J. Salk, M. Kosta, P. Blank, E.M. Röttinger

Department of Radiation Oncology, University of Ulm, Germany

IDASAn Extensible Framework for

IMRT Verification

INTRODUCTION:

Patient specific dosimetric verification is evidently an essential part in clinical implementation of IMRT. However, due to the inherent complexity of IMRT, the verification procedure is more demanding and time consuming as compared to conventional conformal therapy. Furthermore, data processing and storage becomes an important aspect for IMRT verification since the volume of data to be handled is much larger than for conventional treatments. Therefore, our aim was to develop an analysis tool kit, that provides a framework for automatizing common tasks in IMRT verification, thus optimizing speed.

METHODS AND MATERIAL:

Dosimetric verification in IMRT typically consists of comparative evaluation of calculated (predicted) and measured (delivered) planar dose distributions. For this purpose we have utilized the data storage and processing package HBOOK in conjunction with the Physics Analysis Workstation (PAW) system, developed at the European Organization for Nuclear Research (CERN) in Ge-neva/Switzerland. These software packages are part of the CERNLIB program library, which is offered under the terms of the GNU General Public License (GPL) for a large number of platforms.

We have customized PAW for IMRT verification by adding an intuitive graphical user interface and a macro package allowing automatization of common repetitive tasks (fig. 1). This "IMRT Dose Analysis System (IDAS)" is a suite of scripts and C code for computationally-intensive tasks, but the core of the system is itself written in the macro dialect of PAW, which makes it easy for users to add additional functionality by defining new functions.

Measured and calculated dose distributions can be visualized by isodose lines, colored two-dimensional maps or three-dimensional surface plots. Isodose overlays, dose difference and gamma index maps can be generated from the data. Horizontal and vertical line profiles of these quantities can be captured interactively through any specified point of the dose distribution, the dose-difference map and the map of the gamma index. Statistical analysis is available in terms of dose-difference and gamma-volume histograms.

The gamma index introduced by Daniel Low et al. is the minimum multidimensional distance between the measure-ment and calculation points in a space composed of dose and physical distance coordinates, scaled by preselected tolerance limits for dose difference and distance to agreement (DTA). Regions where the gamma index exceeds a value of unity correspond to locations where the calculation does not meet the given criteria.

IMRT plans have been generated by the Varian/Dosetek CadPlan/Helios treatment planning system (TPS). The TPS has an option for recalculating patient specific IMRT fields in a phantom for verification.

The measurements were carried out at a Varian Clinac 2300C/D equipped with an 80 Leaf Dynamic MLC. Film dosimetry was used to obtain two-dimensional dose distri-butions corresponding to the calculated verification plans. Films were digitized with a Vidar VXR-12 scanner. Calibration and optical density to dose conversion was performed using the commercial Scanditronix/Wellhöfer OmniPro-Accept software.

The IDAS system provides import filters for reading dose matrices from CadPlan plan storage files and from OmniPro-Accept exported multiprofile data. Import filters for other data sources (e.g. EPID) can be implemented as additional modules.

All sample analysis and graphical presentations shown here were generated solely by using the built-in modules of the IDAS software on a standard 900 MHz Intel Pentium III personal computer running GNU/Linux.

DISCUSSION:

The IDAS software is currently being used routinely for IMRT verification at the Department of Radiation Oncology, University of Ulm, and has proven to be a valuable and robust framework for comparative quantitative analysis of two-dimensional dose distributions in a variety of clinical cases. One of the strengths of this software is the ease with which the user can extend the functionality and automatize common repetitive tasks by adding new macros (e.g. for batch processing). This feature makes the IDAS system particularly suitable for large scale IMRT analysis tasks.

The software is still (and hopefully will always be) work-in-progress. Current version snapshots are available under the terms of the GPL upon request to the corresponding author: <juergen.salk@medizin.uni-ulm.de>.

Fig. 2: Single field IMRT verification for a head & neck treatment. Calculated (a) and measured data (b) is presented as two-dimensional color map. Computed dose differences (c) and gamma index (d) are shown as well. Statistical analysis are displayed in terms of differential dose-difference (e) and cumulative gamma-volume-histograms (f). Tolerance limits of 3.0 % in dose difference and 3 mm in distance-to-agreement were chosen for gamma evaluation.

Fig. 4: Shaded surface plots of calculated (a) and measured (b) dose distribution. Line profiles of calculated (black line) and measured (red line) doses along (c) and across (d) the leaf trajectories. Corresponding gamma index values (green) are also included. The effects of interleaf-leakage are clearly visible as ridges in measured but not in calculated data.

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RESULTS:

In figure 2 sample analysis of a single IMRT beam are shown. Dose distributions, dose-differences and gamma index values are presented as color maps varying from blue to red representing lower and higher levels (fig. 2 (a)-(d)). Computed differential dose-difference histograms (DDH) and cumu-lative gamma-volume histograms (GVH) are shown in figures 2 (e) and 2 (f). A threshold filter has been applied to prevent peripheral dose regions from contributing to the DDH.

For relative dose distributions the gamma formalism entails strong dependence from the normalization point position. In order to minimize the impact of normalization on gamma evaluation, doses has been rescaled to center the DDH peak at zero. Hence, DDH turned out to be an important prerequisite for meaningful and unambiguous gamma analysis.

An example of a composite IMRT plan analysis is shown as isodose overlay in figure 3. Composite IMRT plan measure-ments in axial plane are largely affected only by the leaf pair corresponding to this plane and the adjacent leaf pairs. Hence, we have established individual beam measurements in Beam´s Eye View plane as the preferred method for routine IMRT dosimetry, because it provides verification for all leaf pairs by means of a single measurement for each beam.

Significant discrepancies observed so far can mostly be traced back to interleaf effects (inter- versus intraleaf transmission, tongue and groove), which are not modelled by the TPS (fig. 4). Although these effects may not be of clinical relevance, they can be obstructive in dosimetric verification, especially for single beam verification, which is more sensitive to small deviations than composite dose verification.

Fig. 3: Example of composite 5 field IMRT prostate plan (SMART technique) analysis. Calculated (a) and measured (b) doses are displayed individually by color maps in axial slice view, isodose overlay (c) with solid and dashed lines representing measured and calculated doses, respectively.

Fig. 1: Screenshot of the IDAS software running on a Linux desktop computer. The graphics window and the main control window are shown. Calculated dose distribution is re-presented by colored surface plot. Some optional dialog menus are displayed as well. A progress meter indicates a gamma analysis running in the background.

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