Improved method for the design of tissue compensators

Radiological properties of a wax-gypsum compensator material

In this paper the radiological properties of a compensator material consisting of wax and gypsum is presented. Effective attenuation coefficients (EACs) have been determined from transmission measurements with an ion chamber in a Perspex phantom. Measurements were made at 80 and 100 cm source-to-skin distance (SSD) for beam energies of 6, 8, and 15 MV, for field sizes ranging from narrow beam geometries up to 40 x 40 cm2, and at measurement depths of maximum dose build-up, 5 and 10 cm. A parametrization equation could be constructed to predict the EAC values within 4% uncertainty as a function of field size and depth of measurement. The EAC dependence on off-axis position was also quantified at each beam energy and SSD. It was found that the compensator material reduced the required thickness for compensation by 26% at 8 MV when compared to pure paraffin wax for a 10 x 10 cm2 field. Relative surface ionization (RSI) measurements have been made to quantify the effect of scattered electrons from the wax-gypsum compensator. Results indicated that for 80 cm SSD the RSI would exceed 50% for fields larger than 15 x 15 cm2. At 100 cm SSD the RSI values were below 50% for all field sizes used.

Compensators for intensity-modulated beams

This study describes the importance of attenuator scatter in the construction of compensators. The attenuator used in this study was Lipowitz metal, commonly referred to as cerrobend. Linear attenuation coefficients of cerrobend were measured in air for different thickness of cerrobend sheets and different field sizes for a 6-MV photon beam. The magnitude of the dose contribution from photons scattered by the attenuator was measured. The variations of beam hardening and the scatter to primary ratio as a function of the thickness of cerrobend and varying field size were investigated. The compensators in this study were produced using a simple exponential attenuation model and the measured linear attenuation coefficients. It was found that the beam hardening effect was significant, and can lead to an error of 6.2% in the transmission, for 6 cm of cerrobend in the beam. The maximum scatter contribution to the measured fluence was 19.8% of the transmitted primary dose for a 20 x 20-cm2 field size, and 6 cm of cerrobend in the beam. For a simple wedge-step compensator; there was a maximum deviation of 6% between the measured and our predicted fluence profile. For simple compensators, this deviation can be attributed to scatter.

Optimal flattening filter shape of a surface brachytherapy applicator

As an alternative to standard treatment of superficial lesions with surgery and/or superficial/orthovoltage x-ray irradiation, radioactive moulds can be used. However, close proximity of the source to the treatment region leads to significant dose inhomogeneities in the tissue. Analytical and fast numerical methods have been developed that can calculate a corresponding flattening filter that evens out the dose distribution at the surface. Monte Carlo transport calculations were used to enable accurate treatment of the transport through the geometry. It was found that the use of pre-calculated transport parameters like dependence of the attenuation coefficient and scattering with depth is required for fast calculation of the flattening filter shape. Only 1-2 iterations were needed for a successful convergence to the filter shape that flattens out the dose distribution at the surface to within 1%. The developed methods are very general and could be applied with some minor modifications to other problems, where shaping of the irradiation field is required.

Dosimetric verification of modulated photon fields by means of compensators for a kernel model

The approach in treatment planning of applying beam quality correction factors to model compensator-induced depth-hardening effects is investigated and the present work comprises a dosimetric verification of the model for a common compensator material. Lead sheet modulators for four different phantom shapes were designed using a treatment planning system based on the model. The modulators were designed to yield homogeneous dose in a plane. The calculated modulation created by the lead sheets was re-imported into the treatment planning system and applied to a water phantom geometry for verification purposes. Comparing measurements, a total of 31 different geometries were measured, with calculations in this geometry showing good agreement for depth doses, dose profiles and output data with a maximum deviation of 4% except locally in the penumbra region and close to the edges of the cut lead sheets.

Accuracy of dose calculation methods for retraced tissue compensators

Uncertainties arise in dose calculations involving retracted tissue compensators due to the effects of the compensator upon the scatter component of the dose. Many commercial treatment-planning systems cannot allow directly for the presence of a compensator in isodose calculation or else use simple 2D methods. We present data to test calculation accuracy for a wax compensation system by comparing retraction factors measured along central-axis and off-axis raylines for a variety of compensator shapes, with those derived using effective attenuation coefficient and 3D analytical calculations. The accuracy of using measured uniform-thickness retraction factors for non-uniform shapes and the dose uniformity achievable using a simple compensation system are also discussed. We conclude that the accuracy of simple calculation methods is shape dependent and that calculation errors and dose variations can exceed +/-5% where missing-tissue thickness variations are large. The analytical method is shown to give good agreement with experiment and indicates that it should be possible to adapt algorithms that calculate scatter from the patient for use with compensators.

Evaluation of optimized compensators on a 3D planning system

A commercially available treatment planning system contains several functions that allow for the automation of missing tissue and optimized compensators, where the former retracts the bolus toward the source, and the latter attempts, by iteration, to establish a uniform dose at some user defined depth. The intent of this paper is to report on the compensators designed by the system and to compare them to those devised through conventional techniques. It is demonstrated that the system can model the dosimetric effects of compensators with a high degree of accuracy; measured and predicted doses agree to within 3%. Optimized compensators show slightly improved dose uniformity over thickness reduced compensators. Both show significantly improved uniformity over compensators that simply retract the bolus geometry. In cases where internal inhomogeneities exist, however, the dose uniformity from the optimized compensators vary by as much as 6% at the target depth. These deviations are comparable to the errors of the inhomogeneity algorithm itself. The pathlength reduction technique has been applied to both missing tissue and inhomogeneity compensation, and it has been found that for inhomogeneity compensation, the pathlength reduced compensators produce more uniform distributions than those generated by the optimization algorithm.

Compensation in three-dimensional non-coplanar treatment planning

This paper presents a technique for producing uniform dose distributions within the target volume with non-coplanar field arrangements. The method is based upon the principle used for producing homogeneous dose distributions for a pair of fields in two dimensions, namely, that if the isodose distributions for the fields are made parallel to one another, the combined dose distribution will be uniform. For the three-dimensional non-coplanar case, homogeneous target dose distributions are obtained by designing field modifiers which produce a uniform dose distribution on the perpendicular bisector plane for each pair of fields. It is demonstrated that for three non-coplanar fields irradiating a spherical phantom with spherical target volumes, the target dose distribution will be homogeneous for any arbitrary non-coplanar field arrangement. Furthermore, this technique can be extended to any number of fields. Therefore, target dose distributions for non-coplanar plans can be as homogeneous as the coplanar case. An example of the application of the method to the treatment of rectal carcinoma with non-coplanar fields is given. Furthermore, it is demonstrated that the therapeutic ratio is improved over traditional methods for this clinical example.

An algorithm for design of beam compensators

Beam compensators are optimally designed to give a uniform dose to any plane or midsurface that intersects a single beam, or to give a uniform dose to the volume defined by the intersection of two or more beams. The primary and scatter components are taken into account separately, as well as the patient's shape and internal heterogeneities. The design of the beam compensators is formulated as a linear programming problem and solved with a variation of the Simplex Method. Beam weighting factors are also obtained as part of the solution.

Limitations of retracted missing tissue compensators: an experimental analysis

The introduction of an attenuating medium into a photon beam serves both to reduce the intensity of the primary beam and to create secondary radiation due to scatter. When retracted missing tissue compensators are employed to compensate for irregular surface geometry or internal inhomogeneities they are almost always fabricated without regard to the scattered radiation that they introduce to the system. Analysis of such a retracted compensator designed for use with an anthropomorphic phantom reveals an inability to provide true compensation. Subsequent analysis of a mathematically describable conical geometry demonstrates the need for improved compensator design.

The use of posterior wedge portals as an alternative to tissue compensators

In a regional oncology center, where the patient census makes routine construction of tissue compensators prohibitive, a simple alternative can be successfully implemented. In this instance, a posterior wedge is used to alter isodose levels. This allows us to minimize critical structure doses, and optimize tumor dose levels. The following chest isodose distribution is presented for discussion.

A comprehensive three-dimensional radiation treatment planning system

A comprehensive software system has been developed to allow 3-dimensional planning of radiation therapy treatments using the extensive anatomical information made available by imaging modalities such as CT and MR. Biological structures of interest and tumor volumes are defined by outlines drawn on a sequence of CT slices. Beam set-ups may then be determined in three dimensions by displaying the structure contours in a beam's eye view, or in two dimensions using a single CT cut. Each beam defined may be shaped by the specification of block aperture contours, and its intensity may be modified with the use of planar compensators. 3D dose calculation algorithms are discussed. To evaluate the calculation results, dose volume histograms are provided, as well as various types of displays in two and three dimensions, including dose on arbitrarily oriented planes, dose on the surface of anatomical objects, and isodose surfaces. Computer generated beam films are also available as an aid in patient set-up verification. These tools, and others, provide the basis for a comprehensive 3D system that can be used throughout the treatment planning process.

Dosimetry considerations for a Lipowitz metal tissue compensator system

The present report describes the fabrication technique and the dosimetry aspects of a new compensator system that uses the low melting point allow Lipowitz metal. Compensating ratios (CR, mm of tissue compensated per mm of cerrobend) were determined for various field sizes and depths for 60Co, and X ray energies of 4, 6, 18, and 25 MV. Typical CR for 10 cm X 10 cm field and 10 cm depth were: 60Co, 4 MV, and 6 MV, 1:15; 18 and 25 MV, 1:20. Verification of these CR were performed at 6 MV using polystyrene phantoms. Ionization measurements were made for various field sizes and depths and normalized to central axis full-phantom readings for both compensated and non-compensated fields. Without compensation, percent differences ranged as high as 40% for a tissue deficit of 10 cm. With the compensating filters (CF) in place, this difference was reduced to 2-4%. Overall, the CF system was capable of producing dose uniformity to within +/- 10% for a wide range of depths and field sizes.