摘要:

An imaging device has been developed to acquire images during external photon‐beam radiotherapy treatments. It consists of a linear array of 128 zinc tungstate (ZnWO4) scintillation crystals each of which is individually optically coupled to a photodiode and associated electronics. The image is formed by scanning the linear array across the radiation field using a stepping motor under the control of a microcomputer. Image archive, display, and analysis are performed using a microVAX II computer. Results from a general theoretical analysis are presented before a detailed description of the particular detector construction. The mechanical design of the detector is such that the detector is automatically positioned to within a millimeter relative to the treatment source. This simplifies procedures for analyzing setup variations when comparing a treatment image to any other treatment, or planning, images. Image acquisition takes under 4 s with a contrast resolution of better than 1% at a spatial resolution of 2.5 mm in the object plane. The primary dose used to form these images is 0.55 cGy although the dose received by the patient will be closer to 25 cGy due to the linear scanning geometry and 3.8‐s scan time that is used.

ISSN:0269-3127    

DOI:10.1118/1.596661

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The charge collection in a liquid‐filled matrix ionization chamber system has been investigated. This system, which is used for megavoltage radiography, is scanned electronically by switching the polarizing voltage. In such a system the charge collection cannot be described by the classical ionization chamber theory since this is only valid for a fixed polarizing voltage. A model is presented for an ideal plane‐parallel ionization chamber with switched polarizing voltage that gives a good qualitative description of the physical phenomena in our nonideal system. The new model predicts the amount of charge collected and the signal‐to‐noise ratio as a function of several parameters such as electrode distance, polarizing voltage, radiation intensity, and some liquid characteristics. An important result is that the quantum noise contribution can be made quite small. This situation occurs in liquids with a low ion mobility and long ion lifetimes by a charge‐integrating effect in the liquid. Experiments were performed to test various aspects of the model. A reasonable agreement is found between theoretical and experimental results. The possible use of ultrahigh‐mobility liquids is also discussed.

ISSN:0269-3127    

DOI:10.1118/1.596662

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A treatment planning methodology was developed to investigate the invariant kernel form of conformal therapy proposed by Brahme. Three‐dimensional dose distributions were calculated by convolving a rotationally symmetric, invariant kernel with weighting distributions. Fourier transform convolution techniques implemented on an array processor were used to achieve high calculation speeds, thereby allowing iterative techniques in the spatial and frequency domains for computing dose distributions that asymptotically approach a desired dose distribution. To use rotationally symmetric kernels, the generality of the solution is traded for a fast, deterministic, inverse planning approach. The limitations imposed on the dose distributions by this loss of generality are characterized and tentative conclusions are drawn about the potentials and limits of clinical application of this form of the methodology. Further developments of the concept are suggested.

ISSN:0269-3127    

DOI:10.1118/1.596663

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A model is presented for estimating the contribution of annihilation radiation to the dose perturbation at interfaces between high and low atomic number materials. The contribution is small, but not negligible relative to the total interface dose perturbation. The maximum contribution occurs for photon beams of about 8 MeV in energy. For an 8‐MeV beam passing first through lead, then through polystyrene, the annihilation radiation contribution to the interface dose perturbation is about 8%, at a copper/polystyrene interface, the contribution is about 7%, and at an aluminum/polystyrene interface, the contribution is about 3%.

ISSN:0269-3127    

DOI:10.1118/1.596664

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Batho power‐law formula is in common use in many treatment planning systems to correct for the presence of lungs and other inhomogeneities. While giving excellent agreement with measurement for Cobalt‐60 radiation, it tends to underestimate the lung correction required for higher energy x rays and is undefined for distances beyond an interface less than the buildup distance. This paper suggests a simple modification that greatly improves the agreement with measured data and gives a continuously defined function at all depths. Measurements have been made in a polystyrene and cork phantom to simulate the effects of lung; data are presented for beams of 8‐MV x rays, 16‐MV x rays, and Cobalt‐60 γ rays.

