Verification of treatment parameter transfer by means of electronic portal dosimetry.

Electronic portal imaging devices (EPIDs) are mainly used for patient setup verification during treatment but other geometric properties like block shape and leaf positions are also determined. Electronic portal dosimetry allows dosimetric treatment verification. By combining geometric and dosimetric information, the data transfer between treatment planning system (TPS) and linear accelerator can be verified which in particular is important when this transfer is not carried out electronically. We have developed a pretreatment verification procedure of geometric and dosimetric treatment parameters of a 10 MV photon beam using an EPID. Measurements were performed with a CCD camera-based iView EPID, calibrated to convert a greyscale EPID image into a two-dimensional absolute dose distribution. Central field dose calculations, independent of the TPS, are made to predict dose values at a focus-EPID distance of 157.5 cm. In the same EPID image, the presence of a wedge, its direction, and the field size defined by the collimating jaws were determined. The accuracy of the procedure was determined for open and wedged fields for various field sizes. Ionization chamber measurements were performed to determine the accuracy of the dose values measured with the EPID and calculated by the central field dose calculation. The mean difference between ionization chamber and EPID dose at the center of the fields was 0.8 +/- 1.2% (1 s.d.). Deviations larger than 2.5% were found for half fields and fields with a jaw in overtravel. The mean difference between ionization chamber results and the independent dose calculation was -0.21 +/- 0.6% (1 s.d.). For all wedged fields, the presence of the wedge was detected and the mean difference in actual and measured wedge direction was 0 +/- 3 degrees (1 s.d.). The mean field size differences in X and Y directions were 0.1 +/- 0.1 cm and 0.0 +/- 0.1 cm (1 s.d.), respectively. Pretreatment monitor unit verification is possible with high accuracy and also geometric parameters can be verified using the same EPID image.

An improved technique for breast cancer irradiation including the locoregional lymph nodes.

PURPOSE: To find an irradiation technique for locoregional irradiation of breast cancer patients which, compared with a standard technique, improves the dose distribution to the internal mammary-medial supraclavicular (IM-MS) lymph nodes. The improved technique is intended to minimize the lung dose and reduce the dose to the heart. METHODS AND MATERIALS: The standard technique consists of an anterior mixed electron/photon IM-MS field. In the improved technique, an oblique electron and an oblique asymmetric photon field are combined to irradiate the IM lymph nodes. To irradiate the MS lymph nodes, a combination of an anterior electron and an anterior asymmetric photon field is used. For both the standard and the improved technique, tangential photon fields are used to irradiate the breast. Three-dimensional (3D) treatment planning was performed for 8 patients with various breast sizes for these two techniques. Dose-volume histograms (DVHs) and normal tissue complication probabilities (NTCPs) were compared for both techniques. The field dimensions and energy of the standard technique were determined at simulation, whereas for the improved technique the fields were designed by CT-based treatment planning. RESULTS: The dose in the breast planning target volume was essentially the same for both techniques. For the improved technique, combined with 3D localization information, an improvement in the IM-MS planning target coverage is seen. The volume within the 95% isodose surface was on average 25% (range, 0-64%) and 74% (range, 43-90%) for the standard and improved technique, respectively. The heart generally receives less dose with the improved technique. However, sometimes a small but acceptable increase in lung dose is found. CONCLUSION: The improved technique, combined with localization information of the IM-MS lymph nodes, greatly improves the dose distribution in the planning target volume for a large group of patients without significantly increasing the dose to organs at risk.

A treatment planning method to correct dose distributions distorted by setup verification fields.

PURPOSE: Portal images of conformal treatment fields are often not suitable for setup verification purposes because they contain insufficient bony structures. Therefore, additional rectangular fields are frequently applied for setup verification purposes. It is the aim of this study to reduce the dose distortions induced by these extra fields by appropriately adjusting the beam weights and wedge angles of the treatment fields. METHODS AND MATERIALS: A second treatment plan for the setup verification session is generated, with an identical beam setup as the original plan, but which also includes two orthogonal setup verification fields. An algorithm has been developed, based on vector analysis methods, that adjusts the beam weights and wedge angles of the treatment fields in such a way that both the dose at the isocenter and the dose homogeneity over the planning target volume (PTV) are conserved. RESULTS: The algorithm has been applied to three clinical cases. The number of MUs for the setup verification fields, using a liquid-filled electronic portal imaging device, varied between 16 MU in the head and neck region up to 34 MU for lateral images in the pelvic region. In all cases, the method yielded a treatment plan including two orthogonal setup verification fields with a similar dose distribution over the PTV as the original treatment plan without the setup verification fields. CONCLUSION: The dose distortions resulting from the acquisition of orthogonal verification imaging can be neutralized by modifying the original beam weights and wedge angles of the treatment fields.

A semi-empirical method for calculating the effect of air gap space on output factors on electron beam dose distributions.

BACKGROUND: A simple approach to calculate the effect of air gap on output factors on electron beam dose distribution is presented. METHODS: The method accounts for variations of pencil beam parameters using a model developed by Bruinvis et al. [4,5]. The evaluation of this method is based on measurements of the output factors at various distances between the final collimating device and the phantom surface. RESULTS: Comparison of calculations and measurements of output factors for various cone sizes and 0, 2 and 4 cm air gaps show agreement to within approximately 1.5% for electron energies of 6-13.5 MeV and field sizes of 5.3-10 cm in diameter. CONCLUSION: The accuracy of this semi-empirical method can be considered clinically acceptable and reduces the amount of experimental work needed.

A fast and accurate treatment planning method for boron neutron capture therapy.

BACKGROUND AND PURPOSE: The aim of this study was to test the applicability of conventional semi-empirical algorithms for the treatment planning of boron neutron capture therapy (BNCT). MATERIALS AND METHODS: Beam data of a clinical epithermal BNCT beam obtained in a large cuboid water phantom were introduced into a commercial treatment planning system (TPS). For the calculation of thermal neutron fluence distributions, the Gaussian pencil beam model of the electron beam treatment planning algorithm was used. A simple photon beam algorithm was used for the calculation of the gamma-ray and fast neutron dose distribution. The calculated dose and fluence distributions in the central plane of an anthropomorphic head phantom were compared with measurements for various field sizes. The calculation time was less than 1 min. RESULTS: At the normalization point in the head phantom, the absolute dose and fluence values agreed within the measurement uncertainty of approximately 2-3% (1 SD) with those at the same depth in a cuboid phantom of approximately the same size. Excellent agreement of within 2-3% (1 SD) was obtained between measured and calculated relative fluence and dose values on the central beam axis and at most off-axis positions in the head phantom. At positions near the phantom boundaries, generally in low dose regions, local differences of approximately 30% were observed. CONCLUSIONS: A fast and accurate treatment planning method has been developed for BNCT. This is the first treatment planning method that may allow the same interactive optimization procedures for BNCT as applied clinically for conventional radiotherapy.

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.

Electron dose calculations using the Method of Moments.

The Method of Moments is generalized to predict the dose deposited by a prescribed source of electrons in a homogeneous medium. The essence of this method is (i) to determine, directly from the linear Boltzmann equation, the exact mean fluence, mean spatial displacements, and mean-squared spatial displacements, as functions of energy; and (ii) to represent the fluence and dose distributions accurately using this information. Unlike the Fermi-Eyges theory, the Method of Moments is not limited to small-angle scattering and small angle of flight, nor does it require that all electrons at any specified depth z have one specified energy E(z). The sole approximation in the present application is that for each electron energy E, the scalar fluence is represented as a spatial Gaussian, whose moments agree with those of the linear Boltzmann solution. Numerical comparisons with Monte Carlo calculations show that the Method of Moments yields expressions for the depth-dose curve, radial dose profiles, and fluence that are significantly more accurate than those provided by the Fermi-Eyges theory.

Influence of shape on the accuracy of grid-based volume computations.

The influence of the shape of a region of interest (ROI) on the uncertainty in the sampled volume of the ROI is investigated for computations with regular Cartesian grids. Both mathematically defined volumes and clinically relevant ROIs were studied. The sampling uncertainty is shown to depend on the compactness of the ROI and on effects of grid matching and translational symmetry. In clinical ROIs without translational symmetry the estimate of the sampling uncertainty is improved up to a factor of 2.3 by taking the compactness of the ROI into account. In a spherical ROI grid-matching effects were demonstrated by means of Fourier transforms. In this type of ROI, grid-matching effects decrease as well as increase the sampling uncertainty up to a factor of 1.6. Translational symmetry is shown to cause a decrease in the sampling uncertainty convergence power from 2/3 for spherical ROIs, to 1/2 for stringlike or 1/3 for pancakelike cylinders. For clinical ROIs with translational symmetry, similar decreases were found. With the theory derived and these symmetry effects taken into account the experimental uncertainty of volume computation can be estimated for most clinical ROIs within a factor of 2.5. Special care should be taken in grid sampling of volumes inside isodose surfaces of rectangular field techniques. For the volume of a prostate an uncertainty level of 1% or 5% is obtained with less than 1050 or 80 grid points, respectively, while for such an isodose surface up to 16,000 or 500 grid points are required for the same uncertainty levels.

Quantification of patient to patient variation of skin erythema developing as a response to radiotherapy.

A method is described to determine accurately skin redness during a course of radiotherapy using reflectance spectroscopy utilizing information from across the visible spectrum according to the L*a*b* color coordinate system. The method was used to quantify the development of skin erythema during and after electron beam irradiation of the chest wall following mastectomy. A number of factors were identified which could influence the wide variation in response seen between patients. These were: intra- and inter-observer variation; intra- and inter-patient variation and variation in the actual dose delivered. Statistical analysis, including an analysis of variance of inter- and intra-patient variation, revealed that the major factor that accounts for the observed difference between patients is a true inter-patient variation, with a coefficient of variation, corrected for intra-patient variation, of 43%. Within the narrow dose range administered in this study, there was no demonstrable dose-effect relationship, raising questions about the role of cell death in the basal layer of the epidermis in the pathogenesis of radiation induced erythema.

Calculation of electron beam depth-dose curves and output factors for arbitrary field shapes.

A previously presented method to calculate depth-dose curves and output factors for arbitrarily shaped electron beams is evaluated. The method employs a Gaussian pencil model for direct incident and applicator scattered electrons; the parameter values are derived from measured central axis depth-dose distributions. In addition, an empirical model is used to compute the dose due to electrons scattered by field-defining frames. In this way, the properties of the clinical electron beams are taken into account. In this paper, calculations and measurements for electron beams with energies between 6 and 20 MeV, treatment field dimensions between 3 and 14 cm, and various applicator sizes are compared. The results demonstrate the importance of irregular field dose calculations and the scope of the present method. Agreement better than 3% in dose and 0.2 cm in depth is achieved. For electron beams without applicators, the calculations show the same accuracy. Another method in electron treatment planning that derives values for the radial width parameter of the pencil beam from measured broad beam profiles is also investigated. This method gives good results for dose calculations in beams without applicator scatter. It should be used with care, however, for beams that contain such a scatter component. When electrons scattered by the applicator walls and field-defining frames are neglected, differences between measured and calculated dose up to 8% are found.

Comparison of ionisation measurements in water and polystyrene for electron beam dosimetry.

For the determination of absorbed dose to water in electron beams, dosimetry protocols advocate ionisation measurements in plastic phantoms instead of water for practical reasons. The chamber readings in polystyrene at the depth of maximum ionisation must be corrected for the difference in physical properties between the two materials. This correction factor was determined for a Farmer 0.6 cm3 graphite-walled chamber in electron beams with mean energies at the phantom surface between 6 and 19 MeV. Experiments with white polystyrene yielded corrections for the measured ionisation ranging from 0.3 to 2.4%. For clear polystyrene, 0.6-1% higher corrections were found. For beams with the same mean energy at the phantom surface, but with different beam-flattening and collimation systems, variations in this correction up to 1.2% were observed. In contrast to recent reports on electrical charge storage in polystyrene due to electron irradiation, our experiments do not show differences in the chamber readings any larger than 0.5%.

Calculation of electron beam dose distributions for arbitrarily shaped fields.

A method for the calculation of absorbed dose distributions of arbitrarily shaped electron beams is presented. Isodose distributions and output factors of treatment fields can be predicted with good accuracy, without the need for any dose measurement in the actual field. A Gaussian pencil beam model is employed with two different pencil beams for each electron beam energy. The values of the parameters of the pencil beam dose distributions are determined from a set of measurements of broad beam distributions; in this way the influence of electrons scattered by the applicator walls is taken into account. The dose distribution of electrons scattered from high atomic number metal frames, which define the treatment field contour at the skin, is calculated separately and added. This calculation is based on experimentally derived data. The method has been tested for beams with 6, 10, 14 and 20 MeV electron energy. The distance between calculated and measured isodose lines with values between 10 and 90% is under 0.3 cm. The difference between calculated and measured output factors does not exceed 2%.

Dose calculations for arbitrarily shaped electron beams.

A method for the calculation of absorbed dose distributions of arbitrarily shaped electron beams is described. Isodose distributions and the output factor of a newly designed treatment field can be predicted with good accuracy, without the need for any dose measurement in the actual field. Two different Gaussian pencil beams are used as building elements for the treatment beams of each electron energy. The dose distributions of the pencil beams are derived from measurements of broad beam dose distributions; in this way the influence of electrons scattered by the applicator walls is taken into account. The contribution to the dose by electrons scattered from a high Z metal frame which defines the treatment field contour is calculated separately and added. This calculation is based on experimentally derived data. The method has been tested for electron beams with 6, 10, 14 and 20 MeV nominal energy. The distance between calculated and measured isodose lines with values between 90 and 10 per cent of the maximum dose did not exceed a limit of 0.3 cm. The difference between calculated and measured output factors remained within 2 per cent.

Wall-scattering effects in electron beam collimation.

This report describes now a set of applicators, convering fields with dimensions of 4 to 20 cm, for the 6 to 20 MeV electron beams of a MEL SL75-20 linear accelerator was developed. The electron scatter contribution of the applicator walls to the treatment field was investigated, varying the applicator entrance opening and the scattering foil, with the aim of optimizing the resulting field flatness, with a minimum loss of depth dose. Experiments with field defining end frames and additional perspex scatterers for large field sizes are also reported.