Index of content:
Volume 20, Issue 2, March 1993

The x‐ray centennial—Thompsons and Thomsons
View Description Hide DescriptionWhen x rays were discovered by Wilhelm Roentgen in November, 1895, the news spread rapidly through Europe, Great Britain, and the United States and many individuals became involved in their development. Some of the more prominent participants shared the name of Thompson or Thomson, which causes confusion when the history of x rays is discussed because of their similar pronunciation. In Britain they were William Thompson (Lord Kelvin), J. J. Thomson, and Silvanus P. Thompson and in the United States it was Elihu Thomson. In addition, one of the first books on x rays published in the United States was written by Edward Thompson.

Wall‐correction and absorbed‐dose conversion factors for Fricke dosimetry: Monte Carlo calculations and measurements
View Description Hide DescriptionFor megavoltage radiotherapyphoton beams, EGS4 Monte Carlo calculations show, and experimental measurements confirm with an accuracy of 0.2%, that glass or quartz‐walled vials used in Fricke dosimetry increase the dose in the Fricke solution. This is mainly caused by increased electron scattering from the glass which increases the dose to the Fricke solution. The dose perturbation is shown to vary from nothing in a ^{60}Co beam up to 2% in a 24‐MV beam. For plastic vials of similar shapes, calculations demonstrate that the effect is in the opposite direction and even at high energies it is much less (0.2% to 0.5%).

Calibration of high‐energy photon and electron beams for radiotherapy using AAPM 1983 and IAEA 1987 dosimetry protocols
View Description Hide DescriptionTo follow up on the theoretical comparison of the IAEA 1987 and AAPM 1983 protocols for dosimetrycalibration of high‐energy photons and electrons [Med Phys. 18, 26–35 (1991)], results of a set of dosimetric measurements made with a Farmer type PTW and Capintec ionization chambers in solid water, PMMA, and polystyrene phantoms and exposed to a 4 MV photon beam from a Varian Clinac 4S at Yale, a 10 MV photon beam and 6 and 15 MeV electron beams from a Varian Clinac 1800 at Phelps Radiation Center, University of Connecticut, and a 25 MV photon beam from a Sagittaire at Yale, are presented. Because different methods are used for the determination of electron beam energies, the values of mean electron energy determined by the two protocols are different by up to 8%. However, for dose intercomparison, the overall agreement between the two protocols is within 1% in most cases, with a maximum discrepancy of 3.3% in one case. For photons, the IAEA results are smaller than the AAPM results by 0.7% on the average, while maximum discrepancies are in the range of −0.4%–−1%. In the case of 15 MeV electrons, the discrepancies between the two protocols are found to be in the range of −0.1%–1% and have an average value of 0.5%. In contrast to the above, a large discrepancy is observed between the two protocols for 6 MeV electrons. Depending upon the choice of phantom and ion chamber, this discrepancy is found to be in the range of −0.1%–−3.3%. A largest discrepancy of −3.3% is observed when a Capintec chamber is used in a polystyrene phantom. A most likely cause for this discrepancy is in the values of fluence correction factors used in the two protocols. Until better values for these parameters are available, a Farmer type chamber should not be used in a polystyrene phantom for the calibration of low‐energy electrons (6 MeV). In contrast to the theoretical intercomparison reported earlier, the present results of dosimetry measurements show improved agreement (by up to 1.4%) between the two protocols for electron beams, and about the same level of overall agreement (within 0.5%) for photon beams. The standard deviation of dosecalibration for electron beams determined by the two protocols using different chambers and phantoms are about the same, approximately equal to ±1.5% for both protocols. It is recommended that either the AAPM should revise and update its 1983 protocol or adopt the international protocol produced by the IAEA. The IAEA protocol [Technical Report Series No. 277 (IAEA, Vienna, 1987), pp. 1–98] has fewer erroneous equations, uses more recent interaction data, and has an international appeal.

A finite element approach for modeling photon transport in tissue
View Description Hide DescriptionThe use of optical radiation in medical physics is important in several fields for both treatment and diagnosis. In all cases an analytic and computable model of the propagation of radiation in tissue is essential for a meaningful interpretation of the procedures. A finite element method(FEM) for deriving photon density inside an object, and photon flux at its boundary, assuming that the photon transport model is the diffusion approximation to the radiative transfer equation, is introduced herein. Results from the model for a particular case are given: the calculation of the boundary flux as a function of time resulting from a δ‐function input to a two‐dimensional circle (equivalent to a line source in an infinite cylinder) with homogeneous scattering and absorption properties. This models the temporal point spread function of interest in near infrared spectroscopy and imaging. The convergence of the FEM results are demonstrated, as the resolution of the mesh is increased, to the analytical expression for the Green’s function for this system. The diffusion approximation is very commonly adopted as appropriate for cases which are scattering dominated, i.e., where μ_{ s }≫μ_{ a }, and results from other workers have compared it to alternative models. In this article a high degree of agreement with a Monte Carlo method is demonstrated. The principle advantage of the FE method is its speed. It is in all ways as flexible as Monte Carlo methods and in addition can produce photon density everywhere, as well as flux on the boundary. One disadvantage is that there is no means of deriving individual photon histories.

Decomposition of pencil beam kernels for fast dose calculations in three‐dimensional treatment planning
View Description Hide DescriptionA method for the calculation of three‐dimensional dose distributions for high‐energy photon beams is presented. The main features are (i) the calculation is fast enough to allow interactive three‐dimensional treatment planning, and (ii) irregularly shaped or compensated fields, which are required to fit three‐dimensional dose distributions to target volumes, are adequately taken into consideration. The method is based on the pencil beam convolution technique and shares its features concerning accuracy. A considerable gain in speed is achieved by decomposing the pencil beam kernel into three separated terms, thus reducing the required number of two‐dimensional convolutions. The convolutions are performed in the frequency domain via the fast Hartley transform. Using these techniques, the calculation time for the convolutions is only about 8 s on a DEC VAX station 3100. This is one‐fourth to one‐third of the calculation time for the ray tracing through the three‐dimensional CT data set, which has to be performed in any case. Results of the calculation are compared with measurements in a homogeneous phantom for 15 MV photons. Two irregular fields shaped with a multileaf collimator are considered. The deviations between measured and calculated absolute dose values are smaller than ±2%.

How accurately can EGS4/PRESTA calculate ion‐chamber response?
View Description Hide DescriptionIon‐chamber responses are calculated for graphite, PMMA, and aluminum‐walled ion chambers free‐in‐air in ^{60}Co beams and 200‐keV beams for a graphite chamber. The EGS4 Monte Carlo system is used with various electron step‐size algorithms, in particular the PRESTA algorithm, the much simpler ESTEPE constraint on the energy loss per step, and in combination. Contrary to previous reports, it is found that there are variations in the calculated ion chamber response of up to 3% in ^{60}Co beams and up to 8% in the 200‐keV beam. It is recommended that all ion chamber calculations be done with the PRESTA algorithm plus an additional constraint on the energy losses per step of 1% (or less for higher‐Z materials). The systematic uncertainty in the calculations for ^{60}Co beams is found to be 1% (1σ) and somewhat higher in 200‐keV beams because of sensitivity to transport parameters such as AE and ECUT.

Experimental verification of a three‐dimensional dose calculation algorithm using a specially designed heterogeneous phantom
View Description Hide DescriptionA solid heterogeneous phantom made up of 25‐ and 50‐mm cubes of materials with different electron densities was used to verify the accuracy of a three‐dimensional (3‐D) dose calculation algorithm. This algorithm uses 3‐D information obtained from contiguous CT(computed tomography) slices, spaced 5 mm apart. Primary and scatter doses at a point are calculated by using information from ray‐tracing CT voxels. The algorithm was developed on a Stardent model 1500 Supergraphic workstation. Cubes of materials with different electron densities were stacked up to simulate finite heterogeneities in three dimensions. This design allows verification of the algorithm for surface contour corrections and finite heterogeneities in the treatment field. Thermoluminescent lithium fluoride chips were placed in grooves milled on the cubes for dose measurement at various points. Different experiments were performed to investigate both the accuracy of the dose calculation algorithm and the utility of the versatile test phantom.

Experimental determination of fluence perturbation factors for five parallel‐plate ionization chambers
View Description Hide DescriptionThe calibration of parallel‐plate chambers for absolute dosimetry is an unsettled matter. The medical physics community has not yet agreed on a practical method of obtaining N _{gas}, although several researchers are working on this problem. If the photon and electron fluence perturbation factors, K _{wall} K _{comp}, were known for chambers of standard construction with full buildup provision, then an in‐air Co‐60 calibration could be applied to these, as is done with cylindrical chambers. We have obtained such correction factors for five commercially available chambers based on measurements in air and in homogeneous phantoms relative to matched cylindrical chambers of known dosimetric parameters. For three of the chambers (Markus, Holt and Exradin) we find that K _{wall} K _{comp}=1.000±0.008, in excellent agreement with available results from Monte Carlo calculations. The values for the other two chambers (NACP and Capintec) are different than 1. Our results are compared to recently published values, both calculated and measured.

Recombination correction factors for an ionization chamber exposed to discrete patterned pulsed swept beams
View Description Hide DescriptionAn equation for a recombination correction factor for a pulsed swept beam of electrons was derived by Boag. This equation is based on an integration technique, which assumes that a large number of spot beams cover the radiation field, that the field size is much larger than the spot beam size, and that the spot beam size is much larger than the chamber size. However, for computer‐controlled pulsed swept beams of electrons, the spot beam pattern can be altered, may not cover all of the field area, and the locations are reproducible. In this report, a summation method is proposed for this type of beam. Calculations for two such beams are demonstrated with the chamber located in the center of the field, and with the chamber half‐way to and at the edge of the field for a linear accelerator. The results lie between the integration pulsed swept and pulsed beam curves. Moving the ionization chamber from the center toward the edge of the field produces curves closer to the pulsed swept beam curve. Increasing spot beam size produces curves closer to the pulsed beam curves. It is therefore concluded that the pulsed swept beam cannot be characterized by a single recombination correction factor curve. The actual curve will be bounded by the integration pulsed swept and pulsed beam curves.

Evaluations of two solid water parallel‐plate chambers in high‐energy photon and electron beams
View Description Hide DescriptionUsing high‐energy photon and electron beams, the response of two solid water parallel‐plate chambers is evaluated against a PTW (Physicallsch‐Technische Werkstatten) Farmer‐type chamber. These two chambers are (1) a Memorial ‘‘Holt’’ chamber which was further modified by this author to take advantage of electrically conductive solid water, and (2) a thin window parallel‐plate chamber designed by Attix. The evaluations are made for (1) stability and reproducibility, (2) polarity effect, (3) ion recombination correction, (4) field size dependence on output, (5) dose rate dependence on output, and (6) absorbed dose comparison in high‐energy photon and electron beams using the American Association of Physicists in Medicine Task Group 21 (TG21) protocol.

Three‐dimensional verification of patient placement during radiotherapy using portal images
View Description Hide DescriptionThe use of a single portal image for the three‐dimensional verification of patient placement during radiotherapy has been investigated. In this study, a deviation in the patient positioning is quantified by a three‐dimensional translation and rotation of the patient. The parameter values are obtained by fitting the projections of anatomical match points in the simulator and the portal image, using the three‐dimensional gantry coordinates of these points in the intended treatment setup. Two methods for finding an analytical solution for the fitting problem are presented. One method yields a solution from the affine transformation of the portal image (shift, rotation, magnification, and stretching in a specified direction) that fits the projections of the match points on the simulator image. The second and more accurate method yields a solution that expresses the estimated parameter values and their covariances as functions of image and three‐dimensional gantry coordinates of the selected matchpoints. The robustness and sensitivity of the solution is implicitly given by these expressions. The applications of these methods are illustrated by experiments with a human pelvic bone.

Scatter factors for a 25‐MV x‐ray beam
View Description Hide DescriptionThe scatter factors, i.e., the ratios between the primary and scatter components of absorbed dose, have been determined for a 25‐MV x‐ray beam. The primary dose is described by attenuation and head‐scatter factors. The attenuation coefficient decreased from 0.028 cm^{−1} at the surface to 0.025 cm^{−1} at 30 cm depth. The head‐scatter factor varied about 8% for collimator settings from 5×5 to 40×40 cm^{2}. To determine the scatter factors, measured tissue‐output ratios were divided by the attenuation and head‐scatter factors for the same depth and field size. The results were fitted, with agreement within ±1%, to a model which assumes a linear relation between the scatter factor and the field size if the ratio between field size and depth is constant. The model requires transient electron equilibrium, which is present when the depth exceeds 6 cm and the field size 6×6 cm^{2}. The scatter factor increases to 1.33 at 30‐cm depth and 40×40 cm^{2} field size. The peak scatter factor (PSF) at 3.5‐cm depth reaches 1.06 for the largest field. Both PSF and NPSF, the normalized peak scatter factor, are affected by electron disequilibrium, which causes some conceptual ambiguities and numerical uncertainties.

Backscatter into the beam monitor chamber: Implications for dosimetry of asymmetric collimators
View Description Hide DescriptionBackscatter from the asymmetric collimators of a linac into the beam monitor chamber (BMC) has been investigated for two accelerators having different BMC configurations. The effect has been quantified as a function of field size and collimator jaw position for 6 and 18 MV beams. The results indicate a maximum 2.5% (6 MV) and 4% (18 MV) decrease in output in one case and a negligible effect in the other case. The experiments indicate that the difference can be attributed to the different construction of the BMC’s for the two accelerators.

Dosimetry of large wedged high‐energy photon beams
View Description Hide DescriptionThe dependence of the wedge factor and central axis depth dose on field size was evaluated for 6‐, 10‐, and 24‐MV wedged photon beams for field sizes up to 40×40 cm^{2}. The wedge factor for 60°, 45°, 30°, and 15° wedges in a 24‐MV beam was found to vary by as much as 25%, 12%, 9%, and 5%, respectively, over a field size range of 5×5 to 40×40 cm^{2}. For 10 and 6 MV wedged beams, the wedge factors varied by up to 17% and 15%, respectively, over the same field size range. The depth dose curves for the wedged beams differed significantly from the open beam profiles. At 6 MV, the wedges caused beam hardening while at 24 MV, with the exception of the 15° wedge, all wedged beams were softer than the open beams, for all field sizes. At 10 MV, wedged fields of size less than 20×20 cm^{2} were hardened relative to the open beam, whereas larger wedged fields had depth dose values within ±1% of the 10‐MV open‐beam depth dose data. Accurate treatment planning for large wedged fields and high‐energy photon beams thus requires the use of wedged beam depth dose curves and field size specific wedge factors. It was established that an equivalent square field for a rectangular wedged field can be determined using the standard open beam formulation. The largest difference between the wedge factor for a rectangular beam and its equivalent square beam was 2.5% and occurred for 24‐MV elongated fields. The depth dose values for a rectangular wedged field and its equivalent square beam were within ±1% at all energies studied.

Improving agreement between radiation‐delineated field edges on simulation and portal films: The edge tolerance test tool
View Description Hide DescriptionThe anatomy at the edge of a treatment machine portal film may differ from that shown by the delineator lines on a simulator film by up to 10 mm, even if both the simulator and the treatment machine meet accepted criteria for mechanical tolerances. To assure that this possibility is minimized requires some form of overall alignment check between the simulator and the treatment machine. A new test device, the edge tolerance test tool (ET^{3}), has been designed to permit a quick and accurate check on whether portal film disagreement with simulator films is due to an accumulation of tolerances. Its use should eliminate one source of this common problem.

Surface dose perturbation due to air gap between patient and bolus for electron beams
View Description Hide DescriptionThe effect of air gaps under bolus material on the surface dose for electron beams is investigated. Dose measurements were performed for air gaps from 0.0 to 3.0 cm and bolus thicknesses of 0.5 and 1.0 cm using the various electron energies and cone sizes available on an electron linear accelerator. Our results show that the surface dose decreases for lower electron energies, smaller field sizes, thicker boluses, and larger air gaps.

CT‐based simulation with laser patient marking
View Description Hide DescriptionA CT‐based simulator has been assembled based on a commercial CT scanner, virtual simulation software developed at the University of North Carolina and a laser drawing device to transfer the radiation portals from the virtual simulator to the patient. The simulation process can be completed in approximately 1 h; under most cases, the treatment portals can be designed and the patient marked in one session. The device has an inherent accuracy of ±1 mm. The portal projection accuracy in clinical cases is observed to be better than 2 mm.

Evaluation of a diode detector array for measurement of dynamic wedge dose distributions
View Description Hide Description

Dosimetry for ^{125}I seed (model 6711) in eye plaques
View Description Hide DescriptionThe effect of eye plaque materials (gold backing and silastic seed‐carrier insert) on the dose distribution around a single ^{125}I seed has been measured, using cubic lithium fluoride thermoluminescent dosimeters(TLDs) 1 mm on an edge, in a solid water eye phantom embedded in a solid water head phantom. With an ^{125}I seed (model 6711) positioned in the center slot of the silastic insert for a 20‐mm plaque of the design used in the collaborative ocular melanoma study (COMS), dose was measured at 2‐mm intervals along the plaque central axis (the seed’s transverse axis) and at various off‐axis points, both with and without the COMS gold backing placed over the insert. Monte Carlo calculations (MORSE code) were performed, as well, for these configurations and closely the same geometry but assuming a large natural water phantom. Additional Monte Carlo calculations treated the case, both for 20‐ and 12‐mm gold plaques, where the silastic insert is replaced by natural water. Relative to previous measurements taken in homogeneous medium of the same material (without the eye plaque), the dose reduction found by both Monte Carlo and TLD methods was greater at points farther from the seed along the central axis and, for a given central‐axis depth, at larger off‐axis distances. Removal of the gold backing from the plaque did not make measurable difference in the dose reduction results (10% at 1 cm).

A simple test phantom for stereotactic computed tomography
View Description Hide DescriptionAn easy‐to‐use phantom has been constructed for checking the accuracy of a stereotactic computed tomography localization system. This phantom has been used on a commercially available stereotactic radiosurgery system. With this system, the phantom reference point, whose location is established by means of a precision measuring implement, can be localized by a computed tomography(CT) scanner with a standard deviation of measurement that is less than 0.3 mm in three orthogonal axes.