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At least 577 records · Page 32Linked to original sources
PubMedDate not supplied

Electron beam characteristics on a Philips SL25.

Dosimetry measurements at nominal electron energies of 4, 6, 8, 10, 12, 15, 17, 20, and 22 MeV were made for different sized, open-sided applicators on two Philips SL25 linear accelerators. Measurements include beam flatness, percentage depth dose, surface dose, isodose curves, field size dependence, output at extended distances, virtual source position, and required low melting point alloy thickness for field shaping. These measurements are presented to document the characteristics of electron beams with a new type of applicator design on this series of Philips accelerators.

Electrons↗
PubMedDate not supplied

Albedos for 4-, 10-, and 18-MV bremsstrahlung x-ray beams on concrete, iron, and lead--normally incident.

Backscattering of radiation incident on a solid medium has been treated with some success as a reflection phenomenon. The concept of albedo has been used in the literature to deal with this problem. A Monte Carlo program has been used in this study to simulate the generation of bremsstrahlung beams from medical linear accelerators. The accuracy of the calculated spectra have been checked indirectly by calculating TPRs and comparing them with measured values. The dose albedos for 60Co and 137Cs beams normally incident on concrete have been calculated and compared with experimentally determined values in the literature. The generated energy spectra are then used to calculate the albedo factors for 4-, 10-, and 18-MV bremsstrahlung x-ray beams normally incident on concrete, iron, and lead. The number albedo decreases with increase in the atomic number of the reflecting medium. However, for high-energy photons, the dose albedo increases with increase in the atomic number of the medium. This is an important factor that needs to be considered in the design of shielding. The calculated energy spectra can be used in other problems and the albedo factors can be applied in practical shielding design for medical linear accelerators.

Cesium Radioisotopes↗
PubMed1996

Optimization of conformal electron beam therapy using energy- and fluence-modulated beams.

Fluence modulation of multiple electron beams of various energies has been used to optimize the delivered dose distribution during electron beam radiation therapy. By maximizing the probability of achieving tumor control without causing severe complications electron beam fluence profiles have been optimized for superficial target volumes. It is possible to use several equiportal fluence-modulated electron beams to modify the energy deposition with depth in a controlled manner making it possible to use the technique as an alternative to bolus. The technique was tested in two representative phantom geometries and in three clinical patient geometries using a set of five and two different energies. The local maxima in dose for the plans with five energies were typically lower than with the conventional or advanced bolus techniques. The principles for how the technique could be carried out in the future with a fourth generation radiotherapy accelerator are also indicated.

Electrons↗
PubMed1998

Calculation of photon energy and dose distributions in a 50 MV scanned photon beam for different target configurations and scan patterns.

A method to characterize the energy distribution in the whole photon field is valuable when designing an accelerator for choosing target and flattening filter or scan pattern. Another field of application is beam characterization for treatment planning systems or other dosimetric purposes. This work is focused on the energy distribution in different 50 MV bremsstrahlung beams with different scanning of electrons on three different targets. Fluence differential in energy and angle at the exit of each target has been determined by Monte Carlo calculations for a narrow beam. Data for broad beams were obtained by convolution of the narrow beams with different scan patterns. Photon energy fluence differential in energy at SSD 100 were thus found to be rather different for the targets studied. The results are presented as mean energy profiles and narrow beam half-value layer (HVL) in water. Two different experimental setups were used to measure HVL at the central axis and at off-axis positions. The two methods gave results which differ by 5%-6% and the calculated data where within these experimental results. In conclusion, the presented method for characterization of the photon field energy distribution is well within the experimental results and can thus be used to improve accelerator design or dosimetric calculations, e.g., for treatment planning.

Biophysical Phenomena↗
PubMed1998

Linear accelerator output variations and their consequences for megavoltage imaging.

An experimental study of radiation output intensity fluctuations of a Philips SL25 linear accelerator is presented. Measurements are obtained using an electronic portal imaging device, and the consequences of the measured fluctuations for various different applications of megavoltage imaging including portal imaging, transit dosimetry and megavoltage computed tomography (MVCT) are discussed with examples. Fluctuations in output of +/- 0.7% (1 SD) are seen on every radiation pulse after photon noise and uncertainties caused by the detection system have been accounted for. Large fluctuations are also seen during the initial beam stabilization period (15%), during normal accelerator operation after the beam has been on for more than 1 min (4.5%) and during are therapy as a repeatable function of gantry angle (9%). Such output intensity fluctuations are shown to produce image artifacts in portal imaging devices with scanned detector readout and can also produce systematic errors in detector calibration that would lead to uncertainty in transit dose calculations. The propagation of these intensity fluctuations through MVCT image reconstruction is shown to produce ring artifacts in the reconstructed image. Sample portal and MVCT images are presented. All observed fluctuations in accelerator output are well within the manufacturer's specifications and do not affect the total dose delivered during normal treatment. Finally, megavoltage imaging is shown to be a powerful tool for accelerator quality assurance and treatment verification.

Calibration↗
PubMed2005

Accelerated partial breast irradiation: technically feasible but who will benefit?

Modern breast cancer radiotherapy aims to increase uncomplicated cure rates. A priority is reduction of late effects which include chronic chest wall or breast pain, poor cosmesis, and cardiac toxicity. As breast screening detects early cancers we may be able to safely restrict irradiation postlumpectomy to the tumour bed with a margin, defining a 'partial breast' target volume for treatment. Differing technical approaches to partial breast irradiation are being evaluated in phase III studies with standard whole breast irradiation. These include intra-operative single doses, hypo-fractionated accelerated brachytherapy, and LINAC (linear accelerator)-based three-dimensional external beam therapy.

