Volume 18, Issue 1, January 1991
Index of content:
18(1991); http://dx.doi.org/10.1118/1.596747View Description Hide Description
A restricted angular scattering model for electron penetration in dense media is presented. In the model, the Fermi–Eyges transport equation is modified through the addition of an extra term which may be interpreted as representing an apparent force opposing the scattering of electrons into wider angles. The introduction of this extra term allows the modeling of the measured saturation in the mean square angular spread of electrons with depth. The restricted scattering model retains the Gaussian features of the Fermi–Eyges model and, therefore, may be readily incorporated into existing dose computation algorithms. Good agreement is obtained with measured angular electron distribution data for a point monodirectional beam over a wide range of incident electron energies (5–20 MeV) and scattering media (atomic numbers of 6 to 82). Also, a comparison of the restricted scattering model predictions with measurements of the lateral pencil beam spread shows an improvement over the predictions of Fermi–Eyges model close to the end of the electron range. Broad beam profiles were generated using both the Fermi–Eyges and restricted scattering models. A comparison of predicted and measuredbeam profiles shows that the restricted scattering model is a significant improvement over the Fermi–Eyges model for the prediction of beam penumbra shape in homogeneous media.
18(1991); http://dx.doi.org/10.1118/1.596697View Description Hide Description
A pencil‐beam redefinition algorithm has been developed for the calculation of electron‐beam dose distributions on a three‐dimensional grid utilizing 3‐D inhomogeneity correction. The concept of redefinition was first used for both fixed and arced electron beams by Hogstrom e t a l. but was limited to a single redefinition. The success of those works stimulated the development of the pencil‐beam redefinition algorithm, the aim of which is to solve the dosimetry problems presented by deep inhomogeneities through development of a model that redefines the pencil beams continuously with depth. This type of algorithm was developed independently by Storchi and Huizenga who termed it the ‘‘moments method.’’ Such a pencil beam within the patient is characterized by a complex angular distribution, which is approximated by a Gaussian distribution having the same first three moments as the actual distribution. Three physical quantities required for dose calculation and subsequent radiation transport—namely planar fluence, mean direction, and root‐mean‐square spread about the mean direction—are obtained from these moments. The primary difference between the moments method and the redefinition algorithm is that the latter subdivides the pencil beams into multiple energy bins. The algorithm then becomes a macroscopic method for transporting the complete phase space of the beam and allows the calculation of physical quantities such as fluence, dose, and energy distribution. Comparison of calculated dose distributions with measured dose distributions for a homogeneous water phantom, and for phantoms with inhomogeneities deep relative to the surface, show agreement superior to that achieved with the pencil‐beam algorithm of Hogstrom e t a l. in the penumbral region and beneath the edges of air and bone inhomogeneities. The accuracy of the redefinition algorithm is within 4% and appears sufficient for clinical use, and the algorithm is structured for further expansion of the physical model if required for site‐specific treatment planning problems. Key words: redefinition algorithm, pencil beam, 3‐D inhomogeneity correction, electron dose calculations
Energy constancy checking for electron beams using a wedge‐shaped solid phantom combined with a beam profile scanner18(1991); http://dx.doi.org/10.1118/1.596719View Description Hide Description
An energy constancy checking method is presented which involves a specially designed wedge‐shaped solid phantom in combination with a multiple channel ionization chamber array known as the Thebes device. Once the phantom/beam scanner combination is set up, measurements for all electron energies can be made and evaluated without re‐entering the treatment room. This is also valid for the readjustment of beam energies which are found to deviate from required settings. The immediate presentation of the measurements is in the form of crossplots which resemble depth dose profiles. The evaluation of the measured data can be performed using a hand‐held calculator, but processing of the measured signals through a PC‐type computer is advisable. The method is insensitive to usual fluctuations in beam flatness. The sensitivity and reproducibility of the method are more than adequate. The method may also be used in modified form for photon beams.
18(1991); http://dx.doi.org/10.1118/1.596720View Description Hide Description
The IAEA 1987 protocol is an international protocol which has made a number of improvements over the AAPM 1983 protocol for calibration of high‐energy photon and electron beams. We present a detailed numerical comparison between the two protocols by calculating (i) N gas and N D for PTW (PMMA wall), Capintec (air‐equivalent plastic wall) and NEL (graphite wall) Farmer type ionization chambers for 60Co γ rays; (ii) dose‐to‐water with chamber in waterirradiated by 4‐ or 25‐MV x rays; (iii) dose‐to‐water with chamber in water, PMMA, and polystyrene phantoms irradiated by 5‐ and 10‐MeV electrons; and (iv) dose‐to‐water with chamber in waterirradiated by 20‐MeV electrons. For photons, the IAEA protocol gives results which are in good agreement with the AAPM protocol; on average the IAEA results are 0.6% smaller than the AAPM results while discrepancies between the two are in the range of −0.4% to −1.2%. For 10‐MeV electrons also, the IAEA protocol gives results which are in excellent agreement with the AAPM protocol; on average the IAEA results are 0.3% smaller than the AAPM results while discrepancies between the two are in the range of −1.0% to +0.5%. In contrast to the above, for 5‐MeV electrons, the IAEA protocols give results smaller than the AAPM protocol by 2.0% on average with discrepancies between protocols ranging from −4.1% to −0.7% depending upon the ionization chamber and phantom material used. For 5‐MeV electrons, the discrepancies are particularly large for polystyrene phantom; the average discrepancies being −1.4%, −1.1%, and −3.6% for water, PMMA, and polystyrene, respectively. If data for 5‐MeV electrons with polystyrene phantom are excluded, then the overall agreement between the two protocols for photons and electrons is within the range of −1.9% to +0.5%. Principal reasons for the observed discrepancies are (i) IAEA uses the correct expression for N D resulting in up to +0.8% correction; (ii) IAEA uses the most recent stopping power ratio for graphite‐to‐air resulting in up to +0.5% correction; (iii) IAEA uses a correction of up to +0.8% for the central electrode which AAPM ignores; (iv) the present estimates of the percent depth doses which arise from the differences in measurement depths in the two protocols; and (v) IAEA uses measured values of the fluence correction factor while AAPM uses a theoretical estimate resulting in corrections of up to −2.2%.
18(1991); http://dx.doi.org/10.1118/1.596696View Description Hide Description
A new beam line for radiotherapy and radiosurgery with accelerated helium‐ion beams has been set up at the Bevalac. The new treatment room has been equipped with a very precise patient positioner in order to utilize the superior dose localization properties of light‐ion beams. The beam spreading and shaping system is described, the trade‐offs involved in positioning the beam modifying devices are discussed, and the physical properties of the generated radiation fields are reported. The Bragg peak modulation by axial beam stacking employing a variable range shifter is explained and the control system including beam monitoring and dosimetry is presented.
Demonstration of three‐dimensional deterministic radiation transport theory dose distribution analysis for boron neutron capture therapy18(1991); http://dx.doi.org/10.1118/1.596721View Description Hide Description
The Monte Carlo stochastic simulation technique has traditionally been the only well‐recognized method for computing three‐dimensional radiation dose distributions in connection with boron neutron capture therapy (BNCT) research. A deterministic approach to this problem would offer some advantages over the Monte Carlo method. This paper describes an application of a deterministic method to analytically simulate BNCT treatment of a canine head phantom using the epithermal neutron beam at the Brookhaven medical research reactor (BMRR). Calculations were performed with the TORT code from Oak Ridge National Laboratory (ORNL), an implementation of the discrete ordinates, or S n method. Calculations were from first principles and used no empirical correction factors. The phantom surface was modeled by flat facets of approximately 1 cm2. The phantom interior was homogeneous. Energy‐dependent neutron and photon scalar fluxes were calculated on a 32×16×22 mesh structure with 96 discrete directions in angular phase space. The calculation took 670 min on an Apollo DN10000 workstation. The results were subsequently integrated over energy to obtain full three‐dimensional dose distributions. Isodose contours and depth‐dose curves were plotted for several separate dose components of interest. Phantom measurements were made by measuring neutron activation (and therefore neutron flux) as a function of depth in copper–gold alloy wires that were inserted through catheters placed in holes drilled in the phantom. Measurements agreed with calculations to within about 15%. The calculations took about an order of magnitude longer than comparable Monte Carlo calculations but provided various conveniences, as well as a useful check.
Neutron measurements around medical electron accelerators by active and passive detection techniques18(1991); http://dx.doi.org/10.1118/1.596751View Description Hide Description
Passive and active detection techniques have been employed in order to measure neutron fluence rates and corresponding exposure rates around medical electron accelerators operating at energies well above the neutron binding energies of the structural materials. In these conditions from the treatment head, in the direct photon flux and from the shielded region, a fast neutron flux emerges which is partly absorbed and partly scattered by the walls, eventually establishing a nearly uniform thermal and epithermal flux in the room. Both direct and scattered flux contribute to the dose to the patient. A smaller neutron dose rate can also be found outside the treatment room, where the therapy staff works. Passive detectors, of moderation type, have been employed in the treatment room and 3He active detectors in the external zones. For the treatment room the activation data were compared with results of Monte Carlo simulation of the neutron transport in the room. Technical features of the two measures are briefly presented and results obtained around three different types of accelerators are reported. At the higher beam energies, i.e., 25 MV, a neutron dose of 0.36 Sv was estimated in the treatment field in addition to a therapeutic x‐ray dose of 50 Gy. At lower energies or out of the treatment field the neutron dose drops significantly. In the external zones the dose rates everywhere are below the recommended limits and normally very low, the highest values being recorded in positions very close to the access door of the treatment room.
18(1991); http://dx.doi.org/10.1118/1.596746View Description Hide Description
The tables of the mean restricted collision masss stopping power ratios for water, polystyrene and acrylic relative to air given in the AAPMTG-21 protocol have been fitted to an expression with 20 coefficients using the depth in the phantom and the mean incident electron energy as two independent variables. Using these expressions, the calculatd values agree with the tabulated values with in of the cases and within in of the cases. For each of the four cylindrical chamber inner diameters, given in the protocol, the electron fluence correction has been fitted to an expression with four coefficients using the mean electon energy at depth z as an independent variable.
Polarity effect for various ionization chambers with multiple irradiation conditions in electron beams18(1991); http://dx.doi.org/10.1118/1.596694View Description Hide Description
The effect of reversing the voltage polarity applied to an ionization chamber has been investigated in electron beams for several types of chambers and several irradiation conditions. It has been found that differences in readings can be significant for cylindrical chambers (about 10%) as well as for plane parallel chambers (20%). The effect is larger for large field sizes than small ones. It generally includes an appreciable stem and cable effect. Differences in readings with both polarities are related to the energy distribution of the electron beam and are greater for lower electron energies than higher. Polarity effect and charge deposit within the chamber wall material appear to be closely connected. This charge deposit, expressed as a proportion of the total collected charge, can be directly derived from double polarities measurements. Careful investigation of the effect should be made to avoid significant error (over 5%) in the determination of the absorbed dose. Key words: electrons, polarity effect, ionization chambers
18(1991); http://dx.doi.org/10.1118/1.596695View Description Hide Description