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At least 721 records · Page 40Linked to original sources

PubMed2001

A tool to measure radiotherapy complexity and workload: derivation from the basic treatment equivalent (BTE) concept.

Radiotherapy workload is poorly represented by simple parameters of patients, fractions or fields treated because these do not contain any measure of treatment complexity. However, complexity is increasing and there is an urgent need to quantify this. We have evaluated the basic treatment equivalent (BTE) model as a measure of radiotherapy workload and complexity. Radiotherapy treatment times, from the patient entering to exiting the treatment room maze, were measured for 1298 treatment sessions on 269 patients. The data were used to assess the original model and derive three new models for predicting treatment duration. The most complicated, the 'Addenbrooke's complex model', contained two additional predictor variables, including 'site/technique', in a linear additive form. Before the study, the department used a standard treatment appointment time of 10 minutes. However, 50% of the measured treatments took longer than 10 minutes, (mean 10.9). Summed over the working day, this discrepancy indicates that a standard 10-minute appointment is a poor basis for scheduling radiotherapy. The original BTE model was effective in predicting treatment times, although this was improved by refinement of the model. The Addenbrooke's complex model correctly predicted 70% of treatment times to within 2 minutes (55% for the original BTE model), 80% to within 2.5 minutes and 95% to within 4.7 minutes. The percentage of the variation in observed times accounted for by the model is 59.4%. The models can represent radiotherapy complexity, can improve scheduling on linear accelerators, and are likely to be applicable to other departments. They are thus tools to assess the impact of changes in complexity from new techniques, trial protocols (e.g. the Medical Research Council prostate radiotherapy trial RTO1), and possible time saving from advanced technology such as multileaf collimators (MLCs) or automated machine set-up. The replacement of manually-lifted shielding blocks by MLCs should save 1.1-1.5 minutes for a three- or four-field pelvic plan (i.e. 12%-13%). The models could also be used to aid planning for future linear accelerator provision and for costing radiotherapy treatment.

Efficiency, Organizational↗

PubMed1998

X-ray quantum limited portal imaging using amorphous silicon flat-panel arrays.

We have measured the linearity, spatial resolution (MTF), noise (NPS), and signal-to-noise characteristics (DQE) of an electronic portal imaging device (EPID) based on an amorphous silicon flat-panel array. The array has a 128 x 128-pixel matrix and each pixel is 0.75 x 0.75 mm2 in dimension so the array covers an area of 96 x 96 mm2. The array acts like a large area light sensor and records the optical signals generated in a metal plate/phosphor screen x-ray detector when the detector is irradiated by a megavoltage x-ray beam. In addition, approximately 0.5% of the total signal is generated by nonoptical processes. The noise measurements show that the device is quantum noise limited with the noise power generated by the x-ray quanta being up to 100 times greater than the noise added by the external readout electronics and flat-panel light sensor itself. However, the flat-panel light sensor does reduce the spatial resolution (compared to a perfect optical sensor with infinitesimal pixel size) because of its moderate pixel size and because optical spread can occur in the transparent glues used to attach the phosphor screen to the flat-panel light sensor. The response of the sensor is very linear and does not suffer from the glare phenomenon associated with TV camera-based EPIDs--characteristics which suggest that the amorphous silicon EPID will be well suited to transit dosimetry. Nevertheless, some limitations need to be overcome before these devices can be used clinically. These include developing larger flat-panel light sensors, the elimination of "noisy" pixels with high dark signal, and improvements in the uniform sensitivity of the sensors. This last requirement is only needed for transit dosimetry applications where it would greatly simplify calibration of the device. In addition, an image acquisition scheme must be developed to eliminate artifacts created by the pulsed x-ray beam generated by linear accelerators. Despite these limitations, our studies suggest that the amorphous silicon EPIDs are very well suited to portal imaging.

Artifacts↗

PubMed2004

Shielding evaluation and acceptance testing of a prefabricated, modular, temporary radiation therapy treatment facility.

We have recently commissioned a temporary radiation therapy facility that is novel in two aspects: it was constructed using modular components, and the LINAC was installed in one of the modular sections before it was lifted into position. Additional steel and granular fill was added to the modular sections on-site during construction. The building will be disassembled and removed when no longer needed. This paper describes the radiation shielding specifications and survey of the facility, as well as the ramifications for acceptance testing occasioned by the novel installation procedure. The LINAC is a Varian 21EX operating at 6 MV and 18 MV. The radiation levels outside the vault satisfied the design criteria, and no anomalous leakage was detected along the joints of the modular structure. At 18 MV and 600 monitor units (MU) per minute, the radiation level outside the primary barrier walls was 8.5 micro Sv/h of photons; there were no detectable neutrons. Outside the direct-shielded door, the levels were 0.4 micro Sv/h of photons and 3.0 micro Sv/h of neutrons. The isocentricity of the accelerator met the acceptance criteria and was not affected by its preinstallation into an integrated baseframe and subsequent transport to the building site.

Computer-Aided Design↗

PubMed2004

Stereotactic linear accelerator radiotherapy for pituitary tumors.

Last decade has seen important advances in radiotherapy technology which combine precise tumor localization with accurate targeted delivery of radiation. This technique of high precision conformal radiotherapy, described as stereotactic radiotherapy or radiosurgery, uses modern linear accelerators available in most radiation oncology departments. The article describes the new technique as applied to the treatment of pituitary adenoma and reviews published clinical results.

Adenoma↗

PubMed2001

60Cobalt vs. linear accelerator in the treatment of locally advanced cervix carcinoma: a comparison of survival and recurrence patterns.

OBJECTIVE: To compare the survival and recurrence patterns of patients with locally advanced cervical carcinoma treated with 60cobalt radiotherapy units and linear accelerators. METHODS: Two hundred and forty-eight patients with cervical carcinoma stages IIB-IVA who were treated with primary irradiation between the years 1985 and 1988 comprised the study group. The median survival of patients treated with 60cobalt units and linear accelerators was calculated using the method of Kaplan and Meier and compared using the log-rank test. Recurrence patterns were compared using chi-square analysis; p < .05 was considered significant for all tests. RESULTS: One hundred and ninety-five patients were treated with 60cobalt units (Group 1) and 53 patients were treated with a linear accelerator (Group 2). Group 1 and 2 were similar with regard to mean age and weight, stage distribution, and mean dose to point A. The rate of recurrence was comparable between Group 1 and 2 (65.6% vs. 64.2%) and no significant difference was found in overall survival between the groups (20 months vs. 21 months. p = 81). There was a trend toward increasing pelvic recurrence in Group 1 (50.8%) compared to Group 2 (35.8%, p = .08). CONCLUSIONS: 60Cobalt units and linear accelerators offer comparable rates of overall survival in patients with locally advanced cervix carcinoma.

Cobalt Radioisotopes↗

PubMedDate not supplied

Usefulness and problems of stereotactic radiosurgery using a linear accelerator.

Since the introduction of linac radiosurgery in October 1994, we have treated 27 patients with 36 lesions. We treated nine AVM, 12 metastatic brain tumors, two malignant lymphomas, one anaplastic astrocytoma, two meningiomas, and one brain tumor of unknown pathology. In the follow-up examinations at least five months after treatment, the local control rate was 83% for the metastatic tumors, and two malignant lymphomas disappeared completely. In addition, satisfactory results have been obtained with AVM and other brain tumors without any side effects. In comparison with gamma-knife radiosurgery, linac radiosurgery has some disadvantages such as longer treatment time and cumbersome accuracy control, but if accuracy control is performed periodically, accuracies of 1 mm or less can be obtained. There is some strengths of linac radiosurgery as follow. 1) The acquisition cost is relatively low. 2) Dose distribution are equivalent to gamma-knife. 3) There is no field size limitation. 4) There is great flexibility in beam delivery and linac systems. Radiosurgery using linear accelerators seems to become widely accepted in the future.

Brain↗

PubMed2004

Measurement of depth distributions of (3)H and (14)C induced in concrete shielding of an electron accelerator facility.

The estimation of radioactivity induced in concrete shielding is important for the decommissioning of accelerator facilities. Concentrations of (3)H and (14)C in the concrete shielding of an electron linear accelerator were measured, and the depth distributions of (3)H and (14)C and gamma-ray emitters were discussed in relation to their formation reactions.

Carbon Radioisotopes↗

PubMed1998

Modification of a linear accelerator table top for non-coplanar conformal brain radiotherapy.

The use of non-coplanar conformal therapy necessitates the use of unusual beam projections that may not be accomplished with a conventional linear accelerator table top. Modification of the table top can increase the available combinations of gantry and couch rotation. A standard Philips table top, supplied with an SL 75-5 linear accelerator, was modified to increase available combinations of gantry and couch rotation. This was accomplished by shortening the length and decreasing the width of the table top. The modified table top increases the combinations of gantry and couch angles significantly, simplifying the delivery of non-coplanar conformal therapy without significant compromise to routine treatment. The modification of a standard linear accelerator table top has increased the available combinations of gantry and couch rotation to accommodate non-coplanar conforrmal radiotherapy.

Brain Neoplasms↗

PubMed1991

Reduction of the Bremsstrahlung component of clinical electron beams: implications for electron arc therapy and total skin electron irradiation.

The dose due to Bremsstrahlung in stationary electron beams of nominal energies in the range 6-20 MeV is typically between 1-7% of the maximum dose and is usually not clinically significant. However, in treatments using rotational or multiple electron beams where the x-ray dose from several beams is added the x-ray dose will reach much higher proportions and will be of clinical significance. Moreover, this dose often is located in normal tissue beyond the target volume. Reduction of this x-ray dose is therefore desirable. In the present study a reduction of the x-ray component of electron beams produced by a Clinac 2100C accelerator by a change of the transmission ion chamber and scattering foils is reported. A reduction in Bremsstrahlung of up to 50% can be achieved.

Electrons↗

PubMed2006

Monte Carlo study of backscatter in a flattening filter free clinical accelerator.

In conventional linear accelerators, the flattening filter provides a uniform lateral dose profile. In intensity modulated radiation therapy applications, however, the flatness of the photon field and hence the presence of a flattening filter, is not necessary. Removing the filter may provide some advantages, such as faster treatments and smaller out-of-field doses to the patients. In clinical accelerators the backscattered radiation dose from the collimators must be taken into account when the dose to the target volume in the patient is being determined. In the case of a conventional machine, this backscatter is known to great precision. In a flattening filter free accelerator, however, the amount of backscatter may be different. In this study we determined the backscatter contribution to the monitor chamber signal in a flattening filter free clinical accelerator (Varian Clinac 21EX) with Monte Carlo simulations. We found that with the exception of very small fields in the 18-MV photon mode, the contribution of backscattered radiation to the monitor signal did not differ from that of conventional machines with a flattening filter. Hence, a flattening filter free clinical accelerator would not necessitate a different backscatter correction.

Computer Simulation↗

PubMedDate not supplied

Neutron fluxes in radiotherapy rooms.

The spatial distribution of the neutron flux, originated in an electron accelerator therapy room when energies above the threshold of (y,n) and (e,e'n) reactions are employed, is physically due to a direct flux, coming from the accelerator head, and to a flux diffused from the walls. In this work, the flux is described to a high degree of approximation by a set of functions whose spatial behavior is univocally determined by the angular distributions of the neutrons emitted from the shield of the accelerator head and diffused from the walls. The analytical results are verified with an extended series of Monte Carlo simulations obtained with the MCNP code.

Monte Carlo Method↗

PubMedDate not supplied

Cell analysis with the new Leipzig high-energy ion nanoprobe.

The high-energy ion nanoprobe LIPSION at the University of Leipzig has been in operation since 1998. The ultrastable, 3.5 MV SINLETRON accelerator supplies the H+ or He+ ion beam. A magnetic scanning system moves the focused beam across the sample. At present, a resolution of 41 +/- 4 nm in the low current mode and 300 nm at 5 pA can be achieved. The experimental chamber is equipped with electron-, energy dispersive X-ray-, and particle detectors. They can be used simultaneously to analyse the sample by means of PIXE (particle induced X-ray emission), RBS (Rutherford backscattering), and in the case of thin sections or monolayer samples STIM (scanning transmission ion microscopy). A goniometer allows the application of channeling measurements in single crystals in combination with these methods. In contrast to previous publication describing microbeam facility at LIPSION, the current biomedical research has concentrated on microscopy and tomography on chondrocytes in pig cartilages and fixed single endothelial cells (HUVEC). For the irradiation of single living cells, an external beam facility with irradiation platform, fast beamgate and mini-Petri dishes is under construction.

Animals↗

PubMedDate not supplied

Characterization of fragmented heavy-ion beams using a three-stage telescope detector: detector configuration and instrumentation.

Accelerated heavy-ion beams used in biological and medical research are often utilized in conjunction with absorbers which lead to the fragmentation of the beam. The BERKLET, initially a two-stage solid-state telescope detector, was designed to make rapid, on-line energy and linear energy transfer (LET) measurements of individual particles in a heavy-ion beam, thus allowing characterization of fragmented beams. From data collected with the BERKLET, one is able to determine a number of important parameters. These include: residual energy and LET histograms for the full beam and for the individual Z components, relative number of particles with a given Z, and dose and track average LET's for the full beam and for the individual Z's. Improvements to the BERKLET design and changes in data analysis are discussed and contrasted with the results of an earlier BERKLET configuration. The most notable improvements are the addition of a thin scintillation detector for improved LET measurement, a tenfold improvement in the dynamic range of the event discriminator, reported here as 1:2000, and dual high-and low-gain amplification of the LET signals, permitting the identification of particles with Z's ranging from 12 down to 1.

Energy Transfer↗

PubMed1993

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.

Female↗

PubMed1991

Current status of radiosurgery for arteriovenous malformations.

Cerebral arteriovenous malformations (AVM), regardless of the mode of discovery, have an annual risk of hemorrhage of approximately 4 percent. A progressive obliterative vasculitis culminating in the occlusion of an AVM may be induced by the administration of radiation doses of approximately 20 Gy given in a single fraction. The process takes about two years and occlusion occurs in approximately 80% of patients so treated. Such a dose may be accurately administered to AVMs up to 3 cm in diameter with very little radiation imparted to the adjacent brain by means of multiple highly collimated radially arranged cobalt sources (the Gamma Knife) or by means of a modified linear accelerator turned through an arc or arcs with the target AVM as the centre of rotation. The Gamma Knife and the modified linear accelerator have nearly equal accuracy. Recent experience with modified linear accelerators indicates efficacy equal to the Gamma Knife. Both devices are effective treatment for small AVMs but the cost of modifying a pre-existing linear accelerator is only a few percent of the acquisition and installation costs of the Gamma Knife.

Evaluation Studies as Topic↗

PubMed2003

Big physics, small doses: the use of AMS and PET in human microdosing of development drugs.

The process of early clinical drug development has changed little over the past 20 years despite an up to 40% failure rate associated with inappropriate drug metabolism and pharmacokinetics of candidate molecules. A new method of obtaining human metabolism data known as microdosing has been developed which will permit smarter candidate selection by taking investigational drugs into humans earlier. Microdosing depends on the availability of two ultrasensitive 'big-physics' techniques: positron emission tomography (PET) can provide pharmacodynamic information, whereas accelerator mass spectrometry (AMS) provides pharmacokinetic information. Microdosing allows safer human studies as well as reducing the use of animals in preclinical toxicology.

Animals↗

PubMed1995

Shielding design and dose assessment for accelerator based neutron capture therapy.

Preparations are ongoing to test the viability and usefulness of an accelerator source of epithermal neutrons for ultimate use in a clinical environment. This feasibility study is to be conducted in a shielded room located on the Massachusetts Institute of Technology campus and will not involve patient irradiations. The accelerator production of neutrons is based on the 7Li(p, n)7Be reaction, and a maximum proton beam current of 4 mA at an energy of 2.5 MeV is anticipated. The resultant 3.58 x 10(12) neutrons s-1 have a maximum energy of 800 keV and will be substantially moderated. This paper describes the Monte Carlo methods used to estimate the neutron and photon dose rates in a variety of locations in the vicinity of the accelerator, as well as the shielding configuration required when the device is run at maximum current. Results indicate that the highest absorbed dose rate to which any individual will be exposed is 3 microSv h-1 (0.3 mrem h-1). The highest possible yearly dose is 0.2 microSv (2 x 10(-2) mrem) to the general public or 0.9 mSv (90 mrem) to a radiation worker in close proximity to the accelerator facility. The shielding necessary to achieve these dose levels is also discussed.

Biophysical Phenomena↗

PubMed2003

Feasibility study on epithermal neutron field for cyclotron-based boron neutron capture therapy.

To realize the accelerator-based boron neutron capture therapy (BNCT) at the Cyclotron and Radioisotope Center of Tohoku University, the feasibility of a cyclotron-based BNCT was evaluated. This study focuses on optimizing the epithermal neutron field with an energy spectrum and intensity suitable for BNCT for various combinations of neutron-producing reactions and moderator materials. Neutrons emitted at 90 degrees from a thick (stopping-length) Ta target, bombarded by 50 MeV protons of 300 microA beam current, were selected as a neutron source, based on the measurement of angular distributions and neutron energy spectra. As assembly composed of iron, AlF3/Al/6LiF, and lead was chosen as moderators, based on the simulation trials using the MCNPX code. The depth dose distributions in a cylindrical phantom, calculated with the MCNPX code, showed that, within 1 h of therapeutic time, the best moderator assembly, which is 30-cm-thick iron, 39-cm-thick AlF3/Al/6LiF, and 1-cm-thick lead, provides an epithermal neutron flux of 0.7 x 10(9) [n cm(-2) s(-1)]. This results in a tumor dose of 20.9 Gy-eq at a depth of 8 cm in the phantom, which is 6.4 Gy-eq higher than that of the Brookhaven Medical Research Reactor at the equivalent condition of maximum normal tissue tolerance. The beam power of the cyclotron is 15 kW, which is much lower than other accelerator-based BNCT proposals.

Aluminum Compounds↗