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At least 685 records · Page 38Linked to original sources
PubMed1996

The measurement of linear accelerator isocenter motion using a three-micrometer device and an adjustable pointer.

PURPOSE: The small motions of the major axes of a linear accelerator observed during gantry and treatment table rotation were measured to improve beam-target alignment during stereotactic radiosurgery (SRS). METHODS AND MATERIALS: Measurements of gantry isocenter motion and table rotational axis wobble were performed with an adjustable front pointer and a three-micrometer device. Nominal gantry and table isocenters were specified. The gantry motion path and table isocenter coordinates were then applied to offset simulated treatment target coordinates so as to compensate for gantry sag. Target simulation films were examined to document improvement of beam-target alignment. RESULTS: The overall precision of the measurement of gantry and table isocenter coordinates was 0.2 mm. Over gantry rotation of 0 to 360 degrees, the gantry isocenter was found to follow a pinched loop with a maximum point to point distance of 1 mm. Table axis motion was found to be negligible relative to the reproducibility of gantry isocenter motion. Thus, a table isocenter was defined that was invariant to table rotation. CONCLUSION: Results indicate that the three-micrometer device and adjustable front pointer are useful tools for three-dimensional (3D) mapping of gantry, collimator and table isocenters and their motions. It is suggested that such measurements may be useful in the quality assurance of linear accelerators, particularly to improve beam-target alignment during SRS and other high dose external beam therapy.

Algorithms↗
PubMed2005

Technology enhancements and changes in radiotherapy throughput in New South Wales.

AIMS: To assess the effect that the age of linear accelerators and recent changes in technology have had on linear accelerator throughput in New South Wales, Australia. MATERIALS AND METHODS: Duration was measured (time of patient entry into the treatment room to time of exit) of each radiotherapy treatment fraction delivered on each linear accelerator over a 5-day period. Patient-, treatment- and equipment-based variables were collected for all treatment fractions, and assessed for their effect on fraction duration. Comparisons were made between these data and similar productivity data collected from a study carried out in 1996. Since the sample sizes for both the study periods were large enough, the distributions of the means were assumed normal (Central Limit Theorem). Specific analyses were carried out to assess the affect that new technologies, such as automatic field-sequencing (AFS) and multi-leaf collimator (MLC), have had on fraction duration. RESULTS: A total of 12 892 treatment fields and 4316 treatment fractions were delivered on 27 linear accelerators over 135 days. Comparison between the 2003 and 1996 productivity data showed an increase in the mean number of patients treated per hour by 11% and fields treated per hour by 31%. The mean number of fields treated per fraction increased by 15%. The mean fraction duration was reduced by 13% for linear accelerators of less than the median age of 7 years that were equipped with MLC/AFS, or both, compared with older linear accelerators without AFS and MLC. This reduction was more obvious for complex techniques, such as four-field breast treatments (27% decrease in fraction duration). The mean number of fields treated per hour was 43% more on the newer machines equipped with AFS and MLC. CONCLUSIONS: An increase in productivity has been observed between the 1996 and 2003 study periods, as measured by patients or fields per hour, despite an increase in treatment complexity as measured by fields per fraction. The application of AFS and MLC, and the use of newer linear accelerators, significantly shortened the mean duration per fraction for the common treatment techniques.

Hospital Departments↗
PubMed2007

A virtual-accelerator-based verification of a Monte Carlo dose calculation algorithm for electron beam treatment planning in clinical situations.

BACKGROUND AND PURPOSE: The introduction of Monte Carlo (MC) techniques for treatment planning and also for verification purposes will have considerable impact on the radiation therapy planning process. The aim of this work was to use a virtual accelerator to study the performance of a MC-based electron dose calculation algorithm, implemented in a commercial treatment planning system. METHODS: The performance in phantoms containing air and bone as well as in patient-specific geometries (thorax wall, nose, parotid gland and spinal cord) has been studied. RESULTS: The agreement between the virtual accelerator and the MC dose calculation algorithm is generally very good. A gamma-evaluation with criteria of 0.03 Gy/3 mm (per Gy at the depth of maximum dose) shows that, even for the worst cases, only a small volume of about 1.5% has gamma>1.0. In the worst case, with the 0.02 Gy/2 mm criteria, about 92% of the volume receiving more than 0.85 Gy per 100 monitor units (MU) has gamma-values <1.0. The corresponding value for the volume receiving more than 0.10 Gy/100 MU is about 98%. For the 18 MeV spinal-cord case, where a 6 x 20 cm2 insert is used, the TPS underestimates the dose outside the primary field due to inadequate modelling of the insert. CONCLUSION: The possibility of dose calculations in typical patient cases makes the virtual accelerator a powerful tool for validation and evaluation of dose calculation algorithms present in treatment planning systems.

Algorithms↗
PubMed1990

Use of mouse tail in the study of irradiated microvasculature.

The use of the mouse tail in the study of microvasculature following fractionated radiotherapy is described. Mice are restrained in a perspex housing and mounted on a perspex tray containing slots for individual mice. The mouse restrainer, irradiation set up, and dosimetry are described in detail. Irradiation was carried out using a 4MV Linear accelerator. The technique described here is reproducible. It can be used to study the volume effects in fractionated radiotherapy.

Animals↗
PubMed2002

Using a tungsten rollbar to characterize the source spot of a megavoltage bremsstrahlung linac.

In photon teletherapy, the size and functional form of the photon source spot affect both the sharpness of the penumbra of treatment fields and the sharpness of portal images. Photon source spot parameters are also used in photon teletherapy dose calculation codes. A simple method for characterizing the source spot would complement the existing, more involved methods that have been described in the medical physics literature. Such a method, using a rollbar made of tungsten or other high-Z metal, is used in industrial radiography. We describe the use of a tungsten rollbar for characterizing the source spot edge spread function (and thereby the source spot size and shape) of a megavoltage bremsstrahlung photon source. We use Monte Carlo simulations to quantify anticipated experimental artifacts of the method, assuming typical spot sizes for circ-function, Gaussian, and Bennett line shapes. We illustrate the use of the rollbar method by characterizing the source spot of a typical 9 MV linac used for industrial radiography. The source spot is analyzed using two approaches: (a) fitting the rollbar image with analytic functions and (b) using Abel inversion to obtain the cylindrically symmetric spot profile consistent with the measured rollbar image. Monte Carlo simulations, based on a 6 MV photon teletherapy accelerator, suggest that aspects of the method are applicable to medical bremsstrahlung sources.

Computer Simulation↗
PubMed1996

A reticle retrofit and dosimetric consideration for a linear accelerator.

PURPOSE: An imperfect reticle system in an accelerator causes uncertainties in source-skin distance (SSD), off-axis distance (OAD), isocenter, and so forth. A reticle was designed and fabricated, and its implications on x-ray and electron beam dosimetry were investigated. METHODS AND MATERIALS: A new reticle frame was dimensioned to fit snugly in the accelerator. The frame was fabricated to carry a pair of adjustable cross wires and to allow the machine operation in the photon and electron modes. The impact of the cross wires on 6 MV photon and 5-10 MeV electron beam parameters such as dose rate (Gy/monitor unit), beam uniformity, surface dose, and so forth, were studied using suitable ion chambers and phantoms. RESULTS: The retrofitted system offered long-term mechanical stability leading to precise SSD, OAD, and isocenter measurements. Changes introduced by the cross wires on the 6 MV photon and 5-10 MeV electron beams are presented. CONCLUSION: Long-term stability of a reticle in an accelerator is important for an accurate patient setup and for making reliable dosimetric measurements. Beam characteristrics have to be studied whenever modifications on a reticle system are made.

Equipment Design↗
PubMed1997

The effect of neutron-moderating materials in high-energy linear accelerator mazes.

A study was undertaken to examine the effect of lining high-energy linear accelerator mazes with neutron-moderating materials in order to reduce scattered neutron dose at the accelerator room door. Polyethylene alone reduced neutron dose by no more than 27% and did not significantly reduce gamma ray dose. Polyethylene combined with flexboron panels reduced neutron dose by 50% at most, reducing gamma ray dose by a maximum of 32%. Much greater reductions in both neutron and gamma ray dose (by 92% and 55% respectively) can be obtained by incorporating polyethylene and boron into either internal or external maze doors. Our results support the conclusion that neutrons directly incident on the maze from the accelerator contribute little to the neutron dose at the door, and that the majority of neutron dose is due to scattered and thermal neutrons.

Boron↗
PubMed1999

A study on virtual source position for electron beams from a Mevatron MD linear accelerator.

The virtual source position (VSP) for electron beams of energies 5, 7, 9 10, 12 and 14 MeV and for the applicators (cones) available in the department have been measured for a Mevatron MD class linear accelerator. Different methods of obtaining the virtual source position for electron beams have been investigated in the present study. The results obtained have been compared with those of other workers. It is observed that the VSP is very much machine dependent and needs to be measured for each linear accelerator. The effect of shielding on virtual source position for the type of applicators available in the department has also been investigated.

Air↗
PubMed2004

Influence of initial electron beam parameters on Monte Carlo calculated absorbed dose distributions for radiotherapy photon beams.

Our aim in the present study was to investigate the effects of initial electron beam characteristics on Monte Carlo calculated absorbed dose distribution for a linac 6 MV photon beam. Moreover, the range of values of these parameters was derived, so that the resulted differences between measured and calculated doses were less than 1%. Mean energy, radial intensity distribution and energy spread of the initial electron beam, were studied. The method is based on absorbed dose comparisons of measured and calculated depth-dose and dose-profile curves. All comparisons were performed at 10.0 cm depth, in the umbral region for dose-profile and for depths past maximum for depth-dose curves. Depth-dose and dose-profile curves were considerably affected by the mean energy of electron beam, with dose profiles to be more sensitive on that parameter. The depth-dose curves were unaffected by the radial intensity of electron beam. In contrast, dose-profile curves were affected by the radial intensity of initial electron beam for a large field size. No influence was observed in dose-profile or depth-dose curves with respect to energy spread variations of electron beam. Conclusively, simulating the radiation source of a photon beam, two of the examined parameters (mean energy and radial intensity) of the electron beam should be tuned accurately, so that the resulting absorbed doses are within acceptable precision. The suggested method of evaluating these crucial but often poorly specified parameters may be of value in the Monte Carlo simulation of linear accelerator photon beams.

Algorithms↗
PubMedDate not supplied

Dose to radiotherapy technologist from air activation.

Production rates of the activation of oxygen and nitrogen by high-energy x-ray beams from medical accelerators were measured as functions of the accelerator energy. A technique was developed in which the air-activation production rates are used to evaluate the concentration of radioactive gas in the treatment room and the dose received by the technologist who operates the accelerator. It was found that for typical operating conditions of medical accelerators, the dose received by personnel entering the treatment room is negligible compared to the maximum permissible dose limit.

Air↗
PubMed1997

Shielding for thermal neutrons.

The problem of calculating the neutron capture gamma-ray dose rate due to thermal neutron capture in a boron or cadmium rectangular shield is considered. An example is given for shielding for a door at the exit of medical accelerator room maze in order to determine the optimum location of lead relative to the borated polyethylene.

Boron↗