Stereotactic radiosurgery with linear accelerator

Stereotactic radiosurgery is a method that applies a radiation dose to a limited and well-defined volume while the irradiation of adjacent healthy tissues is minimized. It is most commonly used in the treatment of intracranial lesions because the skull hardness assures the stable location of its contents. Treatment of the rest of the body has recently been proposed and carried out, using original immobilization systems. Stereotactic radiosurgery was first described in 1951 by the Swedish neurosurgeon Lars Leksell who originally used X-rays and then high-energy protons as a source of radiation. In the '80s photons from linear accelerators were used as radiation source, with various stereotactic systems and computerized treatment planning. The method used with all radiosurgical systems, regardless of the source of irradiation, is similar. The lesion is detected with common diagnostic imaging and adequate location frames. At present, to prevent errors in location, MRI and CT data are matched using an Image Fusion computer program. The objective of stereotactic radiosurgery is to destroy tumor cells or to induce changes in tissues that, as in brain arteriovenous malformations lead to the occlusion of their abnormal vessels. Stereotactic radiosurgery is increasingly used today in the treatment of a variety of intracranial lesions to the patients' benefit.     

New system for linear accelerator radiosurgery with a gantry-mounted video camera

PURPOSE: We developed a positioning method that does not depend on the positioning mechanism originally annexed to the linac and investigated the positioning errors of the system. METHODS AND MATERIALS: A small video camera was placed at a location optically identical to the linac x-ray source. A target pointer comprising a convex lens and bull's eye was attached to the arc of the Leksell stereotactic system so that the lens would form a virtual image of the bull's eye (virtual target) at the position of the center of the arc. The linac gantry and target pointer were placed at the side and top to adjust the arc center to the isocenter by referring the virtual target. Coincidence of the target and the isocenter could be confirmed in any combination of the couch and gantry rotation. In order to evaluate the accuracy of the positioning, a tungsten ball was attached to the stereotactic frame as a simulated target, which was repeatedly localized and repositioned to estimate the magnitude of the error. The center of the circular field defined by the collimator was marked on the film. RESULTS: The differences between the marked centers of the circular field and the centers of the shadow of the simulated target were less than 0.3 mm.     

Treatment of hemangioblastomas in von Hippel-Lindau disease with linear accelerator-based radiosurgery

OBJECTIVE: Stereotactic radiosurgery is increasingly being used to treat hemangioblastomas, particularly those that are in surgically inaccessible locations or that are multiple, as is common in von Hippel-Lindau disease. The purpose of this study was to retrospectively evaluate the effectiveness of radiosurgery in the treatment of hemangioblastomas. METHODS: From 1989 to 1996, 29 hemangioblastomas in 13 patients with von Hippel-Lindau disease were treated with linear accelerator-based radiosurgery. The mean patient age was 40 years (range, 31-57 yr). The radiation dose to the tumor periphery averaged 23.2 Gy (range, 18-40 Gy). The mean tumor volume was 1.6 cm3 (range, 0.07-65.4 cm3). Tumor response was evaluated in serial, contrast-enhanced, computed tomographic and magnetic resonance imaging scans. The mean follow-up period was 43 months (range, 11-84 mo). RESULTS: Only one (3%) of the treated hemangioblastomas progressed. Five tumors (17%) disappeared, 16 (55%) regressed, and 7 (24%) remained unchanged in size. Five of nine patients with symptoms referable to treated hemangioblastomas experienced symptomatic improvement. During the follow-up period, one patient died as a result of progression of untreated hemangioblastomas in the cervical spine. Three patients developed radiation necrosis, two of whom were symptomatic. CONCLUSION: Although follow-up monitoring is limited, stereotactic radiosurgery provides a high likelihood of local control of hemangioblastomas and is an attractive alternative to multiple surgical procedures for patients with von Hippel-Lindau disease.     

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.     

In-phantom dosimetry and spectrometry of photoneutrons from an 18 MV linear accelerator

A combination of three superheated drop detectors with different neutron energy responses was developed to evaluate dose-equivalent and energy distributions of photoneutrons in a phantom irradiated by radiotherapy high-energy x-ray beams. One of the three detectors measures the total neutron dose equivalent and the other two measure the contributions from fast neutrons above 1 and 5.5 MeV, respectively. In order to test the new method, the neutron field produced by the 10 cm X 10 cm x-ray beam of an 18 MV radiotherapy accelerator was studied. Measurements were performed inside a tissue-equivalent liquid phantom, at depths of 1, 5, 10 and 15 cm and at lateral distances of 0, 10, and 20 cm from the central axis. These data were used to calculate the average integral dose to the radiotherapy patient from direct neutrons as well as from neutrons transmitted through the accelerator head. The characteristics of the dosimeters were confirmed by results in excellent agreement with those of prior studies. Track etch detectors were also used and provided an independent verification of the validity of this new technique. Within the primary beam, we measured a neutron entrance dose equivalent of 4.5 mSv per Gy of photons. It was observed that fast neutrons above 1 MeV deliver most of the total neutron dose along the beam axis. Their relative contribution increases with depth, from about 60% at the entrance to over 90% at a depth of 10 cm. Thus, the average energy increases with depth in the phantom as neutron spectra harden.     

Radiosurgery of cerebral arteriovenous malformations with a linear accelerator: preliminary report

The authors present the results obtained in seven patients with arteriovenous malformations treated with radiosurgery on a linear accelerator. In four patients obliteration of the malformation was obtained after one year. Two of the patients had a transient period of acute brain oedema surrounding the radionecrotic nidus with neurological signs. The present position of radiosurgery in the treatment of arteriovenous malformations is discussed.     

Dosimetric aspects of physical and dynamic wedge of Clinac 2100C linear accelerator

AIM: To investigate variation of wedge factors on field size and depth for physical and dynamic wedges of identical wedge angles for Clinac 2100C linear accelerator and its clinical implementation. MATERIAL AND METHODS: A computer controlled water phantom dosimetric system is used to generate profile data for physical wedges, whereas a 0.6 cm3 ion chamber is used for generation of profiles for dynamic wedge and wedge factors for both types of wedges. The method has been discussed to handle the dynamic wedge dosimetry in absence of linear array of detectors or film densitometer. RESULTS: A systematic dependence on wedge factor is observed for physical wedge, with respect to depth and wedge angle but not depending on field size. Whereas dynamic wedge shows strong dependence on field size and is not systematic because the dynamic wedge is controlled by segmented treatment tables depending on field size and energy and no significant variation is observed on depth for various wedge angles. The handling of beam data in a commercially available treatment planning system is discussed and a comparison has been made for iso-doses of both types of wedges. CONCLUSION: The dynamic wedge isodose curves shows rather straight lines than physical wedge but larger hot spots at thin edge which needs careful consideration during planning.     

Technical note: thyratron performance--dependence on mounting in a medical linear accelerator

A thyratron is an important component of a medical linear accelerator. The working life of a thyratron has been found to be dependent on its physical orientation in the accelerator. This observation is a result of maintaining accelerators for over 15 years.     

Monte Carlo calculations of electron beam output factors for a medical linear accelerator

The purpose of this study was to investigate the application of the Monte Carlo technique to the calculation and analysis of output factors for electron beams used in radiotherapy. The code EGS4/BEAM was used to obtain phase-space files for 6, 12 and 20 MeV clinical electron beams from a scattering-foil linac (Varian Clinac 2100C) for a clinically representative range of applicator and square or rectangular insert combinations. The source-to-surface distance used was 100 cm. The field sizes ranged from 1 x 1 cm2 to 20 x 20 cm2. These phase-space files were analysed to study the intrinsic beam characteristics and used as source input for relative dose and output factor computations in homogeneous water phantoms using the code EGS4/DOSXYZ. The calculated relative central-axis depth-dose and transverse dose profiles at various depths of clinical interest agreed with the corresponding measured dose profiles to within 2% of the maximum dose. Calculated output factors for the fields studied agreed with measured output factors to about 2%. This demonstrated that for the Varian Clinac 2100C linear accelerator, electron beam dose calculations in homogeneous water phantoms can be performed accurately at the 2% level using Monte Carlo simulations.     

The costs of radiosurgical treatment: comparison between gamma knife and linear accelerator

Radiosurgical treatment can be carried out by means of a Gamma Knife or a Linear Accelerator. The Linear Accelerator may be either a single-purpose appliance, exclusively employed in radiosurgery, or an adapted appliance, which is used primarily for fractioned radiotherapy, and only additionally for radiosurgical purposes. The first alternative will be referred to briefly as a "dedicated Linac", the latter as an "adapted Linac". Cost accounting data for these alternatives will be discussed under three main categories: investment costs, operating costs, and finally staffing costs. Costs are only considered to the extent that this is necessary to facilitate a comprehensive cost comparison. Factors for which the costs remain the same or at least broadly the same will from the outset not be taken into consideration. These include, for instance, the costs of general or special administration, diagnosis, and image processing. The results and conclusions of this study therefore cannot be employed immediately in the evaluation of cost reimbursement schemes of the type carried out by agencies responsible for social insurance. Here, appropriate complete cost analyses especially for this purpose are required. The final comprehensive cost comparison reveals that the adapted Linac is the most favourable alternative with small annual quantities of patients. With larger numbers of patients the Gamma Knife represents the most favourable from a cost accounting angle. The dedicated Linac accordingly does not have a cost advantage for any of the examined numbers of patients. Clearly the lowest treatment costs per patient can be achieved by employing a Gamma Knife and using it to capacity.     

Risk assessment of dithiocarbamate accelerator residues in latex-based medical devices: genotoxicity considerations

The Medical Devices Agency (MDA) has investigated potential human health hazards arising from the presence of dithiocarbamate vulcanization accelerators in latex products (mainly gloves). After collection of manufacturer's data on usage and residues of these accelerators, an independent investigation of solvent extractable residues and dithiocarbamate migration into aqueous simulants was commissioned, to complement equivalent "in-house" test data from two major manufacturers. The presence of extractable accelerator residues in commercial products was confirmed. Potential human health hazards associated with dithiocarbamates include genotoxicity and possible carcinogenicity: a review of published data was conducted to evaluate the evidence for this, with particular reference to three zinc dithiocarbamates with significant commercial usage (ZDMC, ZDEC and ZDBC: see Fig. 1). Data gaps were identified, and mutagenicity studies commissioned to fill these. These studies comprised tests both in vitro (bacterial and L5178Y cell gene mutation, cultured lymphocyte chromosome aberration) and in vivo (mouse bone marrow micronucleus, rat liver UDS). It is concluded that ZDMC must be considered a genotoxin (and thus a probable carcinogen): residues of this substance in latex medical devices should be minimized. ZDEC proved genotoxic in vitro but was not clearly genotoxic in vivo, and may have activity intermediate between that of ZDMC and that of ZDBC, which showed at most weak activity in a single in vitro (chromosome aberration) test. It is proposed that the use of ZDBC as a vulcanization accelerator in the manufacture of latex gloves, rather than ZDEC, ZDMC or their precursors, would reduce or remove the health concerns arising from accelerator residues.     

Functional radiosurgery

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

Dosimetric parameters of a modified set of wedges for use with asymmetric fields of a 6 MV linear accelerator

A set of standard wedge filters has been modified for use with half-collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half-collimated field coverage (up to 20 x 30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off-axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.