摘要:

A theoretical basis for interstitial laser photocoagulation (ILP) practiced with point‐emitting fiber tips has been established by solving the bioheat transfer equation, using basic Green's function methods, for steady and instantaneous point sources of both optical energy and direct heat. Three combination optical and thermal parameters have been identified that strongly influence temperature distributions during ILP. These are defined here as optothermal heat capacities and an optothermal diffusion length, all of which characterize how a thermal diffusion temperature profile is flattened and reduced when optical diffusion is added. Relevance and limitations of this theory for practical ILP are discussed. A useful result is a mathematical verification of previous empirical observations that point optical sources heat tissues less than point heat sources of the same power. A comparison of normalized theoretical temperature transients with published measurements suggests that in normal liver, blood perfusion cooling may exceed thermal conduction by a factor of 5.6±1.7.

ISSN:0094-2405    

DOI:10.1118/1.597279

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simple formula for the spatial resolution of transmission proton radiography is derived for two different methods of measuring the proton coordinates. The effect of multiple Coulomb scattering and energy loss are taken into account. Experimental measurements of the spatial resolution have been done and are compared with the calculations. The technique of measuring entrance and exit coordinates in coincidence for each single proton improves the spatial resolution by a factor of 8 compared to a single coordinate measurement.

ISSN:0094-2405    

DOI:10.1118/1.597212

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Four‐element dipole microwave antenna arrays with square insertion patterns are commonly used clinically for interstitial hyperthermia. One major disadvantage with this type of antenna array is the presence of a large dead length at the tips because the current gradually decreases from maximum at the junctions to zero at the tips. This dead length is usually 1.5–2 cm along the central axis of a 2×2 cm array of regular dipole antennas. Many attempts to improve the performance of dipole antenna arrays have been made by designing antennas with increased current at the tips. While some dipole antennas of new design show negligible dead lengths at close proximity in the single‐antenna configuration, phantom experiments have demonstrated that these antennas exhibit at least 1.1 cm dead space along the central axis of the four‐antenna array. Therefore, there seems to be a limit to which the array dead length can be reduced by the improvements in the dipole‐type antenna design. The goal of this work is to find the theoretical minimum of the array dead length. This was done by assuming a uniform current distribution along the entire antenna. The specific absorption rate (SAR) patterns were calculated for an array with an insertion depth of 7 cm (resonant length for 915 MHz) and a variable spacing between antennas (1–3 cm). It was found that there is a dead length of 6 mm along the central axis of the 2×2‐cm array with the uniform current distribution, which can be considered as the theoretical limit of the dead length for this array. It was also found that the SAR patterns in the midplane of the array are nearly independent of the separation between antennas and the lateral extensions of the SAR patterns are independent of the current distributions.

ISSN:0094-2405    

DOI:10.1118/1.597213

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Electron beam characteristics were investigated for different applicator cone designs. Low‐energy scattered electrons from the cone housing can contribute to the skin dose. The depth dose was measured as a function of electron energies and field size for different cones. The measurements were compared with calculated 3‐D dose distributions obtained from Monte‐Carlo‐derived energy deposition kernels [Phys. Med. Biol.37, 391 (1992)]. This comparison shows that beam‐defining cones can be fabricated to eliminate low‐energy scattered electrons from the cone housing. Eliminating low‐energy electrons from the cones provides depth dose (DD) characteristics for clinical electron beams that are comparable to those (1) from nearly monoenergetic electrons (as determined from the practical range of the electrons), and (2) for which the DD is nearly independent of source‐to‐skin distance. Measurements of DD for small circular fields were also performed. For very small field sizes, corresponding to the width of the lateral spread of a pencil beam, the theoretical and measured DD were found to depend on field size. Three‐dimensional dose calculations of small and irregularly shaped electron fields are necessary to appreciate these effects.

ISSN:0094-2405    

DOI:10.1118/1.597280

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Commercially available ovoid tubes for gynecological applications used in conjunction with the microSelectron‐HDR (Nucletron International B.V., Waardegelder 1, 3905 TH Veenendaal, The Netherlands) for192Ir sources, allow for shielding. Publications concerning the transmission properties of these 4.5‐mm thick stainless steel (AISI number 303/304) shielding segments are scarce and not compatible for implementation in treatment planning. Therefore the effect of shielding on dose distribution is unknown. The effective transmission factor has been measured and implemented in the planning computations. Screening efficacy was evaluated on 20 actual treatment plans, analyzing dose reduction to critical tissue and comparing dose distribution in planes relevant for this particular application. Due to high transmission (effective transmission factor=0.85), stainless steel screening segments only provide low, local dose reductions of maximum 15%. A new approach with regard to optimization and source configuration is needed to reduce dose to vulnerable tissue, exploiting the screening segments to a maximum extent. Better shielding, especially at the midline (plane bisecting the ovoids) could be expected by using shielding segments with other geometrical characteristics.

ISSN:0094-2405    

DOI:10.1118/1.597281

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Beam profiles along the nonwedged direction of a wedged field produced by a linear accelerator exhibit more “sagging” than that of an open field at the same depth. For large fields, the profiles of open and wedged fields can differ by as much as 7%. The extra “sagging” of wedged profiles is mainly due to the difference in penetration between on‐ and off‐axis rays caused by the variation of beam quality across the field. An algorithm was developed to estimate an “effective” depth such that the profile of a wedged field can be approximated by the open‐field profile at the effective depth. The algorithm was verified by measured beam profiles for 6‐ and 15‐MV x‐ray beams for 15°, 30°, 45°, and 60° wedges.

ISSN:0094-2405    

DOI:10.1118/1.597282

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

More than three years have passed since Loma Linda treated the first cancer patient with the world's first proton accelerator dedicated to radiation therapy. Since that time, over 1000 patients have completed treatments and the facility currently treats more than 45 patients per day. With a typical intensity of 3×1010protons per pulse and 27 pulses per minute, dose rates of 90–100 cGy/min are easily achieved on a 20‐cm diameter field. In most cases, patient treatment times are 2 min, much less than the patient alignment time required before each treatment. Nevertheless, there is considerable medical interest in increasing field sizes up to 40‐cm diameter while keeping dose rates high and treatment times low. In this article, beam measurements relevant to intensity studies are presented and possible accelerator modifications for upgrades are proposed. It is shown that nearly all intensity losses can be ascribed to the large momentum spread of the injected beam and occur at or near the injection energy of 2 MeV. The agreement between calculations and measurements appears quite good. In addition, optimum beam characteristics for a new injector are discussed based upon the momentum acceptance and space charge limits of the Loma Linda synchrotron.

ISSN:0094-2405    

DOI:10.1118/1.597270

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A small, lightweight, single‐focusing magnetic spectrometer was designed, assembled, and tested for analysis of electron beams from radiotherapy electron linacs. The objective was to develop a low cost, simple device that could be easily replicated in other medical centers, and to demonstrate the practicality of individual electron counting for precise analysis of electron spectra. Two methods of spectroscopy have been developed. One method consists of counting electrons individually as a function of magnetic field setting. Electrons are deflected through 90° in the magnetic spectrometer, through an exit slit, and into a scintillation detector. A second method consists of recording the complete spectrum of electron energies from the accelerator on a strip of film at a single magnetic field setting. A critical design element is the 10‐cm long collimator for electrons entering the magnet gap, with defining apertures and scraper slits. The spectrometer's cleanliness of transmission, energy calibration, and resolution were all tested at 10 and 16 MeV using the nearly monoenergetic electron beam of the accelerator at the National Research Council of Canada (NRCC). These accelerator tests, and also Monte Carlo trajectory simulations, both show that contamination of the transmitted spectrum due to scattered or knock‐on electrons is negligible. Low‐energy characteristics were tested using a90Sr+90Y beta‐particle source. The energy calibration of the 90° spectrometer mode was based on mapping the magnetic field and also electron trajectory computer simulations. That calibration agrees with the NRCC's own calibrated scale to 0.8% for the single‐particle counting method and to 1.3% for the film method. The energy resolution was measured to be 2% at 10 MeV, which is adequate for radiotherapy linac measurements. The acceptance half angle is 0.5° or less, depending on the aperture size, which is adequate for electron angular distribution measurements within the forward cone of the electron beam. Used with film, the spectrometer is a simple, accurate, and highly transportable device for measuring radiotherapy electron energy spectra.

ISSN:0094-2405    

DOI:10.1118/1.597271

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Composite thermoluminescent (TL) materials characterized by different TL sensitivity to energy of photon radiation, namely, double‐component mixtures of finely dispersed ingredients: thermoluminescent phosphor (LiF or CaSO4:Dy) and nonluminescent material (salts with variousZeffvalues) have been described. The TL sensitivity of such TL composites was demonstrated to depend uponZeffvalues inherent to separate components and upon their concentration ratio in the mixture. Within the range of x‐ray energy values from 10 to 120 keV, the LiF‐based materials containing Li2CO3as nonluminescent material were dosimetrically equivalent to soft tissues. The availability of powder‐like TLD materials containing 65% LiF+35% Li2CO3can definitely serve the basis for extending the application area for the well‐known procedure adopted by IAEA/WHO for comparative testing of x‐ray therapy units with HVL<0.5 mm Cu and for starting the intercomparison of units intended for superficial x‐ray therapy. The use of the dose detectors based on TLD of 35% LiF+65% Li2CO3composition in orthovoltage x‐ray therapy units excludes the necessity for introducing corrections for the dependence of detector's readings on energy levels. LiF‐ and CaSO4:Dy‐based TL materials containing PbSO4as a nonluminescent material are, by a factor of 10, more sensitive to energy variations than other known thermoluminescent phosphors. With the application of TLD based on above materials into two‐detector systems intended for determiningEeffvalues of x‐ray radiation, the ambiguity of readings in low‐energy region has been eliminated, whereas the range of measurement appeared to be significantly expanded.

ISSN:0094-2405    

DOI:10.1118/1.597272

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Prototype miniature dosimeter probes have been designed, built, and characterized employing a small, radiation sensitive metal oxide semiconductor field effect transistor (MOSFET) chip to measure,invivo, the total accumulated dose and dose rate as a function of time after internal administration of long range beta particle radiolabeled antibodies and in external high energy photon and electron beams. The MOSFET detector is mounted on a long narrow alumina substrate to facilitate electrical connection. The MOSFET, alumina substrate, and lead wires are inserted into a 16 gauge flexineedle, which, in turn, may be inserted into tissue. The radiation dosimeter probe has overall dimensions of 1.6 mm diam and 3.5 cm length. The MOSFET probe signals are read, stored, and analyzed using an automated data collection and analysis system. Initially, we have characterized the probe's response to long range beta particle emission from90Y sources in solution and to high energy photon and electron beams from linear accelerators. Since the prototype has a finite substrate thickness, the angular dependence has been studied using beta particle emission from a90Sr source. Temperature dependence and signal drift have been characterized and may be corrected for. Measurements made in spherical volumes containing90Y with diameters less than the maximum electron range, to simulate anticipated geometries in animal models, agree well with Berger point kernel andegs4Monte Carlo calculations. The results from the prototype probes lead to design requirements for detection of shorter range beta particles used in radioimmunotherapy and lower photon energies used in brachytherapy.

ISSN:0094-2405    

DOI:10.1118/1.597214

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY