Theoretical and experimental analysis of scatter from inhomogeneous slabs in a 60Co beam: the differential tissue-air ratio method (DTAR)

An inhomogeneity correction algorithm for irregular fields of high-energy photon beams based on Clarkson integration and the 3D beam subtraction method

A number of treatment-planning systems still use conventional correction methods for body inhomogeneities. Most of these methods (power law method, tissue-air ratio (TAR), etc.) consider only on-axis points, rectangular fields, and inhomogeneous slabs covering the whole irradiating field. A new method is proposed that overcomes the above limitations. The new method uses the principle of the Clarkson method on sector integration to take into account the position and lateral extent of the inhomogeneity with respect to the point of calculation, as well as the shape of the irradiating field. The field is divided into angular sectors, and each sector is then treated separately for the presence of inhomogeneities using a conventional correction method. Applying this method, we can predict the correction factors for Co-60 and 6-MV photon beams for irregular fields that include inhomogeneities of lower or higher densities relative to water. Validation of the predicted corrections factors was made against Monte Carlo calculations for the same geometries. The agreement between the predicted correction factors and the Monte Carlo calculations was within 1.5%. In addition, the new method was able to predict the behavior of the correction factor when the point of calculation was approaching or moving away from the interface between two materials.

AVS/express for the PC implementation of a 3D visualization module for radiotherapy

Treatment planning systems (TPS) are nowadays of great help in cancer radiotherapy. Basically, they allow the pair physician/physicist to simulate the beams' irradiation effects on tumors as well as healthy tissues in terms of delivered radiation doses and finally to assess the validity of a beam setup. The state of the art in TPS leads to the following evidence concerning the future of such softwares: an access to a 3D visualization at each step of the design and verification of a plan has become necessary. Moreover, the fast increasing performances of personal computers (PC) will make possible in a near future the implementation at a lower cost of a complete 3D TPS. One of the keys of such an ambition is the compatibility between the implementation tools and the needs for power and flexibility. A first implementation on Open VMS of a simple 3D visualization for Institut Curie's TPS ISIS using Advanced Visual Systems' AVS 5 has been achieved. Their next generation tool, AVS/Express, seemed to meet the needs of a wide scale development. The use of AVS/Express working together with Microsoft Visual C++ in the implementation on Windows NT of a 3D visualization module is exposed.

A method for measuring the ionization fraction due to the chamber wall (alpha) and assessing its characteristics

To calibrate a megavoltage therapy beam using an ionization chamber, it is necessary to know the fraction of the ionization arising in the chamber wall when this is made of a material different than the medium. A method for measuring the ionization fraction produced by electrons arising in the chamber wall (alpha) is presented here. The method uses three measurements at the same point in a medium in order to calculate alpha. These measurements are made using the examined chamber with and without a buildup cap and one reference chamber of wall material equivalent to the medium (i.e., in our case, A1 and A-150 were used as wall materials for the examined and the reference chamber, respectively). Using this method, it is possible to calculate alpha in the medium for a series of irradiation conditions and assess its characteristics. Two main conclusions came out of this assessment. The first one is the independence of alpha from the wall material, even if this is aluminum (alpha is only dependent on wall thickness expressed in g cm-2). The second one is that alpha depends on the irradiation conditions; it increases with field size and depth.

Quality control of inhomogeneity correction algorithms used in treatment planning systems

PURPOSE

This quality control program has been carried out under the auspices of S.F.P.H. (Socíete Fraņcaise des Physiciens d'Hôpital), to evaluate the performances of radiotherapy treatment planning systems (RTPS) used by different institutions. The aim of this Quality Assurance Programme was: (a) to set up a methodology to assess globally the capability of a given system to perform inhomogeneity corrections in the irradiated medium with external photon beams; (b) to analyze the limitations of the algorithms presently used and especially the two-dimensional (2D) dose calculation possibilities; (c) to check, on a number of systems in clinical use, the validity of the method and the variation of the results as compared to measurements used as reference.

METHODS AND MATERIALS

Phantom (lung equivalent material placed into polystyrene) measurements, using cobalt-60 radiation, were carried out by the authors. The phantoms were circulated among the participating institutes to be scanned, and used as input to the treatment planning computer.

RESULTS

Ten systems were tested in this study, using seven different inhomogeneity correction algorithms implemented in nine different TPS; four of these algorithms are used in a pixel by pixel basis and five of them in a contour basis. Significant discrepancies or inconsistencies have been observed even for sophisticated models supposed to be mostly accurate.

CONCLUSION

The proposed tests and the experimental data provided are very useful as part of a quality-control program. They should be included in the initial extensive validation of TPS before starting clinical use, and should be repeated at regular intervals and at each updating of the program. They have the merit of including the whole procedure, from patient data acquisition to dose distribution printout.

A 3-D beam subtraction method for inhomogeneity correction in high energy X-ray radiotherapy

Most methods of inhomogeneity correction in high energy X-ray beams, assume an infinite lateral extent of the heterogeneous volumes ("slab models") or require sophisticated time-consuming computer algorithms. We present here a method, developed for parallelepiped inhomogeneities based on a beam subtraction concept combined with a conventional "slab model". Provided that the conventional model is appropriately chosen, this method gives agreement with experimental results better than 1% in most cases. It accounts properly for the scatter modification according to the size and position of the inhomogeneous volume. One example of a computer application is also presented.