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Mutagenic effects of heavy ions in bacteria.

Various mutagenic effects by heavy ions were studied in bacteria, irradiated at accelerators in Dubna, Prague, Berkeley or Darmstadt. Endpoints investigated are histidine reversion (B. subtilis, S. typhimurium), azide resistance (B. subtilis), mutation in the lactose operon (E. coli), SOS chromotest (E. coli) and lambda-prophage induction (E. coli). It was found that the cross sections of the different endpoints show a similar dependence on energy. For light ions (Z < or = 4) the cross section decreases with increasing energy. For ions of Z = 10, it is nearly independent of energy. For heavier ions (Z > or = 26) it increases with energy up to a maximum or saturation. The increment becomes steeper with increasing Z. This dependence on energy suggests a "mutagenic belt" inside the track that is restricted to an area where the density of departed energy is low enough not to kill the cell, but high enough to induce mutations.

Azides↗

Radiation therapy for advanced gastric cancer.

A retrospective study of 75 patients with advanced inoperable gastric cancers, referred to the National Cancer Center Hospital between 1962 and 1982, was performed. According to the Borrmann classification based on X ray findings, Type 1 was found in 3 patients, Type 2 in 5, Type 3 in 40, and Type 4 in 15. Twelve patients could not be classified. The histological type was papillary adenocarcinoma in 7 patients, tubular adenocarcinoma in 23, mucinous carcinoma in 6, poorly differentiated adenocarcinoma in 14, signet ring cell carcinoma in 12 and others in 13. The site of remote metastasis in 19 patients was Virchow's lymph node in 8 patients, Douglas pouch in 3, liver and lung in 2 each and others in 4. All patients were treated by a either telecobalt 60 unit or a linear accelerator using 6 Mv photon and the total dose to primary lesion was 4000 cGy in 5 weeks to 7000 cGy in 8-9 weeks. Complete response (CR) was achieved in 6 patients or 8.0%, partial response (PR) in 46 or 61.3%, and no change (NC) in 23 or 30.7%. The response rate based on the sum of CR and PR was about 70%. The 50% survival period in months was 26.5, 7.3, and 3.2, respectively for patients with CR, PR, and NC. For the response of advanced gastric cancer to chemotherapy in the National Cancer Center Hospital, the combined use of UFT and Mitomycin C gave the highest rate, 46%. As for as local response is concerned, the response rate to radiation was 70%, a better result than that of chemotherapy alone.

Adenocarcinoma↗

Evaluation of radiosurgery techniques with cumulative dose volume histograms in linac-based stereotactic external beam irradiation.

Radiosurgery at UCSF is performed with a 6-MV linear accelerator with tertiary collimation for improved small field definition. The dose delivery to the target relative to normal tissue is influenced by the number of arcs, the arc geometry, field size, and beam energy. The impact of arc number, arc geometry, and field size on the dose distribution from 6-MV X rays in a 16 cm spherical phantom has been evaluated through the use of cumulative dose volume histograms. Dose volume histograms were calculated for a) 1-5 and 10 arcs, and b) collimator sizes of 1.25, 2.0, and 3.0 cm. Differences between techniques were found at the 5-10% level for field sizes from 1.25 to 2.0 cm. It was shown that the finite dimension of the sphere and, by extension, head diminishes the differences between techniques for the larger field sizes. The effect of treating with two isocenters is also analyzed and an approach for improving the dose distribution is presented.

Brain Diseases↗

Total skin electron therapy: a technique which can be implemented on a conventional electron linear accelerator.

PURPOSE: A technique for treating whole-body skin with an electron linear accelerator with nominal energies in the range 4-8 MeV is presented. Stationary fields at an extended source-skin distance are used with the patient treated in a reclined position. METHODS AND MATERIALS: The relative beam data, absolute dosimetry measurements and the patient setup parameters are presented. The calculations required to correct for patient size are discussed. RESULTS: The technique described uses a six field circumferential cycle, with longitudinally matched fields along the length of the patient. Treatment times are reasonable using the standard dose rate of the machine. The uniformity of the skin dose measured on three patients was found to be comparable to that of other total skin treatment techniques. CONCLUSION: A technique for treating conditions like mycosis fungoides is presented requiring relatively simple supporting dosimetry. No modifications to the unit are required and no sophisticated treatment apparatus is necessary, which makes the technique attractive to smaller Institutions, especially in developing countries, where technical support may be limited.

