Characteristics of scattered electron beams shaped with a multileaf collimator

Characteristics of dual-foil scattered electron beams shaped with a multileaf collimator (MLC) (instead of an applicator system) were studied. The electron beams, with energies between 10 and 25 MeV, were produced by a racetrack microtron using a dual-foil scattering system. For a range of field sizes, depth dose curves, profiles, penumbra width, angular spread in air, and effective and virtual source positions were compared. Measurements were made when the MLC alone provided collimation and when an applicator provided collimation. Identical penumbra widths were obtained at a source-to-surface distance of 85 cm for the MLC and 110 cm for the applicator. The MLC-shaped beams had characteristics similar to other machines which use trimmers or applicators to collimate scanned or scattered electron beams. Values of the effective source position and the angular spread parameter for the MLC beams were similar to those of the dual-foil scattered beams of the Varian Clinac 2100 CD and the scanned beams of the Sagittaire linear accelerators. A model, based on Fermi-Eyges multiple scattering theory, was adapted and applied successfully to predict penumbra width as a function of collimator-surface distance.     

Calculating dose and output factors for wedged photon radiotherapy fields using a convolution/superposition method

To account for clinical divergent and polychromatic photon beams, we have developed kernel tilting and kernel hardening correction methods for convolution dose calculation algorithms. The new correction methods were validated by Monte Carlo simulation. The accuracy and computation time of the our kernel tilting and kernel hardening correction methods were also compared to the existing approaches including terma divergence correction, dose divergence correction methods, and the effective mean kernel method with no kernel hardening correction. Treatment fields of 10 x 10-40 x 40 cm2 (field size at source to axis distance (SAD)) with source to source distances (SSDs) of 60, 80, and 100 cm, and photon energies of 6, 10, and 18 MV have been studied. Our results showed that based on the relative dose errors at a depth of 15 cm along the central axis, the terma divergence correction may be used for fields smaller than 10 x 10 cm2 with a SSD larger than 80 cm; the dose divergence correction with an additional kernel hardening correction can reduce dose error and may be more applicable than the terma divergence correction. For both these methods, the dose error increased linearly with the depth in the phantom; the 90% isodose lines at the depth of 15 cm were shifted by about 2%-5% of the field width due to significant underestimation of the penumbra dose. The kernel hardening effect was less prominent than the kernel tilting effect for clinical photon beams. The dose error by using nonhardening corrected kernel is less than 2.0% at a depth of 15 cm along the central axis, yet it increased with a smaller field size and lower photon energy. The kernel hardening correction could be more important to compute dose in the fields with beam modifiers such as wedges when beam hardening is more significant. The kernel tilting correction and kernel hardening correction increased computation time by about 3 times, and 0.5-1 times, respectively. This can be justified by more accurate dose calculations for the majority of clinical treatments.     

Performance evaluation of a diode array for enhanced dynamic wedge dosimetry

The performance of a diode array (Profiler) was evaluated by comparing its enhanced dynamic wedge (EDW) profiles measured at various depths with point measurements using a 0.03 cm3 ionization chamber on a commercial linear accelerator. The Profiler, which covers a 22.5 cm width, was used to measure larger field widths by concatenating three data sets into a larger field. An innovative wide-field calibration technique developed by the manufacturer of the device was used to calibrate the individual diode sensitivity, which can vary by more than 10%. Profiles of EDW measured with this device at several depths were used to construct isodose curves using the percentage depth dose curve measured by the ionization chamber. These isodose curves were used to check those generated by a commercial treatment planning system. The profiles measured with the diode array for both 8 and 18 MV photon beams agreed with those of the ionization chamber within a standard deviation of 0.4% in the field (defined as 80% of the field width) and within a maximum shift of less than 2 mm in the penumbra region. The percentage depth dose generally agreed to within 2% except in the buildup region. The Profiler was extremely useful as a quality assurance tool for EDW and as a dosimetry measurement device with tremendous savings in data acquisition time.     

Utility of three-dimensional planning for axillary node coverage with breast-conserving radiation therapy: early experience

PURPOSE: To examine the dosimetric axillary nodal coverage with standard tangential breast radiation fields and determine the utility of three-dimensional treatment planning for such coverage. MATERIALS AND METHODS: Six consecutive patients who were to undergo whole-breast irradiation underwent computed tomographic scanning with 5-mm sections at the time of treatment simulation. Contours were made with a commercial workstation for the lower axillary tissues, lungs, and heart. Axillary coverage was examined with three-dimensional isodose visualization and dose-volume histograms for four plans for each patient: (a) standard tangential radiation fields designed to cover only the breast, with clinical setup; (b) standard tangential fields with beam's-eye-view optimization of collimator angles for axillary and breast coverage; (c) standard tangential fields with adjustment of field width and collimator angles; and (d) customized fields, by adjusting width, collimator angle, and gantry angle and by using customized blocks. RESULTS: With plan a, only one patient had a simulated mean axillary dose greater than 90% of that prescribed. Underdosing occurred primarily in the posterior-superior axillary nodal region. Plan b improved axillary coverage; five patients had a simulated mean axillary dose of 89% or more of the prescribed dose, with adequate whole-breast coverage and no increased pulmonary or cardiac doses. Adjusting the field width and gantry angle further improved simulated mean axillary doses; however, customized blocking was then required to avoid increased mean pulmonary and cardiac doses and unacceptable contralateral breast doses. CONCLUSION: When coverage of lower axillary nodal tissue is desired at breast irradiation, three-dimensional planning with beam's-eye-view adjustment of tangential fields should be considered.     

An experimental investigation of the tongue and groove effect for the Philips multileaf collimator

The tongue and groove effect is an underdosing effect which can occur in certain applications of multileaf collimators. It results from the need to overlap adjacent leaves of a multileaf collimator in order to limit leakage between leaves. The applications in which the effect can occur are the abutment of fields where the beam edges are defined by the leaf edge and the production of intensity-modulated fields by dynamic collimation. The effect has been measured for the 'worst case' when just two MLC fields are matched along leaf edges which have overlapping steps. Measurements of the dose have been made at d(max) and also at a more clinically relevant depth of 87 mm in Perspex for beam energies of 6 MV, 8 MV and 20 MV on two Philips SL series accelerators. Dose distributions were recorded on radiographic film which was subsequently digitized for analysis. The dose reduction of the tongue and groove effect was found to be 15-28% and spread over a width of 3.8 to 4.2 mm. This is somewhat shallower and wider than would be expected from a simple, idealized model of the effect which would predict a dose reduction of 80% over a width of 1 mm.     

Effect of set-up error on the dose across the junction of matching cranial-spinal fields in the treatment of medulloblastoma

PURPOSE: The effect of systematic and stochastic setup error on the dose delivered to the gap region for the three field radiation treatment of medulloblastoma is studied. The consequences of such setup error is discussed. METHODS AND MATERIALS: The treatment of medulloblastoma is typically a 3 field technique, in which two lateral cranial fields are matched with a spine field. The x-ray dose delivered to the region between the matched fields depends upon the gap size. The choice of the gap width between the cranial and spinal fields is controversial. It is currently a compromise between minimizing the risk of dose hot spots to the spine, and the associated clinical complications, as well as the magnitude of cold spots (underdosing) across the gap, with the associated risk of disease recurrence. In this paper, we examine the effect of gap width with a moving junction, referred to as "field feathering", on the dose across the field junction for a 6MV photon beam. In addition, we have studied 129 portal films and 40 simulation films to assess the accuracy and precision of patient setup during treatment with a plan involving feathered fields. Selected landmarks observable on both portal and simulation films were identified and the variation in the distances to the field edges measured. The distribution of patient setup error was convoluted with the beam profiles for a 6MV linac. These convoluted field edges were used obtain dose profiles across the gap region as a function of gap separation. The consequences for therapy are discussed. In addition, analysis of patient setup error on an alternative treatment involving beam modifiers to broaden the beam penumbra is discussed. RESULTS: The magnitude of the spatial stochastic and systematic setup error was determined to be approximately three and two millimeters respectively. The dosimetric consequences of patient setup error lead to over and under dosing in the spinal gap region for the three field technique. The degree of under or over dose depends on the nature and magnitude of the patient setup error. CONCLUSIONS: The effect of patient setup error can lead to significant dosimetric errors in the dose to the gap region depending on the magnitude of the setup errors. The effective over and under dose can be compensated by the use beams modifiers such as a beam spoiler or vibrating jaws.     

Dosimetric parameters of a modified set of wedges for use with asymmetric fields of a 6 MV linear accelerator

A set of standard wedge filters has been modified for use with half-collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half-collimated field coverage (up to 20 x 30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off-axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.     

The effect of strong longitudinal magnetic fields on dose deposition from electron and photon beams

The use of strong, uniform, longitudinal magnetic fields for external electron and photon beam irradiation is considered. Using the EGS4 Monte Carlo code modified to account for the presence of magnetic fields, dramatic narrowing of penumbra for photon and electron irradiations is demonstrated. In the vicinity of heterogeneities, "hot" and "cold" spots due to multiple scattering in electron beams are reduced substantially. However, in the presence of strong magnetic fields, the effect of inhomogeneities can be observed far from the location of the inhomogeneity due to reduced "washout" caused by lateral multiple scattering. The enhanced "Bragg peak," proposed or calculated by other authors, is not observed on the central axis of broad beams, owing to lateral equilibrium. It is proven that for broad parallel beams, the central axis depth-dose curve is independent of the strength of the external longitudinal magnetic field, as long as it is uniform. However, strong longitudinal magnetic fields can induce enhancements by redirection of the electron fields coming from point sources. Strong uniform longitudinal magnetic fields provide a way of controlling the spreading of electron beams due to multiple scattering, making the electron beams more "geometrical" in character, simplifying dose-deposition patterns, possibly allowing electron beams to be used in new ways for radiotherapy. Photon therapy also benefits from strong uniform longitudinal magnetic fields since the penumbra or other lateral disequilibrium effects associated with lateral electron transport can be eliminated.     

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.     

Dose characteristics of in-house-built collimators for stereotactic radiotherapy with a linear accelerator

Dose characteristics of a stereotactic radiotherapy unit based on a standard Varian Clinac 4/100 4 MV linear accelerator, in-house-built Lipowitz collimators and the SMART stereotactic radiotherapy treatment planning software have been determined. Beam collimation is constituted from the standard collimators of the linear accelerator and a tertiary collimation consisting of a replaceable divergent Lipowitz collimator. Four collimators with isocentre diameters of 15, 25, 35 and 45 mm, respectively, were constructed. Beam characteristics were measured in air, acrylic or water with ionization chamber, photon diode, electron diode, diamond detector and film. Monte Carlo simulation was also applied. The radiation leakage under the collimators was less than 1% at 50 mm depth in water. Specific beam characteristics for each collimator were imported to SMART and dose planning with five non-coplanar converging 140 degrees arcs separated by 36 degrees angles was performed for treatment of a RANDO phantom. Dose verification was made with TLD and radiochromic film. The in-house-built collimators were found to be suitable for stereotactic radiotherapy and patient treatments with this system are in progress.     

The relevancy of tenure in medical schools

PURPOSE: To determine whether the traditional teaching of placing the caudal border of the spinal field at the S2-S3 interspace in children receiving craniospinal irradiation (CSI) is appropriate. METHODS AND MATERIALS: Twenty-three children had magnetic resonance imaging (MRI) of the spine with gadolinium prior to craniospinal irradiation at one institution. Thecal sac termination using MRI was determined by drawing a perpendicular line from the point of convergence of dural margins to the corresponding vertebral body. RESULTS: Location of thecal sac termination varied from mid-S1 to low S3 vertebral body, with the most frequent site at the upper S2 vertebral level. Only 2 of 23 (8.7%) children had thecal sac terminations below the S2-S3 interspace. For the nine patients with neuraxis disease, none had thecal sac terminations below the S2-S3 interspace. In seven of the nine patients who had neuraxis seeding at initial presentation, MRI of the spine after CSI was performed and showed that thecal sac termination was lower after radiation therapy in two children, higher in one, and the same in four. CONCLUSIONS: In 2 of 23 children (8.7%), placement of the inferior border at the bottom of the S2 vertebral body would have missed the entire thecal sac. Treatment to the entire neuraxis with adequate coverage of distal spinal theca can be achieved by using MRI. Individualized spinal fields using the MRI may help minimize radiation scatter to the gonads while adequately covering the target volume.     

The FE-lspd model for electron beam dosimetry

The FE-lspd model is a two-component electron beam model that distinguishes between electrons that can be described by small-angle transport theory and electrons that are too widely scattered for small-angle transport theory to be applicable. The two components are called the primary beam and the laterally scattered primary distribution (lspd). The primary beam component incorporates a simple version of the Fermi-Eyges model and dominates dose calculations at therapeutic depths. The lspd component corrects erros in the lateral spreading of the primary beam component, thereby improving the accuracy by which the FE-lspd model calculates dose distribution in blocked fields. Comparisons were made between dose profiles and central-axis depth dose distributions in small fields calculated by the FE-lspd, Fermi-Eyges and EGS4 Monte Carlo models for a 10 MeV beam in a homogeneous water phantom. The maximum difference between the dose calculated using the FE-lspd model and EGS4 Monte Carlo is about 6% at a field diameter of about 1 cm, and less than 2% for field sizes greater than 3 cm diameter. The maximum difference between the Fermi-Eyges and Monte Carlo calculations is about 18% at a field diameter of about 2.5 cm. A comparison was made with the central-axis depth dose distribution measured in water for a 3 cm diameter field in a 10 MeV clinical electron beam. The errors in the dose distribution were found to be less than 2% using the FE-lspd model but almost 18% using the Fermi-Eyges model. A comparison was also made with pencil beam profiles calculated using the second-order Fermi-Eyges transport model.     

Electron contamination in clinical high energy photon beams

The electron contamination in photon beams has been investigated by means of contaminating lepton depth doses and dose profiles in different geometries with two 20 MV beams. Different components of this contamination have been investigated separately by systematically adding contamination to a "clean" reference field. At 20 MV, the air generated electrons were found to be almost negligible compared to the electrons originating from the accelerator head when measurements were performed in standard fields at SSDs between 80 and 120 cm. The total electron part of the depth dose curve was then almost the same, i.e., independent of SSD, when the collimator opening was held fixed. However, when different accessories such as a shaping block and different attenuating plates were located in the beam path below the collimators, a large SSD dependence of the electron contamination was noticed. A comparison was also made between two machines, one equipped with a multileaf collimator, with similar beam qualities at 20 MV. These measurements indicate that the interior view of the treatment head seen by the detector (mainly the flattening filter, monitor chamber, or other electron generating material) influences the magnitude of the electron contamination. When the collimator opening is decreased the electron contamination will also decrease as parts of the electron source will be shielded by the collimator blocks.     

The application of automatic computing machines to radiation treatment planning. 1954

A new method of determining the dose distribution required in treatment planning has been developed by using punched cards, and sorting and tabulating machines instead of isodose charts. The percentage depth doses produced by an X-ray field are first recorded on several sets of punched cards. The number of sets required for one field depends upon the distances used from the cross point of the multifield axes to the point of entry. Each set consists of 36 cards, with each card recording the percentage depth doses of 15 points in one radial line, taking the cross points of central axes of the fields as the origin. In all cases the points are taken at the distances 0, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20, 24 cm from the origin and along radial lines 10 deg. apart. A special polar co-ordinate dose distribution sheet has been designed, and the contour of the patient's body and the position of the fields designated for treatment are drawn on this sheet. Instead of putting the isodose charts at the proper positions of the fields, the sets of punched cards for the fields to be applied are automatically arranged in the right order. Instead of the time-consuming process of reading off the percentage depth doses from the overlapping isodose charts and adding them up for selected points, the cards are fed into the automatic tabulating machine, which makes the summation of data for all the points mentioned above and tabulates the results. The whole operation is done in 10 to 15 minutes. The tabulated results are then plotted on the special dose distribution sheet. A generalised mathematical treatment of the dose at any point in the irradiated region is discussed, and an equation of "point-tumour dose ratio" is derived. Further application of this equation is made to special cases for treatment using equal maximum doses, equal tumour doses and rotation therapy. The geometrical principles involved are also indicated.     

Contamination of a 15-MV photon beam by electrons and scattered photons

The 15-MV photon beam of a linear accelerator (Siemens Mevatron 20) was studied for electron and scattered photon contamination. The surface dose, attributable almost entirely to contamination electrons, has a Gaussian lateral distribution, a linear dependence on field width for square fields, and an inverse square dependence on distance from the bottom of the fixed head assembly. This geometrical dependence is consistent with the proposal that the field flattening filter is the main source of electron contamination when accessories are absent. A tissue-maximum-ratio curve in the build-up region for the electron and photon contamination was produced utilizing the linearity of dose with respect to field width. The derived contamination curve inside was similar to the measured build-up curve outside the field. The primary photon component, obtained by subtracting the contaminant contribution, showed no dependence on field size, source-to-probe distance, or presence of accessories.     

Dose perturbation caused by high-density inhomogeneities in small beams in stereotactic radiosurgery

The influence of high-density tissue heterogeneities in small-diameter beams used in stereotactic radiosurgery has been investigated. Dose perturbation immediately behind aluminium sheets, used to simulate a high-density tissue inhomogeneity such as bone, was studied in a solid water phantom. Dose reduction factors (DRFs), which are the ratios of the dose in the presence of the inhomogeneity to dose in a uniform density solid water phantom, were measured with a diamond detector for three thicknesses of aluminium. DRFs exhibit dependence on both the inhomogeneity thickness and the beam diameter. The DRF decreases with inhomogeneity thickness. The DRF initially decreases with increase in the beam diameter from 12.5 to 25 mm. For fields greater than 25 mm, the DRFs are nearly constant. The commonly used algorithms such as the TAR ratio method underestimate the magnitude of the measured effect. A good agreement between these measurements and Monte Carlo calculations is obtained. The influence of the high-density inhomogeneity on the tissue maximum ratio (TMR) was also measured with the inhomogeneity at a fixed depth dmax from the entrance surface. The TMR is reduced for all detector-inhomogeneity distances investigated. The dose build-up phenomenon observed in the presence of low-density air inhomogeneity is absent in the presence of a high-density inhomogeneity. The beam width (defined by 50% dose points) immediately beyond the inhomogeneity is unaffected by the high-density inhomogeneity. However, the 90%-10% and 80%-20% dose penumbra widths and the dose outside the beam edge (beyond the 50% dose point) are reduced. This reduction in dose outside the beam edge is caused by the reduced range of the secondary radiation (photons and electrons) in the high-density medium.     

Optimization of parameters for the detection of cerebral aneurysms: CT angiography of a model

PURPOSE: To optimize parameters with computed tomographic angiography for the detection of cerebral aneurysms. MATERIALS AND METHODS: Model aneurysms were placed randomly at various branch points and scanned multiple times with spiral technique. The final analysis included 63 branch points and 22 aneurysms. Each spiral scan used a different parameter combination. Collimation ranged from 1.5 to 4.0 mm and pitch ranged from 1:1 to 1.5:1. Images were constructed with shaded surface display (SSD) and maximum intensity projection (MIP) algorithms and were interpreted by three readers for the presence or absence of aneurysm. RESULTS: The receiver operating characteristic (ROC) curve area for 1.5-mm collimation was greater than those of 3- or 4-mm collimation (P < .01 and P < .001, respectively). There was no statistically significant difference in the ROC curve areas between 3- and 4-mm collimation (P = .37). There was no statistically significant decrease in ROC curve area when increasing pitch from 1:1 to 1.5:1 for any value of collimation (P = .96). For all parameter combinations the ROC curve areas for SSD images was greater than that of MIP images (P < .0001). CONCLUSION: For cerebral aneurysm detection, narrow collimation is superior to wider collimation. Mild increases in pitch do not substantially degrade diagnostic accuracy. SSD offers improved diagnostic accuracy over MIP display in this model.     

