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J Siebers

Publications and source records attributed to J Siebers.

9 recordsLinked to original sources
PubMed1999

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↗
PubMed1998

Proton beam output measurement with an extrapolation chamber.

A variable air-volume, parallel-plate extrapolation chamber forming an integral part of a polystyrene phantom was used in measurement of dose rate in a 250 MeV clinical proton beam. The sensitive air-volume of the extrapolation chamber is controlled through the movement of the chamber piston by means of a micrometer mounted on the phantom body. The relative displacement of the piston is monitored by a calibrated mechanical distance travel indicator. The proton beam dose rate determined with the uncalibrated extrapolation chamber was 5% lower than the dose rate determined with a calibrated Farmer-type thimble chamber at the same depth in the polystyrene phantom. Despite the current 5% discrepancy, uncalibrated extrapolation chambers may offer a simple and practical alternative to current techniques used in output measurements of proton beam machines.

Calibration↗
PubMed1996

Proton dosimetry intercomparison.

BACKGROUND AND PURPOSE: Methods for determining absorbed dose in clinical proton beams are based on dosimetry protocols provided by the AAPM and the ECHED. Both groups recommend the use of air-filled ionization chambers calibrated in terms of exposure or air kerma in a 60Co beam when a calorimeter or Faraday cup dosimeter is not available. The set of input data used in the AAPM and the ECHED protocols, especially proton stopping powers and w-value is different. In order to verify inter-institutional uniformity of proton beam calibration, the AAPM and the ECHED recommend periodic dosimetry intercomparisons. In this paper we report the results of an international proton dosimetry intercomparison which was held at Loma Linda University Medical Center. The goal of the intercomparison was two-fold: first, to estimate the consistency of absorbed dose delivered to patients among the participating facilities, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration protocols. MATERIALS AND METHODS: Thirteen institutions participated in an international proton dosimetry intercomparison. The measurements were performed in a 15-cm square field at a depth of 10 cm in both an unmodulated beam (nominal accelerator energy of 250 MeV) and a 6-cm modulated beam (nominal accelerator energy of 155 MeV), and also in a circular field of diameter 2.6 cm at a depth of 1.14 cm in a beam with 2.4 cm modulation (nominal accelerator energy of 100 MeV). RESULTS: The results of the intercomparison have shown that using ionization chambers with 60Co calibration factors traceable to standard laboratories, and institution-specific conversion factors and dose protocols, the absorbed dose specified to the patient would fall within 3% of the mean value. A single measurement using an ionization chamber with a proton chamber factor determined with a Faraday cup calibration differed from the mean by 8%. CONCLUSION: The adoption of a single ionization chamber dosimetry protocol and uniform conversion factors will establish agreement on proton absorbed dose to approximately 1.5%, consistent with that which has been observed in high-energy photon and electron dosimetry.

Calibration↗
PubMed1995

Application of solid state detectors for dosimetry of therapeutic proton beams.

A PTW Riga diamond detector and LiF TLDs have been evaluated for use in proton beam dosimetry by comparing results of proton beam calibration with those obtained using thimble ionization chambers. The thimble ionization chambers were calibrated in terms of exposure while the TLDs and diamond detector were calibrated in terms of absorbed dose in a 60Co beam. Absorbed doses to muscle in proton beams for ionization chambers were derived using the TG 20 charged particle protocol. Absorbed doses to muscle for solid state detectors were derived using absorbed dose proton beam quality correction factors. Differences between the derived doses for ionization chambers and solid state detectors were found to be within the uncertainties of measurements: 4.5% for ionization chambers and 5% for solid state detectors.

Cobalt Radioisotopes↗
PubMed1979

[Metabolic products of microorganisms. 175. Tetracenomycin C (author's transl)].

Streptomyces glaucescens, strain Tü 49 = ETH 22794, produces hydroxystreptomycin as well as the tetracenomycins, a mixture of several lipophilic antibiotics. The main component and the most active one is tetracenomycin C. Tetracenomycin C has a molecular formula C23H20O11 and is chemically related to tetracyclines and anthracyclinones. The pale yellow antibiotic is active against some gram-positive bacteria, especially against streptomycetes. Gram-negative bacteria and fungi are not inhibited. In considering the differences of biological activity and the functional groups of the molecule, tetracenomycin C is not a member of the tetracycline or anthracyclinone group of antibiotics.

Anti-Bacterial Agents↗
PubMedDate not supplied

A prototype beam delivery system for the proton medical accelerator at Loma Linda.

A variable energy proton accelerator was commissioned at Fermi National Accelerator Laboratory for use in cancer treatment at the Loma Linda University Medical Center. The advantages of precise dose localization by proton therapy, while sparing nearby healthy tissue, are well documented [R. R. Wilson, Radiology 47, 487 (1946); M. Wagner, Med. Phys. 9, 749 (1982); M. Goitein and F. Chen, Med. Phys. 10, 831 (1983)]. One of the components of the proton therapy facility is a beam delivery system capable of delivering precise dose distributions to the target volume in the patient. To this end, a prototype beam delivery system was tested during the accelerator's commissioning period. The beam delivery system consisted of a beam spreading device to produce a large, uniform field, a range modulator to generate a spread out Bragg peak (SOBP), and various beam detectors to measure intensity, beam centering, and dose distributions. The beam delivery system provided a uniform proton dose distribution in a cylindrical volume of 20-cm-diam area and 9-cm depth. The dose variations throughout the target volume were found to be less than +/- 5%. Modifications in the range modulator should reduce this considerably. The central axis dose rate in the region of the SOBP was found to be 0.4 cGy/spill with an incident beam intensity of 6.7 x 10(9) protons/spill. With an accelerator repetition rate of 30 spills/min and expected intensity of 2.5 x 10(10) protons/spill for patient treatment, this system can provide 50 cGy/min for a 20-cm-diam field and 9-cm range modulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Equipment Design↗
PubMedDate not supplied

A beam intensity monitor for the Loma Linda cancer therapy proton accelerator.

A beam intensity monitor was tested in a 230-MeV proton beam at the Loma Linda Proton Therapy Accelerator during its commissioning at Fermi National Accelerator Laboratory. The intensity monitor was designed to regulate the beam intensity extracted from the proton synchrotron. The proton beam is tunable between 70 and 250 MeV with an adjustable intensity between 10(10) and 10(11) protons per spill. A beam spill is typically 1 s long with a 2-s repetition period. The intensity monitor must be radiation hard, expose minimum mass to the beam, and measure intensity to 1% in 1-ms time intervals. To this end, a 5-cm-thick xenon gas scintillator optically coupled to a photomultiplier tube (PMT) was tested to measure its response to the proton beam. The gas cell was operated at 1.2 atm of pressure and has 12.7-microns-thick titanium entrance and exit foils. The total mass exposed to the beam is 0.14 g/cm2 and is dominated by the titanium windows. This mass corresponds to a range attenuation equal to 1.4 mm of water. The energy lost to the xenon gas is about 70 keV per proton. Each passing proton will produce approximately 2000 photons. With a detection efficiency on the order of 0.05% for this UV light, one would anticipate over 10(10) photoelectrons per second. In a 1-ms time bin there will be approximately 10(7) photoelectrons. This yields a resolution limited by systematics. For unregulated 0.4-s proton spills, we observe a response bandwidth in excess of 10(4) Hz. While signal-to-noise and linearity were not easily measured, we estimate as few as 10(3) protons can be observed suggesting a dynamic range in excess of 10(5) is available.

Humans↗