Chromium stable isotope ratios show promise as indicators of Cr(VI) reduction in groundwater, but no published study has yet demonstrated that expected relationships between (53)Cr/(52)Cr and Cr(VI) concentration, position, and time occur in an actual groundwater plume. We present an extensive data set from a point-source plume in Berkeley, CA; data extend over 5 years and 14 locations covering the entire plume. We interpret the data using a Rayleigh distillation model with an effective fractionation factor that incorporates an intrinsic fractionation factor determined from incubations of site sediments and accounts for reservoir effects in the restricted subsurface zones where Cr(VI) reduction is thought to occur. The groundwater (53)Cr/(52)Cr and Cr(VI) concentration data are consistent with a scenario where the system has reached a steady state: Cr(VI) reduction continues, the extent of reduction at any point is constant over time, reduction proceeds to completion at the downgradient edge of the plume, and the plume is no longer advancing. The overall consistency of the results with a reasonable model for the site supports the use of Cr isotope-based estimates of reduction, but we discuss current uncertainties and limitations of the approach as well.

Arsenic released by bottom sediments was determined by experiments in which the sediments were artificially re-suspended using a particle entrainment simulator (PES) to simulate river conditions. Sediment cores were collected from various tributaries to drinking water reservoirs in Connecticut spiked with arsenic, and run in the PES at simulated bed-flow shear stresses from 0.0 to 0.6 N/m(2). Under equilibrium conditions, the dissolved fraction of arsenic was found to range from 8.3 to 22.1 microg/l, which in most cases exceeded EPA Maximum Contaminant Level (MCL) of 10 microg/l. Experimental results from these simulations have shown that bed-flow shear stress causes an increased concentration of dissolved arsenic, most notably at shear stresses of 0.4, 0.5, and 0.6 N/m(2). For the solid phase under equilibrium, the concentrations of arsenic ranged between 71 and 275 mg/kg. The average concentration of arsenic on the solid phase as well as partitioning coefficient values (K (p)) were highest at initial shear stress. This was attributed to the higher fraction of colloidal material and finer organic particles in the suspended solid mixture. Particles of such nature proved to have higher affinity to arsenic. K (p) values were determined from PES data and were found to range from 4,687 to 24,090 l/kg. However, on a mass load basis, the amount of arsenic found in suspended sediment increased with the increase of shear stress. Similarly, the amount of arsenic in the solid phase increased significantly for sites with high Volatile Organic Carbon (VOC) content. Because of the influence of Total Suspended Solids (TSS) and VOC concentrations on K (p), the use of the PES is more appropriate in obtaining K (p) values that would be found under real stream conditions when compared to the traditional way of measuring K (p) using a jar study technique.

The inclusion of organ deformation and movement in radiosurgery treatment planning is of increasing importance as research and clinical applications begin to take into consideration the effects of physiological processes, like breathing, on the shape and position of lesions. In this scenario, the challenge is to localize the target in toto (not only by means of marker sampling) and to calculate the dose distribution as the sum of all the contributions from the positions assumed by the target during the respiratory cycle. The aim of this work is to investigate the use of nonrigid registration for target tracking and dynamic treatment planning, i.e., treatment planning based not on one single CT scan but on multiple CT scans representative of the respiration. Twenty patients were CT scanned at end-inhale and end-exhale. An expert radiation oncologist identified the PTV in both examinations. The two CT data sets per patient were nonrigidly registered using a free-form deformation algorithm based on B-splines. The optimized objective function consisted of a weighted sum of a similarity criterion (Mutual Information) and a regularization factor which constrains the transformation to be locally rigid. Once the transformation was obtained and the registration validated, its parameters were applied to the target only. Finally, the deformed target was compared to the PTV delineated by the radiation oncologist in the other study. The results of this procedure show an agreement between the center of mass as well as volume of the target identified automatically by deformable registration and manually by the radiation oncologist. Moreover, obtained displacements were in agreement with body structure constraints and considerations usually accepted in radiation therapy practice. No significant influence of initial target volume on displacements was found. In conclusion, the proposed method seems to offer the possibility of using nonrigid registrations in radiosurgery treatment planning, even if more cases need to be investigated in order to give a statistical consistency to parameter setup and proposed considerations.

Purpose: To propose dosimetric guidelines specifically designed for the Cyberknife radiosurgery system. Non‐availability of 10×10cm2 field and use of small circular collimators (5mm to 60mm) pose serious problems, that have been faced in this study by means of 8 different detectors and Monte Carlo simulation. This work is oriented to measurement of total scatter factors (Sc,p) and to reference dosimetry, though indications will also be given in view of a comprehensive guideline. Method and Materials: PTW PinPoint 31014, Exradin A16 and T14P microchambers, TN 502RDM micromosfet, PTW 30008 diode and TM60003 diamond, MD55 and EBT radiochromic films were used to measure Sc,p. Monte Carlo simulations (BEAMnrc) were used to produce phase space descriptions at the exit plane of each collimator, to calculate: 1) theoretical Sc,p values in water, and 2) correction factors to be applied to Sc,p as measured by 5 detectors (PinPoint, A16, T14P, diode, diamond), obtained by simulating shape and chemical composition of each detector. BEAMnrc was also used to calculate stopping power ratios and chamber correction factors for the Cyberknife linac, to decide whether values of kQ from the IAEA398 protocol could be applied without using a 10×10cm2 field. Results: Sc,p of the 5mm collimator as measured by simulated detectors averaged 0.653 − 9%+14%. Variation for larger collimators was smaller. After Monte Carlo correction, Sc,p of the 5mm collimator became 0.686 −2%+1%. Pure Monte Carlo calculation gave Sc,p=0.715 +/−1%. Calculation of correction factors showed that kQ values for the investigated chambers could be chosen when using IAEA398, introducing +/−0.2% uncertainty. Conclusion: Pure Monte Carlo calculation gave higher values of Sc,p compared to Monte Carlo‐corrected measurement. The latter is to be preferred because correction factors are less sensitive to beam parameters than pure calculation of Sc,p. For determination of Sc,p use of microchambers and Monte Carlo correction is recommended.