Condensed-history Monte-Carlo simulation for charged particles: what can it do for us?

Condensed-history (CH) Monte-Carlo (MC) groups together the vast number of individual charged-particle collisions using multiple scattering theory for elastic angular changes and stopping power for energy losses. CH codes such as EGS4 have been enormously successful in simulating the transport of electrons, for example, in radiotherapy. MC-derived values of the water-to-air stopping-power ratio, s _w/air, are used in all modern codes of practice for absolute dose determination in radiotherapy clinics. MC can also directly yield the dose ratio D _med/D _det for a dosimeter in a medium, and Correlated Sampling has been exploited to increase the efficiency, e. g., the central electrode in an ion chamber (aluminium vs. graphite). The extremely low density of the gas in an ion chamber poses problems for CH codes. However, multiple scattering can now be combined with single scattering and is expected to finally resolve important chamber perturbation effects. An exciting application of CH MC in radiotherapy is the computation of dose distributions in patients. Currently one can achieve an uncertainty around 1% (1 SD) in mm-sized voxels in several minutes for an electron beam and in around an hour for a photon treatment plan on hardware costing less than $20,000, and thus avoid all the various approximations conventionally used to account for inhomogeneities. In the microdosimetry/track structure field, CH codes have shown that the fluence (dΦ/dE) per unit dose at low electron energies is virtually independent of incident particle energy or depth, which simply explains the negligible RBE variation. Received: 1 April 1999 / Accepted in revised form: 1 July 1999     

Influence of light intensity and selection scheme on regeneration time of transgenic flax plants

This study aimed at establishing a protocol to increase the number of regenerated shoots and to limit the recovery of “escapes” during the regeneration of transgenic flax plants (cv Barbara). Here, we describe how light, adapted media and selection scheme could stimulate the transformation process, the organogenic potentiality of calli (by a factor of 3.2) and accelerate the transgenic shoot regeneration (by a factor of about 2). On comparison of the transformation rate observed while using low light (LL) and high light (HL) a considerable enhancement from 0.12 to 5.7% was evident. The promotive effect of light might also had a direct beneficial effect on transgenic plant production time leading to a reduction of more than 4 months in the time need to obtain transgenic seeds. All data indicate that HL plays a role on growth and on protein, rubisco and pigment contents by stimulating the gene implicated in photosynthetic and Calvin cycle processes.     

Changes in the neurosecretory cells of the freshwater snail,lymnaea acuminata induced by urea administration

Data on biomass, production and metabolism of the biota, gathered during many years in the Rybinsk Reservoir, USSR, were used to calculate functional interconnections between separate components of the ecosystem, neglecting fluctuations of biomass and postulating a stationary state of the system.A matrix of interconnections was constructed and the influence of an increase or decrease of the biomass of one element upon all other elements of the ecosystem was calculated and discussed.