Glass transition temperatures of dental porcelains determined by DSC measurement

The dosimetric data on tissue maximum ratios (TMR), output factors, off axis ratios and beam profiles are presented for small circular fields of diameters ranging from 12.5 to 40 mm for 6 MV radiosurgery beam. It is noticed that dmax increases as the collimator field size increases. Comparison of our data with the published TMR and output factors of similar small circular fields shows that our values are higher than those data. Similarities in trend are noticed with the published isodose volumes for 1-5 and 10 arcs. Not much variation is seen beyond two arcs for 80% isodose volumes for all the field sizes. The variation is small in 20% isodose volumes beyond three arcs. Variations are noticed in 5% isodose volumes for 12.5 mm diameter collimated beam. Our experience has been exclusively with malignant neoplasms. An ideal target volume is covered by 80% isodose volume with 3-4 arcs and a single isocenter. Sixteen patients have been treated to date at our institution, including one patient with brain metastases, two patients with meningiomas, one patient with lymphoma and 12 patients with astrocytomas. The majority of tumors have been treated with single isocenter but some as large as 7 cm have been treated safely with two isocenters.     

Viability of an isocentric cobalt-60 teletherapy unit for stereotactic radiosurgery

The potential for radiosurgery with an isocentric teletherapy cobalt unit was evaluated in three areas: (1) the physical properties of radiosurgical beams, (2) the quality of radiosurgical dose distributions obtained with four to ten noncoplanar converging arcs, and (3) the accuracy with which the radiosurgical dose can be delivered. In each of these areas the cobalt unit provides a viable alternative to an isocentric linear accelerator (linac) as a radiation source for radiosurgery. A 10 MV x-ray beam from a linac used for radiosurgery served as a standard for comparison. The difference between the 80%-20% penumbras of stationary radiosurgical fields in the nominal diameter range from 10 to 40 mm of the cobalt-60 and 10 MV photon beams is remarkably small, with the cobalt-60 beam penumbras, on average, only about 0.7 mm larger than those of the linac beam. Differences between the cobalt-60 and 10 MV radiosurgical treatment plans in terms of dose homogeneity within the target volume, conformity of the prescribed isodose volume to the target volume, and dose falloffs outside the target volume are also minimal, and therefore of essentially no clinical significance. Moreover, measured isodose distributions for a radiosurgical procedure on our Theratron T-780 cobalt unit agreed with calculated distributions to within the +/- 1 mm spatial and +/- 5% numerical dose tolerances, which are generally specified for radiosurgery. The viability of isocentric cobalt units for radiosurgery will be of particular interest to centers in developing countries where cobalt units, because of their relatively low costs, provide the only megavoltage source of radiation for radiotherapy, and could easily and inexpensively be modified for radiosurgery. Of course, the quality assurance protocols and mechanical condition of a particular teletherapy cobalt unit must meet stringent requirements before the use of the unit for radiosurgery can be advocated.     

Monte Carlo dosimetry study of a 6 MV stereotactic radiosurgery unit

Small-field and stereotactic radiosurgery (SRS) dosimetry with radiation detectors, used for clinical practice, have often been questioned due to the lack of lateral electron equilibrium and uncertainty in beam energy. A dosimetry study was performed for a dedicated 6 MV SRS unit, capable of generating circular radiation fields with diameters of 1.25-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. With this code the accelerator was modelled for radiation fields with a diameter as small as 0.5 cm. The radiation fields and dosimetric characteristics (photon spectra, depth doses, lateral dose profiles and cone factors) in a water phantom were evaluated. The cone factor (St) for a specific cone c at depth d is defined as St(d, c) = D(d, c)/D(d, c(ref)), where c(ref) is the reference cone. To verify the Monte Carlo calculations, measurements were performed with detectors commonly used in SRS such as small-volume ion chambers, a diamond detector, TLDs and films. Results show that beam energies vary with cone diameter. For a 6 MV beam, the mean energies in water at the point of maximum dose for a 0.5 cm cone and a 5 cm cone are 2.05 MeV and 1.65 MeV respectively. The values of St obtained by the simulations are in good agreement with the results of the measurements for most detectors. When the lateral resolution of the detectors is taken into account, the results agree within a few per cent for most fields and detectors. The calculations showed a variation of St with depth in the water. Based on calculated electron spectra in water, the validity of the assumption that measured dose ratios are equal to measured detector readings was verified.