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PubMedDate not supplied

Physical characterization of 45-MV photon beams for use in treatment planning.

Central-axis percentage depth doses and tissue-maximum ratios (TMR) for 45-MV photon beams from a betatron have been measured in water. Also the influence of field size and collimator scatter on the dose in the buildup region have been investigated. The maximum dose for TMR has been shown to occur at a point about 2 cm deeper than the maximum dose for percentage depth dose. This difference is significant in the characterization of the photon beam at this high energy. The measured physical data have been computerized for use in routine treatment planning. Computer-gererated beams have been found to be in close agreement with measured isodose curves. Computer-generated isodose distributions for typical clinical irradiation techniques have been verified using RP/V film in an Alderson phantom.

Computers↗
PubMedDate not supplied

Calculations for beam-flattening filters for high-energy x-ray machines.

A flattening filter is an important component in a medical accelerator to modify the photonbeam properties. To simplify the calculations of the flattening-filter profile, we have developed a computer program which sums primary and scatter and then makes iterations in the primary component to produce a desired total-dose profile. The program can account, to first order, for radial spectral changes by using an effective primary attenuation coefficient which varies with radius. Calculations made to model the Clinac-4 dose profiles using the measured variation of half-value layer with radius show good agreement with the measured data. It is shown that the variation of quality within the beam impairs the flatness that can be achieved over a range of depths. Since perfect flatness cannot be achieved for small and large fields with one flattening filter, one may choose a primary profile which is a compromise over a range of field sizes and depths. A compromise profile for a 4-MV beam is discussed.

Elementary Particles↗
PubMedDate not supplied

Fast and slow neutrons in an 18-MV photon beam from a Philips SL/75-20 linear accelerator.

Fast- and slow-neutron contamination in an 18-MV photon beam from a Philips SL/75-20 linear accelerator has been measured. Aluminum and indium foils were activated to determine fast- and slow-neutron fluence, which were largely independent of field sizes. Measured fast-neutron fluences were typically 13.9 X 10(4) and 4.4 X 10(4) neutrons/cm2/rad of x ray inside and 5 cm outside the field, respectively. Slow-neutron fluences, 1.3 X 10(4) neutrons/cm2/rad of x ray, remained relatively constant inside and outside the field. The reported results are about three times higher than neutron fluences recently reported with a betatron operated at the same energy.

Elementary Particles↗
PubMedDate not supplied

Tissue substitutes in experimental radiation physics.

In this review of tissue substitute materials, the historical development of the important systems is traced from the early 1900's. Tabulations of the constituents, elemental compositions, specific gravities, and the photon and electron interaction characteristics of 64 materials are given together with recommendations of systems having useful simulation properties. Formulation and manufacturing procedures are described and possible future developments in both materials and phantom research are outlined.

Elementary Particles↗
PubMedDate not supplied

Improvement of linear accelerator depth-dose curves.

A semiempirical analytic description of the accelerator depth-dose curve is described along with its physical explanation. The results of Monte Carlo calculations are presented and compared with experimental data to test this model. Calculations were made for different atomic number (Z) materials used as x-ray targets and flatteners, with the results showing that medium-Z materials are the logical choice. It is demonstrated empirically that Dmax is a simple function of the average energy (E) of the x-ray spectrum. The variation of E with Z of the target and flattener is demonstrated. As a practical example, Monte Carlo calculations and experimental data for old and new Clinac 35 accelerators are presented.

Electrons↗
PubMedDate not supplied

Electrons as the cause of the observed dmax shift with field size in high energy photon beams.

For megavoltage x-ray beams, it is well known that the percent depth-dose increases considerably with field size in the buildup region, resulting in a significant shift in the apparent position of maximum dose, dmax. The nature of this increase has been investigated using a sweeping magnet placed just below the treatment head of a 25-MV linac. Measurements show that for increasing magnetic fields the dose in the buildup region is continually reduced, until a point is reached beyond which no additional reduction is observed. Here the buildup curve is essentially field size independent. These results clearly show that electrons are the primary cause of dose increase with field size in the buildup region, in contrast to a recent publication claiming that scattered photons are the cause. Further measurements were made by blocking out the primary beam at the level of the jaws and measuring the depth dose of the scattered electrons originating from the jaws. The results show that a thickness of approximately 1 gcm-2, of either polystyrene or lead, reduces the dose by a factor of two, providing further evidence that the scattered component of the beam consists of low energy electrons.

Electrons↗
PubMedDate not supplied

Improving the buildup and depth-dose characteristics of high energy photon beams by using electron filters.

The attributes of high energy photon beams, i.e., low surface dose, large dmax and improved %DD, are compromised with increase in field size. This is due to the relative increase with field size of the electron component in the beam, as shown by recent experiments done here using a sweeeping magnet. The present study shows that the advantages can be partially regained with the use of foils to remove electrons. Various thicknesses of Al, Cu, Sn, and Pb were placed in a 25-MV linac x-ray beam at several sites in the treatment head. Buildup curves were measured with a "pancake" chamber for various SSD and field sizes. The magnitude of improvement achieved is dependent upon field size, SSD, the atomic number of the foil material, and foil thickness. Pb foil (0.55 gcm-2) provided the best overall improvement. Surface dose reduction of 10%-20% can be achieved along with significant increase in dmax and %DD. These findings suggest a new design feature for the next generation of high energy linacs.

Aluminum↗
PubMedDate not supplied

High-LET pion dose contours under the LAMPF biomedical channel by plastic track detector scanning.

Cellulose-nitrate plastic track detectors have been tested as high spatial-resolution dosimeters at the Biomedical Channel of the Clinton P. Anderson Meson Physics Facility (LAMPF) at the Los Alamos Scientific Laboratory during pilot clinical trials with negative pi mesons (pions). Six plots of dose contours are presented. Reliable macrodosimetry with track detectors would in principle require detector calibration for the particular conditions of geometry and beam parameters in every case but it is interesting that absolute dose values, estimated from data taken with a quite different calibration geometry, were within 20% of agreement with dose values obtained by electronic dosimetry methods. The track detectors measure only the high linear energy transfer (LET) components produced by the pion beam; it is these components which are of special interest in the negative-pion therapy program. Plastic track detectors appear to have many favorable properties for pion dosimetry.

Elementary Particles↗
PubMedDate not supplied

Determination of bone density by coherent-Compton scattering.

Bone density previously has been determined by counting the number of Compton and coherently scattered photons using incident radiation from 241Am (60 keV). We have improved this technique by using more nearly optimal scattering angles and a higher incident beam energy. Three factors make this method clinically attractive; (a) sensitivity to changes in both bone density and elemental composition, (b) elimination of attenuation corrections for overlaying structures, and (c) the ability to measure the axial skeleton. An intrinsic Ge detector with high resolution (500 eV at 100 keV) allowed measurement of forward scatter at angles as low as 30 degrees (from the incident beam), thereby maximizing the fluence of the coherent photons which are highly forward directed. Preliminary results showed high correlations (r = 0.97-0.99) between the coherent-Compton ratio and density of bone phantoms using both 241Am and 153Gd sources. The skin dose for a measurement using 153Gd was calculated to be 1.5 cGy at 3% counting precision.

Americium↗
PubMedDate not supplied

Calculation of the average energy absorbed in photon interactions.

High energy electrons set into motion by photon interactions with matter lose some of their energy by bremsstrahlung. This loss must be evaluated before energy absorption coefficients may be calculated. Recent extensive tables of data published by Plechaty et al. contain an appreciable error in this quantity. The error results from two simplifying assumptions and for the case of very high photon energies interacting with high atomic number materials can be as much as a factor of two. This has important implications for the evaluation of quantities used in radiation dosimetry.

Elementary Particles↗
PubMedDate not supplied

Pion in vivo dosimetry using aluminum activation.

The method of aluminum activation to 24Na has been shown feasible as a high-LET, in vivo dosimeter for clinical pion beams at the Clinton P. Anderson Meson Physics Facility in Los Alamos. A 3 X 3 in. phi NaI (Tl) well detector measures the 24Na activity following exposure by windowing the 2.75 MeV photopeak. Calculations of the 24Na activity agree well with experiment if one assumes a production ratio of 0.075 24Na/stopped pi- in aluminum, and an in-flight cross section of 26 mb. The activity is produced primarily by stopping pions although 15-25% of the activity is the result of neutrons. Thus, the induced activation is a good measure of high-LET dose. By comparison with high-LET dose measured by a 7.6 mu silicon detector and a Rossi chamber, the amount of high-LET dose per activation is found to be 1.35 X 10(-6) rad/(24Na/gm Al). A clinical setup has been installed and a sample patient measurement is compared with high-LET dose calculated by treatment planning programs.

Aluminum↗
PubMedDate not supplied

Calculation of pion dose distributions in water.

Techniques for calculating negative pion beam depth and off-axis dose distributions in a water phantom have been developed at the Clinton P. Anderson Meson Physics Facility in Los Alamos. The superposition of the unmodulated depth-dose curve produced modulated depth-dose curves. The addition of the collimator neutron dose, which has been shown to depend on field size, to the modulated depth-dose curve yields the collimated depth-dose distributions. Off-axis dose distributions under a collimator are produced by calculating the distortion of the uncollimated beam caused by multiple Coulomb scattering and beam phase space. Several comparisons of calculated and measured distributions are shown with agreement of normally +/- 3% of peak dose of +/- 3 mm for a particular dose contour. These distribution are them modified by computerized tomographic data to give patient isodose distributions.

Elementary Particles↗
PubMedDate not supplied

Preclinical studies of dynamic treatment modes in pion therapy.

Preliminary results on a system for delivering dynamic pion radiotherapy treatments are reported. The desired treatment volume is scanned across a small, focused pion beam using a computer-controlled treatment couch. A computer-controlled rangeshifting device modulates the stopping pion depth distribution in coordination with the couch motion to conform the dose to the shape of the treatment volume. For certain shaped tumors, the system can result in substantial normal tissue dose sparing and better field flatness as compared to irradiation by static treatments with broad terms. The characteristics of this new system, plus preliminary results for typical dose distributions as measured with thermoluminescent dosimeters (TLDs), are presented.

