Comparison of dose statistics for bladder wall and rectum wall vs whole organs for VMAT prostate treatment.

Dose-wall histograms (DWHs) have been used as alternatives to dose-volume histograms (DVHs) for hollow organs, with the rationale that the dose delivered to the interior of a hollow organ would be unrelated to the level of radiation damage. The purpose of this study is to conduct a statistical comparison of dose statistics for both walled and solid structure contours for both bladder and rectum in the treatment of intermediate risk prostate cancer with volumetric arc therapy (VMAT). Ten intermediate risk prostate cases were randomly selected. Rectum and bladder were first contoured as solid structures, and then the corresponding wall structures were generated using either a slice-by-slice cropping (2D method), or with a full 3D cropping tool (3D method). Each case was then inverse planned using a 2-arc VMAT technique. Two plans per case were created, 1 with a hypofractionated treatment and 1 with a standard fractionated treatment. DVHs were calculated for solid structure contours, and DWHs were calculated for the walled structure contours generated using 2D and 3D contouring tools. A nonparametric Spearman statistic correlation test was used to compare a large number of relevant dose histogram points, and to establish the relationship between dose statistics for walled and solid structures. Several notable relationships were observed. Maximum rectal dose was strongly correlated between the solid structure and both the 2D-generated (Spearman's correlation rs = 0.988, p < 0.01) and 3D-generated (rs = 0.952 p< 0.01) wall structures. This indicates that the rectal hot spot occurred in or near the wall for all cases, suggesting that both structure types give similar maximum dose information for rectum. Maximum bladder dose was not significantly correlated between solid structures and the 2D (rs = 0.596, p= 0.069) and 3D-generated (rs = 0.681, p= 0.03) counterparts. This suggests that the maximum dose is not consistently in or near the bladder wall. This favors the use of bladder wall contours when considering bladder toxicity, with the maximum dose to the wall potentially being more relevant radiobiologically. This analysis was extended to many other relevant points on the rectum and bladder histogram curves. Where correlations are strong, equations of best-fit are presented. This work establishes several statistically-significant relationships between bladder and rectum DVHs and DWHs for VMAT irradiation of intermediate-risk prostate cancer. This information may be used to inform contouring requirements for clinical trial design as well as for standard patient care.

Edaphic races and phylogenetic taxa in the Lasthenia californica complex (Asteraceae: Heliantheae): an hypothesis of parallel evolution.

Lasthenia californica sensu Ornduff consists of two races that differ in their flavonoid pigments and edaphic tolerances. Recent phylogenetic studies of Lasthenia have revealed that members of L. californica sensu Ornduff belong to two phylogenetic species. The relationship of the edaphic races to these new species and to each other is the focus of this study. Characterization of flavonoid profiles and phylogenetic placement of 33 populations demonstrates that races and phylogenetic taxa are not concordant, suggesting that one or both edaphic races evolved in parallel in the two clades. We hypothesize an edaphically linked ecological role for flavonoid differences that first revealed the existence of two races.

PATTERNS OF MATING IN WILD SUNFLOWER HYBRID ZONES.

Theory predicts that homoploid hybrid speciation will be facilitated by selfing, yet most well-documented hybrid species are outcrossers. One possible explanation for this puzzle is that conditions in hybrid populations may favor selfing, even in otherwise outcrossing species. For example, in self-incompatible plants, mixtures of self and interspecific pollen often induce selfing. Here, we examine patterns of mating in three hybrid zones and four "pure" populations of Helianthus annuus and H. petiolaris, wild, self-incompatible sunflower species that are thought to have parented three homoploid hybrid species. Fourteen to 16 maternal families from each pure population and 44-46 maternal families from each hybrid zone were analyzed for seven polymorphic isozyme loci. Maximum-likelihood (ML) methods were used to estimate multilocus outcrossing rates (Tm ) and hybridization frequencies for each maternal family, each phenotypic group within each hybrid zone (annuus-like, hybrid, and petiolaris-like), and each population. As predicted for self-incompatible species, all four parental populations have outcrossing rate ML estimates of 1.0. Within the hybrid zones, outcrossing rates were lowest in the H. annuus-like fraction of the population (0.73, 0.72, and 0.74 in the three hybrid zones, respectively), largely intermediate in the H. petiolaris-like group (0.94, 0.90, and 0.94), and highest in the hybrid group (0.97, 0.93, and 0.97). Although outcrossing rates are lower in hybrid zones than in pure populations, it is unlikely that the observed decrease facilitates hybrid speciation because outcrossing rates in the critical hybrid fraction of the population do not differ significantly from 1.0. Dividing the outcrossed pollen pool into intraspecific and interspecific components revealed that maternal plants are largely fertilized by conspecific pollen, confirming an important role for pollen competition as a reproductive barrier. Highly sterile hybrid plants do not appear to discriminate between parental species pollen, but hybrids with higher fertility tend to be fertilized by pollen from the parental group they resemble genetically. Thus, gametic selection leads to substantial assortative mating in these hybrid zones.