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At least 847 records · Page 47Linked to original sources
PubMed1989

Measurement of neutron leakage spectra at a 500-MeV proton accelerator.

We have measured the neutron leakage spectra at a number of representative locations outside the shielding of a 500-MeV proton accelerator using a multisphere technique and the 12C(n,2n)11C reaction in a plastic scintillator. The inclusion of the 12C(n,2n)11C reaction in the set of responses has a pronounced effect on the shape of the unfolded neutron spectra and the ambient dose equivalent rate calculated from them.

Neutrons↗
PubMed1996

Dose equivalent measurements at a 2.7 GeV proton accelerator and comparison with the Moyer model.

The Moyer model, based on a semi-empirical method validated in the 7.4 to 350 GeV energy range, is generally used to calculate lateral shielding for high energy proton accelerators. Measurements made for the Saclay Synchrocyclotron, Saturne, have enabled the parameters corresponding to a 2.7 GeV model to be studied for different target thicknesses and angles of observation. These studies show how new data have been used to modify the equations of the model.

Particle Accelerators↗
PubMedDate not supplied

Accelerator-based epithermal neutron beam design for neutron capture therapy.

Recent interest in the production of epithermal neutrons for use in boron neutron capture therapy (BNCT) has promoted an investigation into the feasibility of generating such neutrons with a high current proton accelerator. Energetic protons (2.5 MeV) on a 7Li target produce a spectrum of neutrons with maximum energy of roughly 800 keV. A number of combinations of D2O moderator, lead reflector, 6Li thermal neutron filtration, and D2O/6Li shielding will result in a useful epithermal flux of 1.6 x 10(8) n/s at the patient position. The neutron beam is capable of delivering 3000 RBE-cGy to a tumor at a depth of 7.5 cm in a total treatment time of 60-93 min (depending on RBE values used and based on a 24-cm diameter x 19-cm length D2O moderator). Treatment of deeper tumors with therapeutic advantage would also be possible. Maximum advantage depths (RBE weighted) of 8.2-9.2 (again depending on RBE values and precise moderator configuration) are obtained in a right-circular cylindrical phantom composed of brain-equivalent material with an advantage ratio of 4.7-6.3. A tandem cascade accelerator (TCA), designed and constructed at Science Research Laboratory (SRL) in Somerville MA, can provide the required proton beam parameters for BNCT of deep-seated tumors. An optimized configuration of materials required to shift the accelerator neutron spectrum down to therapeutically useful energies has been designed using Monte Carlo simulation in the Whitaker College Biomedical Imaging and Computation Laboratory at MIT. Actual construction of the moderator/reflector assembly is currently underway.

Boron↗
PubMed1999

Comparative measurements on a series of accelerators by the same vendor.

A series of comparative measurements were conducted during a 12-month period on a series of linear accelerators made by the same vendor. These measurements were conducted during acceptance and commissioning of six Varian model 2100C linear accelerators which had identical beam energies and beam modifiers. Nominal accelerating potentials, scatter measurements, depth doses, beam profiles, wedge and electron beam descriptors are presented. The results show highly similar beam parameters with observed variations being approximately equivalent to the precision of measurement. This similarity implies that a standard data set can be used to validate new commissioning data.

Electrons↗
PubMed2005

Linear accelerator radiosurgery for trigeminal neuralgia.

OBJECTIVE: To report the clinical outcomes following treatment of trigeminal neuralgia with linear accelerator-based radiosurgery. METHODS: Twenty-eight patients with medication refractory idiopathic trigeminal neuralgia were treated with a single fraction of 80 Gy to the trigeminal nerve root. For treatment delivery, a 4-mm collimator and a 7-arc technique were delivered using a stereotactic floor stand system with an isocenter stability of 0.2 +/- 0.1 mm to minimize dose to the brainstem. Treatment delivery time was approximately 55 minutes. RESULTS: With a median follow-up of 12 months (range, 1-40 mo), 57% of patients achieved complete pain relief and 75% (exact 95% confidence interval, 55 to 89%) had their pain reduced to 3 or less on a 10-point pain scale. Median time to pain relief was 1 month. Four patients did not respond to treatment. The actuarial mean time to pain recurrence in responders was 14 months, and the actuarial mean response duration in major to complete responders was 16 months. Women had significantly longer mean time to pain recurrence than men (16 versus 7 months; P = 0.05). Three patients reported new mild facial numbness after radiosurgery and one patient developed neurotrophic keratopathy. CONCLUSION: Linear accelerator-based radiosurgery for medication refractory trigeminal neuralgia provides effective pain relief with a low complication rate.

Aged↗
PubMed2005

Linear accelerator surgery for meningiomas.

OBJECT: In this paper the authors review the results of a single-center experience in the use of linear accelerator (LINAC) surgery for radiosurgical treatment of meningiomas. METHODS: A retrospective analysis of all patients treated with LINAC surgery for meningiomas between May 1989 and December 2001 was performed. All patients participated in follow-up review for a minimum of 2 years, and no patients were excluded. Two hundred ten patients were treated during the study interval. The actuarial local control rate for benign tumors was 100% at both 1 and 2 years, and 96% at 5 years. The actuarial local control rate for atypical tumors was 100% at 1 year, 92% at 2 years, and 77% at 5 years; and that for malignant tumors was 100% at both 1 and 2 years, and only 19% at 5 years. Of the 210 patients 13 (6.2%) experienced temporary radiation-induced complications, and only five (2.3%) experienced permanent complications. In all patients with a permanent complication the histological characteristics of the meningioma were malignant. CONCLUSIONS: Linear accelerator surgery produced high local control rates and very low rates of permanent morbidity in patients harboring benign meningiomas.

Adult↗
PubMed1997

[Motion control system in BJ14 medical electron accelerator].

A dual computer based control mode was used in 14 Mev medical electron accelerator. The special non-linear velocity transformation method makes the positioning operation very fast and convenient. The independent driving mode of four blades are suitable to clinical uses. A multi-interlock function provides a safe environment for user's operation.

Computers↗
PubMedDate not supplied

[Formation and study of the dosimetric characteristics of wide fields of fast electrons used to irradiate laboratory animals using the LUE-25 accelerator].

A mode of formation of wide fields of fast electrons at LUE-25 accelerator is proposed to expose uniformly the extended 4 cm thick biological objects. Dosimetric characteristics of the electron beams have been studied by film and ferrous-sulfate dosimeters. The potencies and limitations of using the dosimeters as a "witness" are discussed.

Animals↗
PubMedDate not supplied

Linear accelerator radiosurgery. A clinical experience.

A clinical experience obtained by the employ of a radiosurgical technique utilizing a standard linear accelerator and a multiple non coplanar arc irradiation modality is presented. The series consists of 155 patients treated from November 1982 until March 1988. These patients harbored intracranial lesions deemed not suitable to open craniotomy procedures.

Adolescent↗
PubMed1984

Experimental evidence for beta-decay as a source of chirality by enantiomer analysis.

Earlier experiments testing the Vester-Ulbricht beta-decay hypothesis for the origin of molecular chirality are reviewed, followed by descriptions of our own experiments involving attempted asymmetric radiolysis of DL-amino acids using quantitative gas chromotography as a probe for optical activity. Our radiation sources included 90Sr-90Y, 14C and 32P Bremsstrahlen, longitudinally polarized electrons from a linear accelerator and longitudinally polarized protons from a cyclotron. With the possible exception of the linear accelerator irradiations, these experiments failed to produce g.c.-detectable enantiomeric excesses, even at 50-70% gross radiolysis. We thus find no unambiguous support for the Vester-Ulbricht hypothesis in any of the attempted asymmetric radiolyses performed to date. Radioracemization, a possible reason for these failures, is discussed.

