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of these limitations are limited and practical guidelines for QA of TPS have become available only recently. From previous ESTRO projects on quality assurance (QA) aspects in radiotherapy that include the treatment planning system (e.g. ESTRO-EQUAL or QUASIMODO) it can be concluded that there are uncertainties related to the dose calculation models [4,7,9]. Another example is the study of Venselaar and Welleweerd [21], where it is shown that especially asymmetric wedged beams are not properly modelled in some commercial treatment planning systems. On the other hand, it is generally accepted that deviations larger than 5% between delivered and prescribed doses may influence the clinical outcome of the treatment [2,14]. For specific tumours and situations, such as in clinical trials, even higher demands might be required in terms of dosimetric accuracy. Because errors and large uncertainties in dose calculations reduce the quality of a treatment, independent dose or MU calculations have been recommended and also used for a long time as a routine quality assurance (QA) procedure when verifying individual treatment plans [3,11,15,16]. During the last years commercial products providing independent dose calculations that can handle all advanced treatment techniques have become available. However, to our knowledge no reports have been published that describes their accuracy or other aspects of their clinical application. Verification calculations are traditionally performed by applying empirical algorithms in a manual calculation procedure, or utilizing software based on fairly simple dose calculation algorithms. With the advent of advanced treatment techniques that are based on the availability of multileaf collimators, asymmetric jaws and dynamic or virtual wedges, empirical dose calculation procedures become much more complex and their accuracy is often limited by the simplicity of the model itself [6]. To achieve high accuracy with an independent dose calculation tool also for the most complex treatment techniques, more sophisticated models are required [12,13,22]. On the other hand, only models that allow detailed investigations of the dosimetric accuracy, and consequently enable more strict action levels (AL), will be effective in catching systematic uncertainties of the TPS. Moreover, misinterpretations of TPS results, and the occurrence of errors associated with the limited accuracy of empirical verification models can be greatly reduced. As a general requirement, an ideal verification dose calculation model should be as independent as possible of the TPS, and should be based on physical effects which are accurately described and tuned by an independent set of algorithm input data [10]. Additionally, the verification software implementation should provide high accuracy uncorrelated to the radiotherapy equipment in use, i.e. its accuracy should neither depend on the type of linear accelerator nor the photon beam energy for which it has been commissioned. To be able to verify multiple beams in an efficient way, it should manage to import treatment plan data (e.g. MLC settings) directly from the TPS or the record and verify (R&V) system. Ideally, a full QA-procedure should include verification of all parameters including the patient anatomy and positioning. In the procedure suggested here the patient geometry D. Georg et al. / Radiotherapy and Oncology 85 (2007) 306–315 307 is verified separately on an everyday basis. The MU verification tool will thus be used only prior to the treatment start, provided that the integrity of the database can be secured by other methods during the duration of the treatment period. The overall intention in modern radiation therapy is to keep the dose delivered to the patient as close as possible to the prescribed dose, i.e. to fulfil a tight dosimetric tolerance level. Such a dosimetric tolerance level should be based on clinical considerations related to radiobiological parameters. In order to secure that a treatment falls within this dosimetric tolerance level at a given probability an action level must be derived. This action level will then also be dependent on the accuracy of the verification tool. Hence, for a given dosimetric tolerance level and a known uncertainty in the verification, the action level that should be applied for that individual treatment plan can be estimated. Correctly applied, this concept will always result in an action level that is narrower than the dosimetric tolerance level. An MU software should therefore be designed to give a high degree of accuracy, including an estimation of the overall uncertainty in the dose calculation. The aim of the present work was to test and evaluate a fluence-based independent dose calculation (or MU) algorithm (for point dose calculations) against calculations from various commercial 3D treatment planning systems under clinical conditions. For that purpose clinically accepted and approved treatment plans were considered. For a subgroup, deviations were further analysed by measurements. The clinical applications covered open beams, wedged beams with both physical and dynamic wedges, and stepand-shoot as well as dynamic IMRT. The underlying model for independent calculations was implemented in such a way that only a few input data are needed for commissioning and tuning; thus the workload for basic algorithm input data acquisition and commissioning is reduced to a minimum. Furthermore, the risk for commissioning errors is largely reduced with a minimum number of input data. By analysing the resulting deviations between dose calculations performed with a TPS and MUV we have tried to assess the application of action levels for independent dose checks for advanced conformal radiotherapy. Methods and materials Independent dose calculation software MUV The semi-analytical model implemented in the software MUV, which stands for monitor unit verification, was based on a two step procedure: (i) calculation of the energy fluence per monitor unit exiting the treatment head, and (ii) calculation of the dose deposition in the phantom resulting from the incident energy fluence. The energy fluence part in the dose calculation takes into account the direct energy fluence, scattered radiation from an extra-focal source and scattered fluence from secondary collimators, as well as backscattered radiation to the monitor chamber [19,20]. The dose deposition was based on a pencil beam model, with a radial parameterisation [1,17,18]. For dose calculations, collimator transmission and the effect of rounded leaf ends
308 Clinical evaluation of fluence based MU software are taken into account. These geometries were modelled equally if they occurred in IMRT, or in asymmetric fields. The model and the software were developed with the intention of creating a dose per MU verification tool that requires a minimum of commissioned input data. For tuning the fluence model for open beams, only 10 measured output factors in air are recommended and the pencil beam model is solely based on the beam quality index TPR20,10 [17,19]. For physical wedges, the wedge angle needs to be specified by the user. In addition, the position of the wedge in the treatment head (i.e. distance from the focal source) needs to be given and two wedge factors need to be measured at 10 cm depth for two different field sizes, e.g. for 10 · 10 cm 2 and 20 · 20 cm 2 . From all this information a lateral wedge modulation matrix, a lateral wedge scatter matrix, and a lateral shift of beam quality are derived and used in the calculations. Another novel feature of the dose verification tool MUV is its uncertainty estimation. The uncertainty prediction was based on thorough testing of the individual components of the model against measurements under different treatment conditions for a variety of existing treatment machines. For example, for open beams the calculation model for the total energy fluence provided results within 1% (2 SD) [19] and for irregular MLC shaped beams the total output factor (OF total) had an average absolute deviation of 0.4% and a maximum deviation of 1.7% [20]. This was lower than the deviations found in a comparable investigation of a commercial treatment planning system [8]. The pencil beam model in MUV includes effects of lateral beam quality variations, which is not the case for most commercial TPSs according to the knowledge of the authors [18]. A more detailed description of the experimental benchmarking tests, which included head scatter calculations on and off axis in open fields, dose calculations on axis in irregular MLC shaped fields at different depths, and the influence of offaxis beam softening (including pencil beam corrections) can be found elsewhere e.g. [17–20]. The actual uncertainty estimation implemented in MUV was based on collimator/MLC settings and treatment depth. Uncertainty estimations are desirable from any dose calculation module because they can be helpful when defining action levels for clinical QA routine. However, the clinical implementation of action levels depending on a semi-analytical uncertainty analysis and subsequent automatic warning of the user requires further investigations and is beyond the scope of the present study. A DICOM interface allows extracting and importing of all dosimetric (e.g. energy, MU) and geometric (e.g. leaf and jaw positions) treatment data for an entire treatment plan instantaneously and automatically, which is a pre-requisite for an efficient QA process for more complex treatment techniques, such as conformal therapy utilizing an MLC or IMRT. The final result of the dose calculation in the MUV software is the dose to water in a pre-defined point of interest in a homogeneous phantom, for a composite treatment plan as well as for each individual beam. The depth of the dose specification (normalisation) point in a treatment plan that is automatically exported by a TPS is the geometric depth. If a simple inhomogeneity correction is desirable because of the actual treatment situation, the calculation depth can be modified, e.g. by using some sort of radiological path length correction with respect to the dose specification point. In cases where it was obvious that depth corrections were needed, the radiological depth was used for calculations with MUV. The radiological depth (or equivalent path length – EPL) was calculated manually by measuring distances and dimensions of bulk densities directly on the final treatment plan, or could be taken from the TPS if verified. If taken from the TPS, spot checks were made and the agreement between manual and computerized calculation was generally within 1–2 mm, which did not influence the final results. The point of interest in the patient can be modified in all directions in MUV by the user (anterior–posterior, lateral and longitudinal) so that finally multiple points can be verified if needed. Radiotherapy equipment The independent monitor unit verification software ‘‘MUV’’ was distributed to five well-established radiotherapy departments across Europe (Medical University Vienna, Medical University Graz, University Hospital Basel, University Hospital Copenhagen, and University Umea˚). Algorithm input data were measured for dedicated accelerators in various photon beams. For beam configuration in MUV, the treatment head configuration (i.e. distances and thickness of field defining elements, position of the physical wedge, etc.) could be selected from a pre-defined linac database which covers all state-of-the-art accelerator brands. The database was established taking vendor information into account and by measuring certain distances directly on the respective treatment machine. The centres and the respective equipment (treatment planning systems including applied algorithms and linear accelerators) used are listed in Table 1. All linear accelerators were equipped with an MLC and represent the most common types used clinically today. For verification of calculations in homogeneous conditions measurements were performed with ionisation chambers in water phantoms or solid verification phantoms. At the Medical University Vienna, measurements were carried out in a Solid Water TM phantom with a calibrated ionisation chamber (Nuclear Enterprises, type 2611A, Volume 0.3 cm 3 ) connected to a NE electrometer (Nuclear Enterprises, type 2620). In Basel a Farmer type chamber (volume 0.6 cm 3 PTW Freiburg, Germany) connected to PTW Unidos electrometer was used in a scanning water phantom, while in Graz a small volume ionisation chamber (PTW, type 31002, 0.125 cm 3 ) was used for verification measurements in water. The Copenhagen centre used the same type of chamber but placed in a Solid Water TM phantom. Clinical testing of MUV In each centre, MUV was evaluated as a QA tool in clinical routine, i.e. MUV calculations were compared with calculations performed by the local TPS. As typically done for such independent verification calculations, either a semi-infinite water phantom was assumed or the calculations were performed in a dedicated solid verification phantom. Note that for the involved TPS, only full 3D treatment plans based on CT information were considered in this study.
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