Test procedures for verification of an electron pencil beam algorithm implemented for treatment planning

The calculation of an electron dose distribution in a patient is a difficult problem because of the presence of tissue and surface inhomogeneities. Verification of the dose planning system is therefore essential. In this investigation, a novel method is used to evaluate a commercially available system (Helax-TMS), at electron energies between 10 and 50 MeV, both for a conventional treatment unit and an MLC-collimated scanned beam unit with a helium-filled treatment head. First, the experiments were designed to verify the local beam database and some fundamental characteristics of the electron beam calculations. Secondly, a number of generalised situations that would be encountered in the clinical treatment planning were evaluated oblique incidence, field shaping with multi-leaf collimator, bolus edges, and air cavities. Dose distributions in two generalised anatomical phantoms simulating a neck and a nose were also analysed. The results have, when so possible, been presented as the dose ratio within the 'flattened area' for dose profiles and down to the 'treatment depth' (80% dose level) for depth doses. In the penumbra region and in the dose fall-off region, the comparison has been represented by the distance deviation between calculated and measured dose profiles or depth doses. A new tool, 'volume integration', was used to evaluate the deviations from a more clinical point of view. Most results were within +/- 2% in dose for volumes larger than a sphere with a diameter of 15 mm, or +/- 2 mm in position. Dose deviations were generally found for oblique incidences and below heterogeneities such as small air cavities and bolus edges in limited volumes.     

Correction factors for parallel-plate chambers used in plastic phantoms in electron dosimetry

In electron beam dosimetry using parallel-plate chambers solid phantoms are sometimes necessary. To obtain the dose to water from the ionization obtained in the solid phantom, fluence correction factors and perturbation factors have to be applied. In this study fluence factors in a perturbation free geometry have been determined experimentally for common phantom materials. Wall perturbation factors for simulated Attix, NACP, and Roos chambers have also been determined for the same materials. Comparative Monte Carlo calculations have been performed using the EGS4 Monte Carlo code. Comparison with data in newly published protocols such as IAEA and IPEMB shows an agreement with the results obtained in this paper to within 1%, demonstrating that the data published in these protocols may be used with reasonable accuracy if recommended phantoms are used. The results also show that if unsuitable phantom materials are used, the wall perturbation factors may differ for different chambers and for different phantom materials by more than 3% and perturbation factors have to be considered in order to obtain a high accuracy in the dose determination.     

An evaluation of epoxy resin phantom materials for electron dosimetry

The use of epoxy resin 'solid water' (water substitute) phantoms is becoming increasingly common in radiotherapy dosimetry, and depth ionization curves and conversion factors from ionization to dose identical to water have often been assumed. Fluence ratios of water to solid water for WTe (produced by Radiation Physics, St Bartholomew's Hospital, London) and RMI 457 (produced by Radiation Measurements Inc., Middleton, Wisconsin) have therefore been determined and have been found to decrease with energy, which, within measurement uncertainty, can be described with a linear function dependent on mean electron beam energy at the depth of measurement, Ed. The fluence ratios for WTe are very close to unity (i.e. within the measuring uncertainty) for most of the energies examined, the exception being a nominal 20 MeV beam. The results also show that an assumption of unity for the fluence ratios of RMI 457 may introduce a systematic error of the order of 1% in electron beam dosimetry at lower energies. As regards the depth ionization curves measured in the respective solid water materials, these are shown to be in agreement with those measured in water within the limits of the measuring uncertainty.     

On compensator design for photon beam intensity-modulated conformal therapy

Recently the compensator has been shown to be an in expensive and reliable dose delivery device for photon beam intensity-modulated radiation therapy (IMRT). The goal of IMRT compensator design is to produce an optimized primary fluence profile at the patient's surface obtained from the optimization procedure. In this paper some of the problems associated with IMRT compensator design, specifically the beam perturbations caused by the compensator, are discussed. A simple formula is derived to calculate the optimal compensator thickness profile from an optimized primary fluence profile. The change of characteristics of a 6 MV beam caused by the introduction of cerrobend compensators in the beam is investigated using OMEGA Monte Carlo codes. It is found that the compensator significantly changes the energy spectrum and the mean energy of the primary photons at the patient's surface. However, beam hardening does not have as significant an effect on the percent depth dose as it does on the energy spectrum. We conclude that in most situations the beam hardening effect can be ignored during compensator design and dose calculation. The influence of the compensator on the contaminant electron buildup dose is found to be small and independent of the compensator thickness of interest. Therefore, it can be ignored in the compensator design and included as a correction into the final dose distribution. The scattered photons from the compensator are found to have no effect on the surface dose. These photons produce a uniform low fluence distribution at the patient's surface, which is independent of compensator shape. This is also true for very large fields and extremely asymmetric and nonuniform compensator thickness profiles. Compared to the primary photons, the scattered photons have much larger angular spread and similar energy spectrum at the patient's surface. These characteristics allow the compensator thickness profile and the dose distribution to be calculated from the optimized fluence profile of primary photons, without considering the scattered photons.     

Intensity modulation with electrons: calculations, measurements and clinical applications

Intensity modulation of electron beams is one step towards truly conformal therapy. This can be realized with the MM50 racetrack microtron that utilizes a scanning beam technique. By adjusting the scan pattern it is possible to obtain arbitrary fluence distributions. Since the monitor chambers in the treatment head are segmented in both x- and y-directions it is possible to verify the fluence distribution to the patient at any time during the treatment. Intensity modulated electron beams have been measured with film and a plane parallel chamber and compared with calculations. The calculations were based on a pencil beam method. An intensity distribution at the multileaf collimator (MLC) level was calculated by superposition of measured pencil beams over scan patterns. By convolving this distribution with a Gaussian pencil beam, which has propagated from the MLC to the isocentre, a fluence distribution at isocentre level was obtained. The agreement between calculations and measurements was within 2% in dose or 1 mm in distance in the penumbra zones. A standard set of intensity modulated electron beams has been developed. These beams have been implemented in a treatment planning system and are used for manual optimization. A clinical example (prostate) of such an application is presented and compared with a standard irradiation technique.     

Monte Carlo calculation of dose enhancement by neutron capture of 10B in fast neutron therapy

Since 1978 the Essen Medical Cyclotron Facility has been used for fast neutron therapy. The treatment of deep-seated tumours by d(14) + Be neutron beam therapy (mean energy = 5.8 MeV) is still limited because of the steep decrease in depth-dose distribution. The interactions of fast neutrons in tissue leads to a thermal neutron distribution. These partially thermalized neutrons can be used to produce neutron capture reactions with 10B. Thus incorporation of 10B in tumours treated with fast neutrons will increase the relative local tumour dose due to the reaction 10B (n, alpha) 7Li. The magnitude of dose enhancement by 10B depends on the distribution of the thermal neutron fluence, 10B concentration, field size of the neutron beam, beam energy and the specific phantom geometry. The slowing down of the fast neutrons, resulting in a thermal neutron distribution in a phantom, has been computed using a Monte Carlo model. This model, which includes a deep-seated tumour, was experimentally verified by measurements of the thermal neutron fluence rate in a phantom using neutron activation of gold foil. When non-boronated water phantoms were irradiated with a total dose of 1 Gy at a depth of 6 cm, the thermal fluencies at this depth were found to be 2 x 10(10) cm-2. The absorbed dose in a tumour with 100 ppm 10B, at the same depth, was enhanced by 15%.     

Beam characteristics and clinical possibilities of a new compact treatment unit design combining narrow pencil beam scanning and segmental multileaf collimation

Not until the last decade has flexible intensity modulated three-dimensional dose delivery techniques with photon beams become a clinical reality, first in the form of heavy metal transmission blocks and other beam compensators, then in dynamic and segmented multileaf collimation, and most recently by scanning high-energy narrow electron and photon beams. The merits of various treatment unit and bremsstrahlung target designs for high-energy photon therapy are investigated theoretically for two clinically relevant target sites, a cervix and a larynx cancer both in late stages. With an optimized bremsstrahlung target it is possible to generate photon beams with a half-width of about 3 cm at a source to axis distance (SAD) of 100 cm and an initial electron energy of 50 MeV. By making a more compact treatment head and shortening the SAD, it is possible to reduce the half-width even further to about 2 cm at a SAD of 70 cm and still have sufficient clearance between the collimator head and the patient. One advantage of a reduced SAD is that the divergence of the beam for a given field size on the patient is increased, and thus the exit dose is lowered by as much as 1%/cm of the patient cross section. A second advantage of a reduced SAD is that the electron beam on the patient surface will be only about 8 mm wide and very suitable for precision spot beam scanning. It may also be possible to reduce the beamwidth further by increasing the electron energy up to about 60 MeV to get a photon beam of around 15 mm half-width and an electron beam as narrow as 5 mm. The compact machine will be more efficient and easy to work with, due to the small gantry and the reduced isocentric height. For a given target volume and optimally selected static multileaf collimator, it is no surprise that the narrowest possible scanned elementary bremsstrahlung beam generates the best possible treatment outcome. In fact, by delivering a few static field segments with individually optimized scan patterns, it is possible to combine the advantage of being able to fine tune the fluence distribution by the scanning system with the steeper dose gradients that can be delivered by a few static multileaf collimator segments. It is demonstrated that in most cases a few collimator segments are sufficient and often a single segment per beam portal may suffice when narrow scanned photon beams are employed, and they can be delivered sequentially with a negligible time delay. A further advantage is the increase of therapeutically useful photons and improved patient protection, since the pencil beam is only scanned where the leaf collimator is open. Consequently, some of the problems associated with dynamic multileaf collimation such as the tongue and groove and edge leakage effects are significantly reduced. Fast scanning beam techniques combined with good treatment verification systems allow interesting future possibilities to counteract patient and internal organ motions in real time.     

A stereotactic radiation therapy device for retinoblastoma using a noncircular collimator and intensity filter

The proximity of the lens to the retina makes the treatment of retinoblastoma a challenge for external beam radiation therapy. The approximately 1 mm separation between the posterior edge of the lens and the anterior region of the retina causes a trade-off between coverage of the entire retina and excessive dose to the lens. A stereotactic, LINAC based, lens sparing technique for treating retinoblastoma is presented. The technique uses noncoplanar arcs with the lens at isocenter. A special noncircular collimator blocks the lens but it also causes the dose distribution to vary across the retina. A fluence modulation filter is used to reduce the dose inhomogeneity across the target. The resulting dose distribution is roughly hemispheric, providing both anterior coverage of the retina and lens blocking unlike conventional techniques. The method used to develop the collimator and filter assembly is presented. Dosimetry of the assembly was carried out using radiochromic film, and the results were entered in a treatment planning system. The dose distribution as measured in a phantom is provided and compared to calculations.     

An adaptive control algorithm for optimization of intensity modulated radiotherapy considering uncertainties in beam profiles, patient set-up and internal organ motion

A new general beam optimization algorithm for inverse treatment planning is presented. It utilizes a new formulation of the probability to achieve complication-free tumour control. The new formulation explicitly describes the dependence of the treatment outcome on the incident fluence distribution, the patient geometry, the radiobiological properties of the patient and the fractionation schedule. In order to account for both measured and non-measured positioning uncertainties, the algorithm is based on a combination of dynamic and stochastic optimization techniques. Because of the difficulty in measuring all aspects of the intra- and interfractional variations in the patient geometry, such as internal organ displacements and deformations, these uncertainties are primarily accounted for in the treatment planning process by intensity modulation using stochastic optimization. The information about the deviations from the nominal fluence profiles and the nominal position of the patient relative to the beam that is obtained by portal imaging during treatment delivery, is used in a feedback loop to automatically adjust the profiles and the location of the patient for all subsequent treatments. Based on the treatment delivered in previous fractions, the algorithm furnishes optimal corrections for the remaining dose delivery both with regard to the fluence profile and its position relative to the patient. By dynamically refining the beam configuration from fraction to fraction, the algorithm generates an optimal sequence of treatments that very effectively reduces the influence of systematic and random set-up uncertainties to minimize and almost eliminate their overall effect on the treatment. Computer simulations have shown that the present algorithm leads to a significant increase in the probability of uncomplicated tumour control compared with the simple classical approach of adding fixed set-up margins to the internal target volume.     

A method for implementing dynamic photon beam intensity modulation using independent jaws and a multileaf collimator

