摘要:

An algorithm, which combines the scanning beam with dynamic collimation to generate any arbitrary intensity profile, is presented. The desired intensity profile is assumed to be piecewise linear. The dynamic collimation method used is the “sliding window.” The algorithm can be used either for a given scanning beam profile or to simultaneously determine the scanning beam profile and the leaf motions required to generate the desired intensity profile, which minimize the total treatment time. The limitations imposed by the physics of an elementary beam are taken into account. The algorithm is an iterative one, with typical calculation times being of the order of a few milliseconds.

ISSN:0094-2405    

DOI:10.1118/1.597672

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The dose along the radial direction located at the midplane of a radioactive stent, simulated by a uniform cylinder of32P, is represented by an analytical function consisting of the sum of two modified exponentials. This procedure reproduces values obtained from numerical integration, for which no closed form exists, to within 5% for distances up to 6 mm from the wall and for stent diameters from 2–6 mm.

ISSN:0094-2405    

DOI:10.1118/1.597674

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Ferrous‐sulfate‐doped gelatin gel dosimeters are useful tools for the measurement of three‐dimensional absorbed radiation dose distributions. The diffusion of ferric ions through these gels causes degradation with time of the dose distribution image. It would be useful to reduce ferric ion diffusion without decreasing gel sensitivity. The amount of ferric ion diffusion is a function of the time delay after radiation, the gel temperature, and the gel concentration. These effects can be quantified by measuring the ferric ion diffusion coefficient. Determination of the diffusion coefficient by irradiating the lower section of a cylinder of gel, which was then imaged repeatedly over time with a clinical magnetic resonance imager, is described. Analysis of the edge spread function formed at each of several times after irradiation by drawing a profile over the imaged junction between the irradiated and unirradiated halves of the cylinder, gave estimates of the variance of the edge spread function. These variances were used to obtain an estimate of the ferric ion diffusion coefficient for the gel. A method of reducing ferric ion diffusion by adding a chelator and the cross linkage agent formaldehyde is suggested. The chelators investigated were 1,10 phenanthroline, xylenol orange, and bathophenanthroline disulfonic acid. These reduced diffusion to varying extents, and influenced the gel sensitivity. The diffusion coefficient in gels containing xylenol orange was found to be 0.44 mm2h−1. The gel sensitivity was 0.0093 s−1Gy−1. This compared with a diffusion coefficient of 0.82 mm2h−1for the base line gel that did not contain formaldehyde or chelators. The sensitivity of this base line gel was 0.0129 s−1Gy−1. The addition of xylenol orange produced the most improved gel dosimeter of the gels studied. This gel had a decreased ferric ion diffusion coefficient and a decreased sensitivity. It was still sensitive enough to be useful.

ISSN:0094-2405    

DOI:10.1118/1.597787

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We discuss a formalism for clinical proton beam dosimetry based on the use of ionization chamber absorbed dose‐to‐water calibration and beam quality correction factors. A quantitykQ, the beam quality correction factor, is defined which corrects the absorbed dose‐to‐water calibration factorND,win a reference beam of qualityQ0to that in a user's beam of qualityQ1. This study of proton beam quality correction factors used60Co (kQγ) and proton (kQp) reference beams. ThekQγfactors were measured using combined water calorimetry and ionometry for PTW and Capintec–Farmer‐type ionization chambers, and were computed from standard dosimetry protocols. Agreement between measured and calculatedkQγvalues for both chambers was found within 1.2% in the plateau region for a monoenergetic 250‐MeV beam and within 1.8% at the spread‐out Bragg peak for a 155‐MeV range‐modulated beam. Comparison of absorbed doses to water determined in the range‐modulated 155‐MeV beam was performed with the PTW chamber using three calibration methods:Ngascalibration (AAPM Report 16),ND,w,γcalibration in a60Co beam in conjunction with akQγfactor, andND,w,pcalibration in a proton beam in conjunction with akQpfactor. Absorbed doses to water obtained with the three methods agreed within 2% when ionization chamber dosimetry data were analyzed using the protonW‐value for air from the AAPM Report 16 and the ICRU 49 proton stopping powers. The use of the proton‐calibrated reference ionization chamber, in conjunction with the beam quality correction factorkQp, significantly reduced the systematic uncertainty of the absorbed dose determination.

