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At least 793 records · Page 44Linked to original sources
PubMed2002

Measurement of DNA damage induced by irradiation with gamma-rays from a TRIGA Mark II research reactor in human cells using Fast Micromethod.

The Fast Micromethod is a novel quick and convenient microplate assay for determination of DNA single-strand breaks. This method measures the rate of unwinding of cellular DNA upon exposure to alkaline conditions using a fluorescent dye which preferentially binds to double-stranded DNA. Here we applied this method to determine the levels of DNA single-strand breaks in HeLa cells induced by y-irradiation deriving from fission isotopes and activation products at the TRIGA Mark II research reactor in Mainz. An increased strand scission factor (SSF) value, which is indicative for DNA damage, was found at doses of 1 Gy and higher. A similar increase in SSF value, which further increased in a dose-dependent manner, was found in human peripheral blood mononuclear cells after irradiation with 6 MV X-rays from a linear accelerator to give a total exposure of 0.5 to 10 Gy.

4-Nitroquinoline-1-oxide↗
PubMedDate not supplied

An isodose shift technique for obliquely incident electron beams.

It is well known that when an electron beam is incident obliquely on the surface of a phantom, the depth dose curve measured normal to the surface is shifted toward the surface. Based on geometrical arguments alone, the depth of the nth isodose line for an electron beam incident at an angle theta should be equal to the product of cos theta and the depth of the nth isodose line at normal incidence. This method, however, ignores the effects of scatter and can lead to significant errors in isodose placement for beams at large angles of incidence. A semi-empirical functional relationship and a table of isodose shift factors have been developed with which one may easily calculate the depth of any isodose line for beams at incident angles of 0 degrees to 60 degrees. The isodose shift factors are tabulated in terms of beam energy (6-22 MeV) and isodose line (10%-90%) and are shown to be relatively independent of beam size and incident angle for angles less than 60 degrees. Extensive measurements have been made on a Varian Clinac 2500 linear accelerator with a parallel-plate chamber and polystyrene phantom. The dependence of the chamber response on beam angulation has been checked, and the scaling factor of the polystyrene phantom has been determined to be equal to 1.00.

Electrons↗
PubMedDate not supplied

Shaping of photon beams from electron linear accelerators in radiation therapy.

The method of calculation of the bremmstrahlung yield and angular distributions based on the semianalytical model described by Kovalev [Vtoricnoe izlucenie uskoritelej elektronov (Atomizdat, Moscow, 1979)] was analyzed. The specially written computer program was used to optimize the geometry of the collimators and the flattening filter design for radiation therapy purposes. The x-ray beam shaping system thus designed was installed in the 4-MeV linear Limex accelerator working at the Cancer Center in Warsaw (Poland). By comparing the calculated beam distributions with the experimental data, the agreement between theory and measurements was found to be within about +/- 3%.

Biophysical Phenomena↗
PubMed1989

Variations in dose distributions by the use of independent asymmetric jaws. A new technique for irradiation of localized carcinoma of the prostate.

The use of independent jaws set asymmetrically in the moving field technique with bilateral arcs allows the entirely new dose distributions be obtained. The basic principle of this technique is that, with the use of moving field technique with bilateral arcs, the independent jaw more proximate to the central ray (axis) of beam must always be on the side of the critical organ. In this area the characteristic deformation of isodoses and the formation of very steep dose slope occur. We consider as optimum technique for the target irradiation of the prostatic carcinoma the moving field technique with bilateral 105 degrees arcs with 20 MeV X-ray beam of linear accelerator and with independent jaws set asymmetrically, when the independent jaw more proximate to the central ray is always + 1.5 cm from the central ray of the beam.

Carcinoma↗
PubMed2002

A practical method of measuring electron isodose curves using a linear detector array for validation of treatment planning system data.

A practical method of measuring electron isodose curves for validation of treatment planning system data using a linear detector array in a water phantom is described. The detector array was used to measure both the profile and depth dose data required. A depth-dependent correction was required, determined from a comparison with diode-measured depth dose. Application of this correction enabled accurate reconstruction of isodose curves. Isodose curves were also measured with diodes, to verify the array measurements. The two were found to be in good agreement. The advantage of the array-based method is a substantial saving in linear accelerator time when compared with point-by-point detector measurements. Although the data reproduced here are specific to the linear detector array of one manufacturer, the overall measurement technique is generally applicable.

Algorithms↗
PubMed1997

Accuracy in target localization in stereotactic radiosurgery.

The accuracy in target localization of CT, MRI, and digital angiography and the isocentric deviation of linear accelerator were investigated for stereotactic radiosurgery. Twenty-five slice images using CT and MRI were obtained out of geometrical phantom which was designed to produce exact 3-dimensional coordinates of several points within a 0.1 mm error range. These diagnostic images were transferred to a 3-D treatment planning system through ethernet. Measured 3-D coordinates of these images from the planning system were compared to known values by geometrical phantom. Anterior-posterior and lateral films were taken by digital angiography for measurement of spatial accuracy. The accuracy of gantry isocenter aligned by laser localizer was also measured. Lead ball was located at the isocenter of linear accelerator and x-ray films were taken by 4-MV photon beam with gantry angles of 0, 90, 180 and 270 degrees, Overall procedure from CT scanning to treatment was carried out using the geometrical phantom. The target accuracy was verified by high energy x-ray portal film. The accuracy of diagnostic machines were within 2.1 mm except MR-axial images. In case of linear accelerator, the deviation of isocenter was within 0.7 mm. Finally, the total isocentric deviation of overall procedure was 1.3 +/- 0.5 mm (using CT localization).

Angiography, Digital Subtraction↗
PubMed1999

Stereotactic proton radiosurgery.

The technique of stereotactic proton radiosurgery is discussed in depth in this article. The physics of the proton beam in radiosurgery is explained, and the different factors of beam delivery are examined. These key factors (correspondence to shape, accuracy of delineation of volume, correspondence to volume, and accuracy of delivery vary) with each of the radiosurgical techniques, from Gamma Knife surgery to linear accelerator therapy. Clinical series in the use of proton radiosurgery are also presented, with an emphasis on efficacy and uses.

Biophysical Phenomena↗
PubMed1992

Light ion accelerators for cancer therapy.

The paper concentrates on the accelerator choices for light ion cancer irradiation facilities, as outlined in several proposals over the last 15 years and concludes with the description of a modern synchrotron facility. Further summary papers on the same subject are cited in references [14, 19].

Equipment Design↗
PubMed1993

EORTC radiotherapy group quality assurance: mechanical checks and beam alignments of megavoltage equipment.

In 1987 mechanical checks of megavoltage units and simulators were included in the on-site physics program of the EORTC. The results reported were obtained in 16 different centres and concern 23 accelerators, 14 cobalt units and 14 simulators. In general, the deviations observed for accelerators and simulators are smaller than for cobalt units. A single score, based on the deviations observed for the mechanical checks, is attributed to each centre.

Cobalt Radioisotopes↗
PubMed1997

Effect of beam blocking on linac head scatter factors.

PURPOSE: This study aimed to investigate the influence of beam blocking on head scatter within the context of a two-component x-ray source model. METHODS AND MATERIALS: Head scatter factors were measured for open rectangular fields defined by X and Y jaws and for fields blocked by a multileaf collimator (MLC) for a 6-MV photon beam from a Varian 2300CD accelerator. A simple formula based on consideration of linac head geometry and an x-ray source model was derived to determine when the head scatter factor for a shaped field depends only on jaw settings and when it is influenced by the blocking (cerrobend blocks or MLC) itself. RESULTS: Experimental data demonstrate that the assumption that head scatter is unaffected by the introduction of beam blocking is not always acceptable, particularly for fields blocked by an MLC. The ratios of open to blocked field dimensions were compared with simple functions characterizing the accelerator head geometry to predict changes in the behavior of head scatter factors observed experimentally. CONCLUSION: A simple formula can be used to determine when head scatter factors are influenced by beam blocking (cerrobend blocks or MLC).

Equipment Design↗
PubMedDate not supplied

Computer interface for a linear accelerator.

A computer interface for the Clinac-18 linear accelerator has been developed, using a standard CAMAC interface plus buffer amplifiers to isolate the CAMAC from the accelerator electronics. Buffer amplifiers are employed because direct connection of the CAMAC system to the accelerator was found to affect accelerator operation. The total interface accommodates the four gantry position analog signals and thirteen digital signals describing all available treatment options. The interface also allows the computer to inhibit beam operation.

Computers↗
PubMedDate not supplied

Measurement of accelerator bremsstrahlung spectra with a high-efficiency Ge detector.

A Compton spectrometer with a high-efficiency Ge detector was used to measure photon spectra from clinical accelerators. The response-function matrix for the detector system was determined using ten radioactive sources that emitted gamma rays of single energy ranging from 279-1525 keV. The system was tested by measuring the spectrum from a 60Co teletherapy machine. The energy distribution of tungsten target bremsstrahlung produced by 2.2, 2.5, 15, 20, and 25 MeV electrons in two clinical accelerators were determined. Theoretical thin-target calculations overestimate the number of photons in the high-energy regions of the spectra, as was expected. However, theoretical thick-target calculations underestimate the numbers of high-energy photons.

Germanium↗
PubMedDate not supplied

Assessment of a linear accelerator for segmented conformal radiation therapy.

Segmented conformal radiation therapy is a new computer-controlled treatment technique under investigation in which the target volume is subdivided into thick transverse segments each of which is then treated individually by rectangular transverse abutting fields. In order to obtain uniform dose at abutments, the machine isocenter remains fixed in the patient and field edges are defined by independently moving focused collimator jaws to give matching geometric divergence. Mechanical variation in jaw and gantry positioning will create some dose variation at field abutments. Film dosimetry was used to study the radiation field positioning accuracy and precision of a commercial linear accelerator. A method of field position calibration was developed using multiple nonabutting fields exposed on the same radiograph. Verification of collimator jaw calibration measurements was performed using multiple abutting fields exposed on a single radiograph. Measurements taken over 5 months of clinical accelerator operation studied the effects of simple jaw motion, simple gantry motion, and combined jaw/gantry motion on jaw position precision and accuracy. The inherent precision and accuracy of radiation field positioning was found to be better than +/- 0.3 mm for both jaws with all types of motions except for the Y2 jaw under combined jaw/gantry motion. When the ability to deliver abutting beams was verified in clinical mode, the average dose variation at abutments was less than 6% at all gantry angles except for one. However, due to accelerator software limitations in clinical mode, the settings for collimator positions could not take advantage of the maximum accuracy of which the hardware is capable.(ABSTRACT TRUNCATED AT 250 WORDS)

Biophysical Phenomena↗
PubMed1994

Comprehensive analysis of electron beam central axis dose for a radiotherapy linear accelerator.

This work evaluates the application of AAPM task group 25 (TG25) methodology for determination of central axis depth dose for a radiotherapy linear accelerator, whose dual scattering foil system and applicators were recently modified. The percent depth dose (%DD) and the dose output factor have been measured for square and rectangular fields at 100- and 110-cm source-to-surface distance (SSDs). At 100-cm SSD, results showed that %DD for a specific energy and field size can vary with applicator, the largest variation being for the 20-MeV, 10 x 10-cm field where a spread of +/- 2.5% or +/- 3 mm about the mean %DD is observed. The square-root method determines rectangular field %DD within 1%. Output factors for rectangular fields are calculated from square field values more accurately using a square-root method than the equivalent-square method recommended by TG25. At 110-cm SSD, the %DD calculated from that at 100-cm SSD using an inverse square factor does not agree with measured values for all fields. The maximum difference observed for the 20-MeV, 6 x 6-cm field was 5.5% or 10 mm. Output data at the 110-cm SSD show that the square-root method is suitable for determination of the air-gap correction factors of rectangular fields. In summary, the recommendations of TG25 work reasonably well for central axis electron beam dosimetry for this version of a radiotherapy linear accelerator, except in limited cases where applicator-scattered electrons apparently cause minor but clinically significant discrepancies.

Biophysical Phenomena↗
PubMed1988

A new stereotactic X-ray knife.

For many years, the irradiation of small volumes of tissue in the brain to necrotizing doses has been investigated as a "non-invasive" alternative to neurosurgery. We propose a new system in which a precisely machined helmet serves as a multi-port focussed X-ray collimator when it is itself irradiated by a conventional medical linear accelerator run in the electron mode. When the collimator is attached to a stereotactic frame, the geometric accuracy of delivering small radiation fields to the brain is limited primarily by the accuracy of the stereotactic localization, and is relatively independent of the positional stability of the accelerator. Field sizes as small as two millimeters are readily achievable. The problem of low dose rate associated with these small fields is overcome by the use of high electron beam currents.

Brain Neoplasms↗
PubMedDate not supplied

[Use of a multileaf collimator for the production of intensity-modulated beams].

In external radiotherapy, the use of intensity modulated fields has been proposed for tissue and non-homogeneity compensation or for the generation of conformal dose distributions. Multileaf collimators can be employed dynamically for the modulation of the X-ray field in two dimensions. Efficient dynamic collimation became possible due to advances in computer and linear accelerator technology. It presents a number of advantages over conventional methods such as the use of compensators. We have developed a program which calculates, from a given intensity distribution, the motion of the MLC leaves as a function of monitor units, and we have applied it on a Varian linear accelerator with a 40 pair multileaf collimator. The analysis of the experimental results demonstrates the feasibility and the potential of the method.

Algorithms↗