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A.T. Reutov home address: kv. 25, korp.2, dom.25, Marshal Jukov prosp., ABSTRACT A laser structure consisting of several coupled injection lasers with saturable absorber, running in a hard mode of turn on, is discussed as optical model of a neuron. The essential properties of two principal neuron components, axon and soma, may be simulated by such a structure. The vertical cavity surface emitting lasers with two parallel p-n junctions are considered as the most convenient active elements for this structure.

Output normalized dose profiles for asymmetric open photon fields have been calculated using a commercial treatment planning system (TPS) based on a dose-to-energyfluence concept. The model does not require any additional measurements for off-axis fields. Calculations are compared with measurements for quadratic fields of 5 cm × 5 cm up to 20 cm × 20 cm, with their geometric field centre positioned 10 cm off-axis in the in-plane direction. The measurements include depth doses and profiles in-plane as well as cross-plane for nominal photon energies of 4, 6 and 18 MV x-rays. Both calculated and measured doses are normalized with respect to a 10 cm × 10 cm reference field, therefore making it possible to compare not only the relative distributions but also the absolute dose levels; that is, calculation of monitor units is included. The calculated depth-dose curves are generally in good agreement with measured data with an accuracy at the absolute dose level of 2% at depths beyond the dose maximum. The cross-plane profiles are calculated with an accuracy better than 3% within the field. The 'tilt' towards the collimator central axis of the in-plane profiles is predicted by the model, but is somewhat overestimated at large depths. The system provides the possibility to separate the primary and scattered parts of the dose and the cause of this tilting was studied by comparing calculated phantom-scattering and head-scattering dose profiles for a symmetric 40 cm × 20 cm field to dose profiles for an asymmetric 20 cm × 20 cm field. The tilting is shown to originate from a change both in phantom scattering and in head scattering compared to the case of symmetrical fields. The results indicate that the investigated TPS can calculate dose distributions in open asymmetric fields with a high degree of accuracy, typically better than 2-3%.

An injection laser logic gate with longitudinal propagation of an information signal and coding of logic variables using two different frequencies of laser generation is considered.