Minimal skin dose increase in longitudinal rotating biplanar linac-MR systems: examination of radiation energy and flattening filter design.

The magnetic fields of linac-MR systems modify the path of contaminant electrons in photon beams, which alters patient entrance skin dose. Also, the increased SSD of linac-MR systems reduces the maximum achievable dose rate. To accurately quantify the changes in entrance skin dose, the authors use EGSnrc Monte Carlo calculations that incorporate 3D magnetic field of the Alberta 0.5 T longitudinal linac-MR system. The Varian 600C linac head geometry assembled on the MRI components is used in the BEAMnrc simulations for 6 MV and 10 MV beam models and skin doses are calculated at an average depth of 70 µm using DOSXYZnrc. 3D modeling shows that magnetic fringe fields decay rapidly and are small at the linac head. SSDs between 100 and 120 cm result in skin-dose increases of between ~6%-19% and ~1%-9% for the 6 and 10 MV beams, respectively. For 6 MV, skin dose increases from ~10.5% to ~1.5% for field-size increases of 5 × 5 cm(2) to 20 × 20 cm(2). For 10 MV, skin dose increases by ~6% for a 5 × 5 cm(2) field, and decreases by ~1.5% for a 20 × 20 cm(2) field. Furthermore, the proposed reshaped flattening filter increases the dose rate from the current 355 MU min(-1) to 529 MU min(-1) (6 MV) or 604 MU min(-1) (10 MV), while the skin-dose increases by only an additional ~2.6% (all percent increases in skin dose are relative to D max). This study suggests that there is minimal increase in the entrance skin dose and minimal/no decrease in the dose rate of the Alberta longitudinal linac-MR system. The even lower skin dose increase at 10 MV offers further advantages in future designs of linac-MR prototypes.

2D lag and signal nonlinearity correction in an amorphous silicon EPID and their impact on pretreatment dosimetric verification.

PURPOSE: This investigation provides measurements of signal lag and nonlinearity separately for the Varian aS500 electronic portal imaging device (EPID), and an algorithm to correct for these effects in 2D; their potential impact on intensity modulated radiation therapy (IMRT) verification is also investigated. The authors quantify lag, as a function of both delivered monitor units (MU) and time, by using a range of MUs delivered at a clinically used rate of 400 MU∕min. Explicit cumulative lag curves are thus determined for a range of MUs and times between the end of irradiation and the end of image acquisition. Signal nonlinearity is also investigated as a function of total MUs delivered. The family of cumulative lag curves and signal nonlinearity are then used to determine their effects on dynamic multileaf collimator (MLC) (IMRT) deliveries, and to correct for theses effects in 2D. METHODS: Images acquired with an aS500 EPID and Varis Portal-Vision software were used to quantify detector lag and signal-nonlinearity. For the signal lag investigation, Portal-Vision's service monitor was used to acquire EPID images at a rate of 8 frames/s. The images were acquired during irradiation and 66 s thereafter, by inhibiting the M-holdoff-In signal of the Linac for a range of 4.5-198.5 MUs. Relative cumulative lag was calculated by integrating the EPID signal for a time after beam-off, and normalizing this to the integrated EPID signal accumulated during radiation. Signal nonlinearity was studied by acquiring 10 × 10 cm(2) open-field EPID images in "integrated image" mode for a range of 2-500 MUs, and normalized to the 100 MU case. All data were incorporated into in-house written software to create a 2D correction map for these effects, using the field's MLC file and a field-specific calculated 2D "time-map," which keeps track of the time elapsed from the last fluence delivered at each given point in the image to the end of the beam delivery. RESULTS: Relative cumulative lag curves reveal that the lag alone can deviate the EPID's perceived dose by as large as 6% (1 MU delivery, 60 s postirradiation). For signal nonlinearity relative to 100 MU, EPID signals per MU of 0.84 and 1.01 were observed for 2 and 500 MUs, respectively. Correction maps were applied to a 1 cm sweeping-window 14 × 14 cm(2) field and clinical head-and-neck IMRT field. A mean correction of 1.028 was implemented in the head-and-neck field, which significantly reduced lag-related asymmetries in the EPID images, and restored linearity to the EPID imager's dose response. Corrections made to the sweeping-field showed good agreement with the treatment planning system-predicted field, yielding an average percent difference of 0.05% ± 0.91%, compared to the -1.32% ± 1.02% before corrections, or 1.75% ± 1.04% when only a signal nonlinearity correction is made. CONCLUSIONS: Lag and signal-nonlinearity have been quantified for an aS500 EPID imager, and an effective 2D correction method has been developed which effectively removes nonlinearity and lag effects. Both of these effects were shown to negatively impact IMRT verifications. Especially fields that involve prolonged irradiation and small overall MUs should be corrected for in 2D.

Skin dose in longitudinal and transverse linac-MRIs using Monte Carlo and realistic 3D MRI field models.

PURPOSE: The magnetic fields of linac-MR systems modify the path of contaminant electrons in photon beams, which alters patient skin dose. To accurately quantify the magnitude of changes in skin dose, the authors use Monte Carlo calculations that incorporate realistic 3D magnetic field models of longitudinal and transverse linac-MR systems. METHODS: Finite element method (FEM) is used to generate complete 3D magnetic field maps for 0.56 T longitudinal and transverse linac-MR magnet assemblies, as well as for representative 0.5 and 1.0 T Helmholtz MRI systems. EGSnrc simulations implementing these 3D magnetic fields are performed. The geometry for the BEAMnrc simulations incorporates the Varian 600C 6 MV linac, magnet poles, the yoke, and the magnetic shields of the linac-MRIs. Resulting phase-space files are used to calculate the central axis percent depth-doses in a water phantom and 2D skin dose distributions for 70 µm entrance and exit layers using DOSXYZnrc. For comparison, skin doses are also calculated in the absence of magnetic field, and using a 1D magnetic field with an unrealistically large fringe field. The effects of photon field size, air gap (longitudinal configuration), and angle of obliquity (transverse configuration) are also investigated. RESULTS: Realistic modeling of the 3D magnetic fields shows that fringe fields decay rapidly and have a very small magnitude at the linac head. As a result, longitudinal linac-MR systems mostly confine contaminant electrons that are generated in the air gap and have an insignificant effect on electrons produced further upstream. The increase in the skin dose for the longitudinal configuration compared to the zero B-field case varies from ∼1% to ∼14% for air gaps of 5-31 cm, respectively. (All dose changes are reported as a % of D(max).) The increase is also field-size dependent, ranging from ∼3% at 20 × 20 cm(2) to ∼11% at 5 × 5 cm(2). The small changes in skin dose are in contrast to significant increases that are calculated for the unrealistic 1D magnetic field. For the transverse configuration, the entrance skin dose is equal or smaller than that of the zero B-field case for perpendicular beams. For a 10 × 10 cm(2) oblique beam the transverse magnetic field decreases the entry skin dose for oblique angles less than ±20° and increases it by no more than 10% for larger angles up to ±45°. The exit skin dose is increased by 42% for a 10 × 10 cm(2) perpendicular beam, but appreciably drops and approaches the zero B-field case for large oblique angles of incidence. CONCLUSIONS: For longitudinal linac-MR systems only a small increase in the entrance skin dose is predicted, due to the rapid decay of the realistic magnetic fringe fields. For transverse linac-MR systems, changes to the entrance skin dose are small for most scenarios. For the same geometry, on the exit side a fairly large increase is observed for perpendicular beams, but significantly drops for large oblique angles of incidence. The observed effects on skin dose are not expected to limit the application of linac-MR systems in either the longitudinal or transverse configuration.

WE-E-BRB-06: Monte Carlo Calculations of the Skin Dose for Longitudinal Linac-MR System Using Realistic Three-Dimensional Magnetic Field Modeling.