Amino Acids↗
PubMed1995

Photoneutrons from medical linear accelerators--radiobiological measurements and risk estimates.

PURPOSE: To assess the oncogenic potential of the photoneutrons produced by high energy medical linear accelerators. METHODS AND MATERIALS: An established line of cells of rodent origin (C3H 10T1/2) was used to assess the oncogenic potential of the radiation dose received in the breast of an anthropomorphic "randoman" phanton, while the cervix received a dose of 70 Gy. Experiments were performed at 6 MV, below the threshold for the production of photoneutrons, and at 20 MV where the dose includes about 0.01 Gy of photoneutrons as well as scattered x-rays. RESULTS: A significantly higher transformation incidence was observed for the 20-MV machine, consistent with the measured neutron dose of about 0.01 Gy and a quality factor of 20. CONCLUSION: An estimate can be made of the additional deaths from second malignancies that might result from the photoneutrons generated by higher energy linear accelerators (Linacs), which must be offset against the possible improvements in survival that might result from the higher tumor doses made possible by the increased percentage depth doses.

Animals↗
PubMed2001

A method for controlling image acquisition in electronic portal imaging devices.

Certain types of camera-based electronic portal imaging devices (EPIDs) which initiate image acquisition based on sensing a change in video level have been observed to trigger unreliably at the beginning of dynamic multileaf collimation sequences. A simple, novel means of controlling image acquisition with an Elekta linear accelerator (Elekta Oncology Systems, Crawley, UK) is proposed which is based on illumination of a photodetector (ORP-12, Silonex Inc., Plattsburgh, NY, USA) by the electron gun of the accelerator. By incorporating a simple trigger circuit it is possible to derive a beam on/off status signal which changes at least 100 ms before any dose is measured by the accelerator. The status signal does not return to the beam-off state until all dose has been delivered and is suitable for accelerator pulse repetition frequencies of 50-400 Hz. The status signal is thus a reliable means of indicating the initiation and termination of radiation exposure, and thus controlling image acquisition of such EPIDs for this application.

Biophysical Phenomena↗
PubMedDate not supplied

An improved technique for aligning the light field with the radiation fields on radiotherapy accelerators.

An improved scheme has been developed for measuring the coincidence of the light field and radiation fields of a radiotherapy accelerator. Scans of the light and radiation fields are made sequentially using a photodiode on standard field plotting apparatus. By superimposing the two plots, differences between the two fields' boundaries can be accurately determined. The technique is simple and avoids uncertainties inherent in common techniques employing film.

Humans↗
PubMed2002

Linear accelerator radiosurgery for meningiomas in and around the cavernous sinus.

OBJECTIVE: A retrospective study to evaluate the efficacy and side effects of linear accelerator radiosurgery in the treatment of cavernous sinus meningiomas. METHODS: Between 1993 and 2001, 42 patients with meningiomas involving the cavernous sinus underwent linear accelerator radiosurgery at our institution. A mean radiation dose of 14 Gy was delivered to the tumor margin. The median tumor volume was 8.2 cm3 (mean, 8.4 cm3). Median follow-up was 36 months (mean, 38 mo). RESULTS: Control of tumor growth was achieved in 97.5% of the patients. There was no mortality or permanent extraocular motor or pituitary dysfunction. Treatment-related complications included new trigeminal neuropathy in 4.7% and a new visual field defect in 2.8%. Two patients required shunt placement after developing hydrocephalus. One patient with symptomatic temporal lobe edema underwent partial excision of the tumor. Improvement of existing cranial neuropathies was noted in 29% of affected trigeminal nerves, in 22% of oculomotor nerves, and in 13% of Cranial Nerves IV and VI. CONCLUSION: This study indicates that linear accelerator radiosurgery can achieve a high control rate of meningiomas involving the cavernous sinus with no mortality and a low incidence of morbidity.

Adult↗
PubMed2002

Egg of the Karner Blue butterfly (Lycaeides melissa samuelis): morphology and elemental analysis.

Most insect eggshells are ornately sculptured; that of the Karner Blue butterfly, Lycaeides melissa samuelis, exhibits a series of interwoven ridges and depressions. Scanning electron microscopic views of the shell show that the patterning resides in the outer chorion, while the inner vitelline membrane is relatively flat and featureless. We here describe the morphology of the egg and introduce a physical technique, use of a Dynamitron accelerator, to identify and localize elements in the eggshell. Most elements present are represented in the chorion, but sulfur appears restricted to the vitelline membrane. The micropyle is particularly rich in calcium and, in unhatched eggs, phosphorus as well.

Animals↗
PubMed2005

Low-dose megavoltage cone-beam CT for radiation therapy.

PURPOSE: The objective of this work was to demonstrate the feasibility of acquiring low-exposure megavoltage cone-beam CT (MV CBCT) three-dimensional (3D) image data of sufficient quality to register the CBCT images to kilovoltage planning CT images for patient alignment and dose verification purposes. METHODS AND MATERIALS: A standard clinical 6-MV Primus linear accelerator, operating in arc therapy mode, and an amorphous-silicon (a-Si) flat-panel electronic portal-imaging device (EPID) were employed. The dose-pulse rate of 6-MV Primus accelerator beam was windowed to expose an a-Si flat panel by using only 0.02 to 0.08 monitor unit (MUs) per image. A triggered image-acquisition mode was designed to produce a high signal-to-noise ratio without pulsing artifacts. Several data sets were acquired for an anthropomorphic head phantom and frozen sheep and pig cadaver head, as well as for a head-and-neck cancer patient on intensity-modulated radiotherapy (IMRT). For each CBCT image, a set of 90 to 180 projection images incremented by 1 degree to 2 degrees was acquired. The two-dimensional (2D) projection images were then synthesized into a 3D image by use of cone-beam CT reconstruction. The resulting MV CBCT image set was used to visualize the 3D bony anatomy and some soft-tissue details. The 3D image registration with the kV planning CT was performed either automatically by application of a maximization of mutual information (MMI) algorithm or manually by aligning multiple 1D slices. RESULTS: Low-noise 3D MV CBCT images without pulsing artifacts were acquired with a total delivered dose that ranged from 5 to 15 cGy. Acquisition times, including image readout, were on the order of 90 seconds for 180 projection images taken through a continuous gantry rotation of 180 degrees. The processing time of the data required an additional 90 seconds for the reconstruction of a 256(3) cube with 1.0-mm voxel size. Implanted gold markers (1 mm x 3 mm) were easily visible or all exposure levels without artifacts. In general, the presence of high Z materials such as tooth fillings or implanted markers did not result in visible streak artifacts. The registration of structures such as the spinal canal and the nasopharynx in the MV CBCT and kV CT data sets was possible with millimeter and degree accuracy as assessed by displacement simulations and subsequent visual evaluation. CONCLUSIONS: We believe that the quality of these images, along with the rapid acquisition and reconstruction times, demonstrates that MV CBCT performed by use of a standard linear accelerator equipped with a flat-panel imager can be applied clinically for patient alignment.

Animals↗
PubMed1990

Proton beam therapy: a promising method of locoregional cancer control.

Proton beams offer superior characteristics for clinical radiation therapy, including the capability to localize precisely the dose to the desired target volume. Such precision enables the radiation therapist to give higher doses to the tumor while avoiding intolerable doses to adjacent normal tissues. Locoregional control is thus increased, and treatment morbidity and side effects are decreased. When it opens in late spring 1990, Loma Linda University Medical Center's proton treatment facility will feature the world's first accelerator and proton therapy system designed for patient care. During the next decade, other similarly-designed proton therapy systems will be built in Canada, England, France, Belgium, Germany, Japan, and South Africa, as well as at Massachusetts General Hospital in the United States.

Facility Design and Construction↗