A mathematical model is derived for digitally controlled linear accelerators to deliver a desired photon intensity distribution by combining collimator motion and machine dose rate variations. It shows that, at any instant, the quotient of the machine dose rate and the speed of collimator motion is proportional to the gradient of the desired in-air photon fluence distribution. The model is applicable for both independently controlled collimator jaws and multileaf collimators and can be implemented by controlling different parameters to accommodate linear accelerators from different manufactures. For independent jaws, each pair of jaws creates photon fluence variations along the direction of the jaw movement. For multileaf collimators, where each leaf is independently controlled, any two-dimensional (2D) photon fluence distribution can be delivered. The model has been implemented for wedged isodose distributions using independent jaws, and 2D intensity modulation using a multileaf collimator. One-dimensional (1D) wedged isodose distributions are created by moving an independent jaw at constant speed while varying machine dose rate. 2D intensity modulation has been implemented using a 'dynamic stepping' scheme, which controls the leaf progression during irradiation at constant machine dose rate. With this automated delivery scheme, the beam delivery time for dynamic intensity modulation, which depends on the complexity of the desired intensity distribution, approaches that of conventional beam modifiers. This paper shows the derivation of the model, its application, and our delivery scheme. Examples of 1D dynamic wedges and 2D intensity modulations will be given to illustrate the versatility of the model, the simplicity of its application, and the efficiency of beam delivery. These features make this approach practical for delivering conformal therapy treatments.     

Monte Carlo calculations of electron beam output factors for a medical linear accelerator

The purpose of this study was to investigate the application of the Monte Carlo technique to the calculation and analysis of output factors for electron beams used in radiotherapy. The code EGS4/BEAM was used to obtain phase-space files for 6, 12 and 20 MeV clinical electron beams from a scattering-foil linac (Varian Clinac 2100C) for a clinically representative range of applicator and square or rectangular insert combinations. The source-to-surface distance used was 100 cm. The field sizes ranged from 1 x 1 cm2 to 20 x 20 cm2. These phase-space files were analysed to study the intrinsic beam characteristics and used as source input for relative dose and output factor computations in homogeneous water phantoms using the code EGS4/DOSXYZ. The calculated relative central-axis depth-dose and transverse dose profiles at various depths of clinical interest agreed with the corresponding measured dose profiles to within 2% of the maximum dose. Calculated output factors for the fields studied agreed with measured output factors to about 2%. This demonstrated that for the Varian Clinac 2100C linear accelerator, electron beam dose calculations in homogeneous water phantoms can be performed accurately at the 2% level using Monte Carlo simulations.     

Respiratory effects in mice exposed to airborne emissions from Stachybotrys chartarum and implications for risk assessment

A Siemens Mevatron KV2 accelerator installed at the Royal Adelaide Hospital employs cylindrical solid-walled electron cones for some electron collimation. The cones being used at present result in treatment fields that do not always conform with the International Electrotechnical Commission (IEC) Standards (particularly at high energies). The aim of this project was to simulate the existing cones using Monte Carlo methods in order to evaluate potential cone modifications required to overcome the field irregularities. Simulations were performed using the EGS4 (Electron Gamma Shower version 4, distribution II) Monte Carlo code installed on a DEC Alpha workstation at the University of South Australia. To rigorously simulate the existing electron cones it was necessary to also simulate various components within the treatment head of the linear accelerator. Results of simulations for existing cones were found to be consistent with experimental data. (obtained from Royal Adelaide Hospital beam quality assurance measurements). Two proposed changes to the cones were then simulated and the effects of these alterations were assessed. This study has shown how treatment head simulation techniques can be used to assess the changes in dose distribution that result from alterations to the treatment head and accessories. Within practical engineering constraints modifications to an existing electron collimation system were proposed and theoretically evaluated.     

Optimized lens-sparing treatment of retinoblastoma with electron beams

PURPOSE: The ideal lens-sparing radiotherapy technique for retinoblastoma calls for 100% dose to the entire retina including the ora serrata and zero dose to the lens. Published techniques, most of which use photons, have not accomplished this ideal treatment. We describe here a technique that approaches this ideal configuration using electron beam therapy. METHODS AND MATERIALS: Dose-modeling calculations were made using a computer program built around a proprietary algorithm. This program calculates 3D dose distribution for electrons and photons and uses the Cimmino feasibility method for the inverse problem of beam weighting to achieve the prescribed dose. The algorithm has been verified in the ocular region by measurements in a RANDO phantom. To search for an ideal lens-sparing beam setup, a stylized phantom of an 8-month-old infant was generated with built-in inhomogeneities, and a phantom of a 5-year-old child was generated from a patient CT series. RESULTS: Of more than 100 different beam setups tested, two 9 MeV electron beams at gantry angles plus and minus 26 degrees from the optic nerve axis achieved the best distribution. Both fields have a lens block and an isocenter between the globe and origin of the optic nerve. When equal doses are given to both fields, the entire extent of the retina (including ora serrata) received 100%, while the lens received 10% or less. CONCLUSION: The two-oblique-electron-beam technique here described appears to meet most of the stringent dosimetry needed to treat retinoblastoma. It is suitable for a range of ages, from infancy to early childhood years.     

Description and evaluation of a new 3-D computerized treatment planning dose compensator system

Evaluation has been performed of compensators generated by means of a computerized three-dimensional treatment planning system that can utilize either digitized slice profiles or CT scans. Two methods of calculating compensator thickness are used: the modified Batho power law (dSAR) method for digitized profiles and the equivalent TAR (eqTAR) method for CT scans. This system not only compensates for patient surface contours but also compensates for internal inhomogeneities. In addition, any required wedging will be incorporated in the compensator generation. This system has been tested for a number of extreme cases with inhomogeneities and sloping contours. Good agreement was obtained between the measured and computer calculated dose profiles especially along the central axis of the beam. A "Profile Uniformity Index" was defined to quantify the goodness of dose compensation in three dimensions. Compensation using this system can achieve good dose uniformity within the target volume in all clinical cases and is definitely an improvement over systems based solely on tissue deficit.     

Quantification of mean energy and photon contamination for accurate dosimetry of high-energy electron beams

The scientific background of the standard procedure for determination of the mean electron energy at the phantom surface (E0) from the half-value depth (R50) has been studied. The influence of energy, angular spread and range straggling on the shape of the depth dose distribution and the R50 and Rp ranges is described using the simple Gaussian range straggling model. The relation between the R50 and Rp ranges is derived in terms of the variance of the range straggling distribution. By describing the mean energy imparted by the electrons both as a surface integral over the incident energy fluence and as a volume integral over the associated absorbed dose distribution, the relation between E0 and different range concepts, such as R50 and the maximum dose and the surface dose related mean energy deposition ranges, Rm and R0, is analysed. In particular the influence of multiple electron scatter and phantom generated bremsstrahlung on R50 is derived. A simple analytical expression is derived for the ratio of the incident electron energy to the half-value depth. Also, an analytical expression is derived for the maximum energy deposition in monoenergetic plane-parallel electron beams in water for energies between 2 and 50 MeV. Simple linear relations describing the relative absorbed dose and mass ionization at the depth of the practical range deposited by the bremsstrahlung photons generated in the phantom are derived as a function of the incident electron energy. With these relations and a measurement of the extrapolated photon background at Rp, the treatment head generated bremsstrahlung distribution can be determined. The identification of this photon contamination allows an accurate calculation of the absorbed dose in electron beams with a high bremsstrahlung contamination by accounting for the difference in stopping power ratios between a clean electron beam and the photon contamination. The absorbed dose determined using ionization chambers in heavily photon contaminated (10%) electron beams may be too low--by as much as 1.5%--without correction.     

The application of correlated sampling to the computation of electron beam dose distributions in heterogeneous phantoms using the Monte Carlo method

Although the Monte Carlo method is capable of computing the dose distribution in heterogeneous phantoms directly, there are some advantages to computing a heterogeneity correction factor. If this approach is adopted there are savings in time using correlated sampling. This technique forces histories to have the same energy, position, direction and random number seed as incident on both the heterogeneous and homogeneous water phantom. This ensures that a history that has, by chance, travelled through only water in the heterogeneous phantom will have the same path as it would have through the homogeneous phantom, resulting in a reduced variance when a ratio of heterogeneous dose to homogeneous dose is formed. Metrics to describe the distributions of uncertainty, efficiency, and degree of correlation are defined. EGS4 Monte Carlo calculation of the dose distribution from a 20 MeV electron beam on water phantoms containing aluminum or air slab heterogeneities illustrate that this technique is the most efficient when the heterogeneity is deep within the phantom, but that improved efficiency can be realized even when the heterogeneity is at or near the surface. This is because some correlation between the two histories is retained despite passage through the heterogeneity.     

Two replica blotting methods for fast immunological analysis of common proteins in two-dimensional electrophoresis

Knowledge of the photon spectrum of a radiotherapy beam is often needed for three-dimensional (3-D) dose calculations using Monte Carlo methods and/or algorithms employing energy deposition kernels. Direct measurement of the x-ray energy fluence spectrum is not feasible for the high-energy photon beams used clinically. In this paper, the spectrum is extracted from basic beam data that are readily obtained for a clinical beam. We describe the photon spectrum using just two parameters. One parameter, which determines the high-energy part of the spectrum, is obtained using the measured dose in the buildup region for a small field, where electron contamination of the beam can be neglected. The other parameter is extracted from the photon beam attenuation in water. The results compare favorably to spectra generated from Monte Carlo simulations.     

