Monte Carlo study of a Cyberknife stereotactic radiosurgery system

This study investigated small-field dosimetry for a Cyberknife stereotactic radiosurgery system using Monte Carlo simulations. The EGSnrc/BEAMnrc Monte Carlo code was used to simulate the Cyberknife treatment head, and the DOSXYZnrc code was implemented to calculate central axis depth-dose curves, off-axis dose profiles, and relative output factors for various circular collimator sizes of 5 to 60 mm. Water-to-air stopping power ratios necessary for clinical reference dosimetry of the Cyberknife system were also evaluated by Monte Carlo simulations. Additionally, a beam quality conversion factor, kQ, for the Cyberknife system was evaluated for cylindrical ion chambers with different wall material. The accuracy of the simulated beam was validated by agreement within 2% between the Monte Carlo calculated and measured central axis depth-dose curves and off-axis dose profiles. The calculated output factors were compared with those measured by a diode detector and an ion chamber in water. The diode output factors agreed within 1% with the calculated values down to a 10 mm collimator. The output factors with the ion chamber decreased rapidly for collimators below 20 mm. These results were confirmed by the comparison to those from Monte Carlo methods with voxel sizes and materials corresponding to both detectors. It was demonstrated that the discrepancy in the 5 and 7.5 mm collimators for the diode detector is due to the water non-equivalence of the silicon material, and the dose fall-off for the ion chamber is due to its large active volume against collimators below 20 mm. The calculated stopping power ratios of the 60 mm collimator from the Cyberknife system (without a flattening filter) agreed within 0.2% with those of a 10 X 10 cm2 field from a conventional linear accelerator with a heavy flattening filter and the incident electron energy, 6 MeV. The difference in the stopping power ratios between 5 and 60 mm collimators was within 0.5% at a 10 cm depth in water. Furthermore, kQ values for the Cyberknife system were in agreement within 0.3% with those of the conventional 6 MV-linear accelerator for the cylindrical ion chambers with different wall material.     

Decoupling initial electron beam parameters for Monte Carlo photon beam modelling by removing beam-modifying filters from the beam path

A new method is presented to decouple the parameters of the incident e(-) beam hitting the target of the linear accelerator, which consists essentially in optimizing the agreement between measurements and calculations when the difference filter, which is an additional filter inserted in the linac head to obtain uniform lateral dose-profile curves for the high energy photon beam, and flattening filter are removed from the beam path. This leads to lateral dose-profile curves, which depend only on the mean energy of the incident electron beam, since the effect of the radial intensity distribution of the incident e- beam is negligible when both filters are absent. The location of the primary collimator and the thickness and density of the target are not considered as adjustable parameters, since a satisfactory working Monte Carlo model is obtained for the low energy photon beam (6 MV) of the linac using the same target and primary collimator. This method was applied to conclude that the mean energy of the incident e- beam for the high energy photon beam (18 MV) of our Elekta SLi Plus linac is equal to 14.9 MeV. After optimizing the mean energy, the modelling of the filters, in accordance with the information provided by the manufacturer, can be verified by positioning only one filter in the linac head while the other is removed. It is also demonstrated that the parameter setting for Bremsstrahlung angular sampling in BEAMnrc ('Simple' using the leading term of the Koch and Motz equation or 'KM' using the full equation) leads to different dose-profile curves for the same incident electron energy for the studied 18 MV beam. It is therefore important to perform the calculations in 'KM' mode. Note that both filters are not physically removed from the linac head. All filters remain present in the linac head and are only rotated out of the beam. This makes the described method applicable for practical usage since no recommissioning process is required.     

Electron beam dose calculations

Electron beam dose distributions in the presence of inhomogeneous tissue are calculated by an algorithm that sums the dose distribution of individual pencil beams. The off-axis dependence of the pencil beam dose distribution is described by the Fermi-Eyges theory of thick-target multiple Coulomb scattering. Measured square-field depth-dose data serve as input for the calculations. Air gap corrections are incorporated and use data from'in-air' measurements in the penumbra of the beam. The effective depth, used to evaluate depth-dose, and the sigma of the off-axis Gaussian spread against depth are calculated by recursion relations from a CT data matrix for the material underlying individual pencil beams. The correlation of CT number with relative linear stopping power and relative linear scattering power for various tissues is shown. The results of calculations are verified by comparison with measurements in a 17 MeV electron beam from the Therac 20 linear accelerator. Calculated isodose lines agree nominally to within 2 mm of measurements in a water phantom. Similar agreement is observed in cork slabs simulating lung. Calculations beneath a bone substitute illustrate a weakness in the calculation. Finally a case of carcinoma in the maxillary antrum is studied. The theory suggests an alternative method for the calculation of depth-dose of rectangular fields.     

Influence of initial electron beam parameters on Monte Carlo calculated absorbed dose distributions for radiotherapy photon beams

Our aim in the present study was to investigate the effects of initial electron beam characteristics on Monte Carlo calculated absorbed dose distribution for a linac 6 MV photon beam. Moreover, the range of values of these parameters was derived, so that the resulted differences between measured and calculated doses were less than 1%. Mean energy, radial intensity distribution and energy spread of the initial electron beam, were studied. The method is based on absorbed dose comparisons of measured and calculated depth-dose and dose-profile curves. All comparisons were performed at 10.0 cm depth, in the umbral region for dose-profile and for depths past maximum for depth-dose curves. Depth-dose and dose-profile curves were considerably affected by the mean energy of electron beam, with dose profiles to be more sensitive on that parameter. The depth-dose curves were unaffected by the radial intensity of electron beam. In contrast, dose-profile curves were affected by the radial intensity of initial electron beam for a large field size. No influence was observed in dose-profile or depth-dose curves with respect to energy spread variations of electron beam. Conclusively, simulating the radiation source of a photon beam, two of the examined parameters (mean energy and radial intensity) of the electron beam should be tuned accurately, so that the resulting absorbed doses are within acceptable precision. The suggested method of evaluating these crucial but often poorly specified parameters may be of value in the Monte Carlo simulation of linear accelerator photon beams.     

Monte Carlo calculation of nine megavoltage photon beam spectra using the BEAM code

A recent paper analyzed the sensitivity to various simulation parameters of the Monte Carlo simulations of nine beams from three major manufacturers of commercial medical linear accelerators, ranging in energy from 4-25 MV. In this work the nine models are used: to calculate photon energy spectra and average energy distributions and compare them to those published by Mohan et al. [Med. Phys. 12, 592-597 (1985)]; to separate the spectra into primary and scatter components from the primary collimator, the flattening filter and the adjustable collimators; and to calculate the contaminant-electron fluence spectra and the electron contribution to the depth-dose curves. Notwithstanding the better precision of the calculated spectra, they are similar to those calculated by Mohan et al. The three photon spectra at 6 MV from the machines of three different manufacturers show differences in their shapes as well as in the efficiency of bremsstrahlung production in the corresponding target and filter combinations. The contribution of direct photons to the photon energy fluence in a 10 x 10 field varies between 92% and 97%, where the primary collimator contributes between 0.6% and 3.4% and the flattening filter contributes between 0.6% and 4.5% to the head-scatter energy fluence. The fluence of the contaminant electrons at 100 cm varies between 5 x 10(-9) and 2.4 x 10(-7) cm(-2) per incident electron on target, and the corresponding spectrum for each beam is relatively invariant inside a 10 x 10 cm2 field. On the surface the dose from electron contamination varies between 5.7% and 11% of maximum dose and, at the depth of maximum dose, between 0.16% and 2.5% of maximum dose. The photon component of the percentage depth-dose at 10 cm depth is compared with the general formula provided by AAPM's task group 51 and confirms the claimed accuracy of 2%.     

Differences in electron depth-dose curves calculated with EGS and ETRAN and improved energy-range relationships

For 1-50 MeV electrons incident on a water phantom there are systematic differences in the depth-dose curves calculated by the Monte Carlo codes EGS and ETRAN (and its descendants SANDYL, CYLTRAN, ACCEPT, and the ITS system). Compared to ETRAN, the EGS code calculates a higher surface dose and a slightly slower dose falloff past the dose maximum. The discrepancy in the surface dose is shown to exist because the modified Landau energy-loss straggling distribution used in ETRAN underestimates the mean energy loss by about 10% since it underestimates the number of large energy-loss events. Comparison to experimental data shows a preference for the EGS depth-dose curves at 10 and 20 MeV. Since various dosimetry protocols assign electron beam energies based on measured depth-dose curves in water, formulas based on these more accurate EGS4 calculations are presented: relating the mean energy of an incident electron beam to R50, the depth at which the dose in a water phantom falls to 50% of its maximum value; and relating the most probable energy of the incident beam to the projected range of the depth-dose curve. A study is presented of the effects of the incident electron spectrum on the calculated depth-dose curve.     

A model to determine the initial phase space of a clinical electron beam from measured beam data

Advanced electron beam dose calculation models for radiation oncology require as input an initial phase space (IPS) that describes a clinical electron beam. The IPS is a distribution in position, energy and direction of electrons and photons in a plane in front of the patient. A method is presented to derive the IPS of a clinical electron beam from a limited set of measured beam data. The electron beam is modelled by a sum of four beam components: a main diverging beam, applicator edge scatter, applicator transmission and a second diverging beam. The two diverging beam components are described by weighted sums of monoenergetic diverging electron and photon beams. The weight factors of these monoenergetic beams are determined by the method of simulated annealing such that a best fit is obtained with depth-dose curves measured for several field sizes at two source-surface distances. The resulting IPSs are applied by the phase-space evolution electron beam dose calculation model to calculate absolute 3D dose distributions. The accuracy of the calculated results is in general within 1.5% or 1.5 mm; worst cases show differences of up to 3% or 3 mm. The method presented here to describe clinical electron beams yields accurate results, requires only a limited set of measurements and might be considered as an alternative to the use of Monte Carlo methods to generate full initial phase spaces.     

Photon beam characterization and modelling for Monte Carlo treatment planning

Photon beams of 4, 6 and 15 MV from Varian Clinac 2100C and 2300C/D accelerators were simulated using the EGS4/BEAM code system. The accelerators were modelled as a combination of component modules (CMs) consisting of a target, primary collimator, exit window, flattening filter, monitor chamber, secondary collimator, ring collimator, photon jaws and protection window. A full phase space file was scored directly above the upper photon jaws and analysed using beam data processing software, BEAMDP, to derive the beam characteristics, such as planar fluence, angular distribution, energy spectrum and the fractional contributions of each individual CM. A multiple-source model has been further developed to reconstruct the original phase space. Separate sources were created with accurate source intensity, energy, fluence and angular distributions for the target, primary collimator and flattening filter. Good agreement (within 2%) between the Monte Carlo calculations with the source model and those with the original phase space was achieved in the dose distributions for field sizes of 4 cm x 4 cm to 40 cm x 40 cm at source surface distances (SSDs) of 80-120 cm. The dose distributions in lung and bone heterogeneous phantoms have also been found to be in good agreement (within 2%) for 4, 6 and 15 MV photon beams for various field sizes between the Monte Carlo calculations with the source model and those with the original phase space.     

Angular correction in reconstruction of electron spectra from depth dose distributions

Techniques for reconstruction of electron spectra from the depth-dose curves used to date have ignored the angular distribution of incident electrons scattered at large angles. The approximation adopted is to employ a database of monoenergetic depth-dose curves generated for normal incidence of electrons at the surface. This approximation is acceptable for direct electrons with small angular spread. However, electrons scattered from the treatment head and collimating system may have large average angles of incidence which affects the depth-dose distribution significantly at shallow depths by increasing energy deposition close to the surface. We show that ignoring the electron incident angular distribution leads to systematic errors in the low energy region of reconstructed electron spectra. We propose a simple 1-D model to correct for these systematic errors using only electron angular distribution at the central beam axis. This model provides reconstructed spectra in excellent agreement with Monte Carlo simulation in the low energy region.     

Functional forms for photon spectra of clinical linacs

Specifying photon spectra of clinical linacs using a functional form is useful for many applications, including virtual source modelling and spectral unfolding from dosimetric measurements such as transmission data or depth-dose curves. In this study, 11 functional forms from the literature are compiled and quantitatively compared. A new function is proposed which offers improvements over existing ones. The proposed function is simple, physics-based and has four free parameters, one of which is the mean incident electron kinetic energy. A comprehensive benchmark set of validated, high-precision Monte Carlo spectra is generated and used to evaluate the strengths and limitations of different functions. The benchmark set has 65 spectra (3.5-30 MV) from Varian, Elekta, Siemens, Tomotherapy, Cyberknife and research linacs. The set includes spectra on- and off-axis from linacs with and without a flattening filter, and in treatment and imaging modes. The proposed function gives the lowest spectral deviations among all functions. It reproduces the energy fluence values in each bin for the benchmark set with a normalized root-mean-square deviation of 1.7%. The mean incident electron kinetic energy, maximum photon energy, most-probable energy and average energy are reproduced, on average, within 1.4%, 4.3%, 3.9% and 0.6% of their true values, respectively. The proposed function is well behaved when used for spectral unfolding from dosimetric data. The contribution of the 511 keV annihilation peak and the energy spread of the incident electron beam can be added as additional free parameters.     

Photon-beam subsource sensitivity to the initial electron-beam parameters

One limitation to the widespread implementation of Monte Carlo (MC) patient dose-calculation algorithms for radiotherapy is the lack of a general and accurate source model of the accelerator radiation source. Our aim in this work is to investigate the sensitivity of the photon-beam subsource distributions in a MC source model (with target, primary collimator, and flattening filter photon subsources and an electron subsource) for 6- and 18-MV photon beams when the energy and radial distributions of initial electrons striking a linac target change. For this purpose, phase-space data (PSD) was calculated for various mean electron energies striking the target, various normally distributed electron energy spread, and various normally distributed electron radial intensity distributions. All PSD was analyzed in terms of energy, fluence, and energy fluence distributions, which were compared between the different parameter sets. The energy spread was found to have a negligible influence on the subsource distributions. The mean energy and radial intensity significantly changed the target subsource distribution shapes and intensities. For the primary collimator and flattening filter subsources, the distribution shapes of the fluence and energy fluence changed little for different mean electron energies striking the target, however, their relative intensity compared with the target subsource change, which can be accounted for by a scaling factor. This study indicates that adjustments to MC source models can likely be limited to adjusting the target subsource in conjunction with scaling the relative intensity and energy spectrum of the primary collimator, flattening filter, and electron subsources when the energy and radial distributions of the initial electron-beam change.     

Characterization of an add-on multileaf collimator for electron beam therapy

An add-on multileaf collimator for electrons (eMLC) has been developed that provides computer-controlled beam collimation and isocentric dose delivery. The design parameters result from the design study by Gauer et al (2006 Phys. Med. Biol. 51 5987-6003) and were configured such that a compact and light-weight eMLC with motorized leaves can be industrially manufactured and stably mounted on a conventional linear accelerator. In the present study, the efficiency of an initial computer-controlled prototype was examined according to the design goals and the performance of energy- and intensity-modulated treatment techniques. This study concentrates on the attachment and gantry stability as well as the dosimetric characteristics of central-axis and off-axis dose, field size dependence, collimator scatter, field abutment, radiation leakage and the setting of the accelerator jaws. To provide isocentric irradiation, the eMLC can be placed either 16 or 28 cm above the isocentre through interchangeable holders. The mechanical implementation of this feature results in a maximum field displacement of less than 0.6 mm at 90 degrees and 270 degrees gantry angles. Compared to a 10 x 10 cm applicator at 6-14 MeV, the beam penumbra of the eMLC at a 16 cm collimator-to-isocentre distance is 0.8-0.4 cm greater and the depth-dose curves show a larger build-up effect. Due to the loss in energy dependence of the therapeutic range and the much lower dose output at small beam sizes, a minimum beam size of 3 x 3 cm is necessary to avoid suboptimal dose delivery. Dose output and beam symmetry are not affected by collimator scatter when the central axis is blocked. As a consequence of the broader beam penumbra, uniform dose distributions were measured in the junction region of adjacent beams at perpendicular and oblique beam incidence. However, adjacent beams with a high difference in a beam energy of 6 to 14 MeV generate cold and hot spots of approximately 15% in the abutting region. In order to improve uniformity, the energy of adjacent beams must be limited to 6 to 10 MeV and 10 to 14 MeV respectively. At the maximum available beam energy of 14 MeV, radiation leakage results mainly from the intraleaf leakage of approximately 2.5% relative dose which could be effectively eliminated at off-axis distances remote from the field edge by adjusting the jaw field size to the respective opening of the eMLC. Additionally, the interleaf and leaf-end leakage could be reduced by using a tongue-and-groove leaf shape and adjoining the leaf-ends off-axis respectively.     

An MCNP-based model of a linear accelerator x-ray beam.

The Monte Carlo N-Particle radiation transport computer code (MCNP) has been employed on a personal computer to develop a simple model simulating the major components within the beam path of a linear accelerator radiation head, namely the electron target, primary conical collimator, beam flattening filter, wedge filter and the secondary collimators. The model was initially used to calculate the energy spectra and angular distributions of the x-ray beam for the Philips SL 75/5 linear accelerator, in a plane immediately beneath the flattening filter. These data were subsequently used as a 'source' of x-rays at the target position, to assess the emergent beam from the secondary collimators. The depth dose distributions and dose profiles at constant depth for various field sizes have been calculated for a nominal operating potential of 4 MV and found to be within acceptable limits. It is concluded that the technique may be used to calculate the energy spectra of any linear accelerator upon specification of the component dimensions, materials and nominal accelerating potential. It is anticipated that this work will serve as the basis of a quality control tool for linear accelerators and treatment planning systems.     

The in-air scattering of clinical electron beams as produced by accelerators with scanning beams and diaphragm collimators

The electron distribution F(x, y, z, theta x, theta y) in air has been evaluated for a clinical electron beam emanating from a scanning beam accelerator in which the collimation of the beam is performed by means of diaphragm collimators. The multiple scattering theory of Fermi turns out to be adequate in describing this electron distribution. In this theory, the only parameter to be determined experimentally is the angular variance at the level of the collimator blocks. Generally, this angular variance features the same energy dependence as the angular scattering power and its value at an arbitrary energy can be derived from measuring the penumbra widths of off-axis profiles in air, at various distances beyond the collimator blocks. Then, the angular variance at the level of a secondary diaphragm collimator can be calculated, as well as off-axis profiles in air at arbitrary distances. In this way, the relative electron distribution at the surface of patients can be calculated easily. This in turn serves adequately as input to the calculation of patient dose distributions in radiation therapy planning.     

Dosimetric characterization of the 18-MV photon beam from the Siemens Mevatron 77 linear accelerator

A comprehensive set of dosimetric measurements has been made on the Mevatron 77.80.67 18-MV photon beam. Percentage depth dose, dose in the buildup region, field size dependence of output, transmission through lead, tray attenuation, and isodose curves for the open and wedged fields were measured using an ionization chamber in water and polystyrene phantoms. These dosimetric measurements sufficiently characterized the beam to permit clinical use. The depth dose at 10-cm depth for a 10 X 10 cm2 field at 100-cm source-to-skin distance (SSD) is 80.9%, which meets design specifications. Central axis depth-dose data were fitted to within 0.5% by a set of polynomial equations utilizing a two-dimensional linear regression analysis. Tissue-maximum ratios calculated from depth-dose data agree with measured data to within 2%. Output differences as large as 2.5% were measured for rectangular fields depending on which collimator jaws defined the long dimension of the field. The field size dependence of output was fit to within +/- 0.1% by a linear regression. The half-value thickness of the beam was measured to be 13 mm of lead.     

Radiation characteristics of helical tomotherapy

Helical tomotherapy is a dedicated intensity modulated radiation therapy (IMRT) system with on-board imaging capability (MVCT) and therefore differs from conventional treatment units. Different design goals resulted in some distinctive radiation field characteristics. The most significant differences in the design are the lack of flattening filter, increased shielding of the collimators, treatment and imaging operation modes and narrow fan beam delivery. Radiation characteristics of the helical tomotherapy system, sensitivity studies of various incident electron beam parameters and radiation safety analyses are presented here. It was determined that the photon beam energy spectrum of helical tomotherapy is similar to that of more conventional radiation treatment units. The two operational modes of the system result in different nominal energies of the incident electron beam with approximately 6 MeV and 3.5 MeV in the treatment and imaging modes, respectively. The off-axis mean energy dependence is much lower than in conventional radiotherapy units with less than 5% variation across the field, which is the consequence of the absent flattening filter. For the same reason the transverse profile exhibits the characteristic conical shape resulting in a 2-fold increase of the beam intensity in the center. The radiation leakage outside the field was found to be negligible at less than 0.05% because of the increased shielding of the collimators. At this level the in-field scattering is a dominant source of the radiation outside the field and thus a narrow field treatment does not result in the increased leakage. The sensitivity studies showed increased sensitivity on the incident electron position because of the narrow fan beam delivery and high sensitivity on the incident electron energy, as common to other treatment systems. All in all, it was determined that helical tomotherapy is a system with some unique radiation characteristics, which have been to a large extent optimized for intensity modulated delivery.     

A flattening filter free photon treatment concept evaluation with Monte Carlo

In principle, the concept of flat initial radiation-dose distribution across the beam is unnecessary for intensity modulated radiation therapy. Dynamic leaf positioning during irradiation could appropriately adjust the fluence distribution of an unflattened beam that is peaked in the center and deliver the desired uniform or nonuniform dose distribution. Removing the flattening filter could lead to reduced treatment time through higher dose rates and reduced scatter, because there would be substantially less material in the beam; and possibly other dosimetric and clinical advantages. This work aims to evaluate the properties of a flattening filter free clinical accelerator and to investigate its possible advantages in clinical intensity modulated radiation therapy applications by simulating a Varian 2100-based treatment delivery system with Monte Carlo techniques. Several depth-dose curves and lateral dose distribution profiles have been created for various field sizes, with and without the flattening filter. Data computed with this model were used to evaluate the overall quality of such a system in terms of changes in dose rate, photon and electron fluence, and reduction in out-of-field stray dose from the scattered components and were compared to the corresponding data for a standard treatment head with a flattening filter. The results of the simulations of the flattening filter free system show that a substantial increase in dose rate can be achieved, which would reduce the beam on time and decrease the out-of-field dose for patients due to reduced head-leakage dose. Also close to the treatment field edge, a significant improvement in out-of-field dose could be observed for small fields, which can be attributed to the change in the photon spectra, when the flattening filter is removed from the beamline.     

Total skin electron therapy treatment verification: Monte Carlo simulation and beam characteristics of large non-standard electron fields

Total skin electron therapy (TSET) is a complex technique which requires non-standard measurements and dosimetric procedures. This paper investigates an essential first step towards TSET Monte Carlo (MC) verification. The non-standard 6 MeV 40 x 40 cm2 electron beam at a source to surface distance (SSD) of 100 cm as well as its horizontal projection behind a polymethylmethacrylate (PMMA) screen to SSD = 380 cm were evaluated. The EGS4 OMEGA-BEAM code package running on a Linux home made 47 PCs cluster was used for the MC simulations. Percentage depth-dose curves and profiles were calculated and measured experimentally for the 40 x 40 cm2 field at both SSD = 100 cm and patient surface SSD = 380 cm. The output factor (OF) between the reference 40 x 40 cm2 open field and its horizontal projection as TSET beam at SSD = 380 cm was also measured for comparison with MC results. The accuracy of the simulated beam was validated by the good agreement to within 2% between measured relative dose distributions, including the beam characteristic parameters (R50, R80, R100, Rp, E0) and the MC calculated results. The energy spectrum, fluence and angular distribution at different stages of the beam (at SSD = 100 cm, at SSD = 364.2 cm, behind the PMMA beam spoiler screen and at treatment surface SSD = 380 cm) were derived from MC simulations. Results showed a final decrease in mean energy of almost 56% from the exit window to the treatment surface. A broader angular distribution (FWHM of the angular distribution increased from 13 degrees at SSD = 100 cm to more than 30 degrees at the treatment surface) was fully attributable to the PMMA beam spoiler screen. OF calculations and measurements agreed to less than 1%. The effect of changing the electron energy cut-off from 0.7 MeV to 0.521 MeV and air density fluctuations in the bunker which could affect the MC results were shown to have a negligible impact on the beam fluence distributions. Results proved the applicability of using MC as a treatment verification tool for complex radiotherapy techniques.     

Characteristics of the high-energy photon beam of a 25-MeV accelerator

The CGR Saturne 25 is an isocentrically mounted standing wave medical linear accelerator that produces dual-energy photon beams and a scanned electron beam with six selectable energies between 4 and 25 MeV. The highest energy photon beam is nominally referred to as 23 MV. For this beam the mean energy of the accelerated electron beam on the 1.3 radiation length (4 mm) tungsten x-ray target is found to be approximately 21 MeV, with the energy acceptance stated to be +/- 5%. The electron beam traverses a 270 degrees bending magnet upstream of the x-ray production target. The resulting bremsstrahlung beam passes through a combination steel and lead flattening filter, 4-cm maximum thickness. Dosimetric data for the 23-MV beam are presented with respect to rectangular field output factor, depth of maximum dose as a function of field size, surface and buildup dose, central axis percent depth dose, tissue-phantom ratios, beam profile, applicability of inverse square, and block transmission. Some data are also presented on the effect of different flattening filter designs on apparent beam energy.