ISSN:0094-2405    

DOI:10.1118/1.597768

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The use of multiple high energy electron beams has been limited in the treatment of deep seated tumors. This is principally because of the rapid increase in the physical electron beam penumbra as a result of the rise in large angle scattering with depth in the patient. This decreases the transverse dose gradient between the target volume and sensitive dose limiting structures and diminishes the ability to conform electron isodose lines to the target volume. If the beam is flat in air, then its profile will become progressively more rounded with depth, due to the increase of scatter out of the beam edges. With a scanning elementary electron beam, such as produced by the Microtron MM50, the characteristics of a broad beam profile are determined by the scan pattern. Using an appropriate scan pattern one can create, at any depth within the range of electrons, various dose profiles with the sharpness not exceeding that of the elementary beam. The objective of this work was to study methods that produce the desired electron beam profile at the depth of the target volume, and to derive the surface fluence profile required. Two approaches were explored to modulate the elementary beam distribution: “amplitude modulation” (AM) and “frequency modulation” (FM). We calculated coordinate and intensity distributions of the 25, 40, and 50 MeV elementary beam pulses at the surface that would yield a flat field at various prescribed depths. The results are in good qualitative agreement with iterative deconvolution calculations by Brahmeetal. [Acta Radiol. Oncol.19, 305–319 (1980)]. The scattering penumbra between the 50%–90% isodose lines can be reduced by up to 40% by beam modulation. The modulation should also enable the combination of multiple electron beams so as to achieve the desired conformal isodose profile as is customarily seen with photon beam planning, but with greater normal tissue sparing due to higher electron longitudinal depth—dose gradients. The results can be also used for electron accelerators that do not use a scanning elementary beam.

ISSN:0094-2405    

DOI:10.1118/1.597786

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A device that generates low‐energy x rays at the tip of a needle‐like probe was developed for stereotactic interstitial radiosurgery. Electrons from a small thermionic gun are accelerated to a final energy of up to 40 keV and directed along a 3 mm outside diameter drift tube to a thin Au target, where the beam size is approximately 0.3 mm. All high‐voltage electronics are in the probe housing, connected by low‐voltage cable to a battery‐operated control box. X‐ray output, which is nearly isotropic, consists of a bremsstrahlung spectrum and several lines between 7 and 14 keV, with characteristic radiation contributing 15% of the total energy output. To date, 14 patients with metastatic brain tumors have been treated with this device.

ISSN:0094-2405    

DOI:10.1118/1.597790

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A miniature, battery operated 40 kV x‐ray device has been developed for the interstitial treatment of small tumors (<3 cm diam) in humans. X rays are emitted from the tip of a 10 cm long, 3 mm diameter probe that is stereotactically inserted into the tumor. The beam, characterized by half‐value layer (HVL), spectrum analysis, and isodose contours, behaves essentially as a point isotropic source with an effective energy of 20 keV at a depth of 10 mm in water. The absolute output from the device was measured using a parallel plate ionization chamber, modified with a platinum aperture. The dose rate in water determined from these chamber measurements was found to be nominally 150 cGy/min at a distance of 10 mm for a beam current of 40 μA and voltage of 40 kV. The dose in water falls off approximately as the third power of the distance. To date, 14 patients have been treated with this device in a phase I clinical trial.

ISSN:0094-2405    

DOI:10.1118/1.597791

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The scattered radiation in 6 MV radiotherapy portal images is analyzed. First, a quantity SPR*is studied, by means of Monte Carlo (MC) modeling. SPR*is defined as the ratio, on the central axis, of the signal due to scattered radiation to that due to the primary radiation. The detector model mimics a high‐energy photon detector in the context of transit dosimetry. Second, a physical model of SPR*has been developed from first principles. For a cylindrical phantom, placed symmetrically about the isocenter, it predicts that SPR*depends on the areaAat the isocenter of the circular field and the phantom thicknessdas follows. SPR*=k0Ad(1+k1d)(1+k2A), wherek0=0.0266(L1+L2)2/(L1L2)2,k2=−[L−21+L−22+(L−11+L−12)2((2/3 +(3κ/2))]/2π,L1is the source‐to‐isocenter distance,L2is the isocenter‐to‐detector distance, and κ is the mean energy of the radiation beam (MeV/0.511). Constantk1, for which there is no simple expression, depends onL2. Comparison to the MC data shows that for 60≤L2≤100 cm the dependence is weak andk1≅2×10−3cm−1. The root mean square (rms) agreement between the MC‐derived values of SPR*and the physical model is better than 0.001 over a wide range ofAanddvalues likely to be encountered in clinical practice forL2≥50 cm. Third, experimental measurements of the scatter‐to‐primary ratio were obtained using our custom built imaging system mounted on a Philips SL25 linear accelerator. In the first experiment,Awas varied from 40 to 400 cm2withL1=L2=100 cm withd=20 cm. In the second experiment water depthdwas varied from 0 to 28 cm withL1=L2=100 cm andA=200 cm2. The rms agreements between the MC data and the experiments were 0.0015 and 0.0045, respectively.

ISSN:0094-2405    

DOI:10.1118/1.597792

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An algorithm for automatic registration of pairs of portal images based on image correlation is presented. It uses a fast‐Fourier‐transform‐based cross‐correlation operator to find the optimal registration, accounting for both in‐plane translations and rotations. Different cross‐correlation operators have been tested: the Pearson linear correlation coefficient has been implemented by fast Fourier transform and its performance has been compared to that of the more conventional normalized cross‐correlation. A sequential approach has been applied to speed up the registration considerably without degrading the performance of the algorithm. The algorithm has also been applied to the automatic registration of portal images to digitally reconstructed radiographs (DRRs), which have been modified to resemble megavoltage images. The results are indicative of the feasibility of this approach to the inspection of patient setup in radiation therapy.

ISSN:0094-2405    

DOI:10.1118/1.597743

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The high resolution and excellent soft tissue contrast of Magnetic Resonance Imaging (MRI) have enabled direct, noninvasive visualization of Multiple Sclerosis (MS) lesionsinvivo. This has allowed the quantification of changes in the appearance of lesions in MR exams to be used as a measure of disease state. Nevertheless, accurate quantification techniques are subject to inter‐ and intra‐operator variability, which may hinder monitoring of disease progression. We have developed a computer program to assist an experienced operator in the quantification of MS lesions in standard spin‐echo MR exams. The accuracy of assisted and manual quantification under known conditions was studied using exams of a test phantom, while inter‐ and intra‐operator reliability and variability were studied using exams of a MS patient. Results from the phantom study show that accuracy is improved by assisted quantification. The patient exam results indicate that assisted quantification reduced inter‐operator variability from 0.34 to 0.17 cm3, and reduced intra‐operator variability from 0.23 to 0.15 cm3. In addition, the minimum significant change between two successive measurements of lesion volume by the same operator was 0.64 cm3for manual quantification and 0.42 cm3for assisted quantification. For two different operators making successive measurements, the minimum significant change was 0.94 cm3for manual quantification, but only 0.47 cm3for assisted quantification. Finally, the number of lesions to be monitored for an average change in volume at a given power and significance level was reduced by a factor of 2–4 by assisted quantification. These results suggest that assisted quantification may have practical applications in clinical trials, especially those that are large, multicenter, or extended over time, and therefore require lesion measurements by one or more operators.

ISSN:0094-2405    

DOI:10.1118/1.597685

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY