Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams.

PURPOSE: To investigate the behavior of plane-parallel ion chambers in high-energy photon beams through measurements and Monte Carlo simulations. METHODS: Ten plane-parallel ion chamber types were obtained from the major ion chamber manufacturers. Absorbed dose-to-water calibration coefficients are measured for these chambers and k(Q) factors are determined. In the process, the behaviors of the chambers are characterized through measurements of leakage currents, chamber settling in cobalt-60, polarity and ion recombination behavior, and long-term stability. Monte Carlo calculations of the absorbed dose to the air in the ion chamber and absorbed dose to water are obtained to calculate k(Q) factors. Systematic uncertainties in Monte Carlo calculated k(Q) factors are investigated by varying material properties and chamber dimensions. RESULTS: Chamber behavior was variable in MV photon beams, especially with regard to chamber leakage and ion recombination. The plane-parallel chambers did not perform as well as cylindrical chambers. Significant differences up to 1.5% were observed in calibration coefficients after a period of eight months although k(Q) factors were consistent on average within 0.17%. Chamber-to-chamber variations in k(Q) factors for chambers of the same type were at the 0.2% level. Systematic uncertainties in Monte Carlo calculated k(Q) factors ranged between 0.34% and 0.50% depending on the chamber type. Average percent differences between measured and calculated k(Q) factors were - 0.02%, 0.18%, and - 0.16% for 6, 10, and 25 MV beams, respectively. CONCLUSIONS: Excellent agreement is observed on average at the 0.2% level between measured and Monte Carlo calculated k(Q) factors. Measurements indicate that the behavior of these chambers is not adequate for their use for reference dosimetry of high-energy photon beams without a more extensive QA program than currently used for cylindrical reference-class ion chambers.

The central electrode correction factor for high-Z electrodes in small ionization chambers.

PURPOSE: Recent Monte Carlo calculations of beam quality conversion factors for ion chambers that use high-Z electrodes [B. R. Muir and D. W. O. Rogers, Med. Phys. 37, 5939-5950 (2010)] have shown large deviations of kQ values from values calculated using the same techniques as the TG-51 and TRS-398 protocols. This report investigates the central electrode correction factor, Pcel, for these chambers. METHODS: Ionization chambers are modeled and Pcel is calculated using the EGSnrc user code egs_chamber for three cases: in photon and electron beams under reference conditions; as a function of distance from an iridium-192 point source in a water phantom; and as a function of depth in a water phantom on which a 200 kVp x-ray source or 6 MV beam is incident. RESULTS: In photon beams, differences of up to 3% between Pcel calculations for a chamber with a high-Z electrode and those used by TG-51 for a 1 mm diameter aluminum electrode are observed. The central electrode correction factor for a given value of the beam quality specifier is different depending on the amount of filtration of the photon beam. However, in an unfiltered 6 MV beam, Pcel, varies by only 0.3% for a chamber with a high-Z electrode as the depth is varied from 1 to 20 cm in water. The difference between Pcel calculations for chambers with high-Z electrodes and TG-51 values for a chamber with an aluminum electrode is up to 0.45% in electron beams. The central electrode correction, which is roughly proportional to the chambers absorbed dose sensitivity, is found to be large and variable as a function of distance for chambers with high-Z and aluminum electrodes in low-energy photon fields. CONCLUSIONS: In this work, ionization chambers that employ high-Z electrodes have been shown to be problematic in various situations. For beam quality conversion factors, the ratio of Pcel in a beam quality Q to that in a Co-60 beam is required; for some chambers, kQ is significantly different from current dosimetry protocol values because of central electrode effects. It would be best for manufacturers to avoid producing ion chambers that use high-Z electrodes.

Measured and Monte Carlo calculated K(Q) factors: accuracy and comparison.

PURPOSE: The journal Medical Physics recently published two papers that determine beam quality conversion factors, k(Q), for large sets of ion chambers. In the first paper [McEwen Med. Phys. 37, 2179-2193 (2010)], k(Q) was determined experimentally, while the second paper [Muir and Rogers Med. Phys. 37, 5939-5950 (2010)] provides k(Q) factors calculated using Monte Carlo simulations. This work investigates a variety of additional consistency checks to verify the accuracy of the k(Q) factors determined in each publication and a comparison of the two data sets. Uncertainty introduced in calculated k(Q) factors by possible variation of W/e with beam energy is investigated further. METHODS: The validity of the experimental set of k(Q) factors relies on the accuracy of the NE2571 reference chamber measurements to which k(Q) factors for all other ion chambers are correlated. The stability of NE2571 absorbed dose to water calibration coefficients is determined and comparison to other experimental k(Q) factors is analyzed. Reliability of Monte Carlo calculated k(Q) factors is assessed through comparison to other publications that provide Monte Carlo calculations of k(Q) as well as an analysis of the sleeve effect, the effect of cavity length and self-consistencies between graphite-walled Farmer-chambers. Comparison between the two data sets is given in terms of the percent difference between the k(Q) factors presented in both publications. RESULTS: Monitoring of the absorbed dose calibration coefficients for the NE2571 chambers over a period of more than 15 yrs exhibit consistency at a level better than 0.1%. Agreement of the NE2571 k(Q) factors with a quadratic fit to all other experimental data from standards labs for the same chamber is observed within 0.3%. Monte Carlo calculated k(Q) factors are in good agreement with most other Monte Carlo calculated k(Q) factors. Expected results are observed for the sleeve effect and the effect of cavity length on k(Q). The mean percent differences between experimental and Monte Carlo calculated k(Q) factors are -0.08, -0.07, and -0.23% for the Elekta 6, 10, and 25 MV nominal beam energies, respectively. An upper limit on the variation of W/e in photon beams from cobalt-60 to 25 MV is determined as 0.4% with 95% confidence. The combined uncertainty on Monte Carlo calculated k(Q) factors is reassessed and amounts to between 0.40 and 0.49% depending on the wall material of the chamber. CONCLUSIONS: Excellent agreement (mean percent difference of only 0.13% for the entire data set) between experimental and calculated k(Q) factors is observed. For some chambers, k(Q) is measured for only one chamber of each type--the level of agreement observed in this study would suggest that for those chambers the measured k(Q) values are generally representative of the chamber type.

Monte Carlo calculations of kQ, the beam quality conversion factor.

PURPOSE: To use EGSnrc Monte Carlo simulations to directly calculate beam quality conversion factors, kQ, for 32 cylindrical ionization chambers over a range of beam qualities and to quantify the effect of systematic uncertainties on Monte Carlo calculations of kQ. These factors are required to use the TG-51 or TRS-398 clinical dosimetry protocols for calibrating external radiotherapy beams. METHODS: Ionization chambers are modeled either from blueprints or manufacturers' user's manuals. The dose-to-air in the chamber is calculated using the EGSnrc user-code egs_chamber using 11 different tabulated clinical photon spectra for the incident beams. The dose to a small volume of water is also calculated in the absence of the chamber at the midpoint of the chamber on its central axis. Using a simple equation, kQ is calculated from these quantities under the assumption that W/e is constant with energy and compared to TG-51 protocol and measured values. RESULTS: Polynomial fits to the Monte Carlo calculated kQ factors as a function of beam quality expressed as %dd(10)x and TPR10(20) are given for each ionization chamber. Differences are explained between Monte Carlo calculated values and values from the TG-51 protocol or calculated using the computer program used for TG-51 calculations. Systematic uncertainties in calculated kQ values are analyzed and amount to a maximum of one standard deviation uncertainty of 0.99% if one assumes that photon cross-section uncertainties are uncorrelated and 0.63% if they are assumed correlated. The largest components of the uncertainty are the constancy of W/e and the uncertainty in the cross-section for photons in water. CONCLUSIONS: It is now possible to calculate kQ directly using Monte Carlo simulations. Monte Carlo calculations for most ionization chambers give results which are comparable to TG-51 values. Discrepancies can be explained using individual Monte Carlo calculations of various correction factors which are more accurate than previously used values. For small ionization chambers with central electrodes composed of high-Z materials, the effect of the central electrode is much larger than that for the aluminum electrodes in Farmer chambers.

Gait characteristics during inadvertent obstacle contacts in young, middle-aged and older adults.

BACKGROUND: When stepping over obstacles, analyses have focused on the successful trials to understand adaptive gait. However, examination of the inadvertent trips that occasionally occur in the laboratory can provide a rich source of information regarding the gait characteristics underlying trip-related falls. RESEARCH QUESTION: What gait variables during obstacle crossing are associated with inadvertent obstacle contacts, and are these variables different across the lifespan? METHODS: Three age groups included: young adults (20-35 years, N = 20), middle-aged adults (50-64 years, N = 15), and older adults (65-79 years, N = 19). A stationary, visible obstacle (26 cm tall) was placed in the middle of a walkway. Foot trajectories and head angles were compared between contact and non-contact trials. RESULTS: Twelve participants contacted the obstacle: seven young adults (3.5% of young adult trials), two middle-aged adults (1.3%), and three older adults (1.6%). Young and middle-aged adults contacted primarily with the trail limb, while older adults contacted primarily with the lead limb. Contacts occurred for different reasons: Most young adult contact trials had appropriate foot placement, but inadequate elevation; middle-aged and older adults demonstrated inappropriate foot placement before the obstacle, leading to foot contact during the swing phase. SIGNIFICANCE: Lower contact rates in the middle-aged and older adults indicates that the cautious strategies adopted during obstacle crossing are effective. Higher contact rates in young adults may indicate trial-and-error exploratory behavior. Inappropriate foot placement in the middle-aged and older adults may indicate impaired ability to gather obstacle position information during the approach phase.

Determination of consensus k Q values for megavoltage photon beams for the update of IAEA TRS-398.

The IAEA is currently coordinating a multi-year project to update the TRS-398 Code of Practice for the dosimetry of external beam radiotherapy based on standards of absorbed dose to water. One major aspect of the project is the determination of new beam quality correction factors, k Q , for megavoltage photon beams consistent with developments in radiotherapy dosimetry and technology since the publication of TRS-398 in 2000. Specifically, all values must be based on, or consistent with, the key data of ICRU Report 90. Data sets obtained from Monte Carlo (MC) calculations by advanced users and measurements at primary standards laboratories have been compiled for 23 cylindrical ionization chamber types, consisting of 725 MC-calculated and 179 experimental data points. These have been used to derive consensus k Q values as a function of the beam quality index TPR20,10 with a combined standard uncertainty of 0.6%. Mean values of MC-derived chamber-specific [Formula: see text] factors for cylindrical and plane-parallel chamber types in 60Co beams have also been obtained with an estimated uncertainty of 0.4%.

Changes in the control of obstacle crossing in middle age become evident as gait task difficulty increases.

BACKGROUND: Age-associated physiological changes result in modified gait, such as slower speed, for older adults. Identifying the onset of age-related gait changes will provide insight into the role of aging on locomotor control. It is expected that a more challenging gait task (obstacle crossing) puts more demands on physiological systems, and may reveal gait modifications in a middle-aged group that are not evident in an easier gait task (level walking). RESEARCH QUESTION: To identify the effect of advancing age on gait as a function of increasing locomotor challenge during an obstacle crossing task. METHODS: Three age groups (young, middle-aged, and older adults) stepped over an obstacle placed in a 15 m walkway. Task challenge ranged from low to high in four conditions: unobstructed gait, 3, 10, and 26 cm obstacles. Gait measures were calculated during the approach and crossing steps. RESULTS: Significant interactions were observed for gait speed (age by height by step, p < 0.01), foot placement variability (age by step, p < 0.01) and foot clearance (age by height, p = 0.05). Relative to young adults, older adults walked slower in all conditions and had higher foot clearances for the 10 and 26 cm obstacles. Middle-aged adults walked with speeds and foot clearances that were not different from young adults in the lower gait challenge conditions, and changed to values that were not different from older adults in the highest gait challenge conditions. Foot placement variability was greater for the middle-aged and older groups, but only in the last two steps before the obstacle. SIGNIFICANCE: Multiple gait changes were observed as early as middle-age, and changes in speed and foot clearance became more evident as task difficulty increased. The increased gait challenge placed more demands on the neuromuscular system, revealing age-related gait modifications that were not evident in the level walking gait task.

Genetic parameters for a multiple-trait multiple-lactation random regression test-day model in Italian Holsteins.

The objectives of this study were to estimate variance components for test-day milk, fat, and protein yields and average daily SCS in 3 subsets of Italian Holsteins using a multiple-trait, multiple-lactation random regression test-day animal model and to determine whether a genetic heterogeneous variance adjustment was necessary. Data were test-day yields of milk, fat, and protein and SCS (on a log2 scale) from the first 3 lactations of Italian Holsteins collected from 1992 to 2002. The 3 subsets of data included 1) a random sample of Holsteins from all herds in Italy, 2) a random sample of Holsteins from herds using a minimum of 75% foreign sires, and 3) a random sample of Holsteins from herds using a maximum of 25% foreign sires. Estimations of variances and covariances for this model were achieved by Bayesian methods using the Gibbs sampler. Estimated 305-d genetic, permanent environmental, and residual variance was higher in herds using a minimum of 75% foreign sires compared with herds using a maximum of 25% foreign sires. Estimated average daily heritability of milk, fat, and protein yields did not differ among subsets. Heritability of SCS in the first lactation differed slightly among subsets and was estimated to be the highest in herds with a maximum of 25% foreign sire use (0.19 +/- 0.01). Genetic correlations across lactations for milk, fat, and protein yields were similar among subsets. Genetic correlations across lactations for SCS were 0.03 to 0.08 higher in herds using a minimum of 75% or a maximum of 25% foreign sires, compared with herds randomly sampled from the entire population. Results indicate that adjustment for heterogeneous variance at the genetic level based on the percentage of foreign sire use should not be necessary with a multiple-trait random regression test-day animal model in Italy.

Small composite field correction factors for the CyberKnife radiosurgery system: clinical and PCSR plans.

A formalism has been proposed for small and non-standard photon fields in which [Formula: see text] correction factors are used to correct dosimeter response in small fields (indiviual or composite) relative to that in a larger machine-specific reference (MSR) field. For clinical plans consisting of several fields, a plan-class specific reference (PCSR) plan can also be defined, serving as an intermediate calibration field between the MSR and clinical plans within a certain plan-class. In this work, the formalism was applied in the calculation of [Formula: see text] for 21 clinical plans delivered by the [Formula: see text] radiosurgery system, each plan employing one or two of the smallest diameter collimators: 5 mm, 7.5 mm, and 10 mm. Three detectors were considered: the Exradin A16 and A26 micro chambers, and the W1 plastic scintillator. The clinical plans were grouped into 7 plan-classes according to commonly shared characteristics. The suitability of using a PCSR plan to represent the detector response of each plan within the plan-class was investigated. Total and intermediate correction factors were calculated using the [Formula: see text] Monte Carlo user code. The corrections for the micro chambers were large, primarily due to the presence of the low-density air cavity and the volume averaging effect. The correction for the scintillator was found to be close to unity for most plans, indicating that this detector may be used to measure small clinical plan correction factors in any plan except for those using the 5 mm collimator. The PCSR plan was shown to be applicable to plan-classes comprising isocentric plans only, with plan-classes divided according to collimator size. For non-isocentric plans, the variation of [Formula: see text] as a function of the point of measurement within a single plan, as well as the high inter-plan-class variability of the correction factor, precludes the use of a PCSR plan.

Investigation of endometrial abnormalities in asymptomatic women treated with tamoxifen and an evaluation of the role of endometrial screening.

PURPOSE: Tamoxifen is the most commonly prescribed adjuvant therapy for women with breast cancer. It has agonist activity on the endometrium and is associated with an increased risk of endometrial cancer. The aim of this study was to evaluate whether screening with transvaginal ultrasound (TV USS) with or without hysteroscopy is worthwhile. PATIENTS AND METHODS: A total of 487 women with breast cancer, 357 treated with tamoxifen and 130 controls, were screened with TV USS, and endometrial thickness was measured. Women with thickened endometrium underwent outpatient hysteroscopy. RESULTS: Length of time on tamoxifen ranged from 5 to 191 months (mean, 66 months), and endometrial thickness ranged from 1 to 38 mm (mean, 7.3 mm). Women treated with tamoxifen had significantly thicker endometrium than did controls (P <.0001). There was a statistically significant (P <.0001) positive correlation between length of time on tamoxifen and endometrial thickness. One hundred forty-five women had endometrium greater than 5 mm on USS, and 134 underwent successful outpatient hysteroscopy, 61 of whom had atrophic endometrium, resulting in a 46% false-positive scan rate. The remaining women all had benign features to explain the USS findings. CONCLUSION: TV USS detects a high incidence (41%) of apparent endometrial thickening in women treated with tamoxifen, although 46% had atrophic endometrium on further assessment, and none of the remaining asymptomatic women had significant lesions. Length of time on tamoxifen relates to endometrial thickening as measured by TV USS. TV USS is a poor screening tool because of the high false-positive rate. The low frequency of significant findings suggests that endometrial screening in asymptomatic women is not worthwhile.

Ion chamber absorbed dose calibration coefficients, N(D,w), measured at ADCLs: distribution analysis and stability.

PURPOSE: To analyze absorbed dose calibration coefficients, ND,w, measured at accredited dosimetry calibration laboratories (ADCLs) for client ionization chambers to study (i) variability among ND,w coefficients for chambers of the same type calibrated at each ADCL to investigate ion chamber volume fluctuations and chamber manufacturing tolerances; (ii) equivalency of ion chamber calibration coefficients measured at different ADCLs by intercomparing ND,w coefficients for chambers of the same type; and (iii) the long-term stability of ND,w coefficients for different chamber types by investigating repeated chamber calibrations. METHODS: Large samples of ND,w coefficients for several chamber types measured over the time period between 1998 and 2014 were obtained from the three ADCLs operating in the United States. These are analyzed using various graphical and numerical statistical tests for the four chamber types with the largest samples of calibration coefficients to investigate (i) and (ii) above. Ratios of calibration coefficients for the same chamber, typically obtained two years apart, are calculated to investigate (iii) above and chambers with standard deviations of old/new ratios less than 0.3% meet stability requirements for accurate reference dosimetry recommended in dosimetry protocols. RESULTS: It is found that ND,w coefficients for a given chamber type compared among different ADCLs may arise from differing probability distributions potentially due to slight differences in calibration procedures and/or the transfer of the primary standard. However, average ND,w coefficients from different ADCLs for given chamber types are very close with percent differences generally less than 0.2% for Farmer-type chambers and are well within reported uncertainties. CONCLUSIONS: The close agreement among calibrations performed at different ADCLs reaffirms the Calibration Laboratory Accreditation Subcommittee process of ensuring ADCL conformance with National Institute of Standards and Technology standards. This study shows that ND,w coefficients measured at different ADCLs are statistically equivalent, especially considering reasonable uncertainties. This analysis of ND,w coefficients also allows identification of chamber types that can be considered stable enough for accurate reference dosimetry.

Peto's paradox and the hallmarks of cancer: constructing an evolutionary framework for understanding the incidence of cancer.

An evolutionary perspective can help unify disparate observations and make testable predictions. We consider an evolutionary model in relation to two mechanistic frameworks of cancer biology: multistage carcinogenesis and the hallmarks of cancer. The multistage model predicts that cancer risk increases with body size and longevity; however, this is not observed across species (Peto's paradox), but the paradox is resolved by invoking the evolution of additional genetic mechanisms to suppress cancer in large, long-lived species. It is when cancer cells overcome these defence mechanisms that they exhibit the hallmarks of cancer, driving the ongoing evolution of these defences, which in turn is expected to create the differences observed in the genetics of cancer across species and tissues. To illustrate the utility of an evolutionary model we examined some recently published data linking stem-cell divisions and cancer incidence across a range of tissues and show why the original analysis was faulty, and demonstrate that the data are consistent with a multistage model varying from three to seven mutational hits across different tissues. Finally, we demonstrate how an evolutionary model can both define patterns of inherited (familial) cancer and explain the prevalence of cancer in post-reproductive years, including the dominance of epithelial cancers.

Proactive gait strategies to mitigate risk of obstacle contact are more prevalent with advancing age.

The purposes of this study were to determine if healthy older adults adopt strategies to decrease the likelihood of obstacle contact, and to determine how these strategies are modified as a function of advancing age. Three age groups were examined: 20-25 yo (N = 19), 65-79 yo (N = 11), and 80-91 yo (N = 18). Participants stepped over a stationary, visible obstacle on a walkway. Step length and gait speed progressively decreased with advancing age; the shorter step length resulted in closer foot placement to the obstacle and an associated increased risk of obstacle contact. Lead (first limb to cross the obstacle) and trail (second) limb trajectories were examined for behavior that mitigated the risk of contact. (1) Consistent trail foot placement before the obstacle across all ages allowed space and time for the trail foot to clear the obstacle. (2) To avoid lead limb contact due to closer foot placement before and after the obstacle, the lead toe was raised more vertically after toe-off, and then the foot was extended beyond the landing position (termed lead overshoot) and retracted backwards to achieve the shortened step length. Lead overshoot progressively increased with advancing age. (3) Head angle was progressively lower with advancing age, an apparent attempt to gather more visual information during approach. Overall, a series of proactive strategies were adopted to mitigate risk of contact. However, the larger, more abrupt movements associated with a more vertical foot trajectory and lead overshoot may compromise whole body balance, indicating a possible trade-off between risk of contact and stability.

The control of Staphylococcus epidermidis biofilm formation and in vivo infection rates by covalently bound furanones.

In order to overcome the continuing infection rate associated with biomaterials, the use of covalently bound furanones as an antibiofilm coating for biomaterials has been investigated. Furanones have previously been shown to inhibit growth of Gram-positive and Gram-negative bacteria. The aim of these studies were to covalently bind furanones to polymers and to test their efficacy for inhibiting biofilm formation of Staphylococcus epidermidis and in vivo infection rate. Two methods of covalent attachment of furanones were used. The first, a co-polymerisation with a styrene polymer, and second, a plasma-1-ethyl-3-(dimethylaminopropyl) carbodiimide (EDC) reaction to produce furanone-coated catheters. Biofilm formation by S. epidermidis in vitro was inhibited by 89% for polystryene-furanone disks and by 78% by furanone-coated catheters (p<0.01). In an in vivo sheep model we found furanones were effective at controlling infection for up to 65 days. Furanones have potential to be used as a coating for biomaterials to control infection caused by S. epidermidis.

Use of risk factors to allocate schedules for breast cancer screening.

In an effort to reduce the cost of breast cancer screening several studies have explored the possibility of using risk factors to select a high-risk group of women and then restrict screening to that group. The results of these studies have been almost entirely negative and so it is not possible at present to classify any woman as at such low risk that she need not be screened. Nevertheless it is well known that some groups of women can be identified as being at higher risk than the general population. In this study it is assumed that each woman will be offered one screen at which risk factor information will be collected. The usual screening policy is then one of uniform intervention in which the interval to the next screen is the same for all women: the interval that is currently recommended in the UK is three years. An alternative is a risk strategy in which the time to the next screen depends on the woman's risk status; thus the total number of screens available to the population are distributed according to risk status. Using data from the Edinburgh randomised trial of breast cancer screening these policies have been compared. It is estimated that the proportion of cases detected by screening in the three years following the completion of the initial screening round could be raised from 60% to 67% by adopting a risk strategy. Lead time benefits are also quantified as are the comparisons for an established screening programme.

Osteogenic sarcoma--a rare primary tumour of the breast.

A case of primary osteogenic sarcoma of the breast is reported and the histological and mammographic features demonstrated. A role for technetium 99 diphosphonate bone scanning in the diagnosis of this condition and measurement of serum alkaline phosphatase activity for monitoring progress of the disease is suggested.

Retrospective identification of chemical warfare agents by high-temperature automatic thermal desorption-gas chromatography-mass spectrometry.

An automated thermal desorption (ATD)-gas chromatography-mass spectrometry method was developed for the analysis of selected chemical warfare (CW) agents. Suitable methods were developed for analytes of high volatility to low volatility. The less volatile CW agents required the purchase and installation of a high-temperature valve upgrade kit allowing valve temperatures of up to 260 degrees C to be reached. The limit of detection was 50 ng on the tube for most CW agents in full scan. Chloropicrin exhibited some temperature dependence, with detection limits improving as ATD temperatures were decreased below 150 degrees C. A sample storage trial was undertaken to establish the most suitable storage environment for CW agents adsorbed onto Tenax TA. Temperature and time of storage were found to influence recovery of analytes with best recoveries being observed after 1 day storage in a freezer (-12 degrees C). This method was evaluated during a trial of procedures for sampling and identification of chemical agents at Porton Down, UK. Sulfur mustard was detected downwind of a simulated exploded munition.

Extraction of thiodiglycol from soil using pressurised liquid extraction.

Thiodiglycol (TDG) is the predominant hydrolysis product of the chemical warfare agent sulfur mustard. The extraction of TDG was investigated using pressurised liquid extraction and the results compared for a variety of different solvents and soils. TDG was analysed underivatised by gas chromatography with flame photometric detection. A mixture of methanol-water (9:1), proved to be the most efficient extracting solvent for TDG at a temperature of 150 degrees C and 10 MPa.

Ultrasound evaluation of ectopic pregnancy including correlation with human chorionic gonadotrophin levels.

A total of 267 patients referred for ultrasound examination because of clinical suspicion of ectopic pregnancy were studied prospectively. The outcome was correlated with the ultrasound findings and human chorionic gonadotrophin levels using a quantitative serum assay. The value of the following in the diagnosis of ectopic pregnancy are assessed: ultrasound criteria for early diagnosis of intrauterine pregnancy before visualization of a viable fetus, the discriminatory zone of hCG and extrauterine abnormalities.

Umbilical artery Doppler flow velocity waveform: the outcome of pregnancies with absent end diastolic flow.

We have studied doppler recordings of flow velocity waveform of the umbilical artery in 380 high-risk pregnancies and 160 patients with uncomplicated pregnancies. The results were not used in clinical management. In 24 cases, all in high-risk pregnancies, there was absence of end diastolic flow (AEDF). Four babies died, 22 out of 24 weighted less than the 5th centile for gestation and 20 were delivered by Caesarean Section. With only four exceptions, all fetuses showed other evidence of antenatal fetal compromise, though this was delayed in some cases for up to 24 days. AEDF does not appear to prejudice neonatal outcome. All six babies born weighing 750-999 g survived. In addition, 4 patients continued their pregnancy for 3-6 weeks after the first doppler with AEDF. Only one of the 4 developed other evidence of fetal compromise and two had vaginal deliveries. In our experience, AEDF is found only in abnormal pregnancy, and is a very serious sign of likely fetal compromise. It is an indication for extremely careful surveillance but not necessarily an indication for delivery.