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Purpose: This paper presents initial experimental results from a prototype of high dose rate (HDR) BrachyView, a novel in-body source tracking system for HDR brachytherapy based on a multipinhole tungsten collimator and a high resolution pixellated silicon detector array. The probe and its associated position estimation algorithms are validated and a comprehensive evaluation of the accuracy of its position estimation capabilities is presented. Methods: The HDR brachytherapy source is moved through a sequence of positions in a prostate phantom, for various displacements in x, y, and z. For each position, multiple image acquisitions are performed, and source positions are reconstructed. Error estimates in each dimension are calculated at each source position and combined to calculate overall positioning errors. Gafchromic film is used to validate the accuracy of source placement within the phantom. Results: More than 90% of evaluated source positions were estimated with an error of less than one millimeter, with the worst-case error being 1.3 mm. Experimental results were in close agreement with previously published Monte Carlo simulation results. Conclusions: The prototype of HDR BrachyView demonstrates a satisfactory level of accuracy in its source position estimation, and additional improvements are achievable with further refinement of HDR BrachyView’s image processingmore » algorithms.« less

This paper presents initial experimental results from a prototype of high dose rate (HDR) BrachyView, a novel in-body source tracking system for HDR brachytherapy based on a multipinhole tungsten collimator and a high resolution pixellated silicon detector array. The probe and its associated position estimation algorithms are validated and a comprehensive evaluation of the accuracy of its position estimation capabilities is presented.The HDR brachytherapy source is moved through a sequence of positions in a prostate phantom, for various displacements in x, y, and z. For each position, multiple image acquisitions are performed, and source positions are reconstructed. Error estimates in each dimension are calculated at each source position and combined to calculate overall positioning errors. Gafchromic film is used to validate the accuracy of source placement within the phantom.More than 90% of evaluated source positions were estimated with an error of less than one millimeter, with the worst-case error being 1.3 mm. Experimental results were in close agreement with previously published Monte Carlo simulation results.The prototype of HDR BrachyView demonstrates a satisfactory level of accuracy in its source position estimation, and additional improvements are achievable with further refinement of HDR BrachyView's image processing algorithms.

Treatment of ocular cancers using eye plaque brachytherapy is now an established medical procedure. However, current QA for these eye plaques is quite rudimentary, limiting the opportunities for precise pre-tumour plaque customisation. This paper proposes and experimentally validates a new technique for imaging of eye plaque dose distributions using a high-resolution pixelated silicon detector. Results are presented demonstrating the 2D and 3D isodose surfaces produced using experimental data collected using this method. © 2010 Elsevier B.V. All rights reserved.

© 2014 Institute of Physics and Engineering in Medicine. HDR BrachyView is a novel in-body dosimetric imaging system for real-time monitoring and verification of the source position in high dose rate (HDR) prostate brachytherapy treatment. It is based on a high-resolution pixelated detector array with a semi-cylindrical multi-pinhole tungsten collimator and is designed to fit inside a compact rectal probe, and is able to resolve the 3D position of the source with a maximum error of 1.5 mm. This paper presents an evaluation of the additional dose that will be delivered to the patient as a result of backscatter radiation from the collimator. Monte Carlo simulations of planar and cylindrical collimators embedded in a tissue-equivalent phantom were performed using Geant4, with an 192Ir source placed at two different source-collimator distances. The planar configuration was replicated experimentally to validate the simulations, with a MOSkin dosimetry probe used to measure dose at three distances from the collimator. For the cylindrical collimator simulation, backscatter dose enhancement was calculated as a function of axial and azimuthal displacement, and dose distribution maps were generated at three distances from the collimator surface. Although significant backscatter dose enhancement was observed for both geometries immediately adjacent to the collimator, simulations and experiments indicate that backscatter dose is negligible at distances beyond 1 mm from the collimator. Since HDR BrachyView is enclosed within a 1 mm thick tissue-equivalent plastic shell, all backscatter radiation resulting from its use will therefore be absorbed before reaching the rectal wall or other tissues. dosimetry, brachytherapy, HDR

In this communication, we examine the evidence (if any) for a nonlinear dose response in relation to leukemia mortality in the Japanese A-bomb population. Specifically, we seek an estimate of the probability that, at low doses of radiation, the relative risk (RR) is smaller than one. Using Bayesian bootstrap techniques, we find that there is a 90% probability that-for at least one dose group at or below 200 mSv-there is a reduction of effect relative to the control group. We take this as evidence for radiation hormesis (end point: leukemia mortality) at low doses of radiation.

A transrectal real-time source tracking system, BrachyView, is developed by Centre for Medical Radiation Physics at University of Wollongong for treatment quality control during the high dose rate prostate brachytherapy procedure (HDR-PBT). The probe consists of a 15 × 60 mm2 pixellated silicon detector (four Timepix detectors) encased in a cylindrical seven pinhole tungsten collimator. The source position is determined by back-projecting the source projection images through the collimator pinholes. A prototype BrachyView probe consisting of a single Timepix detector (15 × 15 mm2) and a planar seven pinhole collimator is evaluated in this paper. The source position can be estimated with a maximum error of 0.2 mm on the detector plane, which translates into a maximum error of 1.42 mm within the prostate volume. This result is in good agreement with the results obtained in previously published simulation studies. © 2013 IEEE.

High dose rate (HDR) brachytherapy is a form of radiation therapy for treating prostate cancer whereby a high activity radiation source is moved between predefined positions inside applicators inserted within the treatment volume. Accurate positioning of the source is essential in delivering the desired dose to the target area while avoiding radiation injury to the surrounding tissue. In this paper, HDR BrachyView, a novel inbody dosimetric imaging system for real time monitoring and verification of the radioactive seed position in HDR prostate brachytherapy treatment is introduced. The current prototype consists of a 15 × 60 mm(2) silicon pixel detector with a multipinhole tungsten collimator placed 6.5 mm above the detector. Seven identical pinholes allow full imaging coverage of the entire treatment volume. The combined pinhole and pixel sensor arrangement is geometrically designed to be able to resolve the three-dimensional location of the source. The probe may be rotated to keep the whole prostate within the transverse plane. The purpose of this paper is to demonstrate the efficacy of the design through computer simulation, and to estimate the accuracy in resolving the source position (in detector plane and in 3D space) as part of the feasibility study for the BrachyView project.Monte Carlo simulations were performed using the GEANT4 radiation transport model, with a (192)Ir source placed in different locations within a prostate phantom. A geometrically accurate model of the detector and collimator were constructed. Simulations were conducted with a single pinhole to evaluate the pinhole design and the signal to background ratio obtained. Second, a pair of adjacent pinholes were simulated to evaluate the error in calculated source location.Simulation results show that accurate determination of the true source position is easily obtainable within the typical one second source dwell time. The maximum error in the estimated projection position was found to be 0.95 mm in the imaging (detector) plane, resulting in a maximum source positioning estimation error of 1.48 mm.HDR BrachyView is a feasible design for real-time source tracking in HDR prostate brachytherapy. It is capable of resolving the source position within a subsecond dwell time. In combination with anatomical information obtained from transrectal ultrasound imaging, HDR BrachyView adds a significant quality assurance capability to HDR brachytherapy treatment systems.

The conformity of the achieved dose distribution to the treatment plan strongly correlates with the accuracy of seed implantation in a prostate brachytherapy treatment procedure. Incorrect seed placement leads to both short and long term complications, including urethral and rectal toxicity. The authors present BrachyView, a novel concept of a fast intraoperative treatment planning system, to provide real-time seed placement information based on in-body gamma camera data. BrachyView combines the high spatial resolution of a pixellated silicon detector (Medipix2) with the volumetric information acquired by a transrectal ultrasound (TRUS). The two systems will be embedded in the same probe so as to provide anatomically correct seed positions for intraoperative planning and postimplant dosimetry. Dosimetric calculations are based on the TG-43 method using the real position of the seeds. The purpose of this paper is to demonstrate the feasibility of BrachyView using the Medipix2 pixel detector and a pinhole collimator to reconstruct the real-time 3D position of low dose-rate brachytherapy seeds in a phantom.BrachyView incorporates three Medipix2 detectors coupled to a multipinhole collimator. Three-dimensionally triangulated seed positions from multiple planar images are used to determine the seed placement in a PMMA prostate phantom in real time. MATLAB codes were used to test the reconstruction method and to optimize the device geometry.The results presented in this paper show a 3D position reconstruction accuracy of the seed in the range of 0.5-3 mm for a 10-60 mm seed-to-detector distance interval (Z direction), respectively. The BrachyView system also demonstrates a spatial resolution of 0.25 mm in the XY plane for sources at 10 mm distance from Medipix2 detector plane, comparable to the theoretical value calculated for an equivalent gamma camera arrangement. The authors successfully demonstrated the capability of BrachyView for real-time imaging (using a 3 s data acquisition time) of different brachytherapy seed configurations (with an activity of 0.05 U) throughout a 60 × 60 × 60 mm(3) Perspex prostate phantom.The newly developed miniature gamma camera component of BrachyView, with its high spatial resolution and real time capability, allows accurate 3D localization of seeds in a prostate phantom. Combination of the gamma camera with TRUS in a single probe will complete the BrachyView system.

In this paper calculated cross sections are presented for the interaction of electrons with poly(CSP), a single-stranded chain that contains one cytosine sugar phosphate unit in the elementary cell. To model a single strand of helical DNA (e.g. the base stacking), the Watson-Crick model for the geometry of poly(CSP) has been used. The use, for computational simplicity, of a single, rather than a double stranded polynucleotide may be justified on the basis of previous calculations indicating that - to a good approximation - the electronic structure (other than excitation states) of complementary base pairs may be described as a superposition of the corresponding structures of the individual components. The cross sections are obtained from the dielectric response function of the system, _(q,?), which is calculated with the aid of a semi-empirical Hamiltonian, the parameters of which have been adjusted to fit available optical and electron scattering data on DNA and related systems.

A mathematical formalism that calculates survival probabilities for cells exposed to ionising radiation while progressing through the mitotic cycle is described. The formalism, based on the linear quadratic expression -InS(D) = aD + ßD2, includes phase-specific cellular parameters and recovery constants for sublethal damage, the interaction between sublesions produced at different stages of the cell cycle, and the time spent by cells in each phase of the cycle while being irradiated. Survival curves calculated with this formalism are compared with model calculations where the overall survival probability is obtained (in the authors' view, incorrectly) as a simple product of phase specific probabilities.

The authors report calculations of the excitonic spectra for trans-polyacetylene obtained in the Hartree-Fock, Tamm-Danoff, and random-phase approximations. In the first case, in terms of two-particle propagator theory, the interaction between the excited electron and the hole is neglected. In the latter two cases, this interaction is considered in the first order. In this framework, the interaction between excitations of different bands and k-vectors has been included. They discuss the bandwidths and density of states for [pi]-[pi]*, [sigma]-[pi]*, [pi]-[sigma]*, and [sigma]-[sigma]*. 15 refs., 2 figs., 2 tabs.

Ab initio Hartree–Fock crystal orbital results were used as input for the calculation of exciton energies in the Tamm–Dancoff and random-phase approximation for polymers with helical symmetry. The calculations were applied to a hydrogen fluoride chain. We show that the Tamm–Dancoff method is a good approximation to the random-phase theory. Furthermore, the influence of the band–band interaction in the exciton calculation has been investigated.

Microdosimetric spectra for energetic beams of Fe, Ar, Ne, N, and C with incident energies between 500 and 600 MeV/amu have been measured as a function of absorber thickness. The experiments were performed at the Bevalac Accelerator of the Lawrence Berkeley Laboratory, CA. Data obtained with two different types of detectors are compared in order to examine the production of delta rays by energetic heavy ions and their effect on distributions of energy deposition. Recent controversies in the literature are examined in terms of the present observations. Consequent limitations of LET distributions and/or single-hit-particle concepts in reference to personnel and equipment in space are discussed. >

We examine, as a function of a particle type and energy as well as the geometrical configuration of the sensitive site, the extent to which a description of the track in terms of itsradial dose distribution can be used as a substitute for a full microdosimetric description. The critical parameter appears to be the ratio between the site diameter and the maximum lateral extension of the track (penumbra): whenever this ratio is significantly larger than 1 the radial-distribution-derived microdosimetric spectrum is accurate. While in general, for ratios smaller than 1, the spectra thus calculated are not correct, one can obtain significant improvement by considering tracks of restricted radial extent (yet comprising most of the delivered dose). In terms of a microdosimetric-based quality factor the radial dose distributions generally lead to correct estimates.

We describe an ab initio calculation of the properties of energy loss by electrons in crystalline water using its dielectric response function, e(q), ?), where q and ? are, respectively, the wave vector and frequency. The calculation was performed on a model system (cubic ice) in order to take advantage of its ordered structure (i.e. Bloch's theorem), but also because of evidence that liquid water in biological systems ('structured' water) contains residues with tetrahedral structure (i.e. ice) over time scales of at least 10-11 s. The main features of the calculation are: (a) e(q,w) is evaluated in the random phase approximation (we used the expression given by Ehrenreich and Cohen), (b) the crystal potential is expressed as a sum of water-molecule self-consistent potentials, and (c) wave functions are expanded using tight binding functions (ultimately employing a Gaussian base set). A total of seven states (bands), five occupied and two conduction, are considered. We report the band structure and the density of states of the crystal, as well as values of e(q,w) at selected values of q and w. Results are compared with energy loss measurements and with absorption spectra (XPS, UPS, and VUV data). The possibility of using an empirical combination of molecular potentials as a phenomenological Hamiltonian is also examined.

This paper describes analytic tools in support of a paradigm shift in brachytherapy treatment planning for prostate cancer--a shift from standard pre-planning to intraoperative planning using dosimetric feedback based on the actual deposited seed positions within the prostate. The method proposed is guided by several desiderata: (a) bringing both planning and evaluation in the operating room (i.e. make post-implant evaluation superfluous) therefore making rectifications--if necessary--still achievable; (b) making planning and implant evaluation consistent by using the same imaging system (ultrasound); and (c) using only equipment commonly found in a hospital operating room. The intraoperative dosimetric evaluation is based on the fusion between ultrasound images and 3D seed coordinates reconstructed from fluoroscopic projections. Automatic seed detection and registration of the fluoroscopic and ultrasound information, two of the three key ingredients needed for the intraoperative dynamic dosimetry optimization (IDDO), are explained in detail. The third one, the reconstruction of 3D coordinates from projections, was reported in a previous article. The algorithms were validated using a custom-designed phantom with non-radioactive (dummy) seeds. Also, fluoroscopic images were taken at the conclusion of an actual permanent prostate implant and compared with data on the same patient obtained from radiographic-based post-implant evaluation. To offset the effect of organ motion the comparison was performed in terms of the proximity function of the two seed distributions. The agreement between the intra- and post-operative seed distributions was excellent.

The finite element method (FEM) is well suited for use in the non-rigid registration of magnetic resonance spectroscopy images (MRSI) with intraoperative ultrasound images of the prostate because FEM provides a principled method for modeling the physical deformation caused when the MRSI intra-rectal imaging probe compresses the prostate. However, FEM requires significant labor and computational time to construct a finite element model and solve the resulting large system of equations. In particular, any finite element based registration method must address the questions of how to generate a mesh from an image and how to solve the system of finite element equations efficiently. This paper focuses on how m-rep image segmentations can be used to generate high quality multi-scale hexahedral meshes for use with FEM. Results from the application of this method to the registration of CT images of a prostate phantom with implanted brachytherapy seeds are presented.

Treatment planning for prostate brachytherapy is guided largely by the desire to determine a seed configuration that yields a prescription isodose surface that conforms with the prostate boundary. Currently, planning is based on the state of the prostate at a particular instant in time; typically, when the prostate is swollen. Unfortunately, a plan that conforms to the swollen prostate will not match the gland when shrinkage and seed displacement occur. Furthermore, these factors may directly affect the dose received by urethra and rectum over an extended period of time. The authors examine the possibility of defining an "effective planning volume" (EPV) that compensates for information that is unavailable during initial planning (e.g. changes in prostate volume, seed displacement).

PURPOSE: To compare the course and magnitude of change on the horizontal and vertical meridians of the cornea after 5 different incisions for cataract: extracapsular cataract extraction (ECCE), 6 mm superior scleral tunnel (6Sup), 3 mm superior scleral tunnel (3Sup), 3 mm temporal scleral tunnel (3Temp), and 3 mm temporal corneal incision (3Cor). METHODS: Retrospective chart review of 665 cases of preoperative regular astigmatism. The preoperative keratometry (K) reading was subtracted from the postoperative K reading to determine mean net change on each meridian at 1 day, 1 week, 2 weeks, 1 month, 1.5 months, 2 months, 4 months, 6 months and 12 months and at 6 month intervals thereafter. After the superior incisions, the temporal changes on each meridian are well described by an analytic model with an initial and final plateau. The changes after the temporal incisions are described by a linear equation. RESULTS: After each superior incision, the steepness and length of the transition from the initial to final plateau for each meridian depend on incision length. Considering the uncertainty of measuring K, the corneal meridians stabilized 4.5 months after ECCE, 1.2 months after 6Sup, and 0.3 months after 3Sup. No significant change was detected on the horizontal and vertical meridians after 3Temp and 3Cor. CONCLUSION: The magnitude and the duration of changes on the horizontal and vertical meridians of the cornea after cataract surgery depend on both incision length and location. Small temporal incisions induce less change than superior incisions.

A critical review of the literature leads to the conclusion that at the radiation doses generally of concern in radiation protection (2 Gy), protracted exposure to low linear energy transfer (LET) radiation (x- or gamma-rays) does not appear to cause lung cancer. There is, in fact, indication of a reduction of the natural incidence.

PURPOSE: This study compares the change over time of the astigmatism caused by "large" incision extracapsular cataract extraction (ECCE) and three smaller incisions for phakoemulsification. Based on this data, a mathematical model that predicts the course of astigmatism after a superior incision of length 3 to 12 mm has been developed. The relationship of axial length and preoperative astigmatism to induced post-operative astigmatism, the recovery of visual acuity, and the rate of YAG laser capsulotomy after each procedure also are documented. METHODS: Induced astigmatic change was calculated using a simple method of vector analysis. The change in induced astigmatism was calculated for 8 years after ECCE (n = 144), for 3 years after 6 mm superior incisions (6SUP) (n = 93), for 2 years after 3 mm superior incisions (3SUP) (n = 120), and for 18 months after 3 mm temporal incisions (3Temp) (n = 65). Plotted semi-logarithmically, the astigmatic change in each group may be represented mathematically. RESULTS: Two weeks after ECCE the mean induced cylinder was +3.47 D, which decayed to about -1.25 D after 6 months. Induced cylinder increased gradually to about -1.6 D after 8 years, although this further change was not significantly different than that at 6 months after surgery. For the phako groups, the net induced cylinder on the first post-operative day was: +1.23 D (6SUP), +0.49 D (3Sup), and -0.19 D (3Temp). After 6Sup the wound was astigmatically stable after approximately 3 months, and 3 years after surgery net induced cylinder was -0.66 D. After 3Sup the wound was astigmatically stable after about 6 weeks, and after 18 months net induced cylinder was -0.35 D. No significant change in astigmatism was detected at any time after 3Temp. Maximum visual acuity was reached after a mean of approximately 6 weeks after ECCE, 2 weeks after 6Sup, and between 1 day and 1 week after 3Sup and 3Temp. The rate of YAG laser capsulotomy was higher after ECCE than after any of the phakoemulsification procedures. No relationship of axial length or preoperative astigmatism to astigmatic change was detected. CONCLUSIONS: Incision size and location affect post-operative astigmatism. Induced astigmatism decreases with wound size, and only the 3 mm temporal incision is astigmatically neutral. The time for visual recovery increases with wound size. There appears to be less need for laser capsulotomy after phakoemulsification with capsulorrhexis than after ECCE. Axial length does not affect induced astigmatism after any of the 4 incisions, and preoperative astigmatism does not affect astigmatic change after ECCE and 6Sup.

