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PubMed1990

[Experience in the use of subtotal irradiation at different stages of therapy in lymphogranulomatosis].

A study was made of a possibility to use subtotal irradiation (STI) as an equivalent of chemotherapy for the treatment of 33 Hodgkin's disease patients aged 17 to 77 (of them 25 were treated during the 1st-19th yrs. of therapy, for 8 patients it was the 1st stage of antitumor therapy). ROKUS apparatus and a linear accelerator of 15 MeV were used for irradiation at a single dose of 1.5 Gy and a total dose of 3-6 Gy, at a 2-4 day interval. STI efficacy was assessed by a response of the peripheral and intrathoracic lymph nodes, tumor foci in the lungs, by a decrease in liver and spleen sizes, a decrease in body temperature and the end of night sweating. Of 25 patients a positive effect was achieved in 19, in 4 patients it was absent, and in 2 patients disease progression was observed. A marked positive effect was noted in 8 previously untreated patients, especially with respect to the elimination of signs of intoxication. A conclusion has been made that STI can be employed for the treatment of Hodgkin's disease patients as a method of general cytostatic action. STI causes a more noticeable and prolonged leukopenia than a cycle of polychemotherapy.

Adolescent↗

PubMed2006

Development of a four-dimensional image-guided radiotherapy system with a gimbaled X-ray head.

PURPOSE: To develop and evaluate a new four-dimensional image-guided radiotherapy system, which enables precise setup, real-time tumor tracking, and pursuit irradiation. METHODS AND MATERIALS: The system has an innovative gimbaled X-ray head that enables small-angle (+/-2.4 degrees ) rotations (pan and tilt) along the two orthogonal gimbals. This design provides for both accurate beam positioning at the isocenter by actively compensating for mechanical distortion and quick pursuit of the target. The X-ray head is composed of an ultralight C-band linear accelerator and a multileaf collimator. The gimbaled X-ray head is mounted on a rigid O-ring structure with an on-board imaging subsystem composed of two sets of kilovoltage X-ray tubes and flat panel detectors, which provides a pair of radiographs, cone beam computed tomography images useful for image guided setup, and real-time fluoroscopic monitoring for pursuit irradiation. RESULTS: The root mean square accuracy of the static beam positioning was 0.1 mm for 360 degrees of O-ring rotation. The dynamic beam response and positioning accuracy was +/-0.6 mm for a 0.75 Hz, 40-mm stroke and +/-0.4 mm for a 2.0 Hz, 8-mm stroke. The quality of the images was encouraging for using the tomography-based setup. Fluoroscopic images were sufficient for monitoring and tracking lung tumors. CONCLUSIONS: Key functions and capabilities of our new system are very promising for precise image-guided setup and for tracking and pursuit irradiation of a moving target.

Equipment Design↗

PubMed1998

Electron wedges for radiation therapy.

PURPOSE: Brain tumors can be advantageously treated with electron over photon radiation, by exploiting the rapid fall-off in dose with depth. This advantage could be further enhanced by utilizing multiple electron beams. However, in some beam configurations, wedged dose profiles would be necessary for the dose uniformity. Unlike photons, shaped pieces of material placed in electron beam severely degrade the energy, give additional scattering and, therefore, are suboptimal. The purpose of this study was to create wedged electron fields, using intensity modulation. The combination of electron wedges enables a more uniform coverage of brain tumors with a reduced dose to normal tissue. METHODS AND MATERIALS: Intensity modulation was performed for 10 to 50 MeV electrons using a narrow scanning elementary beam of a racetrack Microtron accelerator, delivering radiation pulses with coordinates and intensities prescribed by a custom scan matrix. Dispensing more pulses (or longer pulses) within the field to increase the local dose, one can sharpen the penumbra at depth and generate wedged dose distributions of arbitrary angle as well as many other desired profiles. We modulated the electron beams, measured dose distributions using film in an anthropomorphic phantom, and compared the results with conventional techniques. RESULTS: Intensity modulation of electron beams decreases the 50-90% penumbra at depth by 40% and increases the flatness by 80%. Wedged profiles at depth can be created for any angle up to about 70 degrees, depending on the beam energy. Multiple modulated electron beams give smaller 20-70% but larger 70-100% isodose regions than photon beams. CONCLUSIONS: Electron beams can improve dose distributions in brain compared to the same number of photon beams, reducing the 20-70% isodoses region in normal tissue by 30%. Intensity modulation significantly improves the dose distribution from combined electron beams providing a sharper penumbra, better conformity, and reduced margin.

Brain Neoplasms↗

PubMed2002

Tumour dose enhancement using modified megavoltage photon beams and contrast media.

This study examines the magnitude of tumour dose enhancement achieved by injection of gadolinium or iodine contrast media (CM) and treatment using modified x-ray photon spectra from linear accelerators. Monte Carlo modelling of the linear accelerator and patient geometry was used to explore the effect of removing the flattening filter for various beam qualities and the resultant effect on dose enhancement. In addition, ionization measurements were conducted to observe dose enhancement within a phantom containing CM. Simulation results indicate that for flattened 6-24 MV photon beams and realistic CM tumour concentrations, the dose enhancement remains below 5%. However, if the flattening filter is removed, dose enhancement is increased significantly. For a 30 mg ml(-1) gadolinium CM tumour concentration, for example, 8.4%, 10.8%, 13.7% and 23.1% dose enhancements are achieved for 18 MV, 6 MV, 4 MV and 2 MV unflattened beams, respectively. In contrast to the phototherapy technique, which uses the orthovoltage beam from a modified CT scanner to achieve dose enhancement, all unflattened spectra preserve the dose build-up at the surface, and thus the skin and bone are spared.

Chemotherapy, Adjuvant↗

PubMed2003

Using Monte Carlo simulations to commission photon beam output factors--a feasibility study.

This study investigates the feasibility of using Monte Carlo methods to assist the commissioning of photon beam output factors from a medical accelerator. The Monte Carlo code, BEAMnrc, was used to model 6 MV and 18 MV photon beams from a Varian linear accelerator. When excellent agreements were obtained between the Monte Carlo simulated and measured dose distributions in a water phantom, the entire geometry including the accelerator head and the water phantom was simulated to calculate the relative output factors. Simulated output factors were compared with measured data, which consist of a typical commission dataset for the output factors. The measurements were done using an ionization chamber in a water phantom at a depth of 10 cm with a source-detector distance of 100 cm. Square fields and rectangular fields with widths and lengths ranging from 4 cm to 40 cm were studied. The result shows a very good agreement (< 1.5%) between the Monte Carlo calculated and the measured relative output factors for a typical commissioning dataset. The Monte Carlo calculated backscatter factors to the beam monitor chamber agree well with measured data in the literature. Monte Carlo simulations have also been shown to be able to accurately predict the collimator exchange effect and its component for rectangular fields. The information obtained is also useful to develop an algorithm for accurate beam modelling. This investigation indicates that Monte Carlo methods can be used to assist commissioning of output factors for photon beams.

Calibration↗

PubMed1989

Fast neutron therapy clinical trials in the United States.

The National Cancer Institute (NCI) began supporting clinical fast neutron clinical studies in the United States beginning in the early 1970s using physics-based cyclotrons and linear accelerators at a number of locations and facilities. The early work pointed out the handicaps imposed by the limitations of horizontal beams and low energy neutrons. This, combined with some encouraging, clinical results using neutrons and photons in a mixed mode of therapy, prompted the NCI in 1979 to initiate a 10-year contract program to design, develop, and build hospital-based neutron therapy machines and to conduct phase III clinical trials. As we approach the end of the 10-year effort, three hospital-based neutron facilities are currently operational-at the University of Washington, Seattle; University of California at Los Angeles: and M. D. Anderson Hospital at University of Texas System Cancer Center, Houston. Phase III trials are in progress in four sites: head and neck, prostate, and lung tumors, and cancers of radioresistant histologies, such as melanoma, renal cell carcinoma, and sarcomas of the soft tissue and bone. The contractors will continue to receive limited NCI support to complete the clinical studies.

