Ann Van Esch

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 18MV 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%, 1mm in the build-up region, and 1%, 1mm 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%, 1mm). 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 18MV than for 6MV. 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 (18MV) than for low (6MV) energies.

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.

On-line quality assurance of rotational radiotherapy treatment delivery by means of a 2D ion chamber array and the Octavius phantom

For routine pretreatment verification of innovative treatment techniques such as (intensity modulated) dynamic arc therapy and helical TomoTherapy, an on-line and reliable method would be highly desirable. The present solution proposed by TomoTherapy, Inc. (Madison, WI) relies on film dosimetry in combination with up to two simultaneous ion chamber point dose measurements. A new method is proposed using a 2D ion chamber array (Seven29, PTW, Freiburg, Germany) inserted in a dedicated octagonal phantom, called Octavius. The octagonal shape allows easy positioning for measurements in multiple planes. The directional dependence of the response of the detector was primarily investigated on a dual energy (6 and 18 MV) Clinac 21EX (Varian Medical Systems, Palo Alto, CA) as no fixed angle incidences can be calculated in the Hi-Art TPS of TomoTherapy. The array was irradiated from different gantry angles and with different arc deliveries, and the dose distributions at the level of the detector were calculated with the AAA (Analytical Anisotropic Algorithm) photon dose calculation algorithm implemented in Eclipse (Varian). For validation on the 6 MV TomoTherapy unit, rotational treatments were generated, and dose distributions were calculated with the Hi-Art TPS. Multiple cylindrical ion chamber measurements were used to cross-check the dose calculation and dose delivery in Octavius in the absence of the 2D array. To compensate for the directional dependence of the 2D array, additional prototypes of Octavius were manufactured with built-in cylindrically symmetric compensation cavities. When using the Octavius phantom with a 2 cm compensation cavity, measurements with an accuracy comparable to that of single ion chambers can be achieved. The complete Octavius solution for quality assurance of rotational treatments consists of: The 2D array, two octagonal phantoms (with and without compensation layer), an insert for nine cylindrical ion chambers, and a set of inserts of various tissue equivalent materials of different densities. The combination of the 2D array with the Octavius phantom proved to be a fast and reliable method for pretreatment verification of rotational treatments. Quality control of TomoTherapy patients was reduced to a total of approximately 25 min per patient.

Pre-treatment dosimetric verification by means of a liquid-filled electronic portal imaging device during dynamic delivery of intensity modulated treatment fields

BACKGROUND AND PURPOSE Although intensity modulated radiation therapy is characterized by three-dimensional dose distributions which are often superior to those obtained with conventional treatment plans, its routine clinical implementation is partially held back by the complexity of the beam verification. This is even more so when a dynamic multileaf collimator (dMLC) is used instead of a segmented beam delivery. We have therefore investigated the possibility of using a commercially available, liquid-filled electronic portal imaging device (EPID) for the pre-treatment quality assurance of dynamically delivered dose distributions. METHODS AND MATERIALS A special acquisition mode was developed to optimize the image acquisition speed for dosimetry with the liquid-filled EPID. We investigated the accuracy of this mode for 6 and 18 MV photon beams through comparison with film and ion chamber measurements. The impact of leaf speed and pulse rate fluctuations was quantified by means of dMLC plans especially designed for this purpose. Other factors influencing the accuracy of the dosimetry (e.g. the need for build-up, remanence of the ion concentration in the liquid and bulging of the liquid at non-zero gantry angles) were studied as well. We finally compared dosimetric EPID images with the corresponding image prediction delivered without a patient in the beam. RESULTS The dosimetric accuracy of the measured dose distribution is approximately 2% with respect to film and ion chamber measurements. The accuracy declines when leaf speed is increased beyond 2 cm/s, but is fairly insensitive to accelerator pulse rate fluctuations. The memory effect is found to be of no clinical relevance. When comparing the acquired and expected distributions, an overall agreement of 3% can be obtained, except at areas of steep dose gradients where slight positional shifts are translated into large errors. CONCLUSIONS Accurate dosimetric images of intensity modulated beam profiles delivered with a dMLC can be obtained with a commercially available, liquid-filled EPID. The developed acquisition mode is especially suited for fast and accurate pre-treatment verification of the intensity modulated fields.