Calibration↗

Experimental electron beam irradiation of food and the induction of radioactivity.

Samples of chicken, prawns, cheeses and spices were irradiated on the Harwell electron linear accelerator HELIOS at 20 MeV to assess mechanisms for the induction of radioactivity. The induced radioactivity was measured using a lead shielded Ge(Li) gamma-ray spectrometer, and the results were compared with activities calculated on the basis of photoneutron and photoproton reactions induced by real and virtual photons. In general, there was good agreement. Bounds were also placed on the induction of radioactivity by capture of neutrons produced in the food samples themselves. Further, the data were used to assess the effects of a gross malfunction of an electron beam irradiation facility; after 1 day, the specific activity of food samples irradiated to 10 kGy at 20 MeV was approximately 0.01 Bq g-1. In addition, food samples were also irradiated at 10 MeV, and irradiated and control samples were analysed for microbiological burden. Reductions in the microbiological burden of the food samples by factors consistent with those found in previous measurements were found.

Electrons↗

Workload and use factor of medical linear accelerators in radiotherapy.

An important factor in the design of primary protective barriers is the use factor. The present study was aimed at obtaining historical data on the use factor of two dual modality linear accelerators in a radiotherapy department. Gantry angle, field size, and beam modifiers were recorded for all radiation qualities in use at two medical linear accelerators with 6 MV and 18 MV x-rays and multiple electron energies ranging from 4 MeV to 20 MeV. The data for one year of clinical use was extracted from a record and verifying system and an estimate of the physics workload on the machines was obtained by going through the quality assurance records and machine log books. Of the total dose of approximately 37,000 Gy delivered in one year at isocenter on each unit 80% was given as 6 MV x-rays. As can be expected, most x-ray beams were directed at the four cardinal gantry angles with the angular distribution for 6 MV and 18 MV x-rays being very similar. Electron fields were broadly distributed around the gantry pointing down position. Less than 25% of all clinical x-ray treatment fields extended beyond a field size of 200 cm2.

Blood Component Transfusion↗

Dosimetric characteristics of wedges mounted beyond the blocking tray.

In a beam accessory configuration for a linear accelerator using a prototype multileaf collimator, newly designed wedges were mounted beyond the blocking tray. The isodose curves, depth of maximum dose, surface dose, and wedge transmission factors were measured for the wedges designed for this unique configuration. The same set of wedges was used for both 6- and 18-MV x rays. The shape of the wedged isodose curves was essentially unchanged from those produced by the conventional wedges located above the blocking tray. The isodose curves exhibited the desired wedge angles over the range of field sizes from 5 x 5 to 15 x 40 cm. In the 10 x 10-cm field, the average difference between the observed wedge angle and the desired wedge angle was 3.8 degrees. The surface doses ranged from 18% to 35% for the wedged 10 x 10-cm fields as compared with about 15% for the same open field. Dosimetrically the wedges were acceptable for clinical use.

Humans↗

Choice of optimum megavoltage for accelerators for photon beam treatment.

Over three decades ago, the development of megavoltage accelerators revolutionized radiation oncology and provided the therapist with photons and electrons of any desired energy. The initial advantages cited for high energy photon therapy, listed below, have proved valid and accelerators have almost totally replaced orthovoltage units. Initially, it appeared that most of these cited advantages should continue to improve with increasing energy, and there has been an impetus for the production of ever higher megavoltage accelerators. Some of these advantages are reviewed in this paper. Also, recent investigations have indicated increasing diffuseness of the photon beam boundary with increasing energy because of lateral transport of electrons. The impact on treatment planning as a function of energy of the increase in volume dose due to the diffuseness of beam boundaries, "build-down" and "rebuild-up" effects in tissues at cavity and inhomogeneity interfaces, bone absorption, and photoneutron production are discussed. Consideration of the behavior of these parameters indicates that optimum photon energies have been achieved and that the impetus for higher megavoltages is unwarranted for most treatment. For many therapeutic applications, there are major advantages of 4 MV to 8 MV photon beams relative to 60Co gamma rays. For large lesions in the abdomen or pelvis there is an advantage to energies above those provided by 15 MV units. The various considerations above are discussed and summarized as a function of lesion site and megavoltage.

Humans↗

Measurement and analysis of induced activities in concrete irradiated using high-energy neutrons at KENS Neutron Spallation Source Facility.

Precise estimation of induced activities in concrete shields for high-energy accelerator facilities is one of the most important issues that need to be solved, not only for the reduction of exposure for workers, but also for the reduction of radioactive wastes. Irradiation experiments have been performed by using the 500 MeV Neutron Spallation Source Facility in KEK. The large concrete assembly was placed in the direction of 0 degrees to the beamline. Two kinds of samples were placed at several positions in the assembly. The irradiation period was about 1 week and induced activities in the samples were measured until approximately 1.5 y after irradiation. From the comparison between the experiment and the available Monte Carlo calculation code system, good agreement was obtained for 24Na, 47Sc, 47Ca and 54Mn within a factor 2; however, large discrepancies were observed for some other nuclides.

Computer Simulation↗

The Stanford Linear Accelerator Center pulsed x-ray facility.

The Stanford Linear Accelerator Center (SLAC) operates a high-energy (up to 33 GeV) linear accelerator delivering pulses up to a few microseconds wide. The pulsed nature of the electron beam creates problems in the detection and measurement of radiation both from the accelerator beam and the klystrons that provide the radio-frequency power for the accelerator. Hence, a pulsed x-ray facility has been built at SLAC mainly for testing the response of different radiation detection instruments to pulsed radiation fields. The x-ray tube consists of an electron gun with a control grid. This provides a stream of pulsed electrons that can be accelerated towards a confined target window. The window consists of Al 0.051 cm (20 mils) thick, plated on the vacuum side with a layer of Au 0.0006 cm (1/4 mil) thick. The frequency of electron pulses can be varied by an internal pulser from 60 to 360 pulses per second with pulse widths of 360 ns to 5 microseconds. The pulse amplitude can be varied over a wide range of currents. An external pulser can be used to obtain other frequencies or special pulse shapes. The voltage across the gun can be varied from 0 to 100 kV. The maximum absorbed dose rate obtained at 6.35 cm below the target window as measured by an ionization chamber is 258 Gy/h. The major part of the x-ray tube is enclosed in a large walk-in cabinet made of 1.9-cm-thick (3/4-inch-thick) plywood and lined with 0.32-cm-thick (1/8-inch-thick) Pb to make a very versatile facility.

Electrons↗

Accelerator mass spectrometry of 63Ni at the Munich Tandem Laboratory for estimating fast neutron fluences from the Hiroshima atomic bomb.

After the release of the present dosimetry system DS86 in 1987, measurements have shown that DS86 may substantially underestimate thermal neutron fluences at large distances (>1,000 m) from the hypocenter in Hiroshima. This discrepancy casts doubts on the DS86 neutron source term and, consequently, the survivors' estimated neutron doses. However, the doses were caused mainly by fast neutrons. To determine retrospectively fast neutron fluences in Hiroshima, the reaction 63Cu(n, p)63Ni can be used, if adequate copper samples can be found. Measuring 63Ni (half life 100 y) in Hiroshima samples requires a very sensitive technique, such as accelerator mass spectrometry (AMS), because of the relatively small amounts of 63Ni expected (approximately 10(5)-10(6) atoms per gram of copper). Experiments performed at Lawrence Livermore National Laboratory have demonstrated in 1996 that AMS can be used to measure 63Ni in Hiroshima copper samples. Subsequently, a collaboration was established with the Technical University of Munich in view of its potential to perform more sensitive measurements of 63Ni than the Livermore facility and in the interest of interlaboratory validation. This paper presents the progress made at the Munich facility in the measurement of 63Ni by AMS. The Munich accelerator mass spectrometry facility is a combination of a high energy tandem accelerator and a detection system featuring a gas-filled magnet. It is designed for high sensitivity measurements of long-lived radioisotopes. Optimization of the ion source setup has further improved the sensitivity for 63Ni by reducing the background level of the 63Cu isobar interference by about two orders of magnitude. Current background levels correspond to a ratio of 63Ni/Ni<2x10(-14) and suggest that, with adequate copper samples, the assessment of fast neutron fluences in Hiroshima and Nagasaki is possible for ground distances of up to 1500 m, and--under favorable conditions--even beyond. To demonstrate this capability, we have measured successfully 6Ni/Ni ratios as low as (3.5 +/- 0.6) x 10(-13). The latter are, based on DS86, representative of a ratio expected from a typical Hiroshima copper sample at about 1,300-m ground range.