PURPOSE: This study quantifies the effects of the magnetic field of a longitudinal linac-MR system (B-field parallel to beam direction) on skin dose due to the confinement of contaminant electrons, using Monte Carlo calculations and realistic 3-D models of the magnetic field. METHODS: The complete realistic 3-D magnetic fields generated by the bi-planar Linac-MR magnet assembly are calculated with the finite element method using Opera- 3D. EGSnrc simulations are performed in the presence of ∼0.6T and IT MRI fields that have realistic rapid fall-off of the fringe field. The simulation geometry includes a Varian 600C 6MV linac, the yoke and magnetic shields of the MRIs, and features an isocentre distance of 126 cm. Phase spaces at the surface of a water phantom are scored using BEAMnrc; DOSXYZnrc is used to score the resulting CAX percent depth-doses in the phantom and the 2D skin dose distributions in the first 70 urn layer. For comparison, skin doses are also calculated in the absence of magnetic field and using a 1-D magnetic field with an unrealistic fringe field. The effects of field size and air gap (between phantom surface and magnet pole) are also examined. RESULTS: Analysis of the phase-space and dose distributions reveals that significant containment of electrons occurs primarily close to the uniform magnetic field region. The increase in skin dose due to the magnetic field depends on the air gap, varying from 1% to 13% for air gaps of 5 to 31 cm, respectively. The increase is also field-size dependent, varying from 3% at 20×20 cm2 to 11% at 5×5 cm2. CONCLUSIONS: Calculations based on various realistic MRI 3D magnetic-field maps that appropriately account for the rapid decay of the fringe field show that the increase in the patient skin dose of a longitudinal Linac-MR system is clinically insignificant.

Sci-Sat AM: Brachy - 01: Evaluation of dose-volume metrics for microbeam radiation therapy.

Microbeam radiation therapy (MRT) is an experimental technique delivering an array of high dose synchrotron X-ray microbeams. Development of metrics to predict the biological efficacy of MRT dose distributions is needed to guide further MRT research and for potential translation to human trials. The most commonly used metric is the peak-to-valley-dose ratio (PVDR) relating the dose at the microbeam center to that between two microbeams. We investigate three additional metrics that characterize dose distributions from a more volumetric perspective - the peak-to-mean-valley-dose ratio (PMVDR), mean dose, and percentage volume below a threshold. The metrics are evaluated for Monte Carlo simulations of dose distributions in three cubic head phantoms (2, 4 and 8 cm side lengths) for microbeam widths of 25, 50, and 75 µm and centre-to-centre spacings of 100, 200 and 400 µm. The ratio of the PMVDR to the PVDR varied from 0.24 to 0.80 for the different configurations, indicating a difference in the predicted geometric dependence of outcome for these two metrics. The mean dose was 102, 79, and 42 % of the mean skin dose for the 2, 8, and 16 cm head phantoms, respectively. The percentage volume below a 10% dose threshold was highly dependent on geometry, with ranges for the different collimation configurations of 2 - 87% and 33 - 96% for the 2 and 16 cm heads, respectively. Different dose-volume metrics exhibit different dependencies on MRT geometry parameters, suggesting that reliance on PVDR as a predictor of therapeutic outcome may be insufficient.

SU-E-T-469: The Effects of Patient Anatomy and Parallel Magnetic Fields on Beamlet Dose Distributions.

PURPOSE: Beamlets are generated in a patient geometry in the presence of a magnetic field to investigate the effects of tissue density and magnetic field on beamlet dose distributions, which is important for the optimization of photon fluence to be delivered by a linac-MR system. METHODS: 50×50 mm2 fields were placed with isocenter in the middle of a patient's right lung. Each treatment field was decomposed into 100 beamlets (each 5×5 mm2 ). BEAMnrc scored the particle phase space at 100.2 cm from the source in the linac-MR geometry (isocentre at 126 cm) with parallel magnetic fields of 0, 0.56, and 3T. DOSXYZnrc was modified to score the energy deposited by particles from this phase space as a function of the beamlet the particle passed through. The calculation volume of 70×46×64 voxels encompassed the patient with a voxel size of 3×3×3 mm3 . Each beamlet was normalized to the dose calculated to a 3×3×3 mm3 voxel with isocenter at 5cm depth in a flat water tank without a magnetic field. RESULTS: Beamlet files were calculated on Western Canada's high performance computing cluster (Westgrid) using 100 processors, enabling simulation of 109 histories in less than 3 hours. The resulting files, which contained 3D dose distributions for all 100 beamlets, were 81 MB per field. The Monte Carlo uncertainty was also stored. The gyroradii for 1 MeV electron traversing field lines at 20 degrees are 2.9mm and 0.5mm for 0.56 and 3T fields respectively. The 0.56T parallel magnetic field has a small effect compared to the distortion of the beamlet introduced by the presence of lung. CONCLUSIONS: The effect of tissue heterogeneities is more significant than the effect of a 0.56T parallel magnetic field. A 3T field refocuses the dose in lung to the beamlet path and significantly reduces the lateral electron scatter.

Sci-Thurs PM: Delivery-06: 2-D lag and response nonlinearity corrections for dynamic IMRT verification using an EPID.

In recent years, EPIDs have been used for pre-treatment IMRT verification. Although EPID lag and signal nonlinearities have been investigated, they have not been implemented in the verification process. In dynamic sliding-window IMRT delivery, the dose delivered, and the time between the end of dose delivery and the end of image acquisition differ between pixels. The resulting differences in lag and signal-response across the image can cause artificial asymmetries and amplitude changes in measured EPID dose images. These artifacts alter the agreement between measured and predicted images, potentially complicating the assessment of clinical IMRT verifications. A method of 2-D (pixel-by-pixel) correction was developed based on data from sets of experiments performed to independently quantify the lag and nonlinearity characteristics of Varian's aS500 EPID. To test the correction, it was applied to two sweeping window 10×10 cm2 fields that differ only in sweeping direction. The correction resolved discrepancies in the symmetry between these two cases, and the differences between measured and predicted amplitudes evident when small numbers of MUs were delivered. To illustrate its potential use, the correction technique was applied to a measured image of a clinical IMRT field that produced a relatively poor verification result. The correction partially accounted for discrepancies between measured and Eclipse-predicted images of this field, reducing the percentage of pixels failing a Gamma analysis (3 %, 3 mm) from 8.5 to 5.6 %. This correction technique can be used to help resolve the source of discrepancies in troublesome clinical IMRT verifications.

Limitations of a TCP model incorporating population heterogeneity.

The variation between individuals in their dose-response characteristics complicates attempts to extract estimates of radiobiological parameters (e.g. alpha, beta, etc) from fits to clinical dose-response data. The use of 'population' dose-response models that explicitly account for this variability is necessary to avoid obtaining skewed parameter estimates. In this work, we evaluated an example of a 'population' tumour control probability (TCP) model in terms of its ability to provide reliable parameter estimates. This was accomplished by performing fits of this population model to 'pseudo' data sets, which were generated with Monte Carlo techniques and based on preset values for the various radiobiological parameters. The fitting exercises illustrated considerable correlations between the model parameters. Especially significant was the large correlation observed between the parameter mu=alpha/sigmaalpha used to characterize the level of population heterogeneity in radiosensitivity and the alpha/beta parameter typically used to describe the response to fractionation. The results imply that fits to clinical data may not be able to distinguish between tumours exhibiting a high degree of heterogeneity and a strong beta-mechanism and those containing little heterogeneity and having a weak beta-mechanism. One implication is that basing the design of optimal fractionation regimes on such fitting results may be error-prone. If in vitro assays are to be used to independently determine biologically reasonable ranges for parameter values, an accurate knowledge of the relationship between in vitro and in vivo dose-response characteristics is required.

Radiation damage, repopulation and cell recovery analysis of in vitro tumour cell megacolony culture data using a non-Poissonian cell repopulation TCP model.

The effects of radiation damage, tumour repopulation and cell sublethal damage repair and the possibility of extracting information about the model parameters describing them are investigated in this work. Previously published data on two different cultured cell lines were analysed with the help of a tumour control probability (TCP) model that describes tumour cell dynamics properly. Different versions of a TCP model representing the cases of full or partial cell recovery between fractions of radiation, accompanied by repopulation or no repopulation were used to fit the data and were ranked according to statistical criteria. The data analysis shows the importance of the linear-quadratic mechanism of cell damage for the description of the in vitro cell dynamics. In a previous work where in vivo data were analysed, the employment of the single hit model of cell kill and cell repopulation produced the best fit, while ignoring the quadratic term of cell damage in the current analysis leads to poor fits. It is also concluded that more experiments using different fractionation regimes producing diverse data are needed to help model analysis and better ranking of the models.

Investigating the effect of clonogen resensitization on the tumor response to fractionated external radiotherapy.

In this work we further develop the modeling of tumor dynamics by proposing a mechanism of tumor resensitization that is based on the process of reoxygenation. Reoxygenation is modeled using the concept of nonstationary diffusion of oxygen. This leads to the derivation of an explicit expression for the radiosensitivity parameter that predicts a radiosensitivity that increases with time. To account for the resensitization mechanism, the time-dependent expression for the radiosensitivity is then incorporated within a tumor control probability (TCP) model that already includes tumor cell repopulation and repair. We fit a set of experimental animal TCP curves corresponding to several different fractionation regimes using both the modified (with resensitization) and unmodified (without resensitization) versions of the TCP model. In comparison to the unmodified model, the modified model produces statistically superior fits, and is able to describe an "inverse" dose-fractionation behavior present in the data.

Three-dimensional IMRT verification with a flat-panel EPID.

A three-dimensional (3D) intensity-modulated radiotherapy (IMRT) pretreatment verification procedure has been developed based on the measurement of two-dimensional (2D) primary fluence profiles using an amorphous silicon flat-panel electronic portal imaging device (EPID). As described in our previous work, fluence profiles are extracted from EPID images by deconvolution with kernels that represent signal spread in the EPID due to radiation and optical scattering. The deconvolution kernels are derived using Monte Carlo simulations of dose deposition in the EPID and empirical fitting methods, for both 6 and 15 MV photon energies. In our new 3D verification technique, 2D fluence modulation profiles for each IMRT field in a treatment are used as input to a treatment planning system (TPS), which then generates 3D doses. Verification is accomplished by comparing this new EPID-based 3D dose distribution to the planned dose distribution calculated by the TPS. Thermoluminescent dosimeter (TLD) point dose measurements for an IMRT treatment of an anthropomorphic phantom were in good agreement with the EPID-based 3D doses; in contrast, the planned dose under-predicts the TLD measurement in a high-gradient region by approximately 16%. Similarly, large discrepancies between EPID-based and TPS doses were also evident in dose profiles of small fields incident on a water phantom. These results suggest that our 3D EPID-based method is effective in quantifying relevant uncertainties in the dose calculations of our TPS for IMRT treatments. For three clinical head and neck cancer IMRT treatment plans, our TPS was found to underestimate the mean EPID-based doses in the critical structures of the spinal cord and the parotids by approximately 4 Gy (11%-14%). According to radiobiological modeling calculations that were performed, such underestimates can potentially lead to clinically significant underpredictions of normal tissue complication rates.

Inverse treatment planning by physically constrained minimization of a biological objective function.

In the current state-of-the art of clinical inverse planning, the design of clinically acceptable IMRT plans is predominantly based on the optimization of physical rather than biological objective functions. A major impetus for this trend is the unproven predictive power of radiobiological models, which is largely due to the scarcity of data sets for an accurate evaluation of the model parameters. On the other hand, these models do capture the currently known dose-volume effects in tissue dose-response, which should be accounted for in the process of optimization. In order to incorporate radiobiological information in clinical treatment planning optimization, we propose a hybrid physico-biological approach to inverse treatment planning based on the application of a continuous penalty function method to the constrained minimization of a biological objective. The objective is defined as the weighted sum of normal tissue complication probabilities evaluated with the Lyman normal-tissue complication probability model. Physical constraints specify the admissible minimum and maximum target dose. The continuous penalty function method is then used to find an approximate solution of the resulting large-scale constrained minimization problem. Plans generated by our approach are compared to ones produced by a commercial planning system incorporating physical optimization. The comparisons show clinically negligible differences, with the advantage that the hybrid technique does not require specifications of any dose-volume constraints to the normal tissues. This indicates that the proposed hybrid physico-biological method can be used for the generation of clinically acceptable plans.

Investigating the effect of cell repopulation on the tumor response to fractionated external radiotherapy.

In this work we study the descriptive power of the main tumor control probability (TCP) models based on the linear quadratic (LQ) mechanism of cell damage with cell recovery. The Poisson, binomial, and a dynamic TCP model, developed recently by Zaider and Minerbo are considered. The Zaider-Minerbo model takes cell repopulation into account. It is shown that the Poisson approximation incorporating cell repopulation is conceptually incorrect. Based on the Zaider-Minerbo model, an expression for the TCP for fractionated treatments with varying intervals between two consecutive fractions and with cell survival probability that changes from fraction to fraction is derived. The models are fitted to an experimental data set consisting of dose response curves that correspond to different fractionation regimes. The binomial TCP model based on the LQ mechanism of cell damage solely was unable to fit the fractionated response data. It was found that the Zaider-Minerbo model, which takes tumor cell repopulation into account, best fits the data.

Dosimetric IMRT verification with a flat-panel EPID.

A convolution-based calibration procedure has been developed to use an amorphous silicon flat-panel electronic portal imaging device (EPID) for accurate dosimetric verification of intensity-modulated radiotherapy (IMRT) treatments. Raw EPID images were deconvolved to accurate, high-resolution 2-D distributions of primary fluence using a scatter kernel composed of two elements: a Monte Carlo generated kernel describing dose deposition in the EPID phosphor, and an empirically derived kernel describing optical photon spreading. Relative fluence profiles measured with the EPID are in very good agreement with those measured with a diamond detector, and exhibit excellent spatial resolution required for IMRT verification. For dosimetric verification, the EPID-measured primary fluences are convolved with a Monte Carlo kernel describing dose deposition in a solid water phantom, and cross-calibrated with ion chamber measurements. Dose distributions measured using the EPID agree to within 2.1% with those measured with film for open fields of 2 x 2 cm2 and 10 x 10 cm2. Predictions of the EPID phantom scattering factors (SPE) based on our scatter kernels are within 1% of the SPE measured for open field sizes of up to 16 x 16 cm2. Pretreatment verifications of step-and-shoot IMRT treatments using the EPID are in good agreement with those performed with film, with a mean percent difference of 0.2 +/- 1.0% for three IMRT treatments (24 fields).

Derivation of the expressions for gamma50 and D50 for different individual TCP and NTCP models.

This paper presents a complete set of formulae for the position (D50) and the normalized slope (gamma50) of the dose-response relationship based on the most commonly used radiobiological models for tumours as well as for normal tissues. The functional subunit response models (critical element and critical volume) are used in the derivation of the formulae for the normal tissue. Binomial statistics are used to describe the tumour control probability, the functional subunit response as well as the normal tissue complication probability. The formulae are derived for the single hit and linear quadratic models of cell kill in terms of the number of fractions and dose per fraction. It is shown that the functional subunit models predict very steep, almost step-like, normal tissue individual dose-response relationships. Furthermore, the formulae for the normalized gradient depend on the cellular parameters alpha and beta when written in terms of number of fractions, but not when written in terms of dose per fraction.

Diffuse pollution: lessons from soil conservation policies.

Diffuse pollution of water from various land uses requires a control approach different from the uniform regulations that were successful for point sources. Local solutions, designed by local watershed councils, are preferred over national directives. Soil erosion control policy, based on voluntary measures coordinated by local conservation districts, provides a guide for improvement of water quality in watersheds. Local planning and action are supplemented by government provision of free technical advice, economic incentives and public education programs. There are no national standards, because land use decisions are a guarded prerogative of individual landowners. Among the lessons that can be transferred to diffuse pollution control are the practicality of the voluntary approach, the need for continuing financial incentives, local solutions to local problems, the wide range of effectiveness of local groups, the uncertainty whether the most critical problems are resolved and the need for education to foster a normative attitude of what we ought to do for watershed health.

[Fetal development in late gestosis and nicotine consumption]. / Die fetale Entwicklung bei Spätgestose und Nikotinkonsum.

In pre-eclamptic and in smoking women, the foetus often develops growth retardation. Hence, nicotine as vasoconstrictive substance increases the blood pressure, therefore causes a higher incidence for pre-eclamptic toxaemia in smoking pregnant women. By means of perinatal inquiry in Baden-Württemberg, not only the frequency of preeclamptic toxaemia was proven in smoking women, but also the frequency of toxaemia in mothers with low, of normal and high weight of the newborn. Hypertension is more frequent in mothers with overweight babies than in mothers with babies of normal weight. In case of overweight newborns, toxaemia is less often caused by proteinuria than in underweight babies. Hypertension is less frequent in smoking pregnant women than in non-smoking women. These findings can be explained by a new theory, which interprets pre-eclamptic toxaemia as a compensatory mechanism in foetal growth retardation. For the foetus, which is insufficiently supplied by the placenta, this regulatory mechanism enhances the blood supply of the placenta. The higher incidence of toxaemia in pregnant women with overweight babies is explained by an increased demand on the placenta, which causes a better foetal blood supply of the placenta in developing the toxaemia. In this case, the toxaemia is compensated. In the stage of decompensation with foetal growth retardation, all reserves are mobilised by an increased permeability of the vessels, which leads to an improved passage through the placenta, but also to proteinuria and increased incidence of oedema.(ABSTRACT TRUNCATED AT 250 WORDS)

[Why are most children born from I. cephalic presentation?]. / Warum werden die meisten Kinder aus I. Schädellage geboren?

The question, why most children are delivered in I. cephalic position, is unsolved up to now. Most children are positioned with crossed legs in the uterus. Ultrasonography was employed from the 36th week of gestation onwards to find out whether there is a connection between crossing of the legs and the position in utero. From 58 pregnancies with I. position in 47 cases and from 32 pregnancies with II. position in 28 cases, the leg nearer to the back of the mother was crossed over the other one. According to the neuro-cerebral differentiation between both sides, the foetus prefers a crossing in such a manner, that the left leg is folded over the right leg. The foetus finds a more stable position, if the more mobile leg is directed with the foot against the yielding belly walls of the mother. In accordance with the preferred crossing of the left leg over the right leg the I. position in utero is more frequent.

[Origin of the urge to press in the expulsion period]. / Zur Frage der Entstehung des Pressdrangs in der Austreibungsperiode.

So far, the explanation of the bearing-down pains in the expulsion phase has been unsatisfactory. It is assumed that the press reflex is caused by the pressure of the presenting part on the pelvic floor. This is in contradiction to the observation that the press reflex only appears during labour pains and may occur before the presenting part reaches the pelvic floor. In the expulsion phase, the uterine contraction tenses the ligg. cardinaliae, the vagina and the rectal wall in neighbourhood of the vagina. The tension of the rectal wall leads to an urgent need of relieving bowels, which supports the press reflex. By pressing, the woman perceives a pain relief because of a reducement of the tension of the ligg. cardinalia.