 0.2 to 42 cm3. The prescribed doses at the isocenter varied from 50 to 55 Gy and were given as a single fraction in the majority of the patients (31/33). Late complications consisting of intracranial bleeding and/or hemiparesis were observed in three patients. To date, 21 patients underwent repeat angiographic studies at 1 year post-treatment. A complete obliteration of the lesion was achieved in 38% of these patients. For the patients whose arteriovenous malformation nidus was covered by a minimum dose of 25 Gy, the total obliteration rate was 61.5% (8/13), whereas none of the patients who had received less than 25 Gy at the edge of the nidus obtained a total obliteration. Our preliminary analysis at 1 year post-radiosurgery reveals results comparable to those previously reported for other radiosurgical techniques for the same follow-up period.     

Radiosurgery Treatment Planning

The Gamma Knife (Leksell Gamma Unit; Elekta Radiosurgery, Inc, Atlanta, GA) and specially adapted linear accelerators are the systems most commonly used for intracranial stereotactic external beam irradiation. Both systems deliver collimated radiation to a treatment isocenter and a stereotactic head frame is required to align each target or portion of the target with this isocenter. Image acquisition and treatment planning define how the target is to be treated using one or more treatment positions. Diagnostic images are required to link the target volume with the stereotactic frame coordinate system. The number of diagnostic imaging modalities and the corresponding diagnostic technique(s) selected for optimum target localization are determined by the nature of the target and surrounding structures. The diagnostic modalities presented in this article include magnetic resonance imaging, magnetic resonance angiography, computerized tomography, and x-ray angiography. This review covers the fundamentals of radiosurgical treatment planning using the Gamma Knife as the system of reference.     

Stereotactically delivered cranial radiation therapy: a ten-year experience of linac-based radiosurgery in the UK

In 1989, linear accelerator (linac)-based cranial stereotactic radiation therapy ('radiosurgery') was introduced in the UK at St Bartholomew's Hospital; a new, relocatable stereotactic frame was first used at the same time, allowing fractionated stereotactic radiotherapy. In the first decade of clinical practice using this technology, some 200 patients with blood vessel tumours/malformations have been treated, together with another 200 suffering from other conditions. The usefulness of this technique for cerebral arteriovenous malformations (AVM) has been demonstrated, and also a significant cure rate for AVM of >3 cm diameter (which is larger than for those previously reported after treatment on the gamma unit), albeit attended by a higher complication rate. The epilepsy associated with AVM is much improved by successful radiotherapy. The usefulness of radiosurgery for glomus tumours has been confirmed and new data published on the efficacy of the technique for haemangioblastoma, with new radiation therapy strategies designed for patients with von Hippel-Lindau disease. The acoustic neuroma treatment results have included improvements in hearing (a result not reported in the gamma unit literature), which are ascribed to the lower internal dose gradient within the target volume. Fractionation will, it is argued, also lead to sparing of the special sensory cochlear nerve. The risks of radiosurgery to the brainstem for chordoma of the mid-clivus are reduced by using a 'spacer' technique for the prepontine space. For meningiomas involving the cavernous sinus, conventionally fractionated radiotherapy is recommended when the meningeal base diameter exceeds 3.0 cm and radiosurgery (utilizing fractionation where appropriate) is advised for smaller lesions. Thus far, radiosurgery indications for pituitary adenomas have been restricted to recurrences after conventional radiotherapy, usually those in the cavernous sinus. In therapy for recurrent craniopharyngioma, it is argued that fractionation delivered via the relocatable frame will be important, particularly when the disease envelops the optic chiasma. For semicystic/semisolid craniopharyngiomas, the stereotactic delivery of colloidal yttrium-90 into a cystic element is useful, while stereotactic radiosurgery is delivered to the solid component. Staff at this centre consider that radiosurgery for low-grade gliomas, perhaps as boost therapy after conventional fractionation, is worthy of more research. We have been extremely selective in the use of radiosurgery for brain metastases (2% of patients, compared with about 30% in some Gamma Knife units), but future indications may become broader, probably using it as a booster technique after whole-brain conventionally-fractionated radiotherapy. Positron emission tomography scanning, co-registered with magnetic resonance imaging, allows the 'boost' concept in radiosurgery to become a sophisticated and accurate reality. Post-radiosurgical sequelae have been placed within a standard framework classification. New observations are being made with regard to subacute reactions: late-responding intrinsic and extra-axial tumours may swell in the subacute period, prior to shrinkage, and be attended by symptomatic surrounding brain oedema.     

Fractionated stereotactic radiation therapy for intracranial tumors

In stereotactic radio surgery, a single, large dose of radiation is delivered to a small, well-defined, stereotactically localized intracranial lesion. In contrast to conventional radiation therapy, in radio surgery no attempt is made to spare normal cells within the target volume by fractionating the tumor dose. In 1987, the authors began a program of fractionated stereotactic radiation therapy for selected tumors involving sensitive brain structures. Their objective was to improve the therapeutic index and study the feasibility of the fractionated technique. Fifteen patients were treated with a multifraction regimen typically consisting of six fractions of 700 cGy each, given on alternate days for 2 weeks (total tumor dose, 4200 cGy). All patients were treated with the dynamic stereotactic radio surgical technique. A head ring ("halo frame") was used for immobilization and setup during radiation treatments. At a median follow-up time of 27 months, the symptoms of the majority of the patients improved clinically; this improvement usually occurred within a few weeks after completion of the treatment. The radiologic response was much slower. Currently, only two patients have had complete radiologic disappearance of their lesions; the majority of the patients have only had a decrease in tumor size. The treatments were well tolerated by the patients and no acute complications were observed. One patient who had a vasogenic edema 11 months after treatment fully recovered after steroid therapy. Fractionated stereotactic radiation therapy is a feasible treatment technique and may prove to be useful for selected patients with intracranial tumors. Although the preliminary data are encouraging, this technique should still be considered experimental. A larger number of patients and a longer follow-up time are necessary to determine whether the results of this technique are actually better than those of conventional radiation therapy.