ISSN:0269-3127    

DOI:10.1118/1.596665

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Obtaining accurate %DD values for routine treatment calculations is essential in radiation therapy. Many papers have presented expressions to calculate this or related parameters but these expressions often required that the parameters needed by the equation be determined for each individual treatment unit. This paper presents an expression that calculates %DD values with a mean‐square accuracy of ∼1.0% versus measured values. The expression is applicable to beam energies ranging from Co‐60 to 24 MV, field sizes from 4×4 to 40×40 cm2, and depths from 1 cm deeper thandmaxto 30 cm. The only information required by this expression that is machine specific is the ionization ratio.

ISSN:0269-3127    

DOI:10.1118/1.596666

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Bremsstrahlung spectra from thick cylindrical targets of Be, Al, and Pb have been measured at angles of 0°, 1°, 2°, 4°, 10°, 30°, 60°, and 90° relative to the beam axis for electrons of 15‐MeV incident energy. The spectra are absolute (photons per incident electron) and have a 145‐keV lower‐energy cutoff. The target thicknesses were nominally 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20‐cm‐diam by 25‐cm‐long cylindrical NaI detector. Measured spectra were corrected for pile‐up, background, detector response, detector efficiency, attenuation in materials between the target and detector and collimator effects. Spectra were also calculated using the EGS4 Monte Carlo system for simulating the radiation transport. There was excellent agreement between the measured and calculated spectral shapes. The measured yield of photons per incident electron was 9% and 7% greater than the calculated yield for Be and Al, respectively, and 2% less for Pb, all with an uncertainty of ±5%. There was no significant angular variation in the ratio of the measured and calculated yields. The angular distributions of bremsstrahlung calculated using available analytical theories dropped off more quickly with angle than the measured distributions. The predictions of the theories would be improved by including target‐scattered photons.

ISSN:0269-3127    

DOI:10.1118/1.596667

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In a number of clinical situations, the dose to the skin and the superficial tissues is of concern. Both beam obliquity and a beam modifier will modify the dose delivered to these regions due to changes in the scattering geometry, scattered photon and secondary electron production, and changes in the energy spectrum of a polyenergetic beam. Some linear accelerators use a single universal wedge mounted within the treatment head. Because such a wedge is at an extended distance from the patient, its contribution to the beam contaminants incident to the skin will be limited. Measurements of the ionization in the buildup region have been performed in a polystyrene phantom irradiated with a 6‐MV x‐ray beam from a linear accelerator equipped with a universal wedge. The variation of the buildup dose with obliquity, universal wedging, and distance from the source has been measured for angles of incidence between 0° and 60° and for effective wedge angles between 0° and 45°. The results indicate that the percentage buildup has a much stronger dependence upon the angle of incidence than upon the effective wedge angle. For distances approaching the treatment head, it is shown that the universal wedge generates secondary electrons that elevate the surface dose, but that this contribution decreases with distance.

ISSN:0269-3127    

DOI:10.1118/1.596733

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It is shown that in narrow monoenergetic photon beams under conditions of lateral electron disequilibrium, primary absorbed doseP(r) is a simple function of beam radiusr:P(r)=Pλ⋅(1−e−γ⋅r), wherePλis the primary dose in broad beams for which complete lateral electron equilibrium exists, and γ depends on photon energy and absorbing medium. This formula was evaluated using Monte‐Carlo‐generated data for the primary dose in water from monoenergetic photons in the energy range from 2 to 8 MeV. The primary dose was studied in beams of radii 0.006 cm

ISSN:0269-3127    

DOI:10.1118/1.596741

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The cavity‐gas calibration factorNgasfor Memorial parallel‐plate chambers was determined against calibrated Farmer chambers using two different calibration methods as suggested in the TG 21 protocol. These methods are: (a)60Co calibration at 5‐cm depth; and (b) high‐energy electron beam calibration. TheNgasvalues determined from the60Co in‐phantom calibrations were approximately 1.9% higher than the values determined from the high‐energy electron beams.

ISSN:0269-3127    

DOI:10.1118/1.596668

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY