Biomedical subjects

C Kappas

Publications and source records attributed to C Kappas.

At least 19 recordsLinked to original sources

PubMed2006

Assessing the difference between planned and delivered intensity-modulated radiotherapy dose distributions based on radiobiological measures.

AIMS: Because of the highly conformal distributions that can be obtained with intensity-modulated radiotherapy (IMRT), any discrepancy between the intended and delivered distributions would probably affect the clinical outcome. Consequently, there is a need for a measure that would quantify those differences in terms of a change in the expected clinical outcome. MATERIALS AND METHODS: To evaluate such a measure, cancer of the cervix was used, where the bladder and rectum are proximal and partially overlapping with the internal target volume. A solid phantom simulating the pelvic anatomy was fabricated and a treatment plan was developed to deliver the prescribed dose to the phantom. The phantom was then irradiated with films positioned in several transverse planes. The racetrack microtron at 50 MV was used in the treatment planning and delivery processes. The dose distribution delivered was analysed based on the film measurements and compared against the treatment plan. The differences in the measurements were evaluated using both physical and biological criteria. Whereas the physical comparison of dose distributions can assess the geometric accuracy of delivery, it does not reflect the clinical effect of any measured dose discrepancies. RESULTS: It is shown how small inaccuracies in delivered dose can affect the treatment outcome in terms of complication-free tumour cure. CONCLUSIONS: With highly conformal IMRT, the accuracy of the patient set-up and treatment delivery are critical for the success of the treatment. A method is proposed to evaluate the precision of the delivered plan based on changes in complication and control rates as they relate to uncertainties in dose delivery.

Dose-Response Relationship, Radiation↗

PubMed2003

Field flatness and symmetry of photon beams: review of the current recommendations.

Flatness and symmetry are main parameters determining the quality of a photon beam produced by linear accelerators. The quality assurance in routine clinical practice of radiotherapy and consequently the treatment-outcome depend definitely on the physical parameters of treatment-delivery. Several recommendations from national and international associations are reported. By reviewing the current literature and mainly according to the World Health Organization (WHO) report of quality assurance in radiotherapy, we may suggest that for flatness and symmetry, the optimal level of deviation should be within +/- 3%. Flatness and symmetry should be checked monthly or once a year in accordance to the guidelines of national societies.

Calibration↗

PubMed2001

A multiobjective gradient-based dose optimization algorithm for external beam conformal radiotherapy.

A multiobjective gradient-based algorithm has been developed for the purpose of dose distribution optimization in external beam conformal radiotherapy. This algorithm is based on the concept of gathering the values of all objectives into a single value. The weighting factors of the composite objective values are varied in different steps, allowing the reconstruction of the trade-off surfaces (three or more objectives) or curves (two objectives) which define the boundary between the feasible and non-feasible domain regions. The analysis of these curves allows the decision-maker to select the solution that best fits the clinical goals. In contrast to all the other algorithms, our method provides not a single solution but a sample of solutions representing all possible clinical importance factors (weights) for the objectives used. The application of this algorithm to two test cases shows that a correct selection for the importance factors to multiply the individual objectives in the global objective value is not trivial and that the location and shape of the boundary region between the feasible and non-feasible solution regions are case dependent. Provided that the individual objective functions are analytically differentiable and that the number of objectives is the range of two to three, the computation times are acceptable for clinical use. Furthermore, the optimization for a unique combination of importance factors within the aggregate objective function is performed in less than 1 min.

Algorithms↗

PubMed2001

A monte carlo approach to electron contamination sources in the Saturne-25 and -41.

The various components of the accelerator treatment head act as sources of contaminating electrons. The presence of contamination electrons increases the surface dose, which deteriorates the skin-sparing effect. The present study examines the sources of this 'contamination', the influence on the surface dose and the shape of the build-up curve. The Monte Carlo simulation of two linear accelerators, Saturne-25 and -41, allowed us to study the influence of electron contamination in various therapeutic energies and in different geometries. The Saturne-25 and -41 cover a wide range of therapeutic energies with nominal energies 12/23 MV and 6/15 MV, respectively. The analysis of the results shows that at a source-to-surface distance of 100 cm and a wide opening of the collimators, the main sources of contaminating electrons are the flattening filter and the air below it. The contribution of the secondary contamination electrons on the surface dose is 16% for 6 MV and 12 MV, 6% for 15 MV and 17% for 23 MV. The energy spectra of electrons coming from the flattening filter and the air below it are completely different. The air produces electrons of low energies. The mean energies of these spectra vary from 1 MeV to 2 MeV depending on the nominal energy of the photon beam. The secondary electrons generated by the flattening filter produce a wide energy spectrum with mean energies of the same order of the bremsstrahlung spectrum. The flattening filter absorbs the secondary electrons generated in the target, the primary collimator and the air inside the head.

Air↗

PubMed2001

Graphical treatment simulation and automated collision detection for conformal and stereotactic radiotherapy treatment planning.

The purpose of this work is to develop a "room's eye view" graphical simulation program with an automated collision detection option, to assist a treatment planning user to visualize the treatment setup checking at the same time the feasibility of his plan. The program simulates the treatment process using accurate three-dimensional graphical models of the gantry, table, and that of an average patient. This allows the use of any mechanical movement concerning the radiotherapy unit. The simulation of the mechanical movements and their limitations are according to IEC standards and thus compatible with any radiotherapy machine. Graphical methods for collision detection between the models guarantee speed and accuracy. The module simulates the treatment setup with accuracy better than 2 degrees for any tested case. When a collision takes place a warning message is displayed. In this paper a software tool is developed that can be used as a stand-alone program or embedded in any treatment planning system. The visualization of the treatment fields prior to treatment permits the geometric feasibility of the plan, thus adding one more step toward the automation of the treatment process.

Computer Simulation↗

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↗