The radiation therapy specific Voxel Monte Carlo (VMC+ +) dose calculation algorithm achieves a dramatic improvement in MC dose calculation efficiency for radiation therapy treatment planning dose evaluation compared with other MC algorithms. This work aims to validate VMC+ + for radiation therapy photon beam planning. VMC++ was validated with respect to the well-benchmarked EGS-based DOSXYZnrc by comparing depth dose and lateral profiles for field sizes ranging from 1 X 1 to 40 x 40 cm(2) for 6 and 18 MV beams in a homogeneous water phantom and in a simulated bone-lung-bone phantom. Patient treatment plan dose distributions were compared for five prostate plans and five head-and-neck (H/N) plans, all using intensity-modulated radiotherapy beams. For all tests, the same incident particles were used in both codes to isolate differences due to modeling of the radiation source. Voxel-by-voxel observed differences were analyzed to distinguish between systematic and purely statistical differences. Dose-volume-histogram-derived dose indices were compared for the patient plans. For the homogeneous water phantom and the bone-lung-bone phantom, the depth dose curve predicted by VMC+ + agreed with that predicted by DOSXYZnrc within expected statistical uncertainty in all voxels except the surface voxel of the water phantom, where VMC+ + predicted a lower dose. When the electron cutoff parameter was decreased for both codes, the surface voxel agreed within expected statistical uncertainty. For prostate plans, the most severe difference between the codes resulted in 55% of the voxels showing a systematic difference of 0.32% of maximum dose. For H/N plans, the largest difference observed resulted in 2% of the voxels showing a systematic difference of 0.98% of maximum dose. For the prostate plans, the most severe difference in the planning target volume D95 was 0.4%, the rectum D35 was 0.2%, the rectum DI7 was 0.2%, the bladder D50 was 0.3% and the bladder D25 was 0.3%. For the H/N plans, the most severe difference in the gross tumor volume D98 was 0.4%, the clinical target volume D90 was 0.2%, the nodes D90 was 0.2%, the parotids D95 was 0.8%, and the cord D2 was 0.8%. All of these differences are clinically insignificant. VMC++ showed an average efficiency gain over DOSXYZnrc of at least an order of magnitude without introducing significant systematic bias. VMC + + can be used for photon beam MC patient dose computations without a clinically significant loss in accuracy.

The aim of this study is to present an efficient method to generate imager-specific Monte Carlo ͑MC͒-based dose kernels for amorphous silicon-based electronic portal image device dose prediction and determine the effective backscattering thicknesses for such imagers. EPID field sizedependent responses were measured for five matched Varian accelerators from three institutions with 6 MV beams at the source to detector distance ͑SDD͒ of 105 cm. For two imagers, measurements were made with and without the imager mounted on the robotic supporting arm. Monoenergetic energy deposition kernels with 0-2.5 cm of water backscattering thicknesses were simultaneously computed by MC to a high precision. For each imager, the backscattering thickness required to match measured field size responses was determined. The monoenergetic kernel method was validated by comparing measured and predicted field size responses at 150 cm SDD, 10ϫ 10 cm 2 multileaf collimator ͑MLC͒ sliding window fields created with 5, 10, 20, and 50 mm gaps, and a head-and-neck ͑H&N͒ intensity modulated radiation therapy ͑IMRT͒ patient field. Field size responses for the five different imagers deviated by up to 1.3%. When imagers were removed from the robotic arms, response deviations were reduced to 0.2%. All imager field size responses were captured by using between 1.0 and 1.6 cm backscatter. The predicted field size responses by the imager-specific kernels matched measurements for all involved imagers with the maximal deviation of 0.34%. The maximal deviation between the predicted and measured field size responses at 150 cm SDD is 0.39%. The maximal deviation between the predicted and measured MLC sliding window fields is 0.39%. For the patient field, gamma analysis yielded that 99.0% of the pixels have ␥ Ͻ 1 by the 2%, 2 mm criteria with a 3% dose threshold. Tunable imager-specific kernels can be generated rapidly and accurately in a single MC simulation. The resultant kernels are imager position independent and are able to predict fields with varied incident energy spectra and a H&N IMRT patient field. The proposed adaptive EPID dose kernel method provides the necessary infrastructure to build reliable and accurate portal dosimetry systems.

Complexes of Aromatic Amine Thiolates the sum of van der Waals radii (3.78 Á).11,12 No other I-S distances of any type appear to have been reported. Burow's normal-coordinate analysis for IS02-assumed a much shorter I-S distance of 2.5 Á.8 No unusual features were evident in the triphenylbenzyl cation or the crystal packing. ConclusionsThe use of large organo cations allows convenient synthesis of a series of well-defined, easily handled iodosulfinate complexes. The complexes have S02 dissociation pressures in the range 0.5-17 Torr at ambient temperature and S02 dissociation pressures in excess of 1 atm at 100 °C and higher. The geometry of the IS02" anion has been shown by x-ray diffraction to be pyramidal with an I-S distance of 3.251 (3) A.Acknowledgment. Stimulating interaction and assistance by Dr. R. A. Penneman of this laboratory has been invaluable throughout the course of our S02 project (see ref 12-16). We gratefully acknowledge this help. The assistance of Dr. D. T. Cromer of this laboratory in the x-ray data collection is also thankfully acknowledged. This work was performed under the auspices of the U.S.

Purpose: This work uses repeat images of intensity modulated radiation therapy (IMRT) fields to quantify fluence anomalies (i.e., delivery errors) that can be reliably detected in electronic portal images used for IMRT pretreatment quality assurance. Methods: Repeat images of 11 clinical IMRT fields are acquired on a Varian Trilogy linear accelerator at energies of 6 MV and 18 MV. Acquired images are corrected for output variations and registered to minimize the impact of linear accelerator and electronic portal imaging device (EPID) positioning deviations. Detection studies are performed in which rectangular anomalies of various sizes are inserted into the images. The performance of detection strategies based on pixel intensity deviations (PIDs) and gamma indices is evaluated using receiver operating characteristic analysis. Results: Residual differences between registered images are due to interfraction positional deviations of jaws and multileaf collimator leaves, plus imager noise. Positional deviations produce large intensity differences that degrade anomaly detection. Gradient effects are suppressed in PIDs using gradient scaling. Background noise is suppressed using median filtering. In the majority of images, PID-based detection strategies can reliably detect fluence anomalies of ≥5% in ∼1 mm 2 areas and ≥2% in ∼20 mm 2 areas. Conclusions: The ability to detect small dose differences (≤2%) depends strongly on the level of background noise. This in turn depends on the accuracy of image registration, the quality of the reference image, and field properties. The longer term aim of this work is to develop accurate and reliable methods of detecting IMRT delivery errors and variations. The ability to resolve small anomalies will allow the accuracy of advanced treatment techniques, such as image guided, adaptive, and arc therapies, to be quantified.

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