摘要:

An investigation of x‐ray spectral reconstruction from transmission data by direct resolution of the matrix systemA*F=T(using spectral algebra formalism) has been previously presented. The resolution has been done with simulated spectrum. In this paper, the method on a real case of a 12 MV photon beam was tested. A special study of the setup has been made to estimate and reduce the experimental errors that could alter the results. In order to convertF(E) (a fraction of the signal due to a photon of energyE) into photon fluence Φ(E), the chamber energy responseR(E) has been studied and an approximated analytical function for its representation was proposed. Spectra reconstructed from different transmission data using different attenuators, buildup caps, and ionization chambers have been compared to verify the uniqueness of the reconstructed spectra. To test the validity of the results, dosimetric values, such as Depth Dose Data have been calculated, from our spectrum, using a specific code developed by Kosunenetal. The results show a good agreement between the measured and calculated data.

ISSN:0094-2405    

DOI:10.1118/1.597523

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The bremsstrahlung spectra from thick targets of Be, Al, and Pb are calculated using the Monte Carlo code MCNP (Monte Carlo N‐particle). The current version of MCNP (v.4A) incorporates a coupled electron‐photon transport scheme that allows the user to estimate the photon fluence produced from primary electron interactions. The simulation parameters are based on bremsstrahlung measurements of 15 MeV electrons incident on thick targets of Be, Al, and Pb at various angles between 0° and 90°. The integrated yield and mean energy of each bremsstrahlung spectrum is calculated for the three targets at these angles. For angles less than 60° the integrated yield calculated by MCNP4A is within 6% of measured values for the three targets. Furthermore, predicted mean energy is within 7% of the values derived from measurement for all angles tested. Also compared are the performances of two MCNP4A fluence tallies; a next‐event estimator (detector tally) and a track length estimator (cell tally). Timing studies indicate the detector tally will perform the integrated yield calculations to a precision of 1% approximately 10 to 50 times faster than the conventional cell tally for an emission angle of 0°.

ISSN:0094-2405    

DOI:10.1118/1.597528

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

General ion recombination has been studied under irradiation conditions relevant for diagnostic radiology and with four different ionization chambers. When the exposure time is appreciably shorter than the ion transit time, the exposure can be designated as pulsed irradiation. On the contrary, for relatively long irradiation times, the term continuous irradiation can be applied. Recombination was estimated by measuring the collected charge at various collecting potentials of the ionization chamber. This is a well‐known method in radiotherapy, but unfortunately it cannot be used in diagnostic radiology with typical exposure meters, since they do not offer the option of varying the collecting potential. For exposures with diagnostic x‐ray units, an alternative approach is to vary the exposure or exposure rate over a wide range at a constant collecting potential. Experimental and theoretical estimates of ion recombination did not yield similar values. This might be due to several causes, such as differences between the actual and the nominal dimensions and volumes of the ionization chambers, due to errors and uncertainties in the physical parameters used in the theoretical models or due to deviations of the shape of the ionization chambers from the perfect cylindrical or parallel plate geometry. For better accuracy, corrections for recombination losses should therefore be based on experimental verification rather than on theoretical models.

ISSN:0094-2405    

DOI:10.1118/1.597529

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597530

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A one‐dimensional electron transport algorithm based on the ideas of invariant imbedding has been developed and applied to the problem of electron backscattering in electron dosimetry. Results have been compared with experimental and Monte Carlo results. Agreement appears to be reasonable at low incident energies (0.5 to 10 MeV) and lowZ(Z≤65) materials.

ISSN:0094-2405    

DOI:10.1118/1.597595

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A prototype multi‐rod collimator for producing irregular fields in photon radiation therapy has been designed and built. The mechanical details of the design and operation of the multi‐rod collimator are discussed. Beam profiles for an approximately 10×10 cm2field have been measured at various depths in phantom, and compared with profiles obtained using the secondary collimator jaws alone and with cast metal blocks. The ability of the collimator to produce irregular fields is demonstrated with reference to some commonly encountered therapy fields and the ability to produce central blocks and island blocks is discussed. Isodose curves for selected irregular fields are presented.

ISSN:0094-2405    

DOI:10.1118/1.597531

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The new proton therapy facility is being assembled at the Paul Scherrer Institute (PSI). The beam delivered by the PSI sector cyclotron can be split and brought into a new hall where it is degraded from 590 MeV down to an energy in the range of 85–270 MeV. A new beam line following the degrader is used to clean the low‐energetic beam in phase space and momentum band. The analyzed beam is then injected into a compact isocentric gantry, where it is applied to the patient using a new dynamic treatment modality, the so‐called spot‐scanning technique. This technique will permit full three‐dimensional conformation of the dose to the target volume to be realized in a routine way without the need for individualized patient hardware like collimators and compensators. By combining the scanning of the focused pencil beam within the beam optics of the gantry and by mounting the patient table eccentrically on the gantry, the diameter of the rotating structure has been reduced to only 4 m. In the article the degrees of freedom available on the gantry to apply the beam to the patient (with two rotations for head treatments) are also discussed. The devices for the positioning of the patient on the gantry (x rays and proton radiography) and outside the treatment room (the patient transporter system and the modified mechanics of the computer tomograph unit) are briefly presented. The status of the facility and first experimental results are introduced for later reference.

ISSN:0094-2405    

DOI:10.1118/1.597522

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simple approach was developed that calculates the output factors and tissue maximum ratio of an asymmetric field at any point within the open field, and specifically both at the central axis (when the jaws do not shadow it) and at the effective center of the open field, using the existing tables for symmetric fields and the multidepth profile information for the largest available field size (either open or with a wedge present). Day's method was adapted to calculate the effective values of the usual field‐size‐dependent parameters. This approach makes these parameters also dependent on the location of the calculation point relative to the field edges in an asymmetric field. This algorithm was tested by comparing its predictions with measurements of asymmetric and half blocked fields, with and without wedges, in a water phantom at different depths and off‐axis distances. The agreement between calculated and measured dose rate is within 1%–3% even in highly asymmetric fields for both 6‐ and 18‐MV photons.

ISSN:0094-2405    

DOI:10.1118/1.597524

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Approximately 4000 women per year in the United States require radiotherapy during pregnancy. This report presents data and techniques that allow the medical physicist to estimate the radiation dose the fetus will receive and to reduce this dose with appropriate shielding. Out‐of‐beam data are presented for a variety of photon beams, including cobalt‐60 gamma rays and x rays from 4 to 18 MV. Designs for simple and inexpensive to more complex and expensive types of shielding equipment are described. Clinical examples show that proper shielding can reduce the radiation dose to the fetus by 50%. In addition, a review of the biological aspects of irradiation enables estimates of the risks of lethality, growth retardation, mental retardation, malformation, sterility, cancer induction, and genetic defects to the fetus.

ISSN:0094-2405    

DOI:10.1118/1.597525

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Source localization from radiographs can be very difficult for template‐guided implants if the needle images overlap. At UCSF several techniques to make this task easier have been developed. The techniques include selection of an optimum simulator gantry angle, use of different types of dummies, and differential dummy loading. In addition, several modifications have been made in our brachytherapy planning computer program to facilitate source entry. As a result of these improvements, source localization is now accomplished in much less time with improved accuracy.

ISSN:0094-2405    

DOI:10.1118/1.597596

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY