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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. © 2000 Wiley‐Liss, Inc.

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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.