摘要:

This is an introductory tutorial to the field of microdosimetry. Following a brief historical outline, we review the physics of experimental and theoretical microdosimetry and the application of microdosimetry to radiation biology within the so‐called “site” approximation. Modern interpretations of microdosimetry that make use of methods developed in integral geometry are described under the heading “structural microdosimetry;” this also illustrates the conceptual basis of dual radiation action. A discussion concerning some future prospects of this field concludes the review.

ISSN:0269-3127    

DOI:10.1118/1.596616

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A variable energy proton accelerator was commissioned at Fermi National Accelerator Laboratory for use in cancer treatment at the Loma Linda University Medical Center. The advantages of precise dose localization by proton therapy, while sparing nearby healthy tissue, are well documented [R. R. Wilson, Radiology47, 487 (1946); M. Wagner, Med. Phys.9, 749 (1982); M. Goitein and F. Chen, Med. Phys.10, 831 (1983)]. One of the components of the proton therapy facility is a beam delivery system capable of delivering precise dose distributions to the target volume in the patient. To this end, a prototype beam delivery system was tested during the accelerator's commissioning period. The beam delivery system consisted of a beam spreading device to produce a large, uniform field, a range modulator to generate a spread out Bragg peak (SOBP), and various beam detectors to measure intensity, beam centering, and dose distributions. The beam delivery system provided a uniform proton dose distribution in a cylindrical volume of 20‐cm‐diam area and 9‐cm depth. The dose variations throughout the target volume were found to be less then ±5%. Modifications in the range modulator should reduce this considerably. The central axis dose rate in the region of the SOBP was found to be 0.4 cGy/spill with an incident beam intensity of 6.7×109protons/spill. With an accelerator repetition rate of 30 spills/min and expected intensity of 2.5×1010protons/spills for patient treatment, this system can provide 50 cGy/min for a 20‐cm‐diam field and 9‐cm range modulation. The distal edge of the spread out Bragg peak was observed at 27.5‐cm depth with an incident proton energy of 235 MeV. The dose at the distal edge falls from 90% to 10% of peak value in 7 mm. The 90%–10% penumbras as small as 6 mm were measured at 3‐cm depth without range shifter material in the beam and as large as 15 mm when measured at 25‐cm depth with 10 cm of range shifter placed in the beam. The unmodulated Bragg curve had a peak to entrance dose ratio of 3, while the peak to entrance dose ratio for the 9‐cm spread out Bragg peak was observed to be only 1.3.

ISSN:0269-3127    

DOI:10.1118/1.596617

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Time‐resolved acoustic pulses generated in water and in soft tissue by pulsed proton beam irradiation were observed. The spatial resolution of depth dose distribution in the clinically applied beam intensity is estimated about 3 mm by means of TOF measurement. The dependence of the acoustic signal intensity on the temperature of medium was examined. Proportionality of acoustic pulse intensity to absorbed dose per pulse was confirmed as well. These results suggest the possibility of clinical application to monitor dose distribution in the patient's body during irradiation of pulsed proton beam.

ISSN:0269-3127    

DOI:10.1118/1.596618

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A specialized charged‐particle radiotherapy technique developed at Lawrence Berkeley Laboratory (LBL) is applied to patients with lesions abutting or surrounding the spinal cord or brain stem. This technique divides the target into two parts, one partially surrounding the critical structure (brain stem or spinal cord) and a second excluding the critical structure and abutting the first portion of the target. Compensators are used to conform the dose distribution to the distal surface of the target. This technique represents a novel approach in treating unresectable or residual tumors surrounding the spinal cord or brain stem. Since the placement of the patient with respect to beam‐shaping devices is critical for divided‐target treatments, a method for calculating dose distributions reflecting random patient motion is proposed, and the effects of random patient motion are studied for two divided‐target patient examples. Dose‐volume histograms and a normal‐tissue complication probability model are used in this analysis. For the patients considered in this study, the normal‐tissue‐complication probability model predicts that random patient motion less than or equal to 0.2 cm is tolerable in terms of spinal cord complications.

