摘要:

The renewed application of fast neutrons in clinical radiotherapy has been stimulated by fundamental radiobiological findings. The biological effects of high LET radiation, including fast neutrons, are different from those obtained with x rays in at least three respects: the oxygen enhancement ratio, the sensitivity of cells at different phases of the cell cycle, and the contribution of sublethal damage to cell reproductive death. Furthermore, wide variations in relative biological effectiveness (RBE) have been observed for different tumors and normal tissues. Measurements of volume changes in human pulmonary metastases indicate that the RBE for slowly growing tumors which are generally well‐differentiated is higher than that for poorly differentiated lesions. Six thousand patients have now been treated with fast neutron beams. The results of the clinical applications vary according to the method of application and to the type of cancer involved: treatment of inoperable malignancies of the salivary gland is very encouraging; the therapeutic gain is rather small for bladder and rectal cancers, soft tissue sarcomas and advanced carcinomas of the cervix; the responses of brain tumors are very disappointing. Most neutron radiotherapy applications have been less than optimal because of inadequate physical and technical conditions. Despite these difficulties, some interesting clinical data have become available. Due to the technical shortcomings, the possible advantages of fast neutrons are probably underestimated for many tumor sites. Well‐designed clinical trials, preferably performed with high energy cyclotrons in clinical environments, will provide a decisive answer to the question of the usefulness of the new radiation modality. Key words: fast neutrons, radiotherapy, radiobiology

ISSN:0269-3127    

DOI:10.1118/1.594851

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The observation of scattered radiations of larger wavelength than the primary had been repeatedly rejected or explained away by many researchers, including Compton. After years of vacillations, he recognized the effect named after him and was the first to develop a quantal equation predicting the wavelength of scattered radiation. It became one of the most significant contributions to modern radiation physics, opening the doors of quantum mechanics. Key words: scattered radiations, Compton effect, William Duane, Karl Stenström

ISSN:0269-3127    

DOI:10.1118/1.595046

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The majority of radiation treatment planning problems are relatively straightforward, involving only specified gantry angles in a treatment plane which is perpendicular to the patient longitudinal axis. In addition, there are a number of more complex three‐dimensional problems which require combined rotation of the gantry, collimator, and turntable for their solutions. These include, for example, the use of non‐coplanar fields and oblique treatment planes, the matching of field edges in three dimensions, the treatment of the breast with opposing tangential fields, and the treatment of inclined elongated lesions. Unfortunately, there is no general systematic approach to the solution of these more complex problems. One may attempt an analytic solution, but this approach is often too cumbersome and tedious. On the other hand, one may resort to a “trial and error” session with the simulator. This paper, therefore, presents a mathematical method which is easily applied and applicable to a wide variety of complex three‐dimensional treatment planning problems. The method considers the gantry, collimator, and turntable as coordinate systems. These coordinate systems are derivable from each other by specified coordinate transformations, which contain the rotation angles of the gantry, collimator, and turntable. Within this mathematical framework, the treatment planning problems are found to reduce to two general types, of which various clinical examples are then given. Key words: treatment planning, coordinate systems, coordinate transformations, three‐dimensional treatment planning, rotation operators

ISSN:0269-3127    

DOI:10.1118/1.594853

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Semi‐analytic and direct numerical integration approaches have been used to study the contribution of doubly scattered photons to dose at a pointPin a homogeneous water medium irradiated by a cobalt‐60 point source. The semi‐analytic analysis was used for an infinite field radius to calculate the variation of second scatter dose with depth, the angular dependence of the second scatter dose, and the spectra. Direct numerical integration was used to calculate the second scatter dose at a depth of 10 cm for various finite field radii ranging from 0 to 25 cm. Maps were made of the locations of scattering sites that were important to the second scatter dose. These calculations show that the second scatter contribution is generally less than the first scatter dose with the exception of a few sites remote from the point of interest. In many ways, the pattern of dose deposition by second scatter is similar to first scatter with less pronounced features. In other respects, the second scatter appears more isotropic. The implications of these calculations for approximate radiotherapy dose calculations are discussed. Key words: photon scatter, second scatter dose, differential scatter air ratio

ISSN:0269-3127    

DOI:10.1118/1.594854

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of small ring‐shaped cavities on the dose delivered by a cobalt‐60 beam to a homogeneous medium was studied experimentally and theoretically. Changes as small as 1×10−4of the total dose were measured. Experimental results show that, depending on the position of the cavity, replacing water with a small cavity can either increase or decrease scatter dose to a point in the medium. The increase in scatter dose was not anticipated and to our knowledge is not predicted by any of the presently available inhomogeneity dose correction algorithms used in treatment planning. Calculations were based on the perturbations of first and second scatter dose contributions and show that the presence of a cavity in the medium introduces three processes that decrease scatter dose and five that increase it. The calculated net effect is in good agreement with experiment. Additional calculations show that the effect of a single small inhomogeneity cannot be easily extrapolated to larger inhomogeneities and that multiple inhomogeneities do not act independently. Results from this study demonstrate two constraints that must be satisfied by future dose calculation algorithms: (i) they must correctly determine dose in a homogeneous non‐unit density material, and (ii) they must account for the change in dose due to small inhomogeneities in the medium. Key words: computed tomography, dose calculations, small inhomogeneity, first scatter dose, second scatter dose

ISSN:0269-3127    

DOI:10.1118/1.594855

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An ionization chamber method has been developed to measure exposure vs depth in a uniform BR 12 “average breast” phantom. It employs a Memorial mammography chamber for exit exposure measurements; resulting data is then corrected for backscatter as well as for the exceptionally thin window of this chamber. A careful comparison has then been made with relative exposure vs depth curves obtained using TLD at several mammography beam qualities, for identical exposure factors and SSD values. Use of a correction for residual and background TL signals significantly improved agreement between TLD and ion chamber curves in the 28 to 35 kVp/0.03 mm Mo range of beam quality. Agreement was within ±5% for the Mo target tube, but TLD readings were 4%–8% higher than ion chamber values for the W/Mo target tube. At Xeromammography energies (45 kVp/1.6 mm Al), corrected TLD curve readings were 6% higher at depth than ion chamber curve values. TLD measurements with 28 to 35 kVp/0.03 mm Mo beams tend tounderestimatedosage to the midbreast parenchyma. For example, in a 5 cm “average breast”, the underestimation ranges from 2%–10% for corrected, 10%–16% for uncorrected TLD readings.   Key words: Mammography dosimetry, mammography, thermoluminescent dosimetry, ion chamber dosimetry

ISSN:0269-3127    

DOI:10.1118/1.595031

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Implanted surgical metal clips often produce objectionable artifacts in CT reconstructions. The artifacts appear as streaks which emanate radially from the site of the clip. It is shown in this paper that these artifacts stem primarily from motion of the clip during the scan. An algorithm is described which reduces the intensity of these artifacts. The procedure attempts to remove the metal object entirely from the scan data by replacing the measured projection values of rays that passed through a neighborhood of the clip with calculated values consistent with an object whose density is an average of the surround. Examples are given for head and body scans as well as for computer simulations which show substantial reduction of the streak intensity. Key words: computed tomography, artifacts, reconstruction algorithms, surgical clip streaks, starburst artifacts

ISSN:0269-3127    

DOI:10.1118/1.595032

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It is shown that, in the production ofp(42)Be neutron beams for clinical use, the use of semithick targets leads to more desirable beam characteristics when appropriate backstop materials are used. Furthermore, an algebraic representation of beam penetration and of dose per unit charge on target, including hardening by polyethylene filters, provides a method for target optimization. Key words: neutron, target, optimization, clinical, power, depth‐dose

ISSN:0269-3127    

DOI:10.1118/1.595033

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The NSD equation,D= 1850×T0.11×N0.24, is a celebrated transmogrification of Cohen's two well‐known collations of data on response to clinical irradiations (3° erythema and 0.90 ablation of skin cancer) in which the relation ofDandTis fixed by the data selected by Cohen and the additional constraint thatNcorrespond to a schedule of five treatments per week is imposed subsequently by Ellis. The present paper shows that the equation, if correct, would have little clinical significance because the proportion,P, in which the dose,D, elicits the 3° erythema is unspecified:D=D(P) =D(?). Since the two Cohen collations each summarize the measurements on a different set of observational units, it is questionable whether the equation can be correct. This paper further shows that the appropriate (Least Squares) estimates of the three‐parameter equation derived for the Cohen data under the Ellis constraint (five treatments per week) is in fact:D(1.0)=1710×T0.54×N−0.26. The paper shows that the NSD equation is also incorrect because the ad hoc method by which Ellis estimates the exponents is inconsistent with the constraints imposed by Cohen and Ellis upon the parameters of the multivariate frequency distribution of the data set. The paper shows that the method by which the correct LS estimates of the exponents were obtained from the Cohen–Ellis data is consistent with these constraints and, therefore, this equation is a correct graduation of any other set of treatment regimens which is also consistent with the Cohen and Ellis constraints. The paper further shows that for such data sets there are, in fact, only two independent continous variables, eitherDandTorDandN, since the Ellis constraint requires thatNandTbecollinear. Thus, the best linear graduation has the typical form:D≃1900T0.32. This is “best” in the usual sense: both prediction and confidence intervals are provided for the estimates of the conditional “tolerance dose”D; these are not inflated by the presence of a collinear variable. This equation is biased, however, by the absence of the collinear variable. The TDF and the CRE concepts are derived from the NSD and, therefore, the deficiencies of the latter concept which we discuss may be expected to encumber these progeny as well. The two characteristic features of the Cohen (Ellis)‐type data which impede the construction of useful estimates of the putative separate effects ofNandTupon the response of tissues to irradiation are that (1) these data donotinclude specifications of either a tissuedefector itsincidence, and (2) the variablesNandTarecollinear. Appendices I and II describe methods by which the effects of these features may be eliminated (I) or reduced (II).   Key words: NSD, response surface, dose response equations, isoeffect equations, management equations, least squares, constrained least squares, collinearity, factoral experiments, ridge regression, principal components

ISSN:0269-3127    

DOI:10.1118/1.595034

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The paper presents an argument for the wider use of simple and standard statistical methods in the design and evaluation of both laboratory and clinical experiments with toxic radiosensitizers. It also presents an argument against the frequent practice of superposing a curve estimated from laboratory observations upon scattergrams of clinical observations to thereby assist the reader to the (often fallacious) inference that the two disparate sets of observations have enough in common bo be graduated by the same curve. Such a practice achieves a substitution of reasoning by analogy for the more reliable statistical procedures of estimation and of testing a hypothesis.

ISSN:0269-3127    

DOI:10.1118/1.595035

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY