Heliospectra (Sweden)

Plants can quickly and dynamically respond to spectral and intensity variations of the incident light. These responses include activation of developmental processes, morphological changes, and photosynthetic acclimation that ensure optimal energy conversion and minimal photoinhibition. Plant adaptation and acclimation to environmental changes have been extensively studied, but many details surrounding these processes remain elusive. The photosystem II (PSII)-associated protein PSB33 plays a fundamental role in sustaining PSII as well as in the regulation of the light antenna in fluctuating light. We investigated how PSB33 knock-out Arabidopsis plants perform under different light qualities. psb33 plants displayed a reduction of 88% of total fresh weight compared to wild type plants when cultivated at the boundary of UV-A and blue light. The sensitivity towards UV-A light was associated with a lower abundance of PSII proteins, which reduces psb33 plants' capacity for photosynthesis. The UV-A phenotype was found to be linked to altered phytohormone status and changed thylakoid ultrastructure. Our results collectively show that PSB33 is involved in a UV-A light-mediated mechanism to maintain a functional PSII pool in the chloroplast.

Point kernels have been generated and applied for calculation of scatter dose distributions around monoenergetic point sources for photon energies ranging from 28 to 662 keV. Three different approaches for dose calculations have been compared: a single-kernel superposition method, a single-kernel superposition method where the point kernels are approximated as isotropic and a novel `successive-scattering' superposition method for improved modelling of the dose from multiply scattered photons. An extended version of the EGS4 Monte Carlo code was used for generating the kernels and for benchmarking the absorbed dose distributions calculated with the superposition methods. It is shown that dose calculation by superposition at and below 100 keV can be simplified by using isotropic point kernels. Compared to the assumption of full in-scattering made by algorithms currently in clinical use, the single-kernel superposition method improves dose calculations in a half-phantom consisting of air and water. Further improvements are obtained using the successive-scattering superposition method, which reduces the overestimates of dose close to the phantom surface usually associated with kernel superposition methods at brachytherapy photon energies. It is also shown that scatter dose point kernels can be parametrized to biexponential functions, making them suitable for use with an effective implementation of the collapsed cone superposition algorithm.

The suitability of high-Z materials as build-up caps for head-scatter measurements has been investigated. Build-up caps are often used to enable characterization of fields too small for a mini-phantom. We have studied lead and brass build-up caps with sufficiently large wall thicknesses, as compared to the range of contaminating electrons originating in the accelerator head, and compared them with build-up caps made of ionization chamber equivalent materials, i.e. graphite. The results were also compared with measurements taken using square and cylindrical polystyrene mini-phantoms. Field sizes ranging from up to were studied for nominal photon energies of 4, 6, 10 and 18 MV. The results show that the use of lead and brass build-up caps produces normalized head-scatter data slightly different from graphite build-up caps for large fields at high photon energies. At lower energies, however, no significant differences were found. The intercomparison between the two different plastic mini-phantoms and graphite caps showed no differences.

A dose calculation algorithm for brachytherapy is presented that reduces errors in absolute dose calculation and facilitates new techniques for modelling heterogeneity effects from tissues, internal shields and superficially positioned sources. The algorithm is based on Monte Carlo simulations for specific source and applicator combinations. The dose is scored separately, in absolute units, for the primary and different categories of scatter according to the photon scatter generation. Radial dose distributions for the primary dose and the total scatter dose are parametrized using functions based on simple one-dimensional transport theory. The fitted radial parameters are functions of the angle to the long axis of the source to account for the anisotropy of the dose distribution. The kerma in air at the reference point 1 m from the source is also simulated using Monte Carlo techniques and both the dose and kerma are normalized per source emitted radiant energy. The calculated kerma per radiant energy is used together with the measured reference air kerma rate and the ratio of the dose to the kerma to calibrate the calculated absolute dose rate. Data are presented for an cylindrical source, in combination with water, nylon and stainless steel applicators. Values of the radial dose profiles, specific dose rate constants and corrections to the air kerma for attenuation and scatter in air are calculated. Anisotropy functions for the source and a water-equivalent applicator are compared to published values. The effects of the applicator wall material on the radial dose distribution are also discussed.

Optimal tumor treatment is the aim of radiooncology. To achieve the maximal probability of uncomplicated cure for tumors even at complex anatomical sites — such as head and neck tumors — or at later stages of disease, all treatment modalities, all experience in radiooncology, radiobiology, and medical physics as well as all possibilities of diagnostic imaging and modern radiotherapy must be considered. New technical developments can help in the spatial visualization of the complex topographical situation, and all physical knowledge must be included to optimize and realize the spatial distribution of the radiation absorbed dose that is really needed.