A method to estimate the transit dose on the beam axis for verification of dose delivery with portal images

PURPOSE AND BACKGROUND A feasibility study is performed to evaluate the possibility of using the transit dose of portal images on the beam axis to measure the accuracy in dose delivery. The algorithm and the method are tested on a breast phantom and on patients with a breast disease. MATERIALS AND METHODS To estimate the transit dose at various air gaps behind the patient, a method is proposed which applies, for a given air gap, the inverse square law to the primary component of the exit dose and an experimentally determined function for the scatter component of the exit dose. It is assumed that the primary component and the scattered component of the exit dose are given by the treatment planning system. The experimental function for the variation of the scattered component with the air gap, determined by phantom measurements, is modelled by an analytical function which contains only field size, air gap and one energy-dependent parameter. RESULTS The measurements on the breast phantom yield a maximum deviation between measured and estimated transit doses of 4.5%. The mean deviation is 0.9% with a standard deviation of the distribution of 2.3%. In vivo diode measurements on the same phantom yield a maximum deviation of 2.7%. Transit dose measurements on the beam axis for 45 portal images of breast patients show a mean deviation of 0.0% between the measured transit dose and the estimated transit dose. The standard deviation of the distribution is 4.4%. The method seems to be very sensitive to patient positioning and to discrepancies in breast thicknesses used for treatment planning. CONCLUSION Preliminary results on breast patients show that the method proposed to evaluate transit doses on the beam axis from portal images may be a valuable alternative to conventional in vivo exit dosimetry. The method can be implemented in a simple way and does not require additional time during the irradiation session, as exit dosimetry with diodes does. The transit dose is only considered in one point. Nevertheless, in the framework of quality assurance of treatment delivery, this study is an example of the possibilities of monitoring at the same time the visual evaluation of the irradiated volume as well as the dosimetric control (i.e. in Gy) of treatment delivery with portal images.

Experimental techniques for pulsed magnetic fields

Abstract Recent developments at the K.U. Leuven pulsed field laboratory are reviewed. This includes new 60 T user magnets, pulse shaping, decentralized data recording for multiple experiments, low temperature cryogenics, vibration damping, rotatable sample holders and sensitive magnetization measurements. Experimental results (on semiconductors, super-conductors and organic conductors) are shown to illustrate the improvements in the experimental techniques.

Optimized Varian aSi portal dosimetry: development of datasets for collective use

Although much literature has been devoted to portal dosimetry with the Varian amorphous silicon (aSi) portal imager, the majority of the described methods are not routinely adopted because implementation procedures are cumbersome and not within easy reach of most radiotherapy centers. To make improved portal dosimetry solutions more generally available, we have investigated the possibility of converting optimized configurations into ready‐to‐use standardized datasets. Firstly, for all commonly used photon energies (6, 10, 15, 18, and 20 MV), basic beam data acquired on 20 aSi panels were used to assess the interpanel reproducibility. Secondly, a standardized portal dose image prediction (PDIP) algorithm configuration was created for every energy, using a three‐step process to optimize the aSi dose response function and profile correction files for the dosimetric calibration of the imager panel. An approximate correction of the backscatter of the Exact arm was also incorporated. Thirdly, a set of validation fields was assembled to assess the accuracy of the standardized configuration. Variations in the basic beam data measured on different aSi panels very rarely exceeded 2% (2 mm) and are of the same order of magnitude as variations between different Clinacs when measuring in reference conditions in water. All studied aSi panels can hence be regarded as nearly identical. Standardized datasets were successfully created and implemented. The test package proved useful in highlighting possible problems and illustrating remaining limitations, but also in demonstrating the good overall results (95% pass rate for 3%,3 mm) that can be obtained. The dosimetric behavior of all tested aSi panels was found to be nearly identical for all tested energies. The approach of using standardized datasets was then successfully tested through the creation and evaluation of PDIP preconfigured datasets that can be used within the Varian portal dosimetry solution. PACS number: 87.55.km, 87.55.Qr, 87.56.N_

The Octavius1500 2D ion chamber array and its associated phantoms: Dosimetric characterization of a new prototype

PURPOSE The purpose of the study is to characterize the prototype of the new Octavius1500 (PTW, Freiburg, Germany) 2D ion chamber array, covering its use in different phantom setups, from the most basic solid water sandwich setup to the more complex cylindrical Octavius® 4D (Oct4D) (PTW) phantom/detector combination. The new detector houses nearly twice the amount of ion chambers as its predecessors (Seven29 and Octavius729), thereby tackling one of the most important limitations of ion chamber (or diode) arrays, namely the limited detector density. The 0.06 cm3 cubic ion chambers are now arranged in a checkerboard pattern, leaving no lines (neither longitudinally nor laterally) without detectors. METHODS All measurements were performed on a dual energy (6 MV and 18 MV) iX Clinac (Varian Medical Systems, Palo Alto, CA) and all calculations were done in the Eclipse treatment planning system (Varian) with the Anisotropic Analytical Algorithm. First, the basic characteristics of the 2D array, such as measurement stability, dose rate dependence and dose linearity were investigated in the solid water sandwich setup. Second, the directional dependence was assessed to allow the evaluation of the new Octavius2D phantom (Oct2D(1500)) for planar verification measurements of composite plans. Third, measurements were performed in the Oct4D phantom to evaluate the impact of the increased detector density on the accuracy of the volumetric dose reconstruction. RESULTS While showing equally good dose linearity and dose rate independence, the Octavius1500 outperforms the previous models because of its instantaneous measurement stability and its twofold active area coverage. Orthogonal field-by-field measurements immediately benefit from the increased detector density. The 3.9 cm wide compensation cavity in the new Oct2D(1500) phantom prototype adequately corrects for directional dependence from the rear, resulting in good agreement within the target dose. Discrepancies may arise towards the sides of the array because of uncompensated lateral beam incidence. The beneficial impact of the detector density is most prominent in the Oct4D system, for which the average pass rate (PR) is now nearly 100% (99.31±0.37) when using gamma criteria of 2%G,2 mm (10% dose threshold). In search of gamma analysis criteria that are not too lenient to detect possibly relevant deviations, the authors conclude that for our radiotherapy environment, the authors choose to adopt 3%L,3 mm PR97% (threshold 10%) criteria for the Oct2D(1500)/Octavius1500 system and 2%L,3 mm PR97% (threshold 10%) for the Oct4D/Octavius1500 system. These are first line pass/check criteria and plans that fail are not necessarily rejected, but submitted to a more detailed investigation. CONCLUSIONS When irradiated from the front, the Octavius1500 array has two main advantages over its 729 predecessors: its instantaneous measurement stability and--most importantly-its twofold detector density. In the Oct2D1500 phantom, these advantages are counterbalanced by the more pronounced directional dependence. The measurement-based 3D dose reconstruction in the Oct4D system, however, benefits considerably from the higher detector density in the checkerboard panel design.

Quality assurance in radiotherapy by identifying standards and monitoring treatment preparation.

BACKGROUND AND PURPOSE Due to the complexity of the treatment preparation in radiotherapy, a number of errors go undetected until after the first treatment session. Some of these errors could easily have been noticed before treatment if an objective filter existed in addition to human supervision. With this in mind, a conceptually novel extension to conventional quality assurance procedures was explored to create a global platform monitoring treatment preparation by comparison with the existing local standards. MATERIALS AND METHODS The feasibility of developing such a platform was evaluated for a test case on a cohort of 202 patients having received breast irradiation. By statistical analysis of the treatment parameters, mean values and tolerance levels could be defined for most parameters based on the observed standard deviations. Useful correlations were traced providing us with a means to automatically track errors, the detection of which would otherwise solely depend upon the alertness of the supervisor. RESULTS AND CONCLUSIONS Apart from its possibilities as a mere quality control tool, the platform, developed in the framework of EQUART (European Quality Assurance Program in Radiotherapy by Monitoring Treatment Preparation), can be incorporated in the treatment preparation chain, providing standard setup values for the simulation. A crucial achievement of EQUART lies in the fact that filtering out of errors occurs prior to treatment initiation.

The K.U. Leuven pulsed high magnetic field facility

Abstract The pulsed field installation at the K.U. Leuven is upgraded for the development and use of the new generation of high performance coils with peak fields in the 60–70 T range. New equipment includes an improved coil winding installation, capacitor bank switchgear and electrical diagnostics to enable the running of two experiments in parallel, new cryostats for stabilized temperatures in the 300–0.4 K range, sample holders whixh permit rotation of the sample, and facilities for sample preparation.