ISSN:0269-3127    

DOI:10.1118/1.596619

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Electronic portal imaging devices that capture an image of a patient's treatment field electronically by means of a computer data acquisition system operating in real time are becoming available. This paper reports on a study of a field verification correlation algorithm that can compare each treatment portal image to an image of the correct treatment field positioning. The algorithm requires no human intervention or analysis of the images but rather uses fast Fourier transforms to produce a correlation distribution. The position and amplitude of the correlation distribution maximum were tested as objective measures of translational and rotational differences of subject positions between pairs of images. The concept was tested by using a prototype algorithm to obtain the correlation distributions for images of an Alderson Rando™ head phantom. Images of the phantom setup with various errors were compared with an image of the phantom in the initial, correct treatment position. Translations, in‐plane and out‐of‐plane rotations, and combinations of translations and rotations were studied. The algorithm accurately measured translations. The value of the correlation distribution maximum was found to be a reasonable candidate for an alignment parameter for which tolerable error thresholds might be established.

ISSN:0269-3127    

DOI:10.1118/1.596743

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Radio‐opaque markers implanted inside or placed on the skin of patients can be used to detect set‐up errors and patient motion. The effects of imaging geometry accuracy for standard radiotherapy equipment on the precision of calculating the positions of radio‐opaque spherical markers using two orthogonal radiographic film projections is investigated. Inaccuracies in the imaging geometry are computed from the manually digitized positions of the marker images on each film pair. Actual marker locations are calculated with a precision limited only by the variance in manual digitization by incorporating those imaging geometry inaccuracies into their computation. Results of a phantom study using a grid of markers in a plastic block indicate that submillimeter precision can be obtained for the spatial coordinates of individual markers, and that the precision is not sensitive to the small inaccuracies in imaging geometry present within the mechanical tolerances of modern radiotherapy treatment machines and simulators.

ISSN:0269-3127    

DOI:10.1118/1.596620

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A dual sensor Wiener filter (DSWF) technique was used to improve the image quality of a scanning type digital radiographic system. In a digital radiographic system, image quality is evaluated by resolution and SNR (signal‐to‐noise ratio), which are two important parameters representing the objective performance of the system. In this method, when two images are acquired in the same region using two sensors with different characteristics of resolution and SNR, they are processed simultaneously using DSWF, which is the extended concept of a Wiener filter to two dimensions. DSWF uses the cross power spectrum between dual sensor outputs of the same chest radiographic image in the design of filter parameters. It has been implemented with fast algorithm using FFT (fast Fourier transform). The performance of the proposed method is compared with that of conventional methods (Wiener filter and parametric projection filter). In simulation studies, it is shown in 12 cases that this new method has SNR improvement of 1–2 dB better than conventional methods.

ISSN:0269-3127    

DOI:10.1118/1.596621

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The transfer function characteristics of a 1.5Timager have been determined. An edge response function (ERF) was obtained from a water/Plexiglas interface at various pixel widths ranging from 0.312 to 1.0 mm. An SE pulse sequence was used with a 5‐mm transaxial slice. The ERF was smoothed, differentiated, and Fourier transformed to obtain MTF curves. The LSF was analyzed for skewness and kurtosis. The area under the MTF amplitude curves and the equivalent bandpass were calculated. All ERFs, LSFs, and MTFs were well behaved. The resulting LSF was Gaussian. All calculated MTFs had cutoff frequencies slightly less than the theoretical Nyquist limit. The MTF calculated from the theoretical Gaussian LSF is slightly superior to that calculated from experimental data and provides an upper limit to the MTF. Spatial resolution in our MR imager is dominated by the pixel size via the Nyquist sampling theorem. System performance is slightly less than theoretically predicted, possibly due to image processing algorithms during the reconstruction process.

ISSN:0269-3127    

DOI:10.1118/1.596622

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Image intensifier distortion due to photocathode curvature and electron optics is shown to be approximated by a simple two parameter odd‐power polynomial. The accuracy of this fit was found to be far better than that of two other one parameter characterizations of distortion when applied to experimental data from four different model image intensifiers ranging in diameter from 9 to 14 in. The standard errors of the two parameter fits were less than 0.1 mm or 0.03% of the field of the IIs and were within the estimated measurement error.

ISSN:0269-3127    

DOI:10.1118/1.596623

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In TV‐based megavoltage imaging systems it is useful to be able to estimate the geometrical efficiencyg2of the camera lens. It is shown that the appropriate expression isg2=(16n2)−1×[F(1+1/m)]−2, wherenis the refractive index of the scintillator,Fis theF‐number of the lens andmis the optical magnification. This expression yields estimates forg2that are 5 to 10 times smaller than the estimates that have appeared previously in the literature.

ISSN:0269-3127    

DOI:10.1118/1.596624

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY