CPP calculation of multiple scattering distributions for charged particles penetrating compounds or mixtures

Charged particle multiple scattering distributions may be constructed from individual atomic scattering events on the basis of compound Poisson process (CPP) theory. We present a CPP method for computing multiple scattering transition probability densities from charged particles penetrating compounds and mixtures. Water as a scattering medium provides here an example of the calculation method which is applicable to compounds or mixtures. Electrons are chosen as examples of charged particle beams. The Rutherford single scattering cross section and a partial wave analysis single scattering cross section are chosen as example cross sections. Transition probability densities predicted on the basis of CPP theory can be calculated with great accuracy for the improvement of radiation dose calculations. The advantages of the CPP method are (a) an effective atomic number need not be defined for the scattering medium, (b) it can be applied in both spherical and planar coordinate systems, and (c) it does not require any specific form for the single scattering cross section.     

Collimated electron beams and their associated penumbra widths

The Fermi-Eyges multiple-scattering theory for electrons is applied to calculate profiles of collimated electron beams. The dose profile below the collimator is a convolution of the intensity distribution of the electrons at the level of the collimator and the distribution arising from the propagation of a Gaussian point source from the collimator to the level of the calculation. The electrons at the level of the collimator possess an angular distribution characteristic of the configuration of the electron beam at the vacuum window. Hence, the dose profile and its associated penumbra width can be expressed in terms of the angular moments of the distribution of the electrons at the collimator. The dependence of the penumbra width on the configuration-dependent angular spread of the electrons at the collimator accounts for differences in the size of the penumbra between two broad-beam configurations. These differences are also seen experimentally. We have also studied the dependence of the angular moments of the electrons upon scattering foils present above the collimator and the position of the beam-broadening device in the accelerator head.     

Comparison between the lateral penumbra of a collimated double-scattered beam and uncollimated scanning beam in proton radiotherapy

Intensity modulated proton radiotherapy (IMPT) can reduce the dose to critical structures by optimizing the distribution and intensity of individual pencil beams. The IMPT can be delivered by dynamically scanning a pencil beam with variable intensity and energy across the tumor target volume. The lateral penumbra of an uncollimated pencil beam is compromised, however, by the scattering in air between the vacuum window and the patient, and by the initial beam size. In this study, we compare the transversal penumbra of a pencil beam to the one of a collimated Gaussian broad divergent beam, such as the one produced by the double scattering system, for different range compensator thicknesses, collimator-to-surface distances (CSD), proton range and pencil beam sizes (sigma0). The effect of vacuum and helium in the nozzle on the pencil beam lateral profile further downstream is also investigated. The lateral spatial intensity distribution for the collimated Gaussian broad divergent proton beam is modeled using the generalized Fermi-Eyges theory. The model is validated with measurements of the lateral profile in water at different depths for two different ranges (7.7 cm and 22.1 cm, respectively). Nearly 2500 treatment fields are analyzed to establish typical clinical beam configurations, such as the range compensator thicknesses, CSD and range, which we use to predict the 80%-20% lateral penumbra. The penumbra of the collimated broad divergent beam is calculated for fixed source-to-surface distance (SSD) of 220 cm and source size of 2.5 cm (sigma). The results show that the model predicts the penumbra at different water depths with accuracy better than 0.2 mm. At depths larger than 7.6 cm (minimum range of the clinical fields analyzed), the accuracy is better than 3%. The treatment fields feature the following average configuration: the range compensator thickness of 6.5+/-2.8 cm (max 19.4 cm), CSD 11.9+/-3.8 cm (max 29.4 cm) and range of 16.0+/-6.1 cm. The penumbra of a pencil beam at shallow depth is in general larger (i.e., worse) than the penumbra of a collimated beam, but better at larger depths. The depth at which the two penumbras are identical exhibits only a small dependence on the proton range, but is strongly affected by the collimator-to-surface distance. For CSD 10 cm, range compensator thickness 6 cm, SSD 220 cm and source size 2.5 cm, this depth is 11.5 cm for a 5 mm pencil beam, and 9.1 cm for a 3 mm pencil beam. For most of the clinical sites considered, assuming the beam configurations of this study, the pencil beam penumbra is larger (i.e., worse). By moving the vacuum window downstream or by replacing air with helium in the gantry nozzle, the dosimetrical benefit of scanning would be drastically improved, especially for small sigma0 (5 mm or less).     

Beam scanning for heavy charged particle radiotherapy

An important aspect of heavy charged particle radiotherapy is its ability to localize dose to the target volume. Current techniques generally employ beam delivery schemes which use range modulated beams in which the Bragg peak is spread out over a range of depths. The range modulation is constant over the entire beam cross section. This paper examines the potential gain from the use of variable range modulation over the beam cross section which can be achieved through beam scanning. An analytic technique has been developed to calculate a gain factor, related to integral dose, for a number of different charged particle beams. The dependence of the gain factor on tumor depth, diameter, and size has been examined. These analytic results have been compared with computer calculations of treatment plans simulating beam scanning and fixed modulation delivery techniques for actual clinical cases. The rationale for beam scanning, and its potential impact on tumor control is discussed.     

Dose delivery error detection by a computer-controlled linear accelerator

A commercial dual photon energy, computer-controlled linear accelerator has a complex collimation and beam delivery system. For accurate dose delivery, six separate motorized elements must be properly positioned for a given beam selection. Experimentally instituted misalignments of the primary electron scattering foils, the primary collimator, the flattening filter, the scattering foil carousel, and the backscatter shield or shutter produced significant dose delivery errors. The ability of the Mylar window monitor chamber to detect these errors was examined for x-ray beams. The fault detection system failed to interrupt dose delivery in a number of situations where the error in dose per monitor unit delivered ranged from -6% to +270% of the calibrated value.     

Small SRS photon field profile dosimetry performed using a PinPoint air ion chamber, a diamond detector, a novel silicon-diode array (DOSI), and polymer gel dosimetry. Analysis and intercomparison

Small photon fields are increasingly used in modern radiotherapy and especially in IMRT and SRS/SRT treatments. The uncertainties related to small field profile measurements can introduce significant systematic errors to the overall treatment process. These measurements are challenging mainly due to the absence of charged particle equilibrium conditions, detector size and composition effects, and positioning problems. In this work four different dosimetric methods have been used to measure the profiles of three small 6 MV circular fields having diameters of 7.5, 15.0, and 30.0 mm: a small sensitive volume air ion chamber, a diamond detector, a novel silicon-diode array (DOSI), and vinyl-pyrrolidone based polymer gel dosimeter. The results of this work support the validity of previous findings, suggesting that (a) air ion chambers are not suitable for small field dosimetry since they result in penumbra broadening and require significant corrections due to severe charged particle transport alterations; (b) diamond detectors provide high resolution and rather accurate small field profile measurements, as long as positioning problems can be addressed and the necessary dose rate corrections are correctly applied; and (c) the novel silicon-diode array (DOSI) used in this study seems to be adequate for small field profile measurements overcoming positioning problems. Polymer gel data were assumed as reference data to which the other measurement data were compared both qualitatively and quantitatively using the gamma-index concept. Polymer gels are both phantom and dosimeter, hence there are no beam perturbation effects. In addition, polymer gels are tissue equivalent and can provide high-spatial density and high-spatial resolution measurements without positioning problems, which makes them useful for small field dosimetry measurements. This work emphasizes the need to perform beam profile measurements of small fields (for acceptance, commissioning, treatment planning systems data feed, and periodic quality assurance purposes) using more than one dosimetric method. The authors believe this to be a safe way towards the reduction of the overall uncertainty related to SRS/SRT treatments.     

Verification of compensator thicknesses using a fluoroscopic electronic portal imaging device.

A method is presented for verification of compensator thicknesses using a fluoroscopic electronic portal imaging device (EPID). The method is based on the measured transmission through the compensator, defined by the ratio of the portal dose with the compensator in the beam and the portal dose without the compensator in the beam. The transmission is determined with the EPID by dividing two images, acquired with and without compensator inserted, which are only corrected for the nonlinear response of the fluoroscopic system. The transmission has a primary and a scatter component. The primary component is derived from the measured transmission by subtracting the predicted scatter component. The primary component for each point is only related to the radiological thickness of the compensator along the ray line between the focus and that point. Compensator thicknesses are derived from the primary components taking into account off-axis variations in beam quality. The developed method has been tested for various compensators made of a granulate of stainless steel. The compensator thicknesses could be determined with an accuracy of 0.5 mm (1 s.d.), corresponding to a change in the transmitted dose of about 1% for a 10 MV beam. The method is fast, accurate, and insensitive to long-term output and beam profile fluctuations of the linear accelerator.     

Slit x-ray beam primary dose profiles determined by analytical transport of Compton recoil electrons

Accurate measurement of radiation beam penumbras is essential for conformal radiotherapy. For this purpose a detailed knowledge of the dosimeter's spatial response is required. However, experimental determination of detector spatial response is cumbersome and restricted to the specific detector type and beam spectrum used. A model has therefore been developed to calculate in slit beam geometry both dose profiles and detector response profiles. Summations over representative photon beam spectra yield profiles for polyenergetic beams. In the present study the model is described and resulting dose profiles verified. The model combines Compton scattering of incident photons, transport of resulting electrons by Fermi-Eyges small-angle multiple scattering theory, and functions to limit electron transport. This analytic model thus yields line spread kernels of primary dose in a water phantom. It is shown that the spatial response of an ideal point detector to a primary photon beam can be well described by the model; the calculations are verified by measurements with a diamond detector in a telescopic slit geometry in which all dose contributions except for the primary dose can be excluded. Effects of photon detector behavior, source size of the linear accelerator (linac) and detector size are studied. Measurements show that slit dose profiles calculated by means of the kernel are accurate within 0.1 mm of the full-width at half-maximum. For a theoretical point source and point detector combined with a 0.2 mm wide slit, the full-width half-maximum values of the slit beam dose profiles are calculated as 0.37 mm and 0.42 mm in a 6 MV and 25 MV x-ray beam, respectively. The present study shows that the model is adequate to calculate local dose effects that are dominated by approximately mono-directional, primary photon fluence. The analytic model further provides directional electron fluence information and is designed to be applied to various detectors and linac beam spectra.     

Phantom equivalent thicknesses in diagnostic radiology

Some measurements in diagnostic radiology must be done with phantoms. The most frequently used phantoms are water, plexiglass, aluminium and copper. It is interesting to know the equivalent thicknesses between these phantoms in order to be able to relate these values to real anatomical organs of known thicknesses. The use of antiscatter grids, the choice of intensifier screens, the beam size and in general the scattering parameters considerably change the equivalent thicknesses. Many situations have been simulated by the Monte Carlo method and equivalent thicknesses are presented.     

Monte Carlo techniques for scattering foil design and dosimetry in total skin electron irradiations

Total skin electron irradiation (TSEI) with single fields requires large electron beams having good dose uniformity, dmax at the skin surface, and low bremsstrahlung contamination. To satisfy these requirements, energy degraders and scattering foils have to be specially designed for the given accelerator and treatment room. We used Monte Carlo (MC) techniques based on EGS4 user codes (BEAM, DOSXYZ, and DOSRZ) as a guide in the beam modifier design of our TSEI system. The dosimetric characteristics at the treatment distance of 382 cm source-to-surface distance (SSD) were verified experimentally using a linear array of 47 ion chambers, a parallel plate chamber, and radiochromic film. By matching MC simulations to standard beam measurements at 100 cm SSD, the parameters of the electron beam incident on the vacuum window were determined. Best match was achieved assuming that electrons were monoenergetic at 6.72 MeV, parallel, and distributed in a circular pattern having a Gaussian radial distribution with full width at half maximum = 0.13 cm. These parameters were then used to simulate our TSEI unit with various scattering foils. Two of the foils were fabricated and experimentally evaluated by measuring off-axis dose uniformity and depth doses. A scattering foil, consisting of a 12 x 12 cm2 aluminum plate of 0.6 cm thickness and placed at isocenter perpendicular to the beam direction, was considered optimal. It produced a beam that was flat within +/-3% up to 60 cm off-axis distance, dropped by not more than 8% at a distance of 90 cm, and had an x-ray contamination of <3%. For stationary beams, MC-computed dmax, Rp, and R50 agreed with measurements within 0.5 mm. The MC-predicted surface dose of the rotating phantom was 41% of the dose rate at dmax of the stationary phantom, whereas our calculations based on a semiempirical formula in the literature yielded a drop to 42%. The MC simulations provided the guideline of beam modifier design for TSEI and estimated the dosimetric performance for stationary and rotational irradiations.     

Differential-pencil-beam dose calculations for charged particles.

The use of a convolution or differential-pencil-beam (DPB) algorithm has been studied for charged-particle dose calculations as a means of more accurately modeling the effects of multiple scattering. Such effects are not reflected in current charged-particle dose calculations since these calculations rely on depth-dose data measured in homogeneous water-equivalent phantoms and use ray-tracing techniques to calculate the water-equivalent pathlength from patient CT data. In this study, isodose plots were generated from three-dimensional dose calculations using Monte Carlo, DPB, and standard ray-tracing methods for a 4-cm modulated 150-MeV proton beam incident on both homogenous and heterogeneous phantoms. To simulate therapy conditions with charged particles, these studies included cases where compensating boluses were introduced to modify the particle range across the treatment field. Results indicate that multiple-scattering effects, including increased penumbral width as a function of beam penetration and the "smearing" of isodose distributions downstream from complex heterogeneities, are well modeled by the DPB algorithm. The DPB algoirthm may also be used to obtain more useful estimates of the dose uncertainty in regions near the end of the beam's range downstream from complex heterogeneities than can be derived from standard ray-tracing calculations.     

The effects of a universal wedge and beam obliquity upon the central axis dose buildup for 6-MV x rays

In a number of clinical situations, the dose to the skin and the superficial tissues is of concern. Both beam obliquity and a beam modifier will modify the dose delivered to these regions due to changes in the scattering geometry, scattered photon and secondary electron production, and changes in the energy spectrum of a polyenergetic beam. Some linear accelerators use a single universal wedge mounted within the treatment head. Because such a wedge is at an extended distance from the patient, its contribution to the beam contaminants incident to the skin will be limited. Measurements of the ionization in the buildup region have been performed in a polystyrene phantom irradiated with a 6-MV x-ray beam from a linear accelerator equipped with a universal wedge. The variation of the buildup dose with obliquity, universal wedging, and distance from the source has been measured for angles of incidence between 0 degrees and 60 degrees and for effective wedge angles between 0 degrees and 45 degrees. The results indicate that the percentage buildup has a much stronger dependence upon the angle of incidence than upon the effective wedge angle. For distances approaching the treatment head, it is shown that the universal wedge generates secondary electrons that elevate the surface dose, but that this contribution decreases with distance.     

Differential oblique angle spectroscopy of the oral epithelium

Increasing evidence suggests that inflammation may contribute to the process of carcinogenesis. This is the basis of several clinical trials evaluating potential chemopreventive drugs. These trials require quantitative assessments of inflammation, which, for the oral epithelium, are traditionally provided by histopathological evaluation. To reduce patient discomfort and morbidity of tissue biopsy procedures, we develop a noninvasive alternative using diffuse reflectance spectroscopy to measure epithelial thickness as an index of tissue inflammation. Although any optical system has the potential for probing near-surface structures, traditional methods of accounting for scattering of photons are generally invalid for typical epithelial thicknesses. We develop a single-scattering theory that is valid for typical epithelial thicknesses. The theory accurately predicts a distinctive feature that can be used to quantify epithelial thickness given intensity measurements with sources at two different angles relative to the tissue surface. This differential measure approach has acute sensitivity to small, layer-related changes in scattering coefficients. To assess the capability of our method to quantify epithelial thickness, detailed Monte Carlo simulations and measurements on phantom models of a two-layered structure are performed. The results show that the intensity ratio maximum feature can be used to quantify epithelial thickness with an error less than 30% despite fourfold changes in scattering coefficients and 10-fold changes in absorption coefficients. An initial study using a simple two-source, four-detector probe on patients shows that the technique has promise. We believe that this new method will perform well on patients with diverse tissue optical characteristics and therefore be of practical clinical value for quantifying epithelial thickness in vivo.     

A "HOWFARLESS" option to increase efficiency of homogeneous phantom calculations with DOSXYZnrc

This paper describes a "HOWFARLESS" transport option, which has been added to DOSXYZnrc to increase the efficiency of beam commissioning calculations in homogeneous phantoms. The algorithm speeds up charged particle transport by only considering the distance to the extreme outer boundaries of the phantom, thus eliminating the need to stop at voxel boundaries. Dose is deposited by approximating the total curved charged particle steps by two straight-line steps joined at a hinge point. Good agreement with normal simulations is achieved at all beam energies and for all practical maximum step lengths with a 1:1 mixture of approximations based on the initial position/ direction of the particle and on its final position/direction. Use of the "HOWFARLESS" option in phantom calculations for 6 and 18 MV photon beams (10 x 10 cm2 and 40 x 40 cm2 fields) from BEAMnrc-simulated accelerators increases the efficiency at the optimum photon splitting number by a factor of 2.9-5.4 when the exact EGSnrc boundary crossing algorithm (BCA) is used and by 51%-89% when the faster PRESTA-I BCA is employed. The efficiency gain due to the "HOWFARLESS" transport option increases with increasing beam energy and decreases with increasing field/dose voxel size. Efficiency improvement is greater when the efficiency of the particle source itself is not a factor, and in such cases the "HOWFARLESS" option improves the DOSXYZnrc efficiency by up to a factor of 13.1 (exact BCA) or 3.5 (PRESTA-I BCA) for photon beams, and up to a factor of 17.2 (exact BCA) or 5.2 (PRESTA-I BCA) for electron beams.     

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.     

Validity of a closed-form diffusion solution in P1 approximation for reflectance imaging with an oblique beam of arbitrary profile

Determination of optical parameters of turbid media from reflectance image data is an important class of inverse problems due to its potential for noninvasive characterization of materials and biological tissues, which demands rapid modeling tools to generate calculated images. We treat the problem of reflectance imaging with homogeneous semi-infinite turbid media as a boundary-value problem of diffusion type in the P1 approximation to the radiative transfer equation. A closed-form solution has been obtained for an oblique incident beam of arbitrary profile and its accuracy has been examined against a Monte Carlo method and measured data. We find that the diffusion solution provides a sufficiently accurate tool to rapidly calculate reflectance images for samples of large or moderate scattering albedo illuminated by a beam of arbitrary profile as long as the anisotropy factor remains less than 0.7 and single scattering albedo larger than 0.8. The closed-form solution can thus be used as a part of a forward modeling toolbox to determine optical parameters from reflectance image data in combination with other method such as the Monte Carlo simulation.     

The effect of scattering foil parameters on electron-beam Monte Carlo calculations.

The sensitivity of electron-beam Monte Carlo dose calculations to scattering foil geometrical parameters is described. A method for resolving discrepancies between Monte Carlo calculation and measured data in a systematic manner is also described. As part of a project to investigate the utility of Monte Carlo methods for calculating data required for commissioning electron beams, a large discrepancy between measured and calculated 20 MeV cross-beam profiles for the largest field size was found. It was hypothesized that the discrepancy was due to incorrect input data and that better agreement between calculation and measurement could be achieved with small changes in the scattering foil system geometry. Four parameters describing the foil system were varied individually until better agreement between calculation and measurement was achieved, and the percentage change in the parameter was tabulated as an indication of the sensitivity of the model to that parameter. The accelerator model for the 20 MeV electron beam was most sensitive to the distance between the scattering foils and to a slightly lesser extent, to the width of the shaped secondary scattering foil. Changes to the primary or secondary foil thickness also significantly modified the falloff and bremsstrahlung component of depth dose, which was unacceptable for the present case. Therefore, the distance between the two scattering foils was changed in our calculations, which the manufacturer later confirmed was indeed the case. For 6 and 12 MeV electron beams, the change was not nearly as significant. It was concluded that Monte Carlo calculations for higher-energy beams and larger field sizes are most sensitive to the geometric configuration of the scattering foil system and should therefore be calculated first to help verify the accuracy of the geometric information.     

Treatment head design for multileaf collimated high-energy electrons.

This paper describes how a conventional treatment head can be modified for use of multileaf collimated electron beams. Automatic and dynamic beam delivery are possible for both electrons and photons by using the computer controlled multileaf collimator (MLC) for both photon and electron beams. Thereby, the electron beams can be mixed more freely into the treatment to take advantage of the specific depth modulation characteristics of electrons. The investigation was based on Monte Carlo calculations using the software package BEAM. The physical parameters used in this optimization were the beam penumbra and the virtual/effective point source position. These parameters are essential for shaping beams, beam matching and for dosimetry calculations. The optimization was carried out by modifying a number of parameters: replacing the air atmosphere in the treatment head with helium, adding a helium bag below the MLC, changing the position of the scattering foils, modifying the monitor chamber, and adjusting the position of the MLC. The beam characteristics for some of these designs were found to fulfil our criteria for clinically useful beams down to at least 9 MeV.     

Electron beam treatment verification using measured and Monte Carlo predicted portal images

Electron beam treatments may benefit from techniques to verify patient positioning and dose delivery. This is particularly so for complex techniques such as mixed photon and electron beam radiotherapy and electron beam modulated therapy. This study demonstrates that it is possible to use the bremsstrahlung photons in an electron beam from a dual scattering foil linear accelerator to obtain portal images of electron beam treatments. The possibility of using Monte Carlo (MC) simulations to predict the electron beam treatment portal images was explored. The MC code EGSnrc was used to model a Varian CL21EX linear accelerator (linac) and to characterize the bremsstrahlung photon production in the linac head. It was found that the main sources of photons in the electron beam are the scattering foils, the applicator and the beam-shaping cut-out. Images were acquired using the Varian CL21EX linac and the Varian aS500 electronic portal imager (EPI); four electron energies (6, 9, 12, 16 MeV), and different applicator and cut-out sizes were used. It was possible to acquire images with as little as 10.7 MU per image. The contrast, the contrast-to-noise ratio (CNR), the signal-to-noise ratio (SNR), the resolution and an estimate of the modulated transfer function (MTF) of the electron beam portal images were computed using a quality assurance (QA) phantom and were found to be comparable to those of a 6 MV photon beam. Images were also acquired using a Rando anthropomorphic phantom. MC simulations were used to model the aS500 EPID and to obtain predicted portal images of the QA and Rando phantom. The contrast in simulated and measured portal images agrees within +/-5% for both the QA and the Rando phantom. The measured and simulated images allow for a verification of the phantom positioning by making sure that the structure edges are well aligned. This study suggests that the Varian aS500 portal imager can be used to obtain patient portal images of electron beams in the scattering foil linacs.     

On the existence of low-energy photons (<150 keV) in the unflattened x-ray beam from an ordinary radiotherapeutic target in a medical linear accelerator

Low-energy photons (<150 keV) are essential for obtaining high quality x-ray radiographs. These photons are usually produced in the accelerator target, but are effectively absorbed by the flattening filter and, at least partially, by the target itself. Experimental proof is presented for the existence of low-energy photons in the unflattened x-ray beam produced by a 6 MeV electron beam normally incident on the thinner of the two existing ports of the all-Cu radiotherapeutic target of a Clinac 18 (Varian Associates) linear accelerator. A number of one-shot absorption measurements were carried out with 12 foils of Pb absorbers with thicknesses varying from 0.25 to 3 mm in steps of 0.25 mm arranged symmetrically around the central axis on a 7.2 cm radius circumference. A Kodak ECL film-screen-cassette combination was used as a detector in the absorption measurements, in which optical density was measured as a function of the thickness of the Pb absorbers. Two sets of absorption measurements were carried out: the first one with the Clinac 18 6 MV unflattened beam and the second one with the Clinac 600C 6 MV therapeutic counterpart beam. There is a striking difference between the two sets: the optical density versus Pb-absorber thickness curve shows a sharp increase in optical density at small absorber thicknesses in the case of the unflattened 6 MV x-ray beam as compared with a gently sloping dependence in the case of the 6 MV therapeutic beam. A semi-quantitative assessment of the low-energy photon contribution to the whole optical density/contrast is presented. A 0.85 mm thick Pb absorber intercepting the 6 MV unflattened x-ray beam eliminates almost totally the sharp peak in the optical density curve at small Pb-absorber thicknesses. This constitutes additional evidence for the existence of low-energy photons (<150 keV) in the unflattened 6 MV beam from the Cu therapeutic target.