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Purpose: CT images are used for patient specific Monte Carlo treatment planning in radionuclide therapy. The authors investigated the impact of tissue classification, CT image segmentation, and CT errors on Monte Carlo calculated absorbed dose estimates in nuclear medicine. Methods: CT errors as a function of patient size, CT reconstruction, and tube current modulation methods were assessed in a phantom experiment on a clinical CT system. The impact of tissue segmentation methods and CT number variations on EGSnrc Monte Carlo calculated absorbed dose distributions was assessed for 99mTc and 131I in the ICRP/ICRU male phantom and in a patient PET/CT-scanned with 124I prior to radioiodine therapy. Results: CT number variations <20 HU were obtained for whole-body CT examinations at effective CT doses ~2 mSv. Monte Carlo calculated absorbed doses depended on both the number of media types and accurate calibration of the CT number-to-density conversion ramp. Tissue segmentation by a 13-tissue CT conversion ramp, calibrated by a stoichiometric method, resulted in low (<4%) dose errors in selected organs for both isotopes. Conclusions: A calibrated CT scanner specific conversion ramp is required for accurate patient specific dosimetry in nuclear medicine. Accurate dosimetry was obtained with a 13-tissue ramp that included five different bone types. [ABSTRACT FROM AUTHOR]

(1) Demographic patterns are described for two sets of patches of Clintonia borealis, a perennial herb, each studied over a 3-year period. Rates of mortality, clonal growth and net ramet population growth are reported for all sixteen studied patches individually and as two sets of pooled data. (2) Overall ramet mortality rates were size-dependent and low (3.9-6.l%). Rates of clonal growth were low to moderate (5.0-20-6%). Most ramets replaced themselves each year with similar-sized ramets. Although flowering and fruiting occurred, no successful recruitment of new plants from seed was observed. (3) More biomass was allocated to flowers and fruits than to clonal growth. Clonal growth was not reduced by flowering nor vice versa. (4) The environments of individual patches varied widely in terms of light and soil moisture, and there were some differences in the rates of ramet mortality and clonal growth. However, al! patches had annual rates of increase in ramet number of between 2 and 20%. (5) Excavations of entire rhizome systems of small patches showed that such patches often consisted of only one or two genets. Patches of only 2-12 ramets typically lad ages of 20 years or more. Larger patches may be considerably older and composed of only one or a few genets. (6) Established genets have long lives, little selection occurs in established patches, and most genets must have broad ecological tolerances. [ABSTRACT FROM AUTHOR]