Brachytherapy↗
PubMed1996

[Photons and protons in radiation therapy. The prospects for developing a proton therapy based on a new Russian accelerator].

Body distribution of absorbed energy of ionizing radiation determines relations between the desired effect and associated radiation lesions. Common methods of photon radiotherapy are characterized by strong irradiation of healthy tissues. Accelerated protons enables an increase in gradient of doses between the radiation-exposed subject and adjacent tissues. It is thought valid to promote wider use of the new Russian proton accelerator for upgrading radiotherapy.

Equipment Design↗
PubMed2001

Dosimetry of low-energy protons and light ions.

For the vertical beam facility at the 14 MV Munich tandem accelerator, various techniques for dosimetry were tested for radiation fields of low-energy protons and light ions (4He, 12C and 16O). A reference dose was determined from the fluence of particles by counting individual particles. A parallel-plate Markus chamber with a small sensitive air volume was used for beam dosimetry applying the ICRU protocol. The doses measured with the ionization chamber were compared with doses evaluated from the fluence measurements. Alternative dose measurements were performed using MTS-N LiF:Mg, Ti thermoluminescence detectors (TLDs) and a photometrically evaluated Fricke chemical dosimeter. An uncertainty of 8% was found in the determination of the dose relative to the reference method. Effects of an inhomogeneous energy loss and a finite track length of the projectiles in the sensitive detector volume of the dosimeters had to be taken into account.

Biophysical Phenomena↗
PubMed2004

Improved treatment of pelvis and inguinal nodes using modified segmental boost technique: dosimetric evaluation.

PURPOSE: To describe a novel, yet simple, modified segmental boost technique (MSBT) and to compare the dosimetry of our method with that of other traditional methods of treatment for the pelvis and inguinal nodes. METHODS AND MATERIALS: We developed a radiotherapy technique that uses linear accelerators with multileaf collimators to treat the pelvis and sequentially boost the inguinal regions, while minimizing "hot spots" across the match-line. This was achieved by angling the gantry for the inguinal fields so that their medial borders aligned with the divergence of the posterior pelvic field. Film dosimetry was performed to compare the MSBT with the traditional segmental boost technique, partial transmission block, and photon/electron combination techniques. These treatment techniques were scored on the basis of the dose homogeneity index, defined as the ratio of match-line maximum dose to the average dose at a given depth in the groin treatment area. RESULTS: The values of the dose homogeneity index were the same (1.04) for MSBT and partial transmission block, and 1.21, 1.39, and 1.18 for the segmental boost technique, photon pelvis with electron tags, and photon pelvis with electron boost, respectively. CONCLUSION: The MSBT proved to be technically simple while optimizing dose homogeneity compared with the other techniques and allows for maximum use of the features of modern linear accelerators.

Electrons↗
PubMed2003

Evaluation of the veridose QC phantom.

Daily, monthly, and annual quality control (QC) of linear accelerators are part of the major tasks of the medical physicist to verify that patients are receiving proper radiation treatment. The control tests consist of the measurement of beam output, verification of the beam energy, and determination of the beam flatness and symmetry in a linear accelerator. A new device, referred to as QC phantom, was designed and fabricated for the QC of linear accelerators. This device is accompanied by software generating the reports of all measured data, keeping track of day-to-day data, and plotting the results. The accuracy, reproducibility, and linearity of the QC phantom were evaluated in this project. Also, the user friendliness of this device for morning warmup of linear accelerators was tested.

Attitude of Health Personnel↗
PubMed1981

Misonidazole enhancement of radiation-induced growth delay in rat rhabdomyosarcoma tumours exposed to accelerated carbon and neon ions.

The response of a rat rhabdomyosarcoma tumour was assessed by measurements of radiation-induced growth delay resulting from administration of the hypoxic cell sensitizer misonidazole in combination with single and fractionated doses of X-rays and charged-particle radiation. Enhancement ratios of 1.8--2.1 were obtained following single doses of misonidazole (500 mg/kg i.p.) and 225 kV X-rays. Single doses of misonidazole with either carbon-ion or neon-ion radiation in the 4 cm extended-peak ionization region led to enhancement ratios of 1.2--1.3. When combined misonidazole (300 mg/kg i.p.) and X-ray treatments were given in four daily fractions, the enhancement ratios decreased to 1.2--1.5. However, a four-fraction schedule using either carbon-ion or neon-ion radiation in combination with misonidazole gave enhancement ratios of 1.1--1.3, which are similar to the values obtained for single-dose schedules with the sensitizer and charged-particle radiation.

Animals↗
PubMed2003

Introducing a system for automated control of rotation axes, collimator and laser adjustment for a medical linear accelerator.

Mechanical stability and precise adjustment of rotation axes, collimator and room lasers are essential for the success of radiotherapy and particularly stereotactic radiosurgery with a linear accelerator. Quality assurance procedures, at present mainly based on visual tests and radiographic film evaluations, should desirably be little time consuming and highly accurate. We present a method based on segmentation and analysis of digital images acquired with an electronic portal imaging device (EPID) that meets these objectives. The method can be employed for routine quality assurance with a square field formed by the built-in collimator jaws as well as with a circular field using an external drill hole collimator. A number of tests, performed to evaluate accuracy and reproducibility of the algorithm, yielded very satisfying results. Studies performed over a period of 18 months prove the applicability of the inspected accelerator for stereotactic radiosurgery.

Algorithms↗