Stereotactic multiple arc radiotherapy

Importance of CBCT setup verification for optical-guided frameless radiosurgery

The purpose of this study is to quantify the discrepancy between optical guidance platform (OGP) frameless localization system (Varian) and Trilogy on-board imaging (OBI) system (Varian) for setting up phantom and stereotactic radiosurgery (SRS) patient; and to determine whether cone-beam CT (CBCT) is necessary for OGP patient setup, and compare CBCT and orthogonal kV-kV in term of their verification capability. Three different phantoms were used in the study: a custom-made phantom, a Penta-Guide phantom, and a RANDO phantom. Five patients using both OGP and CBCT setup and 14 patients using CBCT setup alone were analyzed. One patient who had big couch shifts discrepancy between OGP and CBCT was selected for further investigation. Same patient's CBCT and planning CT were fused. A RANDO phantom simulation experiment was performed using OGP setup with both CBCT and orthogonal kV-kV verification. For all of three phantom experiments, the shifts performed by CBCT beam and orthogonal kV-kV were all within 1 mm. Among five SRS patients using OGP setup, three had 3D couch corrections more than 3 mm. The image fusion of CBCT and planning CT clearly illustrated a tilt of bite-block in a patient's mouth. For 14 SRS patients using CBCT-guided setup, overall 3D correction was 3.3 ± 1.5 mm. RANDO phantom experiment demonstrated how a tilted bite-block caused isocenter shift. CBCT-calculated shifts are the same as expected, but kV-kV results differed by 1-2 mm if the initial head position is tilted. The bite-block tilting in patient's mouth is a major reason for the cause of positioning error for OGP frameless SRS setup. CBCT verification is necessary. CBCT provides more accurate couch corrections than orthogonal kV-kV when head was tilted. OGP is useful for detecting patient movement, but it does not necessarily imply that the isocenter has moved.

The reproducibility of a HeadFix relocatable fixation system: analysis using the stereotactic coordinates of bilateral incus and the top of the crista galli obtained from a serial CT scan

We analysed the repositioning accuracy of bite-plate fixations from serial QA-CT (quality-assurance CT) taken during the course of stereotactic radiotherapy. A total of 72 series of CT examinations from 15 consecutive patients, who underwent stereotactic radiotherapy for various intracranial tumours, were examined. Three or four CT scans were obtained for the purpose of QA for the right and left incus, as well as the crista galli. The stereotactic coordinates of the centres of the incus and the top of the crista galli were semi-automatically obtained for each QA-CT scan. Positional displacements for these anatomical reference points and the centre of the points were obtained. The mean displacements for these points in the 3D directions ranged from -0.10 to 0.08 mm (standard deviations: 0.44-0.94). The absolute positional displacement ranged from 0.93 to 1.09 mm (standard deviations: 0.52-0.88 mm). The rotations of the head were 0.49+/-0.36 degrees. Our 3D measurement technique using anatomical landmarks revealed excellent stability of the mouthpiece fixation system in terms of translational and rotational displacements. This technique can also be used as a QA method for other fixation devices.

Chemoradiotherapy for locally recurrent nasopharyngeal carcinoma: treatment outcome and prognostic factors

OBJECTIVE

To evaluate the treatment outcome of patients with locally recurrent nasopharyngeal carcinoma (NPC) treated with re-irradiation and chemotherapy.

METHODS

Between 1991 and 2004, 36 patients with locally recurrent NPC received re-irradiation and chemotherapy. The median re-irradiation dose was 37.9 Gy; the median total dose of prior irradiation and re-irradiation was 104.4 Gy. The outcome is studied retrospectively and also evaluated the prognostic factors and toxicities.

RESULTS

With a median follow-up of 40 months, 3-year overall survival (OS) was 58.3% and 3-year progression-free survival (PFS) was 25.0%. Patients aged <50 and of early stage at recurrence had a significantly better OS and PFS. Over Grade 3 of late toxicities were seen in patients received a total dose of >110 Gy.

CONCLUSIONS

Age and stage at recurrence were identified as prognostic factors for OS and PFS. Patients received external beam radiation therapy at a total dose of more than 110 Gy should be careful for severe late toxicities, and it is thought to be the optimal dose for recurrent tumor.

Stereotactic radiosurgery XVI: a treatment for previously irradiated pituitary adenomas

We report the use of stereotactic radiosurgery delivered through an adapted linear accelerator [stereotactic multiple arc radiation therapy (SMART)] for pituitary adenomas not cured by conventional therapy. All 21 patients had undergone conventional radiotherapy (45-50 Gy); 18 had also undergone prior surgery. This cohort comprised 13 patients with somatotrope adenomas, four with corticotrope adenomas, one with a lactotrope adenoma, and three with nonfunctioning pituitary adenomas (median follow-up: 33 months, range: 3-72 months). SMART has proven effective, safe, and rapidly acting. We observed an accelerated reduction in GH and IGF-I levels in acromegaly, with normalization of GH and IGF-I levels in 58%. Mean GH fell from 21.1 mU/liter to 7.9 mU/liter (7 ng/ml to 2.6 ng/ml, P < 0.01, median 25 months) faster than our predicted fall to 50% at 2 yr with conventional radiotherapy. Mean IGF-I fell from 624 ng/ml to 384 ng/ml (P < 0.001). Tumor growth was controlled in two of three nonfunctioning pituitary adenomas, and three of four corticotrope adenomas. There were no adverse effects from SMART. Notably there have been no visual sequelae or further loss of anterior pituitary function in this heavily pretreated group. Our data indicate that SMART is an effective complementary therapy for pituitary adenomas that have displayed a suboptimal response to conventional therapy including external irradiation.

CT verification of isocentre relocatability using stereotactic mask fixation system

AIMS

This study introduces a non-invasive method based on computed tomography (CT) verification to ensure patients are accurately positioned before fractionated stereotactic radiotherapy. It enables quality control of mask positioning with reference to the CT images of the treatment plan.

MATERIALS AND METHODS

A mask system, together with a dental impression moulded mouth bite, was used for patient immobilisation. In order to facilitate relevant image comparison, special alignment during CT localisation was discussed in the study. The accuracy of patient set-up was studied by assessing the isocentre position in relation to the patient's anatomical structure. The planning CT images were applied as a reference and the study was applied to 261 cranial applications.

RESULTS

The results show that the mean and the maximum overall displacements at the isocentre were 0.7 and 2.5 mm, respectively. The mean and the maximum rotational displacement in the axial plane were 0.56 degrees and 2 degrees, respectively. The mean translational displacement and rotational displacement were close to zero when considering the direction of movement.

CONCLUSIONS

The results indicate that the systematic error of the mask system and the verification method are minimal. Advantages of this technique include the simple set-up, three-dimensional quantification and short study time (10-15 min). It is therefore practical to implement on a routine basis. Investigation of the ability to relocate the mask is also recommended to justify the required safety margin between the clinical and planning target volumes.

Accuracy of a relocatable stereotactic radiotherapy head frame evaluated by use of a depth helmet

In high precision radiotherapy, the more accurately the patient can be relocated, the smaller the clinical to planning target volume margin can be, with reduction in the volume of normal tissue irradiated. The Gill-Thomas-Cosman (GTC) relocatable stereotactic head frame provides immobilization of the patient which is highly reproducible. A depth helmet and measuring probe were used to confirm the accuracy of relocation of 31 patients treated in the GTC frame. The measurements were processed in a spreadsheet developed to calculate the size of the patient's displacement as a vector. Twenty-seven patients received fractionated stereotactically-guided conformal radiotherapy, and 4 single fraction stereotactic radiosurgery, amounting to 564 measurement episodes. The accuracy was extremely good, and considerably more accurate than standard thermoplastic head shells. Ninety-two percent of the displacement vectors were less than 2 mm, and 97% less than 2.5 mm. Considering each dimension separately, the largest mean displacement was 0.4 mm in the superior-inferior direction. Accuracy was constant through a fractionated course for most patients, but prediction based on measurements from the first few fractions was not reliable. Results were dependent on patient selection, with worse reproducibility in patients with neurological deficits, or difficulty cooperating. The depth helmet measurements detected a loosened mouth bite in one patient and allowed repositioning to be verified without the need for the simulator. Total treatment time, including use of the depth helmet to verify treatment position, is quicker (mean 15.7 min) than using portal films. The depth helmet, used in conjunction with the vector displacement spreadsheet, provides a simple way to define the CTV-PTV margin. For fractionated stereotactic radiotherapy we use a 3 mm CTV-PTV margin. This system could assist technology transfer to centres starting stereotactic radiotherapy using the GTC frame.

Infrared patient positioning for stereotactic radiosurgery of extracranial tumors

We report on a novel, non-invasive patient positioning system for radiosurgery of extracranial tumors. The system consisted of infrared cameras and reflective markers attached to the skin. Because localization accuracy is critical in radiosurgery, we performed a theoretical analysis of the accuracy of the system. A computer simulation program modeled errors in marker position, and was used to predict errors in targeting and study methods for minimizing errors. The use of redundant markers improved the overall accuracy of targeting. Experimental data was collected using a rigid torso phantom and correlated with theoretical results. The accuracy of the infrared system was compared with existing systems.

Development of a relocatable frame technique for gamma knife radiosurgery

Object. One of the limiting factors in gamma knife radiosurgery is the restriction to one treatment imposed by the fixed stereotactic frame. The ability, in selected cases, to remove the frame and replace it on a subsequent occasion in the same location would facilitate fractionated treatments and provide flexibility in the timing of treatment delivery. It is the purpose of this work to investigate techniques for frame fixation and for essential verification of frame position once it has been reapplied.

Methods. A technique is proposed that requires four surgical self-tapping screws to be inserted into the skull. Aluminum pins are inserted through the frame pillars and are tightened against the head of the screws, providing a firm fixation of the frame. Pin lengths are recorded on gauges to ensure reproducibility of the position. In phantom tests, test objects were localized (using the angiographic localizer) before and after each of five frame removal/reapplications to test the reproducibility of frame position. The mean error in the observed target coordinates was 0.3 mm and the maximum error observed was 0.7 mm, indicating that the frame can be reapplied with some confidence.

Repetition of bubble skull measurements has been investigated as a means of verifying that the frame was repositioned correctly; however, reproducibility of patient measurements was found to be poor even when no frame movement had occurred. In contrast, the use of a radiotherapy simulator to obtain repeated lateral and anteroposterior projections of the head was shown to be capable of detecting frame movements of as little as 1 mm.

Conclusions. Using this technique of frame application facilitates the reapplication of the frame with an accuracy of plus or minus 0.7 mm. Bubble measurements are inadequate for the detection of frame movement. Simple techniques in which a radiotherapy simulator is used can verify correct frame placement and indicate frame movements of less than 1 mm.

Treatment planning algorithm corrections accounting for random setup uncertainties in fractionated stereotactic radiotherapy

A number of relocatable head fixation systems have become commercially available or developed in-house to perform fractionated stereotactic radiotherapy (SRT) treatment. The uncertainty usually quoted for the target repositioning in SRT is over 2 mm, more than twice that of stereotactic radiosurgery (SRS) systems. This setup uncertainty is usually accounted for at treatment planning by outlining extra target margins to form the planning target volume (PTV). It was, however, shown by Lo et al. [Int. J. Radiat. Oncol., Biol., Phys. 34, 1113-1119 (1996)] that these extra margins partly offset the radiobiological advantages of SRT. The present paper considers dose calculations in SRT and shows that the dose predictions could be made at least as accurate as in SRS with no extra margins required. It is shown that the dose distribution from SRT can be calculated using the same algorithms as in SRS, with the measured off-axis ratios (OARs) replaced by "effective" OARs. These are obtained by convolving the probability density distribution of the isocenter positions (assumed to be normal) and the original OARs. An additional output correction factor has also been introduced accounting for the isocenter dose reduction (2.4% for a 7 mm collimator) due to the OARs "blurring." Another correction factor accommodates for the reduced (by 1% for 6 MV beam) dose rate at the isocenter due to x-ray absorption in the relocatable mask. Mean dose profiles and the standard deviations of the dose (STD) were obtained through simulating SRT treatment by a combination of normally distributed isocenters. These dose distributions were compared with those calculated using the convolution approach. Agreement of the dose distributions was within 1%. Since standard deviation reduces with the number of fractions, N, as STD/square root(N), the planning predictions in fractionated stereotactic radiotherapy can be made more accurate than in SRS by increasing N and using "effective" OARs along with corrected dose output.

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.

Stereotactic radiosurgery, X: clinical isodosimetry of gamma knife versus linear accelerator X-knife for pituitary and acoustic tumours

Several review articles have compared gamma unit versus linear accelerator (linac)-based radiosurgery systems, concluding that the dose gradient 'fall-off' at the margin of the target (expressed as the distance between isodoses) is very similar for both techniques as far as single isocentre treatment volumes up to 1.5 cm diameter are concerned, and that the two radiosurgical systems are, in general, comparable. 'Fine tuning' of the gamma unit can be carried out by using multiple isocentre plans, the differential use of small collimator sizes (down to 4 mm) and field weightings, and adroit use of the gamma angle, and selective beam blocking. Multiple isocentre plans, beam modification, restriction of gantry angles and arc lengths, and microcollimation can similarly improve the isodose gradients from linac units. In both instances, the dosimetric advantages occur along selected aspects of the target perimeter border. However, the more frequent use of multiple isocentred 'shots' on the gamma unit achieves greater conformity indices for more complex target volumes, but at the expense of steeper internal dose gradients. We studied two patients with tumours close to or arising from radiosensitive special sensory nerves (optic and cochlear) to compare and contrast fine tuning of the two technologies. In a previously irradiated patient with a pituitary adenoma, the dose gradient achieved at the rostral margin, adjacent to the optic chiasma, was steeper on the gamma unit (due to the concentration of small collimator shots rostrally and beam blocking), which was therefore the dosimetrically preferred technique. In contrast, the vastly smaller internal dose gradient (11% for linac/X-knife versus 100% for Gamma Knife) and the ability to fractionate on the X-knife system, gave a large dosimetric advantage to the X-knife plan in the treatment of an acoustic neuroma, where the intracanalicular component of the cochlear nerve traversed the target volume. This advantage also pertains to the cochlear ramus of the internal auditory (labyrinthine) artery and the facial nerve. Our published work on X-knife radiosurgery of acoustic neuroma has documented improvement of hearing after therapy and may be relevant in this regard. That there are advantages in physical dose distribution and fractionation, producing a reduction in the biological dose in normal tissue, argues for the use of linac technology in acoustic neuromas. Craniopharyngiomas enveloping the optic nerve/chiasma will similarly be better treated by the linac X-knife system. It is apparent that different radiosurgery systems may be indicated in particular neuro-oncological situations.

Development of collimator insert for linac based stereotactic irradiation

The aim of this study is to develop collimator inserts of various sizes which are either not commercially available or are expensive to import. The dosimetry parameters such as tissue maximum ratio (TMR), off-axis ratio (OAR) and output factor of the developed collimator insert are compared with that of the commercial collimator insert (Radionics). In order to check the suitability of the collimator insert developed locally for clinical use and to standardize the method of development, a collimator insert of 15 mm identical to the one supplied by Radionics is developed with low-melting alloy (Cerrobend). Moreover for the clinical use of the developed collimator insert, certain acceptance tests are performed which include a collimator concentricity test, beam size check and radiation leakage test. The dose verification is carried out with a thermoluminescent dosimeter (7LiF rods) and an FBX chemical dosimeter in a human-head-shaped Perspex phantom filled with water. The variation between the calculated and measured dose is found to be within +2.4% for 7LiF rods and -2.0% for the FBX chemical dosimeter thus ensuring the suitability of the developed collimator insert for clinical use. This has encouraged us to standardize the method adapted to develop the collimator insert and to develop collimator inserts of different field sizes.

Factors affecting penumbral shape and 3D dose distributions in stereotactic radiotherapy

Linear accelerator based stereotactic radiotherapy or radiosurgery is usually performed with small collimated circular beams. The penumbral shape of these beams is caused by geometric penumbra and radiation penumbra; the former is affected by collimator position and focal spot size, the latter by energy and field size. This note reports measurements of penumbral width as collimator position, size and energy are varied. Radiation penumbra is a much greater effect than geometric penumbra at the distances used. The profile can also be changed by adding flattening filters to the external collimator. Adding a shaped filter to increase the primary radiation at the edges of the beam improves the shape of the profiles. What is of clinical importance is not the penumbral shape itself, but the effect it has on the three-dimensional dosage distributions. Cumulative dose-volume histograms (DVHS) have been used to compare treatment plans produced by beams with different penumbral shapes. None of the aforementioned factors changes the DVH as much as a change in the field radius of a few mm. Ensuring that sufficient collimators are made, and reducing the interval between successive field sizes, is the single most important thing that can be done to avoid irradiating unnecessarily large volumes of tissue.

Dose characteristics of in-house-built collimators for stereotactic radiotherapy with a linear accelerator

Dose characteristics of a stereotactic radiotherapy unit based on a standard Varian Clinac 4/100 4 MV linear accelerator, in-house-built Lipowitz collimators and the SMART stereotactic radiotherapy treatment planning software have been determined. Beam collimation is constituted from the standard collimators of the linear accelerator and a tertiary collimation consisting of a replaceable divergent Lipowitz collimator. Four collimators with isocentre diameters of 15, 25, 35 and 45 mm, respectively, were constructed. Beam characteristics were measured in air, acrylic or water with ionization chamber, photon diode, electron diode, diamond detector and film. Monte Carlo simulation was also applied. The radiation leakage under the collimators was less than 1% at 50 mm depth in water. Specific beam characteristics for each collimator were imported to SMART and dose planning with five non-coplanar converging 140 degrees arcs separated by 36 degrees angles was performed for treatment of a RANDO phantom. Dose verification was made with TLD and radiochromic film. The in-house-built collimators were found to be suitable for stereotactic radiotherapy and patient treatments with this system are in progress.

The University of Florida frameless high-precision stereotactic radiotherapy system

PURPOSE

To develop and test a system for high precision fractionated stereotactic radiotherapy that separates immobilization and localization devices.

METHODS AND MATERIALS

Patient localization is achieved through detection and digital registration of an independent bite plate system. The bite plate is made and linked to a set of six infrared light emitting diodes (IRLEDs). These IRLEDs are detected by an infrared camera system that identifies the position of each IRLED within 0.1 to 0.15 mm. Calibration of the camera system defines isocenter and translational X, Y, and Z axes of the stereotactic radiosurgery subsystem and thereby digitally defines the virtual treatment room space in a computer linked to the camera system. Positions of the bite plate's IRLEDs are processed digitally using a computer algorithm so that positional differences between an actual bite plate position and a desired position can be resolved within 0.1 mm of translation (X, Y, and Z distance) and 0.1 degree of rotation. Furthermore, bite plate misalignment can be displayed digitally in real time with translational (x, y, and z) and rotational (roll, pitch, and yaw) parameters for an actual bite plate position. Immobilization is achieved by a custom head mold and thermal plastic mask linked by hook-and-loop fastener tape. The head holder system permits rotational and translational movements for daily treatment positioning based on the bite plate localization system. Initial testing of the localization system was performed on 20 patients treated with radiosurgery. The system was used to treat 11 patients with fractionated stereotactic radiotherapy.

RESULTS

Assessment of bite plate localization in radiosurgery patients revealed that the patient's bite plate could be positioned and repositioned within 0.5 +/- 0.3 mm (standard deviation). After adjustments, the first 11 patients were treated with the bite plate repositioning error reduced to 0.2 +/- 0.1 mm.

CONCLUSIONS

High precision stereotactic radiotherapy can be delivered using separate localization and immobilization systems. Treatment setup and delivery can be accomplished in 15 min or less. Advantages compared with standard systems require further study.

Characteristics of a dedicated linear accelerator-based stereotactic radiosurgery-radiotherapy unit

A stereotactic radiosurgery and radiotherapy (SRS/SRT) system on a dedicated Varian Clinac-600SR linear accelerator with Brown-Roberts-Wells and Gill-Thomas-Cosman relocatable frames along with the Radionics (RSA) planning system is evaluated. The Clinac-600SR has a single 6-MV beam with the same beam characteristics as that of the mother unit, the Clinac-600C. The primary collimator is a fixed cone projecting to a 10-cm diameter at isocenter. The secondary collimator is a heavily shielded cylindrical collimator attached to the face plate of the primary collimator. The tertiary collimation consists of the actual treatment cones. The cone sizes vary from 12.5 to 40.0 mm diameter. The mechanical stability of the entire system was verified. The variations in isocenter position with table, gantry, and collimator rotation were found to be < 0.5 mm with a compounded accuracy of < or = 1.0 mm. The radiation leakage under the cones was < 1% measured at a depth of 5 cm in a phantom. The beam profiles of all cones in the x and y directions were within +/- 0.5 mm and match with the physical size of the cone. The dosimetric data such as tissue maximum ratio, off-axis ratio, and cone factor were taken using film, diamond detector, and ion chambers. The mechanical and dosimetric characteristics including dose linearity of this unit are presented and found to be suitable for SRS/SRT. The difficulty in absolute dose measurement for small cone is discussed.

Treatment planning optimization for multiple arcs stereotactic radiosurgery using a linear accelerator

PURPOSE

Multiarc stereotactic radiosurgery is a technique used to irradiate an intracranial tumor with minimal damage to the surrounding normal tissue. The purpose of this paper is to present a method for and the results from optimizing three dimensional (3D) treatment dose for multiarc stereotactic radiosurgery.

METHODS AND MATERIALS

The normal procedure for a physician-physicist team designing a treatment plan for multiarc stereotactic radiosurgery is the trial-and-error approach of changing the collimator size and the isocenter of radiation by viewing the isodose curves on a two dimensional (2D) computed tomography (CT) or magnetic resonance imaging (MRI) image plane. Not only is this time consuming, but the resulting treatment plan is not optimal in most, if not all, cases. One reason for such nonconformal isodose curves is that the same collimator size is used for all arcs. However, it is very difficult to determine manually the different collimator sizes for different arcs. A derivative free optimization method is used to optimize the collimator size for each arc, as well as the 3D coordinates of the isocenter(s).

RESULTS

One spherical and two ellipsoidal artificial tumors, and one actual tumor, were used to show the utilities of the optimization process. The 90% isodose curves resulting from optimization conform very well with the tumor; whereas the 90% isodose curves from the conventional method either do not envelop the entire tumor when the collimator size is too small, or a large volume of normal tissue is also irradiated by the 90% dose when the next larger collimator size is used.

CONCLUSIONS

When the collimator size for each arc and the location of the isocenters(s) are optimized in a multiarc stereotactic surgery treatment plan, the 90% isodose curve conforms to the tumor much better than when the same collimator size is used for all arcs.

The possible role of linac-based stereotactic radiotherapy in the treatment of multifocally and heterochronously recurrent malignant astrocytomas. A case report

We present a 24-year-old woman with multifocal and heterochronous recurrence of malignant astrocytomas that were consecutively treated by linac-based sterotactic radiotherapy. The patient had previously received multimodalities of treatment for malignant astrocytomas in the right occipital lobe consisting of surgical resection on four occasions, conventional external-beam irradiation, immunological therapy, and systemic chemotherapy. Thereafter she developed relatively small and well-demarcated recurrent lesions in eight different sites, most of which were located in eloquent and deep-seated regions. She underwent ten fractionated linac-based stereotactic radiotherapy courses with a median total dose of 42 Gy, seven fractions of 6 Gy each in 15 days, experiencing neither any adverse effects nor neurologic deterioration. Three out of 10 treated lesions responded well with subsequent reduction in size, and 80% of them did not increase at least for 6 months, although 66.7% of the lesions eventually exhibited enlargement of the enhanced areas, some of which were presumed as radiation necrosis. These clinical results suggest that the fractionated linac-based stereotactic radiotherapy had significant effects on tumor growth inhibition and attainment of acceptable patient performance status.

Quality assurance in fractionated stereotactic radiotherapy

The recent development of fractionated stereotactic radiotherapy (SRT), which utilises the relocatable Gill-Thomas-Cosman frame (GTC 'repeat localiser'), requires comprehensive quality assurance (QA). This paper focuses on those QA procedures particularly relevant to fractionated SRT treatments, and which have been derived from the technique used at the Royal Marsden Hospital. They primarily relate to the following: (i) GTC frame fitting, initially in the mould room, and then at each imaging session and treatment fraction; (ii) checking of the linear accelerator beam geometry and alignment lasers; and (iii) setting up of the patient for each fraction of treatment. The precision of the fractionated technique therefore depends on monitoring the GTC frame relocation at each fitting, checking the accuracy of the radiation isocentre of the treatment unit, its coincidence with the patient alignment lasers and the adjustments required to set the patient up accurately. The results of our quality control checks show that setting up to a mean radiation isocentre using precisely set-up alignment lasers can be achievable to within 1 mm accuracy. When this is combined with a mean GTC frame relocatability of 1 mm on the patient, a 2-mm allowance between the prescribed isodose surface and the defined target volume is a realistic safety margin for this technique.

Radiosurgery/stereotactic external beam radiotherapy for malignant brain tumours: the Royal Marsden Hospital experience

SRT is a high-precision technique of radiotherapy which delivers focused irradiation to small target volumes. In the context of external beam radiotherapy it can be described as stereotactically guided conformal radiotherapy. As the technique originated from neurosurgical technology, it has initially been limited to single fraction treatment. However, with the use of relocatable fixation devices the way ahead particularly in its application in the treatment of brain tumours is in fractionated SRT. Currently, single fraction SRT/radiosurgery is of proven value only in the treatment of small inoperable arteriovenous malformations. It is being exploited in the management of brain tumours but so far remains as experimental treatment. We have demonstrated that fractionated SRT in patients with gliomas is a non-invasive equivalent to brachytherapy and in patients with solitary metastases a non-invasive alternative to surgical excision. However, the treatment is not without side effects, and the long-term effectiveness and toxicity of SRT, particularly with the use of unconventional fractionation, is not defined. The future use of SRT in the treatment of brain tumours should not be guided simply by the technical possibilities but by a rational appraisal of all treatment options to achieve the best disease control, survival and toxicity. Although there is potential for benefit in a number of small tumours, SRT cannot at present be recommended as the primary treatment in any tumour. In addition, its use should be discouraged in the treatment of unbiopsied brain lesions and as the major form of treatment of pineal germinomas. The technology of stereotactic radiotherapy is evolving, and it is likely that SRT will be integrated into conventional radiotherapy practice to become simply a high-precision technique of radiotherapy delivery in everyday use.

A frameless method for stereotactic radiotherapy

A frameless method for stereotactic multiple arc radiotherapy (SMART) is described. Three short gold wires are implanted in the scalp approximately 100 mm apart. These are localized in a computed tomographic or angiographic study along with the target. Subsequently the gold markers are localized on beam films and the target position calculated using a computer program ISOLOC. This program provides the couch movements required to move the target to the isocentre and a micropositioner attached to the couch is used to make the adjustment. Beam films are repeated until the movements required are less than 1 mm in any direction. It is shown that the simple procedures of implanting the markers subcutaneously do not provide a stable reference system in about 25% of patients and the markers are now screwed into the cranium. The precision of the method is evaluated by phantom studies and measurements taken during several hundred treatments.

Stereotactic radiotherapy of irregular targets: a comparison between static conformal beams and non-coplanar arcs

Stereotactic radiotherapy using a linear accelerator is usually equated with the technique of delivery using multiple non-coplanar arcs, which achieves a spherical dose distribution. As the majority of intracranial lesions are not spherical, a range of schematized tumour shapes were planned to assess the role of static conformal beams in the treatment of irregular lesions. A sphere and 2 ellipsoids, ranging from 20 to 50 mm maximum diameter located intracranially were planned using 3, 4, and 6 non-coplanar static beams with conformal blocks and were compared with four 120 degree non-coplanar arcs. Comparison of the plans was made by the relative sparing of normal tissue outside the target volume using three-dimensional dose-volume distributions. Non-coplanar arcs spared more normal tissue at low isodoses and achieved the best high dose sparing for spherical targets. For the majority of irregular targets, 3 and 4 static beams spared more tissue at doses > or = 50% and > or = 80% than the arc technique. For all irregular volumes, maximum sparing of normal tissue to isodoses > or = 50% and > or = 80% of the treatment isodose was obtained with 6 static conformal beams. We conclude that irregularly shaped tumours suitable for stereotactic radiotherapy with a linear accelerator are better treated with conformal static non-coplanar beams rather than with the multiple arc technique.

Linac-based small-field radiotherapy for brain tumors

Small-field radiotherapy based on a 6-MeV linac and a conventional head mold is investigated as an alternative to radiosurgery with stereotactic frames. The system requires no additional device and allows fractionated treatment. The dose distributions obtained are comparable to those reported with a Gamma Unit. Overall positioning errors are within 2 mm. Using this approach, seven patients with brain tumors who could not have been treated otherwise, underwent fractionated radiotherapy with total accumulated doses ranging from 70 to 108 Gy. The treatment was tolerated well with no acute toxicity or adverse effect encountered during the follow-up period of 8-14 months. All of the patients remained free from disease progression in the treated volumes. Although the follow-up is brief, the preliminary results suggest that this is a simple and inexpensive but effective system for the treatment of small intracranial malignancies.