Biomedical subjects

Ann Van Esch

Publications and source records attributed to Ann Van Esch.

6 recordsLinked to original sources

PubMed2007

The use of magnetic sensors to monitor moderate deep inspiration breath hold during breast irradiation with dynamic MLC compensators.

BACKGROUND AND PURPOSE: To reduce the dose to the heart during left breast irradiation, a moderate deep breath hold technique (MDIBH) was introduced. Originally, verification of the MDIBH was performed with portal images acquired in movie loop during the treatment delivery. However, this verification method is not compatible with the use of dynamic MLC compensation, recently introduced because of its often superior dose distribution. Magnetic sensors were evaluated as an additional/alternative method to monitor the breath hold. MATERIAL AND METHODS: In a first phase, the reproducibility of MDIBH for breast patients was evaluated by investigating for 19 patients the set-up errors derived from portal images in cine loop acquisition during MDIBH. In a second phase, for 10 patients, the breathing curves recorded by magnetic sensors were used to monitor beam-on and beam-off while portal images were simultaneously recorded in movie loop. In a third phase, breast patients treated with dynamic MLC compensation were trained for MDIBH and monitored with magnetic sensors. RESULTS: The interfraction reproducibility of MDIBH for the initial 19 patients was recorded: the mean set-up error, the systematic and the random deviations are all smaller than 4mm in the anterior-posterior direction and in the cranio-caudal direction and smaller than 2 degrees along the rotation axis. Magnetic sensors provided a reproducible breathing curve: while the mean amplitude recorded for 10 patients varied substantially between patients, the individual standard deviation of the amplitude for each session was smaller than 3mm. For these 10 patients, the intrafraction set-up variation between the first portal image of two consecutive breath holds and the intra-breath hold set-up variation between the first and last portal image of each breath hold is smaller than 2mm in the anterior-posterior direction, smaller than 3mm in the cranio-caudal direction and smaller than 1.5 degrees along the rotation axis. CONCLUSION: Using magnetic sensors to record the breathing curve of left breast patients in MDIBH, a verification method was developed, suitable for combining MDIBH with dynamic MLC compensation.

Breast Neoplasms↗

PubMed2006

Testing of the analytical anisotropic algorithm for photon dose calculation.

The analytical anisotropic algorithm (AAA) was implemented in the Eclipse (Varian Medical Systems) treatment planning system to replace the single pencil beam (SPB) algorithm for the calculation of dose distributions for photon beams. AAA was developed to improve the dose calculation accuracy, especially in heterogeneous media. The total dose deposition is calculated as the superposition of the dose deposited by two photon sources (primary and secondary) and by an electron contamination source. The photon dose is calculated as a three-dimensional convolution of Monte-Carlo precalculated scatter kernels, scaled according to the electron density matrix. For the configuration of AAA, an optimization algorithm determines the parameters characterizing the multiple source model by optimizing the agreement between the calculated and measured depth dose curves and profiles for the basic beam data. We have combined the acceptance tests obtained in three different departments for 6, 15, and 18 MV photon beams. The accuracy of AAA was tested for different field sizes (symmetric and asymmetric) for open fields, wedged fields, and static and dynamic multileaf collimation fields. Depth dose behavior at different source-to-phantom distances was investigated. Measurements were performed on homogeneous, water equivalent phantoms, on simple phantoms containing cork inhomogeneities, and on the thorax of an anthropomorphic phantom. Comparisons were made among measurements, AAA, and SPB calculations. The optimization procedure for the configuration of the algorithm was successful in reproducing the basic beam data with an overall accuracy of 3%, 1 mm in the build-up region, and 1%, 1 mm elsewhere. Testing of the algorithm in more clinical setups showed comparable results for depth dose curves, profiles, and monitor units of symmetric open and wedged beams below dmax. The electron contamination model was found to be suboptimal to model the dose around dmax, especially for physical wedges at smaller source to phantom distances. For the asymmetric field verification, absolute dose difference of up to 4% were observed for the most extreme asymmetries. Compared to the SPB, the penumbra modeling is considerably improved (1%, 1 mm). At the interface between solid water and cork, profiles show a better agreement with AAA. Depth dose curves in the cork are substantially better with AAA than with SPB. Improvements are more pronounced for 18 MV than for 6 MV. Point dose measurements in the thoracic phantom are mostly within 5%. In general, we can conclude that, compared to SPB, AAA improves the accuracy of dose calculations. Particular progress was made with respect to the penumbra and low dose regions. In heterogeneous materials, improvements are substantial and more pronounced for high (18 MV) than for low (6 MV) energies.