Body Burden↗

Validation of Monte Carlo generated phase-space descriptions of medical linear accelerators.

The accuracy of Monte Carlo codes in dose calculation systems relies on the correctness of the input data. Monte Carlo calculations are performed to generate phase-space descriptions of the Varian 2100C accelerator at 6 and 18 MeV. Before these data can be reliably used as the input for dose calculations in patients, they must be properly validated. This validation consists of three different stages: validation of the coding of the geometry, validation of the user code for the Monte Carlo code, and validation of calculated results. Geometric validation is performed by isolating and testing treatment head components independently. The user code is checked by testing for energy conservation and the variance reduction schemes incorporated into the user code are checked by comparison of results calculated with and without their employment. Validation of the phase-space description is performed by calculation of depth dose curves and lateral profiles for dose deposition in phantom, with difference plots used to illustrate any discrepancies. Calculated and experimental in-phantom output is also determined. After complete validation, the calculated data can then be reliably used as the input for dose calculations.

Biophysical Phenomena↗

Physics for radiosurgery with linear accelerators.

Radiosurgery had a long development period and, for more than three decades, was used only in a few specialized centers around the world. The development of LINAC-based radiosurgical techniques combined with the concurrent advances in imaging modalities during the 1980s, however, caused so much interest in this treatment modality that most major radiotherapy centers now offer this service or at least plan to offer it in the near future. When considering a LINAC for radiosurgical use, one should remember that technical and clinical requirements for accurate radiosurgery are far more stringent than those applied to standard radiotherapy. This is because in radiosurgery, the targeted volumes are much smaller and the dose is usually delivered in a single irradiation session, whereas in radiotherapy, the dose is delivered to a relatively large target volume on a fractionated basis. Linear accelerator-based radiosurgery broadens the scope of radiotherapy departments. The impetus to introduce this service at a medical center usually comes from neurosurgeons, however. Even after the service becomes routine at an institution, it is the neurosurgeon who refers the patient and who plays the most important role in determining the target volume and its location within the brain. The decision on the choice of isodose surface and the prescribed dose, however, belongs to the radiotherapist. It is becoming clear that radiosurgery is a complex treatment modality for which a successful outcome requires a collaborative team effort by several hospital-based professionals, including neurosurgeons, radiation oncologists, neuroradiologists, and medical physicists. As in standard radiotherapy, physics plays an important role in radiosurgery, not only in the development of target localization, treatment-planning, and dose delivery techniques, but also in the actual patient contact, from the diagnostic target localization procedures, through treatment planning, to patient preparation on the device and dose delivery.

Brain Neoplasms↗

Peripheral dose from a linear accelerator equipped with multileaf collimation.

In radiation therapy, knowledge of the peripheral dose is important when anatomical structures with very low dose tolerances might be involved. Two of the major sources of peripheral dose, leakage from the linac head, and scatter from secondary collimators, depend strongly on the configuration of the linac head and therefore might be affected by the presence of a multileaf collimator (MLC). In this study, peripheral dose was measured at two depths and two field sizes for 6 and 18 MV photons from a linac with a MLC. The MLC was configured both with leaves fully retracted and with leaves positioned at the field edges defined by the secondary collimator jaws. Comparative measurements were also made for 6 MV photons from a linac without MLC. Peripheral dose was determined as a percentage of the central axis dose for the same energy, field size, and depth using diode detectors in solid phantom material. The data for the 6 MV without MLC agreed with those for the beam with MLC leaves retracted. For both energies at all depths and distances from the field edge, configuring the MLC leaves at the field edge yielded a reduction in peripheral dose of 6%-50% compared to MLC leaves fully retracted.

Equipment Design↗

Investigation of photon beam output factors for conformal radiation therapy--Monte Carlo simulations and measurements.

The purpose of this study was to investigate beam output factors (OFs) for conformal radiation therapy and to compare the OFs measured with different detectors with those simulated with Monte Carlo methods. Four different detectors (diode, diamond, pinpoint and ionization chamber) were used to measure photon beam OFs in a water phantom at a depth of 10 cm with a source-surface distance (SSD) of 100 cm. Square fields with widths ranging from 1 cm to 15 cm were observed; the OF for the different field sizes was normalized to that measured at a 5 cm x 5 cm field size at a depth of 10 cm. The BEAM/EGS4 program was used to simulate the exact geometry of a 6 MV photon beam generated by the linear accelerator, and the DOSXYZ-code was implemented to calculate the OFs for all field sizes. Two resolutions (0.1 cm and 0.5 cm voxel size) were chosen here. In addition, to model the detector four kinds of material, water, air, graphite or silicon, were placed in the corresponding voxels. Profiles and depth dose distributions resulting from the simulation show good agreement with the measurements. Deviations of less than 2% can be observed. The OF measured with different detectors in water vary by more than 35% for 1 cm x 1 cm fields. This result can also be found for the simulated OF with different voxel sizes and materials. For field sizes of at least 2 cm x 2 cm the deviations between all measurements and simulations are below 3%. This demonstrates that very small fields have a bad effect on dosimetric accuracy and precision. Finally, Monte Carlo methods can be significant in determining the OF for small fields.

Humans↗

Silicon diode detectors used in radiological physics measurements. Part II: Measurement of dosimetry data for high-energy photons.

Initial calibration of a linear accelerator requires physics instruments to measure accurately central axis depth-dose and off-axis data, both in and out of the beam. These data for an 8- MeV unit were first measured using film, a Farmer 0.6-cm3 ion chamber, a 0.3-cm3 ion chamber, and a 0.1-cm3 silicon diode. Both small probes and film gave a high response compared to the Farmer probe, which has a uniform energy response. Measurements with the diode interfaced to an X-Y recorder required only a fraction of the time required with the chambers, minimizing error due to change in machine output, and permitted resolution of isodose lines in the penumbra. However, corrections required at points in depth due to nonuniform energy response of the unshielded diode were laborious. Construction of a partially shielded diode which duplicates the response of the Farmer probe eliminated the necessity for corrections, permitting rapid accumulation of a wide range of depth-dose and off-axis data.

Elementary Particles↗

A semianalytical method for the design of a linac x-ray beam flattening filter.

The purpose of this study was to design an improved flattening filter for a Therac 20 medical linear accelerator. Profiles of the 18-MV x-ray beam produced by this accelerator measured along the diagonal of a 40 X 40 cm field at a depth of 5 cm were measured, and it was found that there were regions near the corners of the field where the dose was 109% of the central axis dose. An iterative algorithm for designing flattening filters was developed which required, as input, precise measurements of the following data: the unflattened primary beam profile, the fraction of the beam due to contamination radiation arising from interactions of primary photons with the flattening filter and the collimator assemblies, and the attenuation of the primary photons in water and lead as a function of angle from the central axis of the beam. A new flattening filter was designed and profiles of the beam were measured at a number of depths. These measurements showed that the beam was flattened to within +/- 1% out to 24 cm along the diagonal of a 40 X 40 cm field at a depth of 5 cm.

Equipment Design↗

Stereotactic radiosurgery: dose-volume analysis of linear accelerator techniques.

Stereotactic radiosurgery of the brain may be accomplished with a linear accelerator by performing several noncoplanar arcs of a highly collimated beam focused at a point. The shape of the radiation distribution produced by this technique is affected by the beam energy, field size, and the number and size of the arcs. The influence of these parameters on the resulting radiation distributions was analyzed by computing dose volume histograms for a typical brain. Dose volume functions were computed for: (a) the energy range of 4-24 MV x rays; (b) target sizes of 1-4 cm; and (c) 1-11 arcs and dynamic rotation. The dose volume histograms were found to be dependent on the number of arcs for target sizes of 1-4 cm. However, these differences were minimal for techniques with 4 arcs or more. The influence of beam energy on the dose volume histogram was also found to be minimal.

Brain↗