VIRTUOS – Working modes

In image processing mode VIRTUOS supplies filters for importing image sequences according to the DICOM standard as well as filters for various proprietary formats of different manufacturers.

Registration between multi-modal data sets can be done with different methods:

• Based on automatic detection of stereotactic localizers
• Based on interactively defined landmarks
• Maximization of Mutual Information

After multi-modal data sets are acquired, they can be used for the delineation of therapy relevant structures. VIRTUOS supplies various manual and semi-automatic algorithms:

• Manual drawing (free-hand, polygon, circle and other) and editing functions
• Iso-contour finder, either on original image intensities or on displayed grey values
• Region and volume grower
• Model-based segmentation using atlas information and active contour models
• Random Walker

Before or during segmentation images can be enhanced by various filters (filter for noise reduction and contour enhancements as well as some morphological ones).
There are no restrictions concerning the complexity of delineated structures, of course VIRTUOS is able to deal with branching structures or hollow organs correctly.
Special tools are included e.g. auto-margin to create PTV from CTV and for example to creation and display of three-dimensional surface models of the delineated anatomy.

After the creation of a three-dimensional model of the individual patient’s anatomy radiotherapy plans can be defined and optimized in VIRTUOS.

To allow the easy and intuitive definition of treatment parameters VIRTUOS provide several 2D and 3D views on the patient data set:
The most important tool is the Beam’s Eye View, which presents the three-dimensional patient model from the position of the radiation source. This way a user can easily determine which structures are enclosed in the current beam and can adjust to shape of the beam to the target volume. Of course there are automatic functions for adjusting the beam shape to the tumor, too, which allow the specification of direction dependent security margins.
Another important tool is the Observer’s View which presents the same patient model. In that scene all defined beams are added, therefore this scene shows immediately that partial volume where all beams will intersect. That intersection volume gives a first estimation about that region which will receive necessary therapeutically dose level.
The Linac View (linear accelerator) shows the patient model in relation to the irradiation device. That view is useful to determine whether a desired irradiation direction can be realized or must be refused due to possible collisions of gantry, couch and/or patient.
In a fourth three-dimensional scene volume rendering methods can be applied to the image sequence to generate DRRs (digital reconstructed radiographs) or e.g. MIPs (maximum intensity projections). While the first option is useful to generate images which can be used to assure correct patient setup during therapy the second option can be used to identify and locate vessel malformations.
The Spherical View presents in a two-dimensional map a rough rating of the usefulness of all irradiation directions. Due to its interactivity promising directions can be selected easily.

Special attention was given to mechanisms which allow an easy and efficient definition of treatment strategies. By a hierarchical grouping of beams parameter settings in complex techniques can be applied either to single beams or entire groups. This hierarchical beamgroup concept and the possibility to define various templates for standard techniques can speed up plan generation markedly. By using this technology the definition of a 14 beam technique with irregular shaped beams can be realized by only 4-5 mouse clicks.

As supplement proven treatment techniques for particular locations can be stored in a database. This way it is possible to preserve and interchange therapeutical knowledge.

While there are already international standards concerning coordinate systems in radiotherapy, it is still common practice that linac manufacturers use their own ones. This can raise confusions especially if linacs of different manufacturers are used simultaneously in one department. VIRTUOS allows the description of device dependent coordinate systems and this way it can be tailored comprehensively to local conditions.

Besides high-level report functions (treatment documentation for patient record) VIRTUOS provide data export functionalities to communicate directly with linacs via LANTIS RTP Link protocol or DICOM RT.

At DKFZ there are a series of independent dose calculation algorithms, dedicated to different radiation types (photons, electrons, protons and other hadrons) or with varying precision/performance (pencil-beam techniques, superposition, monte-carlo methods). The development of those dedicated algorithms is task of working group E0401 physical models. For more information refer to the corresponding page. Since dose calculation algorithms are designed as independent processes different subselections can be easily combined with VIRTUOS.

After dose calculation was performed for a given plan, the pre-calculated dose distribution can be examined and evaluated in VIRTUOS with different qualitative and quantitative methods.

Dose information can be superimposed onto the two-dimensional slices, multi-planar reconstructions and arbitrary sections through the image cubes either in colourwash mode and/or as isodose lines. Dose information can be integrated into the Observer’s View as isodose ribbons or as surface dose display projected onto the defined anatomical structures. The qualitative evaluation of dose information in relation to the initially acquired image information is an essential necessity for the rating of a given plan. Furthermore quantitative tools are necessary. VIRTUOS provides dose-volume-histograms, dose statistics and the possibility of TCP/NTCP calculations. Dose-volume-histograms quantify which partial volume of a given structure is burdened with a given dose level. TCP (tumour control probability) and NTCP (normal tissue complication probability) gives estimation about the biological effects of the calculated dose distribution.

Since forward planning in radiotherapy is an iterative process (a plan is modified iteratively until a sufficient dose distribution can be achieved), features are necessary to compare different concurrent treatment plans and dose distributions. VIRTUOS allows the simultaneous evaluation of different dose distributions with the described qualitative tools as well as with quantitative ones. The can be subtracted from each other to show and emphasize small differences between two plan.

Even with the use of fixation systems the patients anatomical conditions may have changed compared to the time of planning. Patients often lose weight over the duration of therapy, whereby fixation is no longer firmly seated as at the beginning of therapy and the patient can move relative to the fixation system. So it is possible that the target point is no longer in the isocenter of the linear accelerator and therefore the beam is not pointed at the target point, which result in underdosage in the target volume and overdosage in healthy tissue.

Through imaging during treatment it can be determined whether the patient's position corresponds to the planned situation. If there are any discrepancies, they are detected automatically as possible and are converted automatically in table movements, so that the position of the patient can be corrected by adjusting the table position.