Algorithms↗

PubMed2004

The use of an aSi-based EPID for routine absolute dosimetric pre-treatment verification of dynamic IMRT fields.

BACKGROUND AND PURPOSE: In parallel with the increased use of intensity modulated radiation treatment (IMRT) fields in radiation therapy, flat panel amorphous silicon (aSi) detectors are becoming the standard for online portal imaging at the linear accelerator. In order to minimise the workload related to the quality assurance of the IMRT fields, we have explored the possibility of using a commercially available aSi portal imager for absolute dosimetric verification of the delivery of dynamic IMRT fields. PATIENTS AND METHODS: We investigated the basic dosimetric characteristics of an aSi portal imager (aS500, Varian Medical Systems), using an acquisition mode especially developed for portal dose (PD) integration during delivery of a-static or dynamic-radiation field. Secondly, the dose calculation algorithm of a commercially available treatment planning system (Cadplan, Varian Medical Systems) was modified to allow prediction of the PD image, i.e. to compare the intended fluence distribution with the fluence distribution as actually delivered by the dynamic multileaf collimator. Absolute rather than relative dose prediction was applied. The PD image prediction was compared to the corresponding acquisition for several clinical IMRT fields by means of the gamma evaluation method. RESULTS AND CONCLUSIONS: The acquisition mode is accurate in integrating all PD over a wide range of monitor units, provided detector saturation is avoided. Although the dose deposition behaviour in the portal image detector is not equivalent to the dose to water measurements, it is reproducible and self-consistent, lending itself to quality assurance measurements. Gamma evaluations of the predicted versus measured PD distribution were within the pre-defined acceptance criteria for all clinical IMRT fields, i.e. allowing a dose difference of 3% of the local field dose in combination with a distance to agreement of 3 mm.

Algorithms↗

PubMed2002

A quantitative evaluation of IMRT dose distributions: refinement and clinical assessment of the gamma evaluation.

BACKGROUND AND PURPOSE: Although intensity modulated radiotherapy (IMRT) is a step forward in comparison to conventional, static beam delivery, quality assurance is more complex and labour intensive, demanding detailed two-dimensional dosimetric verification. Regardless of the technique used for measuring the dose distribution, what is essential to the implementation of routine verification of IMRT fields is the efficient and accurate comparison of the measured versus desired dose distribution. In order to achieve a fast, yet accurate quantitative measure of the correspondence between measured and calculated dose, the theoretical concept of the gamma evaluation method presented by Low et al. (Med. Phys., 25 (1998) 656) was converted into a calculation algorithm, taking into account practical considerations related to the discrete nature of the data. MATERIALS AND METHODS: A filter cascade of multiple levels was designed to obtain fast and accurate comparison of the two dose distributions under evaluation. The actual comparison consists of classification into accepted or rejected datapoints with respect to user-defined acceptance criteria (dose difference and distance to agreement). The presented algorithm was tested on dosimetric images calculated and/or acquired by means of a liquid filled portal imaging device during the course of intensity modulated treatments of prostate cancer, including pre-treatment verification as well as verification during treatment. To assess its ability to intercept possible errors in dose delivery, clinically relevant errors were deliberately introduced into the dose distributions. RESULTS: The developed gamma filter method proves successful in the efficient comparison of calculated versus measured IMRT dose distribution. Secondly, intercomparison of dosimetric images acquired during different treatment sessions illustrate its potential to highlight variations in the dosimetric images. The simulated errors were unmistakably intercepted. CONCLUSIONS: The readily obtained gamma evaluation images are an easy tool for quality control of IMRT fields. To reduce the artefacts related to the discrete nature and limited resolution of the data, a fast and accurate filter cascade was developed, offering the possibility to use the gamma method for day to day evaluation of patient dosimetric portal images with or without comparison to a predicted portal dose distribution.

Algorithms↗

PubMed2002

Quality assurance in intensity modulated radiotherapy by identifying standards and patterns in treatment preparation: a feasibility study on prostate treatments.

BACKGROUND AND PURPOSE: Quality assurance (QA) in intensity modulated treatments is a complex and time-consuming process. In spite of intensive quality control procedures some types of errors still can go undetected through the course of a treatment. This study aims to develop an objective QA filter for fast, automatic detection of errors, based on the creation of a global platform monitoring treatment parameters by comparison with existing local standards. Since such a conceptually new type of QA has already proven successful for conventional treatments, we aim to extend it to intensity modulated radiotherapy (IMRT) treatments. MATERIAL AND METHODS: The feasibility of developing such a QA platform was evaluated on 12 prostate treatment plans. Apart from the classical treatment parameters, the optimised fluence distributions were compared and screened for repetitive patterns. This screening was performed by calculating specific parameters, defined to characterise the fluence maps. RESULTS: Repetitive patterns were found in the beam shapes as well as in the intensity distributions, and useful parameters could be defined to quantify typical field patterns. By statistical analysis of these parameters, mean values and tolerance levels were derived, providing a means to automatically filter out unprobable or erroneous intensity modulated treatment fields during the treatment preparation stage. CONCLUSION: It is possible to identify parameters quantifying the characteristic patterns found in fluence distributions of intensity modulated fields of a specific treatment, allowing the development of a platform for automatic pre-treatment quality control.

Feasibility Studies↗

PubMed2002

Acceptance tests and quality control (QC) procedures for the clinical implementation of intensity modulated radiotherapy (IMRT) using inverse planning and the sliding window technique: experience from five radiotherapy departments.

BACKGROUND AND PURPOSE: An increasing number of radiotherapy centres is now aiming for clinical implementation of intensity modulated radiotherapy (IMRT), but--in contrast to conventional treatment--no national or international guidelines for commissioning of the treatment planning system (TPS) and acceptance tests of treatment equipment have yet been developed. This paper bundles the experience of five radiotherapy departments that have introduced IMRT into their clinical routine. METHODS AND MATERIALS: The five radiotherapy departments are using similar configurations since they adopted the commercially available Varian solution for IMRT, regarding treatment planning as well as treatment delivery. All are using the sliding window technique. Different approaches towards the derivation of the multileaf collimator (MLC) parameters required for the configuration of the TPS are described. A description of the quality control procedures for the dynamic MLC, including their respective frequencies, is given. For the acceptance of the TPS for IMRT multiple quality control plans were developed on a variety of phantoms, testing the flexibility of the inverse planning modules to produce the desired dose pattern as well as assessing the accuracy of the dose calculation. Regarding patient treatment verification, all five centres perform dosimetric pre-treatment verification of the treatment fields, be it on a single field or on a total plan procedure. During the actual treatment, the primary focus is on patient positioning rather than dosimetry. Intracavitary in vivo measurements were performed in special cases. RESULT AND CONCLUSION: The configurational MLC parameters obtained through different methods are not identical for all centres, but the observed variations have shown to be of no significant clinical relevance. The quality control (QC) procedures for the dMLC have not detected any discrepancies since their initiation, demonstrating the reliability of the MLC controller. The development of geometrically simple QC plans to test the inverse planning, the dynamic MLC modules and the final dose calculation has proven to be useful in pointing out the need to remodel the single pencil beam scatter kernels in some centres. The final correspondence between calculated and measured dose was found to be satisfactory by all centres, for QC test plans as well as for pre-treatment verification of clinical IMRT fields. An intercomparison of the man hours needed per patient plan verification reveals a substantial variation depending on the type of measurements performed.

Humans↗