Elementary Particles↗
PubMedDate not supplied

Photon activation-15O decay studies of tumor blood flow.

A direct, noninvasive method for measuring absolute values of specific capillary blood flow in living tissue is described. The method is based on the photon activation, in situ, of tissue elements and the measurement of the subsequent decay of the positron activity induced, employing coincidence detection of the photon pairs produced in positron annihilation. Analysis of the time-dependent coincidence spectrum reveals the contribution to the total signal from the decay of 15O, from which the specific capillary blood flow in the imaged, activated volume is ultimately determined. By virtue of its introduction of the radioisotope of interest (15O) directly and uniformly into the tissue volume under investigation, the method described permits both the nonperfused and well perfused fractions of an activated volume to be estimated and hence, the average specific blood flow within imaged tumor volumes to be computed. The model employed to describe and analyze the data is discussed in detail. Results of application of the technique to measurement of specific blood flow in rhabdomyosarcoma tumors grown in WAG/Rij rats are presented and discussed. The method is shown to be reliable and well suited to studies designed to determined the effects of various agents, such as heat, radiation and drugs, on tumor blood flow.

Animals↗
PubMedDate not supplied

Image information transfer properties of x-ray fluorescent screens.

The image information transfer efficiency for five x-ray fluorescent screens (calcium tungstate, barium halide, and three rare earth screens) has been experimentally determined with monoenergetic x-ray beams at energies of 18, 22, 32, 49, 51, 58, and 69 keV. The transfer efficiency, which is defined by the ratio of the output signal-to-noise ratios, was determined from measurements of (a) the fraction of incident x rays absorbed in the screen and (b) the statistical distribution of the number of light photons emitted from the screen per absorbed x ray which was determined by light photon counting techniques. Comparisons of the information transfer efficiency, the average number of light photons emitted per absorbed x ray, and the light output energy per Roentgen are given for the above screens and x-ray energies.

Elementary Particles↗
PubMedDate not supplied

The physics of cancer therapy with negative pions.

The introduction of negative pions into cancer therapy has required the construction of large new proton accelerators together with special magnetic systems to form and direct the pion beam to a patient. A summary is presented of the fundamental properties of pions and of the methods used to study the therapeutic beams. The dosimetry of these beams requires the use of the older techniques as well as new methods for determining the different LET components. The data for a number of beams is given and the utilization of this data in treatment planning is reviewed. An important problem for therapy is the behavior of inhomogeneities in the pion beam, and experimental methods are described which illuminate this problem. The studies of the effects of inhomogeneities in a beam point the way toward fruitful comparisons with the computerized treatment planning codes known as PION-1 and PIPLAN. A useful step in treatment is the verification of doses in patients during therapy. For this purpose the new methods for measuring the high LET doses in patients are described as well as a timing measurement for checking the stopping effect of the tissues as obtained from the CT scans.

Biophysical Phenomena↗
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The measurement of trabecular bone mineral density using coherent and Compton scattered photons in vitro.

A photon scattering method for measuring the trabecular bone mineral density (TBMD) in vitro is described. This method involves the measurement of the ratio of coherent to Compton 90 degrees scattered photons from Am-241 by using a narrow beam geometry with an intrinsic germanium detector. The feasibility of using smaller scattering angles for better counting efficiency and the associated problems in their application for in-vivo measurements were investigated. Calibration of the system with fresh trabecular bone samples showed a linear relationship between the coherent to Compton ratio R of the detected counts and the TBMD (r = 0.94). The effect of the overlying soft tissue on the R ratio was significant while the effect of self-attenuation by the trabecular bone itself and the cortical layer was negligible. It was found that the marrow fat content could alter the value of the R ratio. Our results show that for a 10% increase in the fat content in the interstices of the trabecular bone there is a 2.5% decrease in the R ratio. This technique together with soft tissue corrections will enable us to measure the TBMD of the calcaneum in vivo, assuming a small variation in the trabecular fat content. The estimated absorbed dose to the bone marrow is about 139 mrad.

Americium↗
PubMedDate not supplied

The photon-fluence scaling theorem for Compton-scattered radiation.

This paper concerns a method of scaling photon fluence from one scattering material to another when the photon energies are such that the dominant mode of interaction is Compton scattering. The theorem establishes a one-to-one correspondence between points in the two scattering media where the spectra of primary and scattered photons have the same distribution in energy and angle, and where the fluence ratio equals the square of the electron density ratio. Experimental tests were made with cobalt-60 gamma radiation using ionization-chamber measurements in graphite, acrylic plastic, polystyrene, and water phantoms. The experimental results are consistent with the equality of photon spectral shapes and angular distributions at corresponding points. The fluence ratios may differ by a few percent from the predicted values, depending on distance from the source.

Acrylic Resins↗