Purpose: High dose rate (HDR) brachytherapy is a form of radiation therapy for treating prostate cancer whereby a high activity radiation source is moved between predefined positions inside applicators inserted within the treatment volume. Accurate positioning of the source is essential in delivering the desired dose to the target area while avoiding radiation injury to the surrounding tissue. In this paper, HDR BrachyView, a novel inbody dosimetric imaging system for real time monitoring and verification of the radioactive seed position in HDR prostate brachytherapy treatment is introduced. The current prototype consists of a 15 Multiplication-Sign 60 mm{sup 2} silicon pixel detector with a multipinhole tungsten collimator placed 6.5 mm above the detector. Seven identical pinholes allow full imaging coverage of the entire treatment volume. The combined pinhole and pixel sensor arrangement is geometrically designed to be able to resolve the three-dimensional location of the source. The probe may be rotated to keep the whole prostate within the transverse plane. The purpose of this paper is to demonstrate the efficacy of the design through computer simulation, and to estimate the accuracy in resolving the source position (in detector plane and in 3D space) as part of the feasibility study for themore » BrachyView project.Methods: Monte Carlo simulations were performed using the GEANT4 radiation transport model, with a {sup 192}Ir source placed in different locations within a prostate phantom. A geometrically accurate model of the detector and collimator were constructed. Simulations were conducted with a single pinhole to evaluate the pinhole design and the signal to background ratio obtained. Second, a pair of adjacent pinholes were simulated to evaluate the error in calculated source location.Results: Simulation results show that accurate determination of the true source position is easily obtainable within the typical one second source dwell time. The maximum error in the estimated projection position was found to be 0.95 mm in the imaging (detector) plane, resulting in a maximum source positioning estimation error of 1.48 mm.Conclusions: HDR BrachyView is a feasible design for real-time source tracking in HDR prostate brachytherapy. It is capable of resolving the source position within a subsecond dwell time. In combination with anatomical information obtained from transrectal ultrasound imaging, HDR BrachyView adds a significant quality assurance capability to HDR brachytherapy treatment systems.« less

An analytical model is presented that describes radiation-induced cellular inactivation in the presence of sublethal damage repair, cellular repopulation and redistribution in the mitotic cycle (the 3 Rs). The parameters of the model are measurable experimentally. Also taken into account are the initial age distribution of the cell population, the fact that subgroups of cells progress through the cycle at different speeds, the effects of a dose of radiation on the duration of the four phases of the cycle (G1, S, G2, M), the possibility that a certain fraction of the cells are quiescent, and cell loss and/or cell removal from the proliferating population. Survival probabilities are expressed as linear-quadratic functions of dose where the coefficient alpha and beta as well as the recovery constant (t0) are taken to depend on the position of the cell in the mitotic cycle. Explicit analytical expressions for inactivation probability are given for clonogenic cells exposed to continuous or fractionated radiation. Two model calculations are used to illustrate the formalism: in one, the redistribution of cells during fractionated therapy is examined. In the other calculation, it is shown that it is sufficient to take into account differences in proliferation rates and the change in the ratio alpha/beta within the generation cycle for cells that may have otherwise equal response to acute exposures to explain that in a fractionated treatment protocol late-responding cells are more sensitive to the dose per fraction than early-responding cells. It is not necessary to invoke differences in radiosensitivity between these two classes of cells.

