Biomedical subjects

C Kappas

Publications and source records attributed to C Kappas.

18 recordsLinked to original sources

PubMed2000

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.

Humans↗

PubMed2000

A phantom for dosimetric characterization of small radiation fields: design and use.

An acrylic phantom was designed and constructed for the acquisition and verification of basic dosimetric data of narrow fields in stereotactic radiotherapy/radiosurgery (SRT/SRS) using thermoluminescent (TL) dosimetry. An array of holes to accommodate up to 426 dosimeters was used to allow the assessment of dose distribution in circular fields with a 1-mm spatial dose resolution with minimal field perturbation. It was found experimentally that there must be a minimum gap of 1 mm between neighboring dosimeters in 6-MV photon fields. Most of the dosimetric characteristics of a 6-MV x-ray SRS/SRT unit assessed using TL dosimetry and ion chamber dosimetry were in good agreement when the longitudinal axis of the chamber was parallel to the central beam axis. TL dosimetry showed that the penumbra width increased with increasing collimator aperture. The low cost of the phantom and the widespread use and familiarity of TL dosimetry in radiotherapy departments offer a significant advantage in the use of the proposed methodology.

Equipment Design↗

PubMed2000

Dose-volume analysis of different stereotactic radiotherapy mono-isocentric techniques.

Several stereotactic irradiation techniques, using Linacs with the patient in lying and sitting position and a Gamma Knife Unit, were compared with regard to mono-isocentric three-dimensional dose distributions. Three types of target volumes, a sphere and two ellipsoids, were used for the comparisons. All three targets were centered on a real head, reconstructed from transversal CT scans. The ARTEMIS 3D Treatment Planning System, developed by the Tenon Hospital, Paris, was used for the dosimetry and the dose-volume histogram (DVH) calculation. For the comparative study, several quantitative parameters were used, derived from the dose-volume histogram calculation. Differential DVHs were plotted for each target volume and beam arrangement. Irradiation techniques were compared by deriving quantitative parameters from the DVHs such as mean and integral dose delivered to the target and normal tissue irradiated, as well as by the relative volume of the examined areas. All techniques used in this study produced very similar dose distributions. The small differences confirm the capability of the studied techniques to produce the same irradiation effects. By changing from the spherical target shape to a more elliptical shape, more of the normal tissue was irradiated with higher doses. For elliptical cases we therefore identified a need for more conformal stereotactic planning.

Brain Neoplasms↗

PubMed2000

Intensity modulated arc therapy (IMAT) with centrally blocked rotational fields.

A new technique for intensity-modulated beam (IMB) delivery that combines the features of intensity modulated arc therapy (IMAT) with the use of 'classical blocks' is proposed. The role of the blocks is to realize the high-gradient modulation of the intensity profile corresponding to the region to be protected within the body contour, while the MLC leaves or the secondary collimator defines the rest of the field and delivers intensity-modulated multiple rotational segments. The centrally blocked radiation fields are applied sequentially, in several rotations. Each rotation of the gantry is responsible for delivering one segment of the optimal intensity profile. The new IMAT technique is applied for a treatment geometry represented by an annular target volume centrally located within a circular body contour. The annulus encompasses a circular critical structure, which is to be protected. The beam opening and corresponding weight of each segment are determined in two ways. The first method applies a linear optimization algorithm to precalculated centrally blocked radial dose profiles. These radial profiles are calculated for a set of beam openings, ranging from the largest field that covers the whole planning target volume (PTV) to the smallest, which is 1 cm larger than the width of the central block. The optimization is subjected to dose homogeneity constraints imposed on a linear combination of these profiles and finally delivers the dimensions and weights of the rotational beams to be used in combination. The second method decomposes into several subfields the fluence profile of a rotational beam known to deliver a constant dose level to PTV. This fluence profile is determined by using the analytical method proposed by Brahme for the case of the annular PTV and the concentric organ at risk (OAR). The proper segmentation of this intensity profile provides the field sizes and corresponding weights of the subfields to be used in combination. Both methods show that for this particular treatment geometry, three to seven segments are sufficient to cover the PTV with the 95% dose level and to keep the dose level to the central critical structure under 30% of the maximum dose. These results were verified by experimentally delivering the calculated segments to radiotherapy verification films sandwiched between two cylindrical pieces of a pressed-wood phantom. The total beam time for a three-field irradiation was 77 s. The predicted and experimental dose profiles along the radius of the phantom agreed to within 5%. Generalization of this technique to real-patient treatment geometry and advantages over other conformal radiotherapy techniques are also discussed.

Algorithms↗

PubMed2000

Comparison of conformal radiation therapy techniques within the dynamic radiotherapy project 'Dynarad'.

The objective of the dynamic radiotherapy project 'Dynarad' within the European Community has been to compare and grade treatment techniques that are currently applied or being developed at the participating institutions. Cervical cancer was selected as the tumour site on the grounds that the involved organs at risk, mainly the rectum and the bladder, are very close to the tumour and partly located inside the internal target volume. In this work, a solid phantom simulating the pelvic anatomy was used by institutions in Belgium, France, Greece, Holland, Italy, Sweden and the United Kingdom. The results were evaluated using both biological and physical criteria. The main purpose of this parallel evaluation is to test the value of biological and physical evaluations in comparing treatment techniques. It is demonstrated that the biological objective functions allow a much higher conformality and a more clinically relevant scoring of the outcome. Often external beam treatment techniques have to be combined with intracavitary therapy to give clinically acceptable results. However, recent developments can reduce or even eliminate this need by delivering more conformal dose distributions using intensity modulated external dose delivery. In these cases the reliability of the patient set-up procedure becomes critical for the effectiveness of the treatment.

European Union↗

PubMed1999

Optimization of the dose level for a given treatment plan to maximize the complication-free tumor cure.

During the past decade, tumor and normal tissue reactions after radiotherapy have been increasingly quantified in radiobiological terms. For this purpose, response models describing the dependence of tumor and normal tissue reactions on the irradiated volume, heterogeneity of the delivered dose distribution and cell sensitivity variations can be taken into account. The probability of achieving a good treatment outcome can be increased by using an objective function such as P+, the probability of complication-free tumor control. A new procedure is presented, which quantifies P+ from the dose delivery on 2D surfaces and 3D volumes and helps the user of any treatment planning system (TPS) to select the best beam orientations, the best beam modalities and the most suitable beam energies. The final step of selecting the prescribed dose level is made by a renormalization of the entire dose plan until the value of P+ is maximized. The index P+ makes use of clinically established dose-response parameters, for tumors and normal tissues of interest, in order to improve its clinical relevance. The results, using P+, are compared against the assessments of experienced medical physicists and radiation oncologists for two clinical cases. It is observed that when the absorbed dose level for a given treatment plan is increased, the treatment outcome first improves rapidly. As the dose approaches the tolerance of normal tissues the complication-free cure begins to drop. The optimal dose level is often just below this point and it depends on the geometry of each patient and target volume. Furthermore, a more conformal dose delivery to the target results in a higher control rate for the same complication level. This effect can be quantified by the increased value of the P+ parameter.

Dose-Response Relationship, Radiation↗

PubMed1999

Calculating shielding requirements in diagnostic X-ray departments.

Structural radiation protection for diagnostic X-ray facilities is most commonly performed following the recommendations of the National Council on Radiation Protection and Measurements Report No. 49. A number of analytical methods have already been developed to improve the design of these facilities. Specifically, these methods reassess shielding calculations in X-ray areas with respect to the methodology of the calculation of the barrier thickness and the number of sources considered in the area. Thus, they generate an overall solution for the cases met at the medical radiation structural design. This paper presents an extension of an existing method for calculating shielding requirements, for multiple X-ray tubes in a room operated at various beam qualities. The methodology computes the required shielding thickness such that the exposure behind it stays below a desired value. The presented method eliminates the overestimation of added shielding thickness which may occur using the other methods already mentioned. A user-friendly windows-based program has also been developed to assist shielding computations.

Algorithms↗

PubMed1998

A new non-invasive and relocatable immobilization frame for fractionated stereotactic radiotherapy.

PURPOSE: A newly developed non-invasive immobilization frame for stereotactic radiotherapy is presented, which is intended to be used for both imaging (computed tomography (CT) and angiography) and radiotherapeutic procedures. MATERIALS AND METHODS: The frame is made of duraluminium so as to be stable and light and it has an elliptical shape. The immobilization is achieved using three stable locations on the patient's head, i.e. the upper dentition, the nose and the back of the neck. The fixation on the three locations ensures complete immobilization in all directions. RESULTS: The immobilization frame can be fitted as many times as is needed to most heads. In order to assess the accuracy of relocation, repeated fittings on two volunteers and on 22 patients undergoing stereotactic treatment were performed (more than 200 mountings in total), which showed maximum anterior-posterior, inferior-superior and lateral reproducibility in positioning of less than 1 mm in all cases. CONCLUSIONS AND DISCUSSION: The in-house-constructed stereotactic frame is simple to use, easily made, non-invasive, relocatable and well tolerated by the patients, providing the possibility of multiple fractions. The major advantage of using such a non-invasive stereotactic frame is the flexibility in timing the different diagnostic procedures (CT and angiography) as well as providing the possibility to extend the use to large brain lesions (treatment without an additional collimator) where a high precision is also required. It also offers significant labour and cost saving over the invasive frames and the majority of the non-invasive frames. To date, 22 patients with ages varying between 12 and 70 years have been treated using this method.

Adolescent↗

PubMed1998

Development in a Windows environment of a radiation treatment planning system for personal computers.

A new personal computer (PC) radiotherapy treatment planning system (RTPS) is presented. The PC-based RTPS is designed to run in the Microsoft Windows 3.11 environment (and later versions), for computers equipped with 486 or Pentium processors. The algorithm used by the new PC-based program for dose calculation belongs to the 'radiological pathlength' category and it was previously implemented on VAX 711 computers at Memorial Sloan-Kettering Cancer Center (MSKCC) in New York, NY, within EXTREP-III RTPS. The EXTREP-III program is a two-dimensional RTPS (with restricted three-dimensional capabilities), developed and used in clinical practice at MSKCC during the 1980s. The PC-based program is implemented in the Visual Basic (version 3.0) language and supports features commonly available in most photon-mode RTPSs: dose calculation for fixed, isocentric and rotational irradiation techniques, dose corrections for both internal inhomogeneities and external inhomogeneities (boluses and compensators), association of machine-specific beams with various wedges and blocks, etc. The graphic interface of the PC-based RTPS is completely new and is designed to meet the requirements of fast and accurate planning. The user interface consists of an event-oriented button-based console which allows users to perform planning and to have isodose charts overlaid on patient computed tomography images initially loaded in the program. The PC-based RTPS tests, performed in order to assess its accuracy and speed of computation, show good results. The acceptable computation times obtained, the good accuracy in dose computation and the user-friendly interface of the program are sufficient reasons to consider the PC-based RTPS a good quality-price ratio tool for radiation treatment planning in cancer therapy.

Algorithms↗

PubMed1998

Quality control of dose volume histogram computation characteristics of 3D treatment planning systems.

Detailed quality control (QC) protocols are a necessity for modern radiotherapy departments. The established QC protocols for treatment planning systems (TPS) do not include recommendations on the advanced features of three-dimensional (3D) treatment planning, like the dose volume histograms (DVH). In this study, a test protocol for DVH characteristics was developed. The protocol assesses the consistency of the DVH computation to the dose distribution calculated by the same TPS by comparing DVH parameters with values obtained by the isodose distributions. The computation parameters (such as the dimension of the computation grid) that are applied to the TPS during the tests are not fixed but set by the user as if the test represents a typical clinical case. Six commercial TPS were examined with this protocol within the frame of the EC project Dynarad (Biomed I). The results of the intercomparison prove the consistency of the DVH results to the isodose values for most of the examined TPS. However, special attention should be paid when working with cases of adverse conditions such as high dose gradient regions. In these cases, higher errors are derived, especially when an insufficient number of dose calculation points are used for the DVH computation.

Europe↗

PubMed1998

Developing a dose-volume histogram computation program for brachytherapy.

A dose-volume histogram (DVH) computation program was developed for brachytherapy treatment planning in an attempt to benefit from the DVH's ability to present graphically information on 3D dose distributions. The program is incorporated into a planning system that utilizes a pair of orthogonal radiographs to localize the radiation sources. DVHs are calculated for the volume of tissue enclosed by an isodose surface (e.g. half the value of the reference isodose). The calculation algorithm is based on a non-uniform random sampling that gives a denser point distribution at the centre of the implants. Our program was tested and proved to be fast enough for clinical use and sufficiently accurate (i.e. computation time of 20 s and less than 2% relative error for one point source, for 100,000 calculation points). The accuracy improves when a larger calculation point number is used, but the computation time also increases proportionally. The DVH is presented in the form of a simple graph or table, or as Anderson's 'natural' DVH graph. The cumulative DVH tables can be used to extract a series of indexes characterizing the homogeneity and the dose levels of the distribution in the treatment volume and the surrounding tissues. If a reference plan is available, the DVH results can be assessed relative to the reference plan's DVH.

Brachytherapy↗

PubMed1997

A simple method for 3D lesion reconstruction from two projected angiographic images: implementation to a stereotactic radiotherapy treatment planning system.

INTRODUCTION: The most used imaging modality for diagnosis and localisation of arteriovenous malformations (AVMs) treated with stereotactic radiotherapy is angiography. The fact that the angiographic images are projected images imposes the need of the 3D reconstruction of the lesion. This, together with the 3D head anatomy from CT images could provide all the necessary information for stereotactic treatment planning. We have developed a method to combine the complementary information provided by angiography and 2D computerized tomography, matching the reconstructed AVM structure with the reconstructed head of the patient. MATERIALS AND METHODS: The ISIS treatment planning system, developed at Institute Curie, has been used for image acquisition, stereotactic localisation and 3D visualisation. A series of CT slices are introduced in the system as well as two orthogonal angiographic projected images of the lesion. A simple computer program has been developed for the 3D reconstruction of the lesion and for the superposition of the target contour on the CT slices of the head. RESULTS AND CONCLUSIONS: In our approach we consider that the reconstruction can be made if the AVM is approximated with a number of adjacent ellipses. We assessed the method comparing the values of the reconstructed and the actual volumes of the target using linear regression analysis. For treatment planning purposes we overlapped the reconstructed AVM on the CT slices of the head. The above feature is to our knowledge a feature that the majority of the commercial stereotactic radiotherapy treatment planning system could not provide. The implementation of the method into ISIS TPS shows that we can reliably approximate and visualize the target volume.

Cerebral Angiography↗

PubMed1997

Simulation with EGS4 code of external beam of radiotherapy apparatus with workstation and PC gives similar results?

This article presents a comparison between two implementations of an EGS4 Monte Carlo simulation of a radiation therapy machine. The first implementation was run on a high performance RISC workstation, and the second was run on an inexpensive PC. The simulation was performed using the MCRAD user code. The photon energy spectra, as measured at a plane transverse to the beam direction and containing the isocenter, were compared. The photons were also binned radially in order to compare the variation of the spectra with radius. With 500,000 photons recorded in each of the two simulations, the running times were 48 h and 116 h for the workstation and the PC, respectively. No significant statistical differences between the two implementations were found.

Computer Simulation↗

PubMed1997

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.

Aluminum↗

PubMed1995

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.

Algorithms↗

PubMed1994

On-axis and off-axis primary dose component in high energy photon beams.

The depth dose of the primary dose component, on axis and off axis of six different x-ray beams, has been determined from transmission measurements in narrow beam geometry with and without flattening filter using a Perspex column of a cross section large enough to ensure electronic equilibrium. In order to derive the primary photon fluence, a correction for the scatter from the column has been applied according to the following method: A number of spectra taken from the literature have been used for computing a scatter coefficient Sc at different depths by convolution of dose spread arrays. Using the relationship between Sc and the single attenuation coefficient mu i to represent each entire spectrum, it has been possible to correct the experimental transmission curves iteratively, until the corresponding values of mu were stabilized and representative of the primary. The measured attenuation coefficients were found to have a linear increase as a function of the distance from the central axis for all the energies and types of linear accelerators. For the same nominal energy, this increase is different from one accelerator to another. The same phenomenon was observed for the attenuation coefficients obtained without the flattening filter in the same experimental conditions. The results are tentatively interpreted considering the angular variation of bremsstrahlung energy spectra with and without a flattening filter as calculated by a Monte Carlo method and they are consistent and useful to take accurately into account the softening of the beam as the off-axis distance increases.

Humans↗

PubMedDate not supplied

A simple method for the correction of distorted digital angiographic images for stereotactic target localization.

The most commonly used imaging modality for the diagnosis and localization of arteriovenous malformations (AVMs) treated with stereotactic radiotherapy is traditional angiography, but it would be desirable to also use digital subtraction angiography (DSA). However, DSA images are distorted due to the electron-optical characteristics of the X-ray image intensifier. For that reason, we have developed a method for the correction of the image distortion. The ISIS II Treatment Planning System (ISIS II TPS), developed at the Curie Institute, has been used for image acquisition and stereotactic localization. A grid phantom has been constructed for determining the distortion of the DSA images. The software developed for the correction has been implemented into the TPS and is based on a correction vector produced by matching the distorted and corrected grid points. The method has been tested for its ability to correct the position of all grid points as well as its effectiveness in real cases as compared to traditional angiography. The maximum displacement of the corrected grid points compared with their original position is measured to be 0.1 mm. The accuracy of the target localization using the corrected DSA images is comparable with traditional angiography localization and falls inside acceptable accuracy limits. In conclusion, this method offers the possibility of using DSA images for stereotactic localization without limiting the requested accuracy.

Angiography, Digital Subtraction↗

PubMedDate not supplied

Multimedia educational services in stereotactic radiotherapy.

The computer-based learning methods in medicine have been well established as stand-alone learning systems. Recently, these systems were enriched with the use of telematics technology to provide distance learning capabilities. Stereotactic radiotherapy is one of the most representative advanced radiotherapy techniques. Due to the multidisciplinary character of the technique and the rapid evolution of technology implemented, the demands in training have increased. The potential of interactive multimedia and Internet technologies for the achievement of distance learning capabilities in this domain are investigated. The realization of a computer-based educational program in stereotactic radiotherapy in a multimedia format is a new application in the computer-aided distance learning field. The system is built according to a client and server architecture, based on the Internet infrastructure, and composed of server nodes. The impact of the system may be described in terms of: time and transportation costs saving, flexibility in training (scheduling, rate and subject selection), online communication and interaction with experts, cost effective access to material (delivery or access by a large number of users and revision of the material by avoiding high costs of computer-based training systems and database development).

Computer-Assisted Instruction↗