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K Platoni

Publications and source records attributed to K Platoni.

8 recordsLinked to original sources
PubMed2000

Linac radiosurgery for cerebral arteriovenous malformations: results in 169 patients.

PURPOSE: To present the SALT group results using Linac radiosurgery (RS) for AVM in 169 evaluable patients treated from January 1990 thru December 1993. METHODS AND MATERIALS: Median age was 33 years (range 6-68 years). Irradiation was the only treatment in 55% patients. Other treatment modalities had been used prior to RS in 45%: one or more embolizations in 36%, surgery in 6%, and embolization and surgery in 3% patients. Nidus were supratentorial in 94% patients, infratentorial in 6% patients. Circular 15 MV x-ray minibeams (6-20 mm) were delivered in coronal arcs by a GE-CGR Saturne 43 Linac. Patient set-up included a Betti arm-chair, a Talairach frame. Prescribed peripheral dose was 25 Gy on the 60%-70% isodose (max dose 100%). Arteriographic results were reassessed in December 1997 at 48 to 96 months follow-up. RESULTS: The overall obliteration rate (OR) was 64% (108/169). AVM volumes ranged from 280 to 19,920 mm(3), median 2460 mm(3). OR was 70% for AVM </= 4200 mm(3) 4200 mm(3) (p 25 mm (p = 0.04). OR was 71%, in the absence of embolization, vs. 54% for previously embolized nidus (p = 0.03). OR was 71% for monocentric RS vs. 54% for multi-isocenters (p 28 Gy vs. 55% for values </= 28 Gy (p 79% vs. 57% for lower values (p 17 Gy, vs. 59% for mLd </= 16 Gy (p 40%, vs. 54% for mLi </= 40% (p 85% vs. 60% for CR </= 84% (NS). For patients treated according to our protocol, i.e., 24-26 Gy on the 60%-70% isodoses, OR was higher (68%) than for other patients (47%) (p = 0.02). After multivariate analysis, absence of previous embolization and mono isocentric-irradiation were independent factors predicting obliteration. Complications were: recurrent hemorrhage, 4 patients (1 patient died); brain necrosis on MRI, 2 patients; subsequent epilepsy, 4 patients; other subsequent neurologic deficits, 3 patients. CONCLUSION: Overall OR was 64% (48-96 months follow-up). After monovariate analysis higher ORs were associated with smaller volumes </= 4200 mm(3), smaller nidus size </= 25 mm, absence of prior embolization, monoisocentric RS, higher values for mean and minimum lesion doses and compliance to our protocol. Higher values for the peripheral dose and isodose tended to give better results. Multivariate analysis showed that the absence of prior embolization and monoisocentric irradiation were independent factors predicting successful irradiation.

Adolescent↗
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↗
PubMed1999

Study of ill-conditioning of Linac stereotactic irradiation subspaces using singular values decomposition analysis.

A Linac stereotactic irradiation space is characterized by different angular separations of beams because of the geometry of the stereotactic irradiation. The regions of the stereotactic space characterized by low angular separations are one of the causes of ill-conditioning of the stereotactic irradiation inverse problem. The singular value decomposition (SVD) is a powerful mathematical analysis that permits the measurement of the ill-conditioning of the stereotactic irradiation problem. This study examines the ill-conditioning of the stereotactic irradiation space, provoked by the different angular separations of beams, using the SVD analysis. We subdivided the maximum irradiation space (MIS: (AA)AP x (AA)RL = 180 degrees x 180 degrees) into irradiation subspaces (ISSs), each characterized by its own angular separation. We studied the influence of ISSs on the SVD analysis and the evolution of the reconstruction quality of well defined three-dimensional dose matrices in each configuration. The more the ISS is characterized by low angular separation the more the condition number and the reconstruction inaccuracy are increased. Based on the above results we created two reduced irradiation spaces (RIS: (AA)AP x (AA)RL = 180 degrees x 140 degrees and (AA)AP x (AA)RL = 180 degrees x 120 degrees) and compared the reconstruction quality of the RISs with respect to the MIS. The more an irradiation space is free of low angular separations the more the irradiation space contains useful singular components.

Humans↗
PubMed1999

A computerized dosimetric database for conformal stereotactic irradiations.

An innovative computerized dosimetric database (DDB) is proposed to enable the analysis of the stereotactic radiosurgical dose distributions; it contains relationships between the irradiation parameters and the dose-volume data. Dose-volume data provide guidance to the physicist-physician team by facilitating the initialization of the irradiation parameters and the treatment planning. The presented DDB contains dose-volume data such as the 70% isodose widths and the 70%-30% isodose penumbra along the right-left, anterior-posterior, and superior-inferior directions as a function of the irradiation parameters defined by the user. In order to demonstrate the usefulness of the DDB, the effects of the collimator diameter, the number of arcs, and their length on the shape of the prescription isodose surface are shown and are related to practical considerations for the treatment plan. However, the presented DDB is one example that can be generated by the DDB system. The planner can define as many different DDBs as he/she wishes, which can then be used for different investigations. This type of DDB enables us to investigate the irradiation technique used, to compare different irradiation techniques, to inspect the feasibility of planning different lesion types, or to define some dosimetric rules. The DDB provides useful interactive guidelines for the treatment planning process and replaces the voluminous dosimetric atlas. It has now been in clinical use for a year in a conformal procedure which automatically proposes collimator diameters, arc positions, and lengths allowing rapid conformal planning.

Algorithms↗
PubMed1998

Use of a general inverse technique for the conformational stereotactic treatment of complex intracranial lesions.

PURPOSE: The stereotactic irradiation of intracranial lesions constitutes an excellent example of conformational therapy whose purpose is to adapt the dose envelope to the target volume with great precision and at the same time to deliver as low a dose as possible to the healthy tissues. We propose the mathematical analysis of the singular values decomposition (SVD) as an inverse planning process to find the optimal minibeam weightings that permit the calculation of the most conformational dose distribution. METHODS: For the radiosurgical treatment of complex lesions, we realize a division of the lesion into several elliptic volumes using the "Associated Target Methodology." This division allows the definition of an irradiation configuration: the number of isocenters, the position of the isocenters, and the diameter of each collimator. For this defined irradiation configuration, we use SVD to find the optimal minibeam weightings. This analysis enables us to understand better the ill-conditioning of the multi-isocentric irradiation and the influence of irradiation parameters on the process of reconstruction minibeam weightings. RESULTS: In this paper, the SVD analysis and the reconstruction technique have been evaluated for the first time on practical cases. We present, as an example, a complex lesion compartmentalized into 3 subvolumes according to our Associated Target Methodology. This analysis allows us to study the ill-conditioning of the example and proposes a large number of solutions from among which we have to choose the most conformational physical solution. This choice is based on the dose-volume histograms. CONCLUSION: We use the SVD procedure as a computer-aided planning system and obtain good solutions, i.e., healthy tissue protection and lesion coverage similar to or better than an experimented planner solution.

Brain Neoplasms↗
PubMedDate not supplied

[Implementation of receiver operating characteristics for the quantitative evaluation of stereotactic radiotherapy treatment plans].

The definition of criteria and of a methodology dedicated to the quantitative evaluation of conformal stereotactic treatment plans is presented. We implemented the 'Receiver Operating Characteristics' (ROC) analysis, already used in medical imaging, for the quantitative evaluation of irradiation treatment plans. This implementation is based on data provided by dose-volume histograms (DVH). Three techniques, each one using a different dosimetric criterion, were defined for the choice of a reference isodose for a given treatment plan. We used this ROC analysis for the selection of the most conformal treatment plan and its reference isodose among the treatment plans proposed for one patient. This study revealed the interest of ROC analysis based on dose-volume histograms for the quantitative evaluation of treatment plans.

Humans↗
PubMedDate not supplied

[Progress in optimizing dosimetry plans in stereotactic radiotherapy in the Salt Group (Saint-Anne-Lariboisière-Tenon].

We began intracranial stereotactic irradiation under the direction of O Betti 11 years ago. At the present time, we believe it is interesting to present the methodologies of the SALT (Saint-Anne-Lariboisière-Tenon) group. Up to the present time we have irradiated 693 patients using a single fraction. Arteriovenous malformations (AVMs) represented the majority (90%) of treated lesions. Irradiation protocol has little changed since 1986, and the localization of the target volume was performed in the neurosurgery department of St Anne Hospital, France. The stereotactic images (computerized tomography [CT], angiography) were sent to the radiotherapy department of Tenon Hospital through the French public digital network NUMERIS. Protocol was realized using the stereotactic ARTEMIS-3D/Dosigray TPS. The lesion volume was filled by one or more spherical or elliptical subvolumes using the "Associated Target Methodology". The interactive adjustment of subvolumes was based on the 3D graphical representations of the lesion. The direct optimization of the irradiation space was performed by managing parameters provided by the DDB (Dosimetric Data Base) such as the number of arcs, their angular position, as well as the starting and the ending point of each arc. The evaluation of the calculated dose distribution was made using quantitative parameters. The second method of optimization was based on the minibeam intensity modulation using a mathematical theory of inverse problems and singular value decomposition (SVD) analysis. At the present time, due to technical reasons, linear accelerators do not permit the modulation of intensity of arctherapy. Thus we transformed the profiles of irregular forms into rectangular profiles of modulated ponderation, with each optimized plan being evaluated before its implementation. The criteria of evaluation were derived from the differential and cumulative dose volume histograms (DVH). The DVHs permitted the evaluation of the volumes of underdosage and overdosage inside the lesion and in the healthy tissue, respectively. Using DVHs, we have defined parameters such as the conform factor and the homogeneity index. We stress that the methodology of protocol optimization is valid for single or multiple fractions as well as for intra- and extra-cranial irradiation.

Humans↗