B Van Duyse

Three-dimensional dosimetry using polymer gel and magnetic resonance imaging applied to the verification of conformal radiation therapy in head-and-neck cancer

BACKGROUND AND PURPOSE It was our aim to investigate NMR-based BANG gel dosimetry as a three-dimensional dosimetry technique in conformal radiotherapy. MATERIALS AND METHODS The BANG gel consisting of gelatin, water and co-monomers was first validated in a cylindrical glass flask for a single standard beam. Next, the gel contained in a human neck-shaped cast was used to verify a treatment plan for the conformal irradiation of a concave tumour in the lower neck. Magnetic resonance relaxation rate images were acquired and, based on an appropriate calibration of the gel, converted to absorbed dose distributions. The resulting maps were compared with dose distributions measured using radiographic film. RESULTS The gel-measured dose profiles of standard beams agreed within 3% (root mean square difference) with the profiles measured with high spatial resolution by a diamond detector. For the multi-beam conformal treatment, the difference map between gel-measured and film-measured dose distributions revealed a noise component and a more systematic deviation including structural or space-coherent patterns. The mean absolute value of the difference amounted to 8%. A number of possible causes for this deviation are designated. CONCLUSIONS Polymer gel dosimetry in combination with magnetic resonance imaging is a promising method for dosimetric verification of conformal radiotherapy.

Validation of MR-Based Polymer Gel Dosimetry as a Preclinical Three-Dimensional Verification Tool in Conformal Radiotherapy

The aim of this work was to investigate MR‐based polymer gel dosimetry as a three‐dimensional (3D) dosimetry technique in conformal radiotherapy. A cylindrical container filled with polymer gel was placed in a water‐filled torso phantom to verify a treatment plan for the conformal irradiation of a mediastinal tumor located near the esophagus. Magnetic resonance spin‐spin relaxation rate images were acquired and, after calibration, converted to absorbed dose distributions. The dose maps were compared with dose distributions measured using radiographic film. The average root‐mean‐square structural deviation, for the complete dose distribution, amounted to less than 3% between gel and film dose maps. It may be expected that MR gel dosimetry will become a valuable tool in the verification of 3D dose distributions. The influence of imaging artifacts arising from eddy currents, temperature drift during scanning, and B1 field inhomogeneity on the dose maps was taken into account and minimized. Magn Reson Med 43:116–125, 2000.

MCDE: a new Monte Carlo dose engine for IMRT

A new accurate Monte Carlo code for IMRT dose computations, MCDE (Monte Carlo dose engine), is introduced. MCDE is based on BEAMnrc/DOSXYZnrc and consequently the accurate EGSnrc electron transport. DOSXYZnrc is reprogrammed as a component module for BEAMnrc. In this way both codes are interconnected elegantly, while maintaining the BEAM structure and only minimal changes to BEAMnrc.mortran are necessary. The treatment head of the Elekta SLiplus linear accelerator is modelled in detail. CT grids consisting of up to 200 slices of 512 x 512 voxels can be introduced and up to 100 beams can be handled simultaneously. The beams and CT data are imported from the treatment planning system GRATIS via a DICOM interface. To enable the handling of up to 50 x 10(6) voxels the system was programmed in Fortran95 to enable dynamic memory management. All region-dependent arrays (dose, statistics, transport arrays) were redefined. A scoring grid was introduced and superimposed on the geometry grid, to be able to limit the number of scoring voxels. The whole system uses approximately 200 MB of RAM and runs on a PC cluster consisting of 38 1.0 GHz processors. A set of in-house made scripts handle the parallellization and the centralization of the Monte Carlo calculations on a server. As an illustration of MCDE, a clinical example is discussed and compared with collapsed cone convolution calculations. At present, the system is still rather slow and is intended to be a tool for reliable verification of IMRT treatment planning in the case of the presence of tissue inhomogeneities such as air cavities.

The importance of accurate linear accelerator head modelling for IMRT Monte Carlo calculations

Two Monte Carlo dose engines for radiotherapy treatment planning, namely a beta release of Peregrine and MCDE (Monte Carlo dose engine), were compared with Helax-TMS (collapsed cone superposition convolution) for a head and neck patient for the Elekta SLi plus linear accelerator. Deviations between the beta release of Peregrine and MCDE up to 10% were obtained in the dose volume histogram of the optical chiasm. It was illustrated that the differences are not caused by the particle transport in the patient, but by the modelling of the Elekta SLi plus accelerator head and more specifically the multileaf collimator (MLC). In MCDE two MLC modules (MLCQ and MLCE) were introduced to study the influence of the tongue-and-groove geometry, leaf bank tilt and leakage on the actual dose volume histograms. Differences in integral dose in the optical chiasm up to 3% between the two modules have been obtained. For single small offset beams though the FWHM of lateral profiles obtained with MLCE can differ by more than 1.5 mm from profiles obtained with MLCQ. Therefore, and because the recent version of MLCE is as fast as MLCQ, we advise to use MLCE for modelling the Elekta MLC. Nevertheless there still remains a large difference (up to 10%) between Peregrine and MCDE. By studying small offset beams we have shown that the profiles obtained with Peregrine are shifted, too wide and too flat compared with MCDE and phantom measurements. The overestimated integral doses for small beam segments explain the deviations observed in the dose volume histograms. The Helax-TMS results are in better agreement with MCDE, although deviations exceeding 5% have been observed in the optical chiasm. Monte Carlo dose deviations of more than 10% as found with Peregrine are unacceptable as an influence on the clinical outcome is possible and as the purpose of Monte Carlo treatment planning is to obtain an accuracy of 2%. We would like to emphasize that only the Elekta MLC has been tested in this work, so it is certainly possible that alpha releases of Peregrine provide more accurate results for other accelerators.

An isocenter position verification device for electronic portal imaging: physical and dosimetrical characteristics.

The physical and dosimetrical characteristics of a device, designed to visualize the isocenter position on electronic portal images, were examined. The device, to be mounted on the gantry head of the accelerator, containing five spheric lead markers, was designed in order to visualize the isocenter position on portal images. A quality control device was designed to check the reliability of this technique. The disturbance of the dose distribution by the markers was studied with gel dosimetry. The use of markers resulted in a precise and accurate method to visualize the isocenter on portal images. A maximum underdosage of 11%, due to attenuation by the markers, was observed. The use of markers to visualize the isocenter position on portal images, is a fast and reliable method when analyzing patient setup errors with online electronic portal imaging.

General Anatomy Based Segmentation for Intensity Modulated Radiotherapy of Head and Neck Cancer

In head and neck cancer, intensity modulated radiotherapy (IMRT) allows to generate concave dose distributions to match the shape of the planning target volume (PTV). In our department, IMRT is delivered by multiple static convergent beams, that consist of superposition of unmodulated segments. The segment shapes were obtained from the beam’s eye view (BEV) projection of PTV(s) and organs at risk (OARs). By making the shape of segments also a function of the patient’s outlines (skin surface), a substantial refinement was obtained.

207 Extension of Sherouse's GRATIS™ three-dimensional treatment planning system for use with a multileaf collimator

In February 1995, a linear accelerator (Philips SL25) was retrofitted with a Philips multileaf collimator (MLC). MLC reduces workload (decreases the numbers of cerrobend blocks) and increases throughput (avoids tray placements). However, programming the leaf settings is a labur intensive procedure on the Philips MLC, as settings have to be derived manually from simulator films. We use the GRATIS 3D planning system by George W. Sherouse and have written an extension to plan the leaf settings in beams eye view mode. From a tool implemented in GRATIS’ last release, which draws a conformal block around the target with a predefined margin, we developed a feature for automatic or interactive leaf setting. After virtual simulation, a file with the leaf settings is transferred to the MLC computer over a local network. The advantages of this system are efficiency, automatization and elimination of transfer errors. Within the philosophy of GRATIS, we will make our tool freely available to the GRATIS users.