Clinical Trials as Topic↗

PubMed2006

Direct aperture optimization for IMRT using Monte Carlo generated beamlets.

This work introduces an EGSnrc-based Monte Carlo (MC) beamlet does distribution matrix into a direct aperture optimization (DAO) algorithm for IMRT inverse planning. The technique is referred to as Monte Carlo-direct aperture optimization (MC-DAO). The goal is to assess if the combination of accurate Monte Carlo tissue inhomogeneity modeling and DAO inverse planning will improve the dose accuracy and treatment efficiency for treatment planning. Several authors have shown that the presence of small fields and/or inhomogeneous materials in IMRT treatment fields can cause dose calculation errors for algorithms that are unable to accurately model electronic disequilibrium. This issue may also affect the IMRT optimization process because the dose calculation algorithm may not properly model difficult geometries such as targets close to low-density regions (lung, air etc.). A clinical linear accelerator head is simulated using BEAMnrc (NRC, Canada). A novel in-house algorithm subdivides the resulting phase space into 2.5 X 5.0 mm2 beamlets. Each beamlet is projected onto a patient-specific phantom. The beamlet dose contribution to each voxel in a structure-of-interest is calculated using DOSXYZnrc. The multileaf collimator (MLC) leaf positions are linked to the location of the beamlet does distributions. The MLC shapes are optimized using direct aperture optimization (DAO). A final Monte Carlo calculation with MLC modeling is used to compute the final dose distribution. Monte Carlo simulation can generate accurate beamlet dose distributions for traditionally difficult-to-calculate geometries, particularly for small fields crossing regions of tissue inhomogeneity. The introduction of DAO results in an additional improvement by increasing the treatment delivery efficiency. For the examples presented in this paper the reduction in the total number of monitor units to deliver is approximately 33% compared to fluence-based optimization methods.

Algorithms↗

PubMed2001

Commissioning of a mobile electron accelerator for intraoperative radiotherapy.

Radiation performance characteristics of a dedicated intraoperative accelerator were determined to prepare the unit for clinical use. The linear accelerator uses standing wave X-band technology (wavelength approximately 3 centimeters) in order to minimize the mass of the accelerator. The injector design, smaller accelerator components, and low electron beam currents minimize radiation leakage. The unit may be used in a standard operating room without additional shielding. The mass of the accelerator gantry is 1250 Kg (weight approximately 2750 lbs) and the unit is transportable between operating rooms. Nominal electron energies are 4, 6, 9, and 12 MeV, and operate at selectable dose rates of 2.5 or 10 Gray per minute. D(max) depths in water for a 10 cm applicator are 0.7, 1.3, 1.7, and 2.0 for these energies, respectively. The depths of 80% dose are 1.2, 2.1, 3.1, and 3.9 cm, respectively. Absolute calibration using the American Association of Physicists in Medicine TG-51 protocol was performed for all electron energies using the 10 cm applicator. Applicator sizes ranged from 3 to 10 cm diameter for flat applicators, and 3 to 6 cm diameter for 30 degrees beveled applicators. Output factors were determined for all energies relative to the 10 cm flat applicator. Central axis depth dose profiles and isodose plots were determined for every applicator and energy combination. A quality assurance protocol, performed each day before patient treatment, was developed for output and energy constancy.

Calibration↗

PubMed2006

[Effects of gantry position on the accuracy of lead block localization and relative quality control methods].

BACKGROUND & OBJECTIVE: To date, the low-melting point lead blocks are commonly used as a beam modifier for the accurate three-dimensional (3D) radiotherapy in China. However, care should be taken when using blocks even though they are verified on the specified simulator or linear accelerator before treatment. This is because that those verifications are only performed under the gantry angle of 0 degree. For the oblique gantry angles, the block may shift in the tray leading to the positioning error. In this study, we explored the influence of gantry position on the accuracy of lead block localization, and the relative quality control methods were also provided. METHODS: After rotating the gantry to 90 degrees and setting the collimator angle to 0 degree on a Siemens linear accelerator, a crosshair was marked on the plastic base of the lead block fitted to the tray according to the crosshair of light field of the linear accelerator. Then the discrepancy between the base crosshair and the light crosshair was investigated for different gantry positions and collimator angles. RESULTS: There was clearance between the plastic base of the lead block and the block tray. As for some collimator angles at oblique gantry positions, the lead block shifted in the tray due to the gravity. This led to the position error of blocked radiation fields from 0 up to 3.6 mm at the isocenter plane. CONCLUSIONS: Accurately marking the crosshair on the plastic base of the lead block is an effective method of correctly fixing the lead block influenced by the plastic base as well as of double checking the accuracy of the localization of the lead block. The positioning error of oblique gantry positions can be minimized by limiting the collimator to some degrees which makes the block tray opening to the ceiling or parallel to the ground while the bisector of the restrictive angle points slanting to the ceiling (that is, the first baseline of the block tray points inclined or right to the ceiling).

Humans↗

PubMed1991

[Experiences with intraoperative radiotherapy in gastric carcinoma (Berlin method)].

The aim of our pilot study is to determine whether intraoperative radiotherapy in gastric cancer cannot only prevent a local relapse but also improve the survival rate. Since November 1987, 26 patients with resectable gastric cancer were irradiated intraoperatively with the linear accelerator using fast electrons (single dose: 12 to 16 Gy). Percutaneous radiotherapy was performed postoperatively with 24 to 38 Gy (4 x 2 Gy per week). For intraoperative and percutaneous radiotherapy the target absorbed dose was selected in a way that their combined effect on the tumor was approximately equivalent to that of a total dose of 60 Gy in the usual fractionating. Up to now, the median survival time for stage III patients (UICC 1987) has been twelve months. In five patients who died of a relapse or of peritoneal carcinosis, histologic evaluation revealed in every case a diffuse tumor type according to Lauren-classification. All relapses occurred within the first eight months. The two-year survival rate according to Kaplan-Meier is 67% for stage III. Advanced resectable gastric cancer of the intestinal tumor type seems to profit from adjuvant intraoperative radiotherapy. The results warrant further research within the framework of a prospective randomized multicenter study.

Adenocarcinoma↗

PubMed1993

Commissioning Varian 2100C medical linear accelerator.

One Varian 2100C medical linear accelerator (LINAC) was installed at Kaohsiung Medical College Hospital last summer. The outputs of this LINAC are 6 and 10 MV X-ray beams, and 6, 9, 12, 16, and 20 MeV electron beams. A commissioning procedure including acceptance tests, dose data, environmental radiation survey was executed when the LINAC installation was completed. The acceptance tests organized as a sequential were performed by the medical physicist and Varian's engineers. Dose determination and environmental radiation survey were measured with ion chamber, phantom and survey meters. Radiation dose was calculated following the protocal of the Task Group 21, Radiation Therapy Committee of the American Association of the Physicists in Medicine (AAPM). After the commissioning procedure, the license application to operate the LINAC was submitted to the Atomic Energy Council (AEC) of Executive Yuan. When the license was authorized by the AEC last November, a quality assurance (QA) program was established and performed continuously in order to keep the Varian LINAC with high quality of radiation therapy.

Humans↗

PubMed1991

Treatment planning for stereotactic radiosurgery of intra-cranial lesions.

Stereotactic radiosurgery of intra-cranial lesions is a treatment modality where a well defined target volume receives a high radiation dose in a single treatment. Our technique delivers this dose using a set of non-coplanar arcs and small circular collimators. We use a standard linear accelerator in our treatments, and the adjustable treatment parameters are: isocenter location, gantry arc rotation interval, couch angle, collimator field size, and dose. The treatment planning phase of the treatment determines these parameters such that the target volume is sufficiently irradiated, and dose to surrounding healthy tissue and critical, dose-limiting structures is minimized. The attachment of a BRW localizing frame to the patient's cranium combined with CT imaging (and optionally MRI or angiography) provides the required accuracy for localizing individual structures in the treatment volume. The treatment is fundamentally 3-dimensional and requires a volumetric assessment of the treatment plan. The selection of treatment arcs relies primarily on geometric constraints and the beam's eye view concept to avoid irradiating critical structures. The assessment of a treatment plan involves isodose distributions throughout the volume and integral dose-volume histograms. We present the essential concepts of our treatment planning approach, and illustrate these in three clinical cases.

Adolescent↗

PubMedDate not supplied

A 4-MV CT scanner for radiation therapy; spectral properties of the therapy beam.

A Clinac-4 therapy accelerator has been modified to function also as a computed tomography (CT) scanner. We report on measurements made to calibrate the beam. 104 Bismuth-germanate silicon-photodiode detectors were placed in a circular arc of radius 180 cm, centered on the source, in the arrangement of a third-generation CT scanner. With the beam collimated to a fan section and just filling the detectors, the signal S(chi), obtained with thickness chi of plastic phantom, closely followed the relationship S(chi)/S(0) = exp [- (alpha chi + beta chi 2)], where coefficients alpha and beta varied systematically over the extent of the beam. This result agrees with results of similar work by others. It may be explained in terms of the spectral changes that are known to be an inherent property of such beams. Expressions relating alpha and beta to the source spectral density are derived; also estimates of the source spectral density are calculated using the method of Laplace inversion. It is shown that scattered radiation arising from the fan beam within the scattering medium does not contribute significantly to the values of alpha and beta.

Mathematics↗

PubMed1990

Stereotactic single high dose radiation therapy of benign intracranial meningiomas.

Seventeen patients with intracranial meningiomas were treated with single high dose irradiation at the German Cancer Research Center in Heidelberg. Indications for radiosurgery included unresected tumors, gross disease remaining despite surgery, and recurrences. Therapy was carried out by a technique using multiple non-coplanar arc irradiations from a 15 MeV linear accelerator. This technique coupled with secondary tungsten collimators allowed a high concentration of the dose in the target volume with an extremely steep dose gradient at the field borders. The patients were treated with a single irradiation dose ranging from 10 to 50 Gy (mean of 29 Gy). Four of 17 patients died: one death was tumor-related and not attributable to the treatment, one died of a treatment related complication, and two patients died of intercurrent diseases. The remaining 13 of the 17 patients with a median follow-up time of 40 months have no evidence of tumor relapse. Late severe side effects include five patients with a large area of brain edema, three of which were concurred with tumor necrosis. We conclude from these initial data that single high doses of irradiation concentrated to the tumor volume by stereotaxic methods can achieve local tumor control. It is also clear from these data that the effective therapeutic dose range must be better defined.

Adult↗

PubMed1994

Static field conformal stereotactic radiosurgery: physical techniques.

PURPOSE: Lesions in the head that are irregularly shaped or large present challenges for radiosurgical treatment by conventional techniques. Single, large circular fields may treat normal tissue volumes. Multiple shot or multiple isocenter treatment plans provide better conformation to the target than a single field, but may be difficult to plan and/or treat. As an alternative to these conventions, we are developing static field, conformal stereotactic radiosurgery. In this technique a finite number of fixed, shaped, linear accelerator fields are used to irradiate the target. METHODS AND MATERIALS: Computer simulations were performed for a four-path arc and fixed field techniques and evaluated with dose distributions and dose volume histograms. Beam geometries are defined with a 3-D treatment planning system with beam's eye view capabilities. Equipment for treatment delivery has been designed, including a head frame/support stand assembly and a method for manufacturing the required custom collimators. RESULTS: Isodose distributions and dose volume histograms show that beam geometries with seven or more fields provide target dose distributions equivalent to the arc treatment, but with small increases in peripheral dose. Dose homogeneity across the target volume increases as the solid angle of irradiation is increased. For a hemispherical target, the four-path arc and shaped, static fields provide equal target coverage while the shaped fields treat a smaller high-dose volume. CONCLUSION: Shaped, static fields are an alternative to single isocenter arc radiosurgery and result in smaller volumes at high dose. This smaller volume could translate into sparing for normal adjacent tissues that would otherwise be treated.

Computer Simulation↗

PubMed1995

A design study for an accelerator-based epithermal neutron beam for BNCT.

An achievable design concept for a boron neutron capture therapy (BNCT) facility, based on a high-current, low-energy proton accelerator, is described. Neutrons are produced within a thick natural lithium target, under bombardment from protons with an initial energy between 2.5 and 3.0 MeV. The proton current will be up to 10 mA. After gamma-ray filtering, the neutrons are partially moderated to epithermal energies within a heavy-water moderator, poisoned with 6Li to remove thermal neutrons. Monte Carlo modelling has been used to predict system performance in terms of neutron fluence rate and neutron and gamma-ray dose at the patient position. The relationship between the system performance and key parameters, such as proton energy, moderator depth and 6Li concentration, has been investigated. With a proton current of 10 mA, the facility is capable of providing a therapy beam with a useful neutron fluence rate of 10(9) cm-2 s-1 and a neutron dose per unit fluence of less than 6 x 10(-13) Gy cm2, with a gamma-ray contamination of the therapy beam of about 10(-13) Gy cm2.

Biophysical Phenomena↗

PubMed1998

Design and characterization of a dynamic multileaf collimator.

The characteristics of a prototype computer-assisted dynamic multileaf collimator (DMLC), specifically designed for small-field conformal radiotherapy, were evaluated at the Istituto Nazionale Tumori of Milan. The collimating device consists of two opposing banks of 16 pairs of 8 cm thick, 3.6 mm wide tungsten leaves and allows shaping of a radiation field up to a size of 10 x 10 cm2 at the isocentre. The screening thickness of each leaf is 6.25 mm at the accelerator gantry isocentre. The leaves have a trapezoidal cross section and move along an arched path, thus providing a 'double focused' collimation system. The DMLC was installed on the head of a Varian Clinac 2100C linear accelerator. Mechanical and dosimetric evaluations were performed to test the stability of the mechanical isocentre and to determine leaf leakage, penumbra width, accuracy of leaf positions and uniformity of leaf speed. Displacement of the mechanical isocentre was less than 1 mm at all gantry angles. Standard radiographic films exposed to 6 MV x-ray radiation were used for dosimetric evaluations. Leakage between leaves was less than 2.5%, and leakage through abutted leaves was less than 5.5%. The penumbra width between 20% and 80% isodose at different positions of leaf banks was 2.7 mm in the direction of the leaf motion and 3.1 mm along the side of the leaf with a standard deviation of 0.2 mm in both directions. Accuracy in the positioning of the leaf was 0.3 mm, whereas the maximum repositioning error was less than 0.2 mm. Finally, during movement of the leaves at the maximum speed of 0.5 mm s(-1), the standard deviation of the leaf positioning error was 0.2 mm, proving an accurate uniformity of leaf speed.

Neoplasms↗

PubMed1985

O6-Methylguanine methyltransferase activity is increased in rat tissues by ionising radiation.

The effect of whole body ionising radiation from a linear accelerator on rat tissue O6-methylguanine (O6-meG) methyltransferase (MT) activity has been assessed using an assay which measures the transfer of 3H-radioactivity from 3H-methylated substrate DNA to protein. The maximal effect occurred 2 days after a 1 krad dose, at which time activity in liver extracts was increased approximately 5-fold in two different rat strains. Activity in lung and kidney was increased approximately 4- and 2-fold, respectively. Similar degrees of enhancement were found in these three tissues using an h.p.l.c. method for measuring MT activity. The increase in activity was reflected in an increased capacity to repair O6-meG produced in liver DNA by administration of [14C]dimethylnitrosamine (DMN): this effect was dose dependent, being detectable after 30 rads and maximal after 600 rads. Incorporation of labelled breakdown products of the DMN into adenine in DNA increased as the dose radiation increased suggesting an inhibition of DNA synthesis. The implications of these results for the mechanism of enhancing O6-meG repair are discussed.

Animals↗