Seasonal variation in urocanic acid isomers in human skin

Dual-energy X-ray absorptiometry (DXA), using a narrow pencil-shaped X-ray beam coupled to a single detector, has been used extensively. More recently, DXA using a fan- shaped X-ray beam coupled to an array of detectors has been introduced. This new generation of scanners causes an inherent magnification of scanned structures as the distance from the X-ray source decreases. This magnification, which occurs in the medial-lateral direction but not in the craniocaudal direction, does not affect bone mineral density (BMD). There are, however, significant changes of bone mineral content (BMC), bone area, and parameters of hip geometry, with varying distance of the bone scanned from the X-ray source. Variability of soft tissue thickness in vivo, by altering the distance of the skeleton from the scanning table and X-ray source, may cause clinically significant errors of BMC, bone area, and proximal femur geometry when measured using fan-beam densitometers. We analyzed the geometry of Lunar and Hologic fan beam scanners to derive equations expressing the true width of scanned structures in terms of the apparent width and machine dimensions. We also showed mathematic ally that performing an additional scan, at a different distance from the X-ray source than the first scan, provides simultaneous equations that can be solved to derive the real width of a scanned bone. This hypothesis was tested on the Lunar Expert using aluminium phantoms scanned at different table heights. There was an excellent correlation, r = 0.99 (p < 0.001), between the predicted phantom width and the measured phantom width. In conclusion, this study shows that the magnification error of fan beam DXA can be corrected using a dual scanning technique. This has important implications in the clinical usefulness of BMC and geometrical measurements obtained from these scanners.     

Measured lung dose correction factors for 50 MV photons

Some clinically relevant measurements of lung tissue/water equivalent interfaces have been performed for a 50 MV therapeutic x-ray beam. The purpose was to investigate the severity of dose perturbation effects in lung tissue and adjacent tissues using an energy well above the common clinical practice in thoracic irradiations. The phantoms were constructed of solid water, PMMA and white polystyrene as soft tissue (water) equivalents, and cork was used as the lung tissue equivalent. Measurements were performed using radiographic film and a cylindrical ionization chamber. The results show that the degradation of the 20/80% beam penumbra in the lung region is severe, up to 2.5 times the penumbra in water for a 10 cm thick lung with a density of 0.30 x 10(3) kg m(-3). The lack of electronic equilibrium in the low-density region can cause underdosage at the lung/tumour interface of up to 30% of maximum target dose, and the build-up depth to 95% of target dose in unit density tissue behind the lung may be as large as 22 mm. It is also shown that these figures strongly depend on patient anatomy and beam size and why a careful calculation of the individual dose distribution is needed for optimal choice of photon beam energy in thoracic treatments.     

Tomotherapeutic portal imaging for radiation treatment verification

In their tomotherapy concept Mackie and co-workers proposed not only a new technique for IMRT but also an appropriate and satisfactory method of treatment verification. This method allows both monitoring of the portal dose distribution and imaging of the patient anatomy during treatment by means of online CT. This would enable the detection of inaccuracies in dose delivery and patient set-up errors. In this paper results are presented showing that a single electronic portal imaging device (EPID) could deliver all data necessary to establish such a complete verification system for tomotherapy and even other IMRT techniques. Consequently it has to be shown that it is able to record both the low-intensity photon fluences encountered in tomographic imaging and the intense photon transmission of each treatment field. The detector under investigation is a video-based EPID, the BIS 710 (manufactured by Wellh?fer Dosimetrie, Schwarzenbruck, Germany). To examine the suitability of the BIS for CT at 6 MV beam quality, different phantoms were scanned and reconstructed. The agreement between a diamond detector and BIS responses is quantitative. Tomographic reconstruction of a complete set of these transmission profiles resulted in images which resolve 3 cm large objects having a (theoretical) contrast to water of less than 9%. Three millimetre objects with a 100% contrast are clearly visible. The BIS signal was shown to measure photon fluence distributions. The reconstructed images possess a spatial and contrast resolution sufficient for accurate imaging of the patient anatomy, needed for treatment verification in many clinical cases.