Index of content:
Volume 8, Issue 6, November 1981

Fast neutron radiotherapy: For equal or for better?
View Description Hide DescriptionThe 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 tissuesarcomas and advanced carcinomas of the cervix; the responses of braintumors are very disappointing. Most neutronradiotherapy 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

The tracks of the Compton effect
View Description Hide DescriptionThe 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

Solution to treatment planning problems using coordinate transformations
View Description Hide DescriptionThe 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

Second scatter contribution to dose in a cobalt‐60 beam
View Description Hide DescriptionSemi‐analytic and direct numerical integration approaches have been used to study the contribution of doubly scatteredphotons to dose at a point P in 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 scatterdose with depth, the angular dependence of the second scatterdose, and the spectra. Direct numerical integration was used to calculate the second scatterdose 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 scatterdose. These calculations show that the second scatter contribution is generally less than the first scatterdose 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 radiotherapydose calculations are discussed. Key words: photonscatter, second scatterdose, differential scatter air ratio

Effect of small inhomogeneities on dose in a cobalt‐60 beam
View Description Hide DescriptionThe 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^{−4} of 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

Comparison of ion chamber and TLD dosimetry in mammography
View Description Hide DescriptionAn 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 to u n d e r e s t i m a t e dosage 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: Mammographydosimetry,mammography, thermoluminescent dosimetry,ion chamberdosimetry

An algorithm for the reduction of metal clip artifacts in CT reconstructions
View Description Hide DescriptionImplanted 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

p(42)Be neutron therapy beams: Dose rate and penetration as a function of target thickness and beam filtration
View Description Hide DescriptionIt is shown that, in the production of p(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

‘‘Battered data’’—Some clinical effects of the abuse of multiple regression methods: The NSD
View Description Hide DescriptionThe NSD equation, D = 1850×T ^{0.11}×N ^{0.24}, 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 of D and T is fixed by the data selected by Cohen and the additional constraint that N correspond 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: . 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, either D and T or D and N, since the Ellis constraint requires that N and T be c o l l i n e a r. Thus, the best linear graduation has the typical form: D≃1900T ^{0.32}. 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 of N and T upon the response of tissues to irradiation are that (1) these data do n o t include specifications of either a tissued e f e c t or its i n c i d e n c e, and (2) the variables N and T are c o l l i n e a r. 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

‘‘Battered data’’—Some clinical effects of the abuse of multiple regression methods: A second look at some observations on Ro07−0582 (Misonidazole)
View Description Hide DescriptionThe 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.

Rotating laminar emission camera with GE‐detector: Experimental results
View Description Hide DescriptionExperimental results of a prototype rotating laminar emission camera (ROLEC) for nuclear medicine imaging are reported. A 11.5 mm thick, 45×45 mm high‐purity germaniumdetector is segmented into 30 1.47 mm wide parallel channels and collimated with 39 mm high parallel plates. Projection data acquired at multiple angular orientations as the detector‐collimator assembly is rotated about its center are mathematically reconstructed to image the activity distribution. The spatial resolution of the ROLEC is at least twice as good, at all distances, as that of gamma cameras with high resolution collimators. The better energy resolution of the germanium enhances the detection and resolution of the ROLEC in comparison to gamma cameras with NaI(T1) crystals, the relative superiority increasing with greater volumes and with greater depths. Adequate sensitivity is maintained while achieving these improvements in spatial resolution and in practice; ROLEC images are acquired in less time than pinhole collimatorimages with gamma cameras. Key words: emission camera, laminar camera,germaniumcamera

Rotating laminar emission camera with GE‐Detector: An analysis
View Description Hide DescriptionDesign criteria for a germanium detector emission camera with laminar collimator‐detector geometry are reported. In this design, parallel plates above a grooved germanium slab form detector channels which accumulate data in multiple projections through 180°. Activity distribution is determined through mathematical reconstruction from the projections. The laminar design has greater efficiency than a hole‐collimated device due to a favorable open area ratio and due to collimation in only one dimension. The relative sensitivity at the center of the detector strip increases with detector length and distance from collimator face. Spatial resolution depends upon the design geometry (septal penetration and scatter within the crystal are of such small magnitude that degradation of the point spread function is minimal). A 30 channel prototype device (ROLEC) shows good correlation with theoretically determined PSF and signal–to–noise ratio (SNR). Measuring times projected for a clinical sized device (300 mm diam) are about 4.6 times that of the Anger camera (with same SNR) and high resolution collimator. Key words: emission camera, laminar camera,germaniumcamera

Analytic representation of electron central‐axis depth dose data
View Description Hide DescriptionWe have examined a number of analytic representations of electron central‐axis depth dose data current in the literature, testing them against sets of standard depth dose data. One of them, a two‐parameter model of Shabason and Hendee, is recommended in situations in which good accuracy (∼2%) is desired, with the values of the parameters determined by an approximation formula which we have developed elsewhere. For higher accuracy, we have developed a polynomial model which gives, typically, a standard deviation of the fitting polynomial from the data points of 1%, and a maximum deviation of 2%. Fitting polynomials obtained with this method possess the property of having zero slope at the position of actual maximum dose, and generally a fifth‐order polynomial (requiring four nonzero coefficients) provided the most acceptable fit. The four parameters involved are determined through inversion of a 4 × 4 matrix, and we have tabulated these four coefficients for the standard data sets. The polynomial model is designed for interpolation in the range between the 100% dose depth and the 10% dose depth, and another fitting curve of the same type can be adjoined to cover depths less than the 100% dose depth. Key words: electron teletherapy, central‐axis depth dose, polynomial model, interpolation

Field separation of adjoining therapy fields
View Description Hide DescriptionThe question of optimal portal separation between two contiguous fields is of importance in radiotherapy. An analytical linear model based on experimental data has been developed for a Co–60 unit (Siemens Gammatron S 65 K). It allows the computation of the separation distance for any combination of field sizes not necessarily having the same source‐surface‐distance, defined either with penumbra trimmers or focused shaping blocks. Key words: dose homogeneity, field junction, gap

Improved method for the design of tissue compensators
View Description Hide DescriptionA program, WBEAM, is described which calculates dose distributions in planes perpendicular to the beam axis, taking into account both field shape and patient contour. WBEAM can be used to design compensators which when placed in the beam will produce uniform dose distributions in the plane of calculation. The program was tested in three different situations: a 30×30 cm field with a flat contour, a ‘‘mantle’’ field with a flat contour, a ‘‘mantle’’ field with a human‐like contour. The program performs as designed: the dose distributions are accurate, and the compensators flatten the radiation beams to the specified limits. Key words: compensator, beam flattener, dose distribution, isodose distribution, mantle field

Performance of a refrigerated charcoal trap for xenon‐133
View Description Hide DescriptionThe impulse response function of a charcoal trap to a bolus of xenon‐133 was determined as a function of the total number of hours run both at room temperature and at 25°C. The peak of the response function for a new trap at room temperature reached a value of 360 MPC at 11 h. After 150 h of operation, the impulse response function was determined at −25°C reaching a value of only 35 MPC at 25 h. The exhaust concentration of a trap in a busy nuclear medicine department using 150 mCi of xenon per week was measured and found to be 1600 MPC. The trap was placed in the freezer and kept there while it continued in use. Over a period of 3 weeks, the concentration of xenon in the exhaust of the trap dropped to a value of 13 MPC, or less than 1% of its value at room temperature. Key words: Xenon, charcoal trap, refrigation, trap

Systematic method of formulating liquid phantoms with a given elemental composition and density
View Description Hide DescriptionA general method of formulating tissue equivalent liquid mixtures of a given chemical composition and density using the technique of linear programming is described. It is used to generate mixtures with equivalent atomic composition as ICRP Standard Man and mammalian muscle (NBS Handbook 85) using eight compounds to cover the range of densities from 1.0 to 1.13. Use of the method to handle other parameters of a mixture is also described. Key words: T.E. liquid, elemental composition, density

Cardiac phase: Amplitude analysis using macro programming
View Description Hide DescriptionThe analysis of EKG gated radionuclide cardiacimaging data with Fourier amplitude and phase images is becoming a valuable clinical technique, demonstrating location, size, and severity of regional ventricular abnormalities. Not all commercially available nuclear medicinecomputer systems offer software for phase and amplitude analysis; however, many systems do have the capability of linear image arithmetic using simple macro commands which can easily be sequenced into stored macro‐strings or programs. Using simple but accurate series approximations for the Fourier operations, macro programs have been written for a Digital Equipment Corporation Gamma‐11 system to obtain phase and amplitude images from routine gated cardiac studies. In addition, dynamic cine‐mode presentation of the onset of mechanical systole is generated from the phase data, using only a second set of macro programs. This approach is easily adapted to different data acquisition protocols, and can be used on any system with macro commands for image arithmetic. Key words: Fourier analysis,cardiac cycle, gated blood pool imaging, amplitude image, phase image

Confirmation of a speculation concerning average tissue–air ratio determinations for rotational Co‐60 radiation therapy treatment planning
View Description Hide DescriptionThe question of the number of radii which are necessary to accurately determine the average tissue–air ratio (TAR) to be used in ^{60}Co rotational radiation therapytreatment planning was studied using actual patient contours. It was found that 12 radii adequately determine the average TAR for clinical purposes. Key words: TAR, ^{60}Co, rotational therapy

Use of a Victoreen 500 electrometer to determine ionization chamber collection efficiencies
View Description Hide DescriptionAn investigation has been made of the Victoreen 500 electrometer’s capability to predict saturation corrections for ionization chambermeasurements. This electrometer has been provided with a switch that reduces the polarizing voltage on the chamber to 40% of its normal value. It can be demonstrated theoretically that ion chambermeasurements at two separate voltages should be sufficient to determine the saturation correction, for both pulsed beams and continuous radiation. Tests were made with the Victoreen 500 which indicated that this is the case. Saturation corrections of up to 10% have been demonstrated for both pulsed and continuous beams and these can be predicted using the Victoreen 500 electrometer to within 1%. Key words: ionization chambers, collection efficiency, pulsed beams, electrometers,dosimetry