In this paper, an analytical formalism, designated the R3 model, is applied to the problem of understanding changes in cellular radiosensitivity as a function of the position of the cell in the generation cycle. The model describes the survival probability for cells exposed to ionizing radiation and incorporates the effects of sublethal damage repair, repopulation and redistribution in the cell cycle. The importance of this problem stems in part from the increasing reliance on in vitro, single-cell model systems to design optimal radiotherapy treatment protocols, and in particular from the fact that arguments used to justify newer methodologies often bear quite heavily on descriptors (e.g. alpha and beta) meant to be employed for cells homogeneous in radiosensitivity--a situation hardly applicable to tissues. From the analysis of two sets of data for Chinese hamster V79 cells (Sinclair and Morton, Nature 205, 247-250, 1964; Gillespie et al., Radiat. Res. 64, 353-364, 1975) the linear-quadratic parameters (alpha, beta) are obtained as a function of cell cycle age. The results indicate substantial variation of these parameters inside each phase of the cycle. Moreover, it is noticeable that alpha and beta show different patterns of variation during the cycle and therefore the ratio alpha/beta will also change. At the regions of the cycle where beta is large (the beginning of G1 phase, at the G1/S-phase border and during G2 phase) dose-rate effects would be expected to be important. The data for V79 cells are consistent with the view that sublethal damage repair occurs predominantly in S phase, with very little (if any) repair in the other phases of the cycle.

Procedures for the assessment of risks or vulnerabilities from radiation in space are evaluated in terms of model-independent inductive approaches. The reliability of risks calculated for space applications on the basis of accelerator-based physical and biological data is examined from a microdosimetric perspective. Probability distributions for energy deposition in biologically significant sites extend over several decades in lineal energy even for monoenergetic high-energy particles of relatively high atomic number. Because the response depends on a large number of variables and because of the difficulty of incorporating all such factors into calculations, a precise correlation between a physical descriptor of the field and observed effects in space is not feasible. For the same reasons, it is equally difficult to estimate the accuracies of such risk assessments. We use recently published microdosimetric spectra for HZE particles and biological weighting functions, including those derived from biological measurements with maximum entropy techniques, to illustrate some problems associated with the evaluations of risks from radiation fields in space.

A review of 15 cases suggests that posterior-lip sclerectomy can be performed safely with extracapsular cataract extraction (ECCE) and posterior chamber lens implantation. The anterior chamber was filled with viscoelastic at the end of each procedure; no case required reoperation for shallow chamber or hypotony. The mean intraocular pressure after 1 year was 12.1 mm Hg. The astigmatism induced by the triple procedure did not differ significantly from that caused by ECCE alone during the initial 2 postoperative years. A new mathematical model that describes the change over time of postoperative astigmatism associated with these procedures is described.

Research at the Center for Radiological Research is a blend of physics, chemistry and biology and epitomizes the multidisciplinary approach towards understanding the mechanisms involved in the health problems resulting from human exposure to ionizing radiations. To an increasing extent, the focus of attention is on biochemistry and the application of the techniques of molecular biology to the problems of radiation biology. Research highlights from the past year are briefly described.

We describe Monte Carlo codes that simulate, event by event, the interaction of energetic electrons with a double-stranded DNA molecule and with the condensed water surrounding it. Both direct and indirect effects are treated explicitly . The cross-sectional input necessary in the transport codes was obtained via quantum-mechanical calculations of the dielectric response function epsilon(q, omega), of polycytidine. For each inelastic event on DNA we score the energy deposited locally, the position of the event and the moiety that underwent that event. This information provides a detailed picture of the spatial disposition of molecular alterations for DNA exposed to ionizing radiation.