Biomedical subjects

S Zefkili

Publications and source records attributed to S Zefkili.

9 recordsLinked to original sources

PubMed2005

The sliding slit test for dynamic IMRT: a useful tool for adjustment of MLC related parameters.

For treatments with dynamic intensity modulated radiotherapy (IMRT), the adjustment of multileaf collimator (MLC) parameters affecting both the optimization algorithm and dose distributions is crucial. The main parameters characterizing the MLC are the transmission (T) and the dosimetric leaf separation (DLS). The aim of this study is twofold: a methodology based on the 'sliding slit' test is proposed to determine (T, DLS) combinations inducing the best conformity between calculations and measurements. Secondly, the effects of the MLC adjustment on measured dose and on optimization are presented for different configurations as the chair test and for the patient dosimetric quality control (DQC). Tests were performed with a Varian 23EX linac operated at 20 MV and equipped with a 120 leaf Millenium dynamic collimator. The treatment planning system was CadPlan/Helios (version 6.3.6). Results demonstrated that the sliding width (SW) strongly depends on the (T, DLS) combinations, and the measured dose is a linear function of the SW. Different (T, DLS) combinations induced a good agreement between calculations and measurements. The influence of the MLC calibration was found to be particularly important on the 'sliding slit' test (11.8% for a gap change of 0.8 mm) but not so much on the chair test and on the DQC. To detect small variations in leaf adjustment and to ensure consistency between calculation and actual dose delivered to patients, a daily check called IMRT MU check is proposed.

Calibration↗

PubMed2004

[Constraints and dosage for prostate cancer patients treated with conformal radiotherapy and intensity modulated radiation therapy].

PURPOSE: Intensity modulated radiation therapy (IMRT) is based on a methodology called inverse planning. Starting from dosimetric objectives, constraints of optimization are fixed and given to the inverse planning system, which in turn calculates the modulated intensity to apply to each beam. Since the algorithms allow the constraints to be violated, the results of optimization may differ from the initial dosimetric objectives. Consequently, the user is compelled to adapt the choice of the constraints according to the type of modulation and until satisfactory results are found. The purpose of this work is to present our experience in the choice of these constraints for prostate cancer treatments, as we moved from conformal radiotherapy to IMRT. Treatments were performed with a Varian 23EX linac and calculations were realized with the Varian CadPlan-Helios planning system. PATIENTS AND METHODS: The approach used for the first 12 patients treated at institut Curie with IMRT from June 2002 was analysed. The treatment always consisted of a combination of conformal radiotherapy with and without intensity modulation. RESULTS AND CONCLUSION: Results showed that, a larger fraction of the treatment performed with IMRT induced a better sparing of the organs at risk for the same homogeneous dose distribution to the target volume. Apart from the dose-volume constraint for the rectum, a fixed set of constraints, slightly more restrictive than the dosimetric objectives, could be used for all patients. Compared with conformal radiotherapy, the conformation factor for IMRT increased up to 16%. A specific study was undertaken in view of treatments completely performed with IMRT. The optimal technique consisted in performing separated IMRT plans for the two target volumes, the prostate volume and the prostate plus seminal vesicles volume respectively. Another satisfactory possibility was to define new constraints on two separated planning target volumes, prostate and seminal vesicles. This last approach is now routinely implemented for our IMRT patients.

Humans↗

PubMed2004

[Recommendations for a head and neck IMRT quality assurance protocol].

Head and neck tumors represent very interesting targets for IMRT techniques because of the complex shape of the structures and the organs at risk close by. The use of this kind of techniques requires a quality assurance protocol. The physicists of the GORTEC group shared their experience to define some recommendations in order to draw up a QA protocol. The dosimetric verification of the treatment plans (in terms of absolute and relative dose), the control of the reproducibility of the patient positioning and the use of a record and verify system to control the different parameters form the main parts of these recommendations. Each chapter comprises a description of the different methods, recommendations concerning the equipment, the adopted tolerances, the frequency of controls. At the end of each chapter, a table summarizes the main actions to carry out. These recommendations will allow to harmonize our practices whatever the softwares and the accelerator that are being used. They will simplify the task of the teams that wish to implement IMRT for head and neck tumors.

Clinical Protocols↗

PubMed2002

[Upper aerodigestive tract cancers: clinical benefits of conformal radiotherapy and intensity modulation].

The conformal radiotherapy approach, three-dimensional conformal radiotherapy (3DCRT) or intensity-modulated radiotherapy (IMRT), is based on modern imaging modalities, efficient 3D treatment planning systems, sophisticated immobilization systems and rigorous quality assurance and treatment verification. The central objective of conformal radiotherapy is to ensure a high dose distribution tailored to the limits of the target volume while reducing exposure of normal tissues. These techniques would then allow further tumor dose escalation. Head-and-neck tumors are some of the most attractive localizations to test conformal radiotherapy. They combine ballistic difficulties due to particularly complex shapes (nasopharynx, ethmoid) and problems due to the number and low tolerance of neighbouring organs like parotids, eyes, brainstem and spinal cord. The therapeutic irradiation of head-and-neck tumors thus remains a challenge for the radiation oncologist. Conformal radiotherapy does have a significant potential for improving local control and reducing toxicity when compared to standard radiotherapy. However, in the absence of prospective randomized trials, it is somewhat difficult at present to evaluate the real benefits drawn from 3DCRT and IMRT. The published clinical reports on the use of conformal radiotherapy are essentially dealing with dosimetric comparisons on relatively small numbers of patients. Recently, a few publications have emphasized the clinical experience of several precursor teams with a suitable follow-up. This paper describes the current state-of-the-art of 3DCRT and IMRT in order to evaluate the impact of these techniques on head-and-neck cancers irradiation.

Central Nervous System↗

PubMed2000

Dose calculation and verification of intensity modulation generated by dynamic multileaf collimators.

While the development of inverse planning tools for optimizing dose distributions has come to a level of maturity, intensity modulation has not yet been widely implemented in clinical use because of problems related to its practical delivery and a lack of verification tools and quality assurance (QA) procedures. One of the prerequisites is a dose calculation algorithm that achieves good accuracy. The purpose of this work was twofold. A primary-scatter separation dose model has been extended to account for intensity modulation generated by a dynamic multileaf collimator (MLC). Then the calculation procedures have been tested by comparison with carefully carried out experiments. Intensity modulation is being accounted for by means of a 2D (two-dimensional) matrix of correction factors that modifies the spatial fluence distribution, incident to the patient. The dose calculation for the corresponding open field is then affected by those correction factors. They are used in order to weight separately the primary and the scatter component of the dose at a given point. In order to verify that the calculated dose distributions are in good agreement with measurements on our machine, we have designed a set of test intensity distributions and performed measurements with 6 and 20 MV photons on a Varian Clinac 2300C/D linear accelerator equipped with a 40 leaf pair dynamic MLC. Comparison between calculated and measured dose distributions for a number of representative cases shows, in general, good agreement (within 3% of the normalization in low dose gradient regions and within 3 mm distance-to-dose in high dose gradient regions). For absolute dose calculations (monitor unit calculations), comparison between calculation and measurement reveals good agreement (within 2%) for all tested cases (with the condition that the prescription point is not located on a high dose gradient region).

Algorithms↗

PubMed2000

A method to check the accuracy of dose computation using quality index: application to scatter contribution in high energy photon beams.

Computerized dose calculation verification is a relevant component of radiotherapy treatment planning quality assurance. The usual procedure is to compare measurements to computations for several standard situations. As cases become more complex, special test phantoms and beam arrangements must be used, and an experimental procedure must be carefully established. In this paper we follow a new methodology to prepare a set of reference data that may be used to verify the accuracy of dose calculations involving changes in the scatter component of photon beams. The advantage of this methodology is that local measurements are not required. A quantitative evaluation of dose modifications was performed by means of correction factors (CF). For this purpose, three geometrical configurations were designed (asymmetric, symmetric, and reference) where the primary component was kept constant and the scatter component was varied by changing the height (h) of lateral columns. Measurements were performed in polystyrene phantoms for seven photon beam energies. CF were derived as the ratio of the absolute dose measured at the point of interest to the absolute dose for the reference configuration, for the asymmetric and symmetric configurations, respectively. They were expressed as a function of beam quality (QI). We have verified that, for all configurations studied, CF decrease with QI. For h = 15 cm, CF remain practically constant, whatever machine technology is used [the mean values of CF for the asymmetric and symmetric cases are CFa= 1.028 (0.2% 1 s.d.) and CFs= 1.058 (0.4% 1 s.d.)]. We have developed a test protocol and we have chosen those configurations corresponding to h = 15 cm because they both present greater values of the CF and lower standard deviations. The direct application of the method is straightforward. The user can reproduce on his local TPS the three experimental configurations described in the test protocol, and then compute CF which can be compared to our reference data set for any beam quality.

Biophysical Phenomena↗

PubMed1999

The 'equivalent wedge' implementation of the Varian Enhanced Dynamic Wedge (EDW) into a treatment planning system.

The purpose of this work was to establish procedures for the implementation of the Varian Enhanced Dynamic Wedge into a treatment planning system (TPS), based as much as possible on simple theoretical considerations and already available data. A method is presented for the calculation (rather than measurement) of off-axis relative wedge transmission curves that are required by the TPS for relative dose calculations. We also present a method for absolute dose (monitor unit) calculations, based on the calculation of an effective wedge factor on the prescription point. A simple formula has been derived for the calculation of the effective wedge factor for the most general case, i.e. an arbitrary effective wedge angle, field size and prescription point. Relative dose calculations have been verified by measurements performed on a Varian Clinac 2300C/D linear accelerator, for 6 MV and 20 MV photon energies. Monitor unit calculations have also been verified experimentally for several cases such as symmetric and asymmetric fields with prescription on the collimator axis or on the geometrical centre of the asymmetric field. The presented technique provides results within 2% for both relative and absolute dose calculations for clinically relevant cases.

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

[Use of a multileaf collimator for the production of intensity-modulated beams].

In external radiotherapy, the use of intensity modulated fields has been proposed for tissue and non-homogeneity compensation or for the generation of conformal dose distributions. Multileaf collimators can be employed dynamically for the modulation of the X-ray field in two dimensions. Efficient dynamic collimation became possible due to advances in computer and linear accelerator technology. It presents a number of advantages over conventional methods such as the use of compensators. We have developed a program which calculates, from a given intensity distribution, the motion of the MLC leaves as a function of monitor units, and we have applied it on a Varian linear accelerator with a 40 pair multileaf collimator. The analysis of the experimental results demonstrates the feasibility and the potential of the method.

Algorithms↗