摘要:

Wall correction factors, which correct ion chamber response for photon attenuation and scatter, can differ by as much as 1.0% for spherical chambers depending on whether they are obtained experimentally by extrapolation measurements or by Monte Carlo simulation. This difference is not explained by experimental or calculational statistics which lie in the range 0.05%–0.2%. In this paper it is demonstrated that linear extrapolation of experimental data for spherical chambers is inappropriate, owing to the curvature of the chamber walls. A simple nonlinear theory is constructed that resolves the difference. The Monte Carlo calculations and the nonlinear theory are compared with extrapolation measurements for the NIST (formerly NBS) spherical chambers. It is concluded that wall correction factors should be obtained by Monte Carlo calculation for spherical chambers and that linear‐extrapolation techniques should be regarded with suspicion for all chambers.

ISSN:0269-3127    

DOI:10.1118/1.596547

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A major deficiency of current photon calculation methods that are based on the concept of primary and scatter separation is their inability to handle the condition of electronic disequilibrium. This deficiency is examined and it is shown that the limitation is not inherent in the algorithms themselves but is, at least in part, in the data which the algorithms use. A new concept of primary and scatter separation is developed to cover the condition of electronic disequilibrium. This new concept requires little change to the existing algorithms and only additional data are required, which are generated using Monte Carlo calculation methods. The new concept is tested using programs in the Theratronics Theraplan treatment‐planning system, and two calculation examples illustrate the ability to model electron transport and also the improvement over the existing algorithms. Close analogy of the extended concept with the convolution/superposition method of dose calculation is also indicated.

ISSN:0269-3127    

DOI:10.1118/1.596577

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Experimental techniques have been developed for obtaining single‐event microdosimetric spectra from hospital based linear accelerators. Therapeutic electrons of 12, 15, 18, and 20 MeV from Clinic 18 and 20 accelerators have been produced at ultralow dose rates. Details of the experimental methods have been described previously by the authors. Single‐event lineal energy spectra for these beams, as measured by a Rossi chamber differ significantly from cobalt‐60 both in shape and dose average lineal energy (ȳd) which, depending on electron energy, can be 20–40% lower. The spectral peaks for electrons are greatly enhanced compared to cobalt. Theȳdfor electrons also differs from previously reported values for 10–15‐MV photon beams. These differences are less pronounced than for differences with cobalt. Spectral peaks and shapes correlate well with the actual electron stopping powers in tissue. The theory of dual radiation action predicts changes in relative biological effectiveness at low doses for megavoltage photon and electrons. Although at clinical doses the predicted differences are not statistically significant.

ISSN:0269-3127    

DOI:10.1118/1.596490

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dose measurements using Fricke and ionization methods were compared for60Co gamma rays, 4–25‐MV photons, and 10–25‐MeV electrons. Fricke derived doses based on a constant yield (εmG) were in good agreement with ionization derived doses based on the American Association of Physicists in Medicine Task Group 21 protocol and the National Research Council of Canada (NRC)NDcalibration or the NRC proposedNx. These measurements also confirmed the validity of the double‐voltage technique in the collection efficiency correction, even for swept electron beams. Assuming the correctness of the ionization derived doses, the radiation yield appeared to be 1% higher and to increase with photon energy when irradiation vessels were made of Pyrex but not with polystyrene cells. These glass wall effects could be due to the scattering perturbation of electrons between inhomogeneous materials and, in particular for photon beams, due to the mismatch in mass energy absorption ratios and mass collision stopping power ratios between the Fricke dosimeter and the wall materials.

ISSN:0269-3127    

DOI:10.1118/1.596491

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simple technique to determine the neutron and photon spectra of a clinical fast neutron beam is described. This technique involves making narrow beam attenuation measurements with a pair of ionization chambers and an iterative fitting program to analyze the data. A method is also described for determining the first‐guess neutron spectrum for input into the iterative program. The results of the analysis yield spectra suitable for use in dose calculation algorithms and dosimetry protocols. Presented here is the first‐known published photon spectrum from a clinical machine.

ISSN:0269-3127    

DOI:10.1118/1.596492

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Maintenance of the controlled temperatures at their target levels in the face of disturbances, a uniform temperature distribution within the treatment region, an acceptable temperature rise outside that volume, a fast temperature rise, and stability are desirable characteristics of an optimal hyperthermia treatment control system. This paper presents a proportional‐integral‐derivative plus bang–bang (power on at either a maximum value or at zero) feedback control system designed to meet the above requirements for a scanned focused ultrasound hyperthermia system. Treatment simulations and analytical results for a first‐order approximation of a tumor show that the controller is stable for a wide range of gains and sampling times. It was also found that there is an optimal controller gain which minimizes the peak temperature overshoot and the settling time when a step function input is applied to the system. Both the simulation results and experimental animal results show that the controlled region can be rapidly heated to the target temperature with a small overshoot and maintained at that level in the face of disturbances. The effects of temperature fluctuations due to both the periodic changes caused by the scanning and due to measurement noise can be reduced by the use of an auto regressive moving average approach.Invitrodog kidney model andinvivodog thigh experiments show that the controller works well in practice, and verify that it can compensate for spatial and temporal blood perfusion variations. As shown in both these experiments and in simulations the controller can be used for controlling a single temperature or multiple temperature points simultaneously, thus allowing relatively uniform temperature fields to be created.

ISSN:0269-3127    

DOI:10.1118/1.596581

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This study investigated the design concepts and development of a multielement intracavitary ultrasound applicator for use in hyperthermia. A necessary condition imposed on these applicators is that each transducer element be separately powered and produce collimated beams. This way, the power deposition within the target volume can be controlled by varying the power to each element. Theoretical computer simulations (acoustic and thermal) and bench experiments were used to determine the constraints on the transducer element size and the spacing between them. These have shown that the length of the cylindrical segments (or subsections of) must be greater than ∼10λ for proper collimation and that the spacing between them must be less than ∼1.5 mm for uniform heating. With these design principles in mind, applicators were constructed using sections of cylindrical transducers (wall‐thickness resonance). These were surrounded by temperature‐controlled circulating water which was enclosed by a latex membrane. This allowed for acoustic coupling and additional control over the depth of the maximum temperature from the cavity wall. This depth could be varied between the cavity surface and up to 1.5 cm for circulating water temperatures between 5 and 42 °C, respectively. These applicators were testedinvivoand were able to induce controlled transrectal heating, at depths of 2–3 cm, in the canine rectum and prostate gland.

ISSN:0269-3127    

DOI:10.1118/1.596582

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Radiotherapy with orthovoltage techniques is often the prime treatment for localized superficial malignancies. Surface doses and depth doses measured with cylindrical and end‐window Farmer chambers are presented for various orthovoltage x‐ray beams in the range from 80 to 300 kVp, both for open beams and beams collimated with commercial intracavitary leaded‐glass cones. For radiation fields collimated by a diaphragm positioned at a distance from the patient surface (open beams) there is a small skin‐sparing effect. On the other hand, the surface doses with commercial leaded‐glass intracavitary cones can exhibit a fivefold increase compared to the open‐beam dose maxima. Beyond a depth of ∼0.2 mm in a tissue‐equivalent phantom, the doses measured for open beams and beams collimated with intracavitary cones are essentially identical. The increase in the surface dose observed with intracavitary cones is attributed to photoelectrons and recoil electrons produced in the cones. The high surface doses are measured by thin‐wall parallel‐plate ionization chambers but cannot be measured with cylindrical Farmer chambers since these chambers have wall thicknesses too large for the transmission of electrons produced in the cone. Since cylindrical Farmer chambers are typically used for calibration of radiation output, the high surface doses produced by the intracavitary cones may be overlooked; they can, however, be reduced to open‐beam values by simple modifications to the cones.

ISSN:0269-3127    

DOI:10.1118/1.596493

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Sr‐90 ophthalmic applicators are commonly used for the treatment of superficial eye disorders. Although a variety of dosimetric devices such as film, thermoluminescent dosimeters (TLD's), ion chambers, and radiochromic foils have been used to measure the peak dose at the applicator surface, there is no internationally agreed upon calibration procedure. Recently, large discrepancies among calibrations of the same applicator at three institutions have been reported. Here we describe a technique to obtain the peak dose rate at the applicator surface using LiF TLD's. The technique can be used for the calibration of flat as well as curved surface applicators. Results for two flat and three concave applicators are presented. Our measurement of the surface dose rate for one of the flat applicators is compared with those obtained by four other institutions, each using different dosimetric devices.

ISSN:0269-3127    

DOI:10.1118/1.596494

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An artificial neural network using input data derived from attenuation measurements was trained to generate spectral profiles (relative number of photons versus energy). Once the relative spectral distribution is reconstructed, absolute spectra (number of photons per unit exposure versus energy) can be calculated. A neural network was trained on spectra generated mathematically using the Birch–Marshall model, combined with attenuation data, calculated from the spectra by numerical integration. Whereas attenuation data can be calculated in a straightforward manner from the x‐ray spectra, the reverse is not true. Several neural networks were successfully taught to reconstruct the spectra, given the attenuation data. The networks were tested using kV/inherent filtration combinations that were not in the training set, and the performance of the reconstruction was excellent. Noise in the attenuation data was simulated to test the effects of noise propagation in the reconstruction. The effects of network architecture and data averaging on noise propagation were investigated. Experimentally determined spectral data compiled by Fewell were also used to train a neural network, and the results of the reconstruction were also found to be excellent.

ISSN:0269-3127    

DOI:10.1118/1.596495

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY