Extrathymic tolerance of mature T cells: clonal elimination as a consequence of immunity

The mechanism by which T lymphocytes are tolerized to self or foreign antigens is still controversial. Clonal deletion is the major mechanism of tolerance for immature thymocytes; for mature T cells, tolerance is considered to reflect anergy rather than deletion, and to be a consequence of defective presentation of antigen. This paper documents a novel form of tolerance resulting when mature T cells encounter antigen in immunogenic form. Evidence is presented that exposure of mature T cells to Mlsa antigens in vivo leads to specific tolerance and disappearance of Mlsa-reactive V beta 6+ T cells. Surprisingly, the clonal elimination of V beta 6+ cells is preceded by marked expansion of these cells. Thus, tolerance induction can be the end result of a powerful immune response. These data raise important questions concerning the relationship of tolerance and memory.

A model for calculating tumour control probability in radiotherapy including the effects of inhomogeneous distributions of dose and clonogenic cell density

Most calculations of the biological effect of radiation on tumours assume that the clonogenic cell density is uniform even if account is taken of non-uniform dose distribution. In practice tumours will almost certainly have a non-uniform clonogenic cell density. This paper extends one particular model of tumour control probability (TCP) to incorporate a variable clonogenic cell density while at the same time assuming a constant 2 Gy fraction size and a uniform radiosensitivity throughout the treatment. Since there are virtually no in vivo data on the variation of density we consider some model situations. One clear conclusion is that a large reduction in clonogenic cell density at the edges of a tumour would permit only a very modest decrease in dose if the TCP is not to be reduced. In general the effect on TCP is a complicated function of the variation in both dose and clonogenic cell density. We give the equations which enable both to be included.

Optimisation of conformal radiotherapy dose distributions by simulated annealing

A method of computing the beam profiles for multi-element multiple-beam radiotherapy treatment is presented. This is applicable to the treatment planning of conformal radiotherapy using an isocentric rotation technique. The method begins with the treatment dose prescription and calculates the beam profiles rather than the reverse 'conventional' planning technique. The method is the iterative optimisation method of simulated annealing. It is shown using published 'difficult' clinical treatment planning problems that the high-dose region can be very precisely tailored to the tumour volume even when this has a re-entrant (concave) periphery. Simultaneously the dose can be constrained in other sensitive regions. The mathematics of the method is explained together with this implementation. The analogy of this optimisation with certain reconstruction problems in medical imaging is drawn and by way of experiment dose distributions are presented in image form and beam profiles as sinograms.

Persistent functional deficit in multiple sclerosis and autosomal dominant cerebellar ataxia is associated with axon loss

Proton magnetic resonance spectroscopy (MRS) and MRI were carried out in 11 patients with multiple sclerosis who had clinical evidence of severe cerebellar involvement, 11 multiple sclerosis patients (of similar age and disease duration) who had minimal or no signs of cerebellar disease, eight patients with autosomal dominant cerebellar ataxia (ADCA) and 11 healthy controls. In all subjects MRS was localized to cerebellar white matter (volumes of interest 3-6 ml). Apparent metabolite concentrations were calculated using the fully relaxed water spectrum as an internal standard of reference. The patients also underwent MRI to assess cerebellar volume and (in the two multiple sclerosis groups) lesion volume within the posterior fossa. Magnetic resonance spectroscopy from cerebellar white matter showed a highly significant reduction in the concentration of N-acetyl groups (NA) [which consists predominantly of N-acetylaspartate (NAA), a neuronal marker] in the multiple sclerosis group with cerebellar deficit compared with the multiple sclerosis group with minimal or no signs of cerebellar involvement, and healthy controls. Follow-up MRS performed in six of the multiple sclerosis patients 9 months later showed no change in the median NA concentration. The ADCA group showed a significant reduction of NA from a region of cerebellar white matter and also a reduction in the concentration of choline-containing compounds. The multiple sclerosis group with severe cerebellar deficit and the ADCA group both had significant cerebellar atrophy (suggesting nerve cell body and axon loss) compared with the multiple sclerosis patients with minimal or no signs of cerebellar deficit and healthy controls. The multiple sclerosis patients with cerebellar deficit had a significantly greater lesion volume in the posterior fossa, although the proportion of the spectroscopic voxel occupied by lesions was small, suggesting that axonal loss from normal appearing white matter also contributes to the observed reduction in NA. These results support the hypothesis that axonal loss is important in the development of persistent clinical disability in multiple sclerosis.

Cardiovascular, skeletal, and renal defects in mice with a targeted disruption of the Pkd1 gene

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by cyst formation in the kidney, liver, and pancreas and is associated often with cardiovascular abnormalities such as hypertension, mitral valve prolapse, and intracranial aneurysms. It is caused by mutations in PKD1 or PKD2, encoding polycystin-1 and -2, which together form a cell surface nonselective cation ion channel. Pkd2-/- mice have cysts in the kidney and pancreas and defects in cardiac septation, whereas Pkd1(del34) -/- and Pkd1(L) -/- mice have cysts but no cardiac abnormalities, although vascular fragility was reported in the latter. Here we describe mice carrying a targeted mutation in Pkd1 (Pkd1(del17-21betageo)), which defines its expression pattern by using a lacZ reporter gene and may identify novel functions for polycystin-1. Although Pkd1(del17-21betageo) +/- adult mice develop renal and hepatic cysts, Pkd1(del17-21betageo) -/- embryos die at embryonic days 13.5-14.5 from a primary cardiovascular defect that includes double outflow right ventricle, disorganized myocardium, and abnormal atrio-ventricular septation. Skeletal development is also severely compromised. These abnormalities correlate with the major sites of Pkd1 expression. During nephrogenesis, Pkd1 is expressed in maturing tubular epithelial cells from embryonic day 15.5. This expression coincides with the onset of cyst formation in Pkd1(del34) -/-, Pkd1(L) -/-, and Pkd2-/- mice, supporting the hypothesis that polycystin-1 and polycystin-2 interact in vivo and that their failure to do so leads to abnormalities in tubule morphology and function.

Fibroproliferation occurs early in the acute respiratory distress syndrome and impacts on outcome

The fibroproliferative phase of acute respiratory distress syndrome (ARDS) has traditionally been regarded as a late event but recent studies that suggest increased lung collagen turnover within 24 h of diagnosis challenge this view. We hypothesized that fibroproliferation is initiated early in ARDS, characterized by the presence of fibroblast growth factor activity in the lung and would relate to clinical outcome. Patients fulfilling American/European Consensus Committee criteria for ARDS and control patients ventilated for non-ARDS respiratory failure underwent bronchoalveolar lavage (BAL) and serum sampling within 24 h of diagnosis and again at 7 d. The ability of BAL fluid (BALF) to stimulate human lung fibroblast proliferation in vitro was examined in relation to concentrations of N-terminal peptide for type III procollagen (N-PCP-III) in BALF/serum and clinical indices. At 24 h, ARDS lavage fluid demonstrated potent mitogenic activity with a median value equivalent to 70% (range 31-164) of the response to serum, and was significantly higher than control lavage (32% of serum response, range 11-42; p < 0.05). At 24 h, serum N-PCP-III concentrations were elevated in the ARDS group compared with control patients (2.8 U/ml; range 0.6-14.8 versus 1.1 U/ml; range 0.4-3.7, p < 0.0001) as were BALF N-PCP-III concentrations (2.9 U/ml; range 0. 6-11.4 versus 0.46 U/ ml; range 0.00-1.63, p < 0.01). In addition, BALF N-PCP-III concentrations at 24 h were significantly elevated in nonsurvivors of ARDS compared with survivors (p < 0.05). At 7 d, the mitogenic activity remained elevated in the ARDS group compared with control (p < 0.05) and was also significantly higher in ARDS nonsurvivors compared with survivors (67%; range 45-120 versus 31%; range 16-64, p < 0.05). These data are consistent with the hypothesis that fibroproliferation is an early response to lung injury and an important therapeutic target.

Indirect human exposure to pharmaceuticals via drinking water

There are numerous observations of pharmaceuticals (or their metabolites) as contaminants in wastewater, surface water and groundwater. This implies a potential for indirect human exposure to pharmaceuticals via drinking water supplies. Various effect benchmarks may be employed in an evaluation of the significance of such indirect exposure. In this study a comparison was made between reported concentrations of pharmaceuticals in German drinking water and therapeutic dose. The margin between potential indirect daily exposure via drinking water and daily therapeutic dose was at least three orders of magnitude and typically much more. For certain compounds it was also possible to benchmark exposure against ADIs derived within the context of meat and food stuff residues following veterinary use. In all cases potential exposure was less than the ADIs, similarly suggesting that for these compounds there are no substantial concerns with regards to indirect exposure via drinking water.

Mechanisms of postnatal neurobiological development: implications for human development

This review focuses on the postnatal neuroanatomical changes that arise during the first years of human life. Development is characterized by 2 major organizational periods. The first period begins at conception and includes the major histogenetic events such as neurulation, proliferation, migration, and differentiation. It has been proposed that these events may be controlled by genetic and epigenetic events, which give rise to neural structures that are amenable to external influence. The second period is a time of reorganization in the human cortex. These events occur during gestation and continue postnatally, possibly through the 2nd decade of life. This stage is characterized by dendritic and axonal growth, synapse production, neuronal and synaptic pruning, and changes in neurotransmitter sensitivity. Although the initiation of these events is influenced by endogenous signals, further neural maturation is primarily influenced by exogenous signals. To illustrate both the progressive and regressive events during the postnatal period, we use examples from the development of the human cortex.

EEG mu rhythm and imitation impairments in individuals with autism spectrum disorder

Imitation ability has consistently been shown to be impaired in individuals with autism. A dysfunctional execution/observation matching system has been proposed to account for this impairment. The EEG mu rhythm is believed to reflect an underlying execution/observation matching system. This study investigated evidence of differential mu rhythm attenuation during the observation, execution, and imitation of movements and examined its relation to behaviorally assessed imitation abilities. Fourteen high-functioning adults with autism spectrum disorder (ASD) and 15 IQ- and age-matched typical adults participated. On the behavioral imitation task, adults with ASD demonstrated significantly poorer performance compared to typical adults in all domains of imitation ability. On the EEG task, both groups demonstrated significant attenuation of the mu rhythm when executing an action. However, when observing movement, the individuals with ASD showed significantly reduced attenuation of the mu wave. Behaviorally assessed imitation skills were correlated with degree of mu wave attenuation during observation of movement. These findings suggest that there is execution/observation matching system dysfunction in individuals with autism and that this matching system is related to degree of impairment in imitation abilities.

Reduction of small and large bowel irradiation using an optimized intensity-modulated pelvic radiotherapy technique in patients with prostate cancer

PURPOSE

To investigate the role of intensity-modulated radiation therapy (IMRT) to irradiate the prostate gland and pelvic lymph nodes while sparing critical pelvic organs, and to optimize the number of beams required.

METHODS AND MATERIALS

Target, small bowel, colon, rectum, and bladder were outlined on CT planning scans of 10 men with prostate cancer. Optimized conventional (RT) and 3-dimensional conformal radiotherapy (3D-CRT) plans were created and compared to inverse-planned IMRT dose distributions using dose-volume histograms. Optimization of beam number was undertaken for the IMRT plans.

RESULTS

With RT the mean percentage volume of small bowel and colon receiving >45 Gy was 21.4 +/- 5.4%. For 3D-CRT it was 18.3 +/- 7.7% (p = 0.0043) and for 9-field IMRT it was 5.3 +/- 1.8% (p < 0.001 compared to 3D-CRT). For 7, 5, and 3 IMRT fields, it was 6.4 +/- 2.9%, 7.2 +/- 2.8%, and 8.4 +/- 3.8% (all p < 0.001 compared to 3D-CRT). The rectal volume irradiated >45 Gy was reduced from 50.5 +/- 16.3% (3D-CRT) to 5.8 +/- 2.1% by 9-field IMRT (p < 0. 001) and bladder from 52.2 +/- 12.8% to 7 +/- 2.8% (p < 0.001). Similar benefits were maintained for 7, 5, and 3 IMRT fields.

CONCLUSIONS

The reduction in critical pelvic organ irradiation seen with IMRT may reduce side effects in patients, and allow modest dose escalation within acceptable complication rates. These reductions were maintained with 3-5 IMRT field plans which potentially allow less complex delivery techniques and shorter delivery times.

The physical basis of IMRT and inverse planning

Intensity-modulated radiation therapy (IMRT) can sculpt the high-dose volume around the site of disease with hitherto unachievable precision. Conformal avoidance of normal tissues goes hand in hand with this. Inhomogeneous dose painting is possible. The technique has become a clinical reality and is likely to be the dominant approach this decade for improving the clinical practice of photon therapy. This Series will explore all aspects of the "IMRT chain". Only 15 years ago just a handful of physicists were working on this subject. IMRT has developed so rapidly that its recent past is also its ancient history. This article will review the history of IMRT with just a glance at precursors. The physical basis of IMRT is then described including an attempt to introduce the concepts of convex and concave dose distributions, ill-conditioning, inverse-problem degeneracy, cost functions and complex solutions all with a minimum of technical jargon or mathematics. The many techniques for inverse planning are described and the review concludes with a look forward to the future of image-guided IMRT (IG-IMRT).

An autoantigenic T cell epitope forms unstable complexes with class II MHC: a novel route for escape from tolerance induction

The peptide rAc1-11 represents the dominant T cell epitope of rat myelin basic protein (MBP) in mice of the H-2u haplotype. Residue 4 has been shown previously to govern binding of the peptide to the class II molecule, I-Au. We have constructed peptide analogues bearing amino acid substitutions at position 4 and have assessed their ability to stimulate an antigen-specific T cell hybridoma when presented by viable antigen presenting cells (APC). Complexes between I-Au and one such analogue, rAc1-11[4A], were rapidly lost from the surface of live APC displaying a half-life (t 1/2) of approximately 10 min. Neither shedding of intact complexes from the cell surface, nor their internalization and recycling through an acidic intracellular compartment were found to account for their loss. The possible dissociation of rAc1-11[4A] from the peptide binding cleft was therefore addressed by comparing the t 1/2 of complexes between I-Au and peptide analogues of higher affinity. The tyrosine-substituted analogue, rAc1-11[4Y], remained stably bound to I-Au for at least 4 h, thereby displaying a t 1/2 far in excess of that evident for rAc1-11[4A]. Significantly, the wild type peptide, rAc1-11, bound so transiently that functional complexes could not be detected on the surface of peptide-pulsed APC. The physiological relevance of these findings was confirmed by extending our studies to an analysis of the homologous epitope of murine MBP; evidence that this epitope likewise displays minimal affinity for I-Au suggests a novel strategy for the escape from tolerance induction by encephalitogenic T cells.

Removal of fragrance materials during U.S. and European wastewater treatment

The concentrations and removals of 16 fragrance materials (EMs) were measured in 17 U.S. and European wastewater treatment plants between 1997 and 2000 and were compared to predicted values. The average FM profile and concentrations in U.S. and European influent were similar. The average FM profile in primary effluent was similar to the average influent profile; however, the concentration of FMs was reduced by 14.6-50.6% in primary effluent. The average FM profile in final effluent was significantly different from the primary effluent profile and was a function of the design of the wastewater treatment plant. In general, the removal of sorptive, nonbiodegradable FMs was correlated with the removal of total suspended solids in the plant, while the removal of nonsorptive, biodegradable FMs was correlated with 5-day Biological Oxidation Demand removal in the plant. The overall plant removal (primary + secondary treatment) of FMs ranged from 87.8 to 99.9% for activated sludge plants, 58.6-99.8% for carousel plants, 88.9-99.9% for oxidation ditch plants, 71.3-98.6% for trickling filter plants, 80.8-99.9% for a rotating biological contactor plant, and 96.7-99.9% for lagoons. The average concentration of FMs in final effluent ranged from the limit of quantitation (1-3 ng/L) to 8 microg/L. Measured FM removal and concentrations were compared to predicted values, which were based on industry volume, per capita water use, octanol-water partition coefficient, and biodegradability.

Targeted disruption of p70(s6k) defines its role in protein synthesis and rapamycin sensitivity

Here, we disrupted the p70 S6 kinase (p70(s6k)) gene in murine embryonic stem cells to determine the role of this kinase in cell growth, protein synthesis, and rapamycin sensitivity. p70(s6k-/-) cells proliferated at a slower rate than parental cells, suggesting that p70(s6k) has a positive influence on cell proliferation but is not essential. In addition, rapamycin inhibited proliferation of p70(s6k-/-) cells, indicating that other events inhibited by the drug, independent of p70(s6k), also are important for both cell proliferation and the action of rapamycin. In p70(s6k-/-) cells, which exhibited no ribosomal S6 phosphorylation, translation of mRNA encoding ribosomal proteins was not increased by serum nor specifically inhibited by rapamycin. In contrast, rapamycin inhibited phosphorylation of initiation factor 4E-binding protein 1 (4E-BP1), general mRNA translation, and overall protein synthesis in p70(s6k-/-) cells, indicating that these events proceed independently of p70(s6k) activity. This study localizes the function of p70(s6k) to ribosomal biogenesis by regulating ribosomal protein synthesis at the level of mRNA translation.

Management of cervical intraepithelial neoplasia during pregnancy with LOOP excision

OBJECTIVE

Management of severe cervical dysplasia/possible microinvasive carcinoma during pregnancy is frequently associated with significant morbidity. The purpose of this study is to determine the efficacy of LOOP excision performed during pregnancy, and also to record the nature and frequency of complications of the procedure.

METHODS

Twenty women underwent LOOP excision during pregnancy. The gestational age range was 8-34 weeks. Data concerning indications, complications, and histopathologic results were recorded.

RESULTS

Fourteen of 20 (70%) had dysplastic changes in the LOOP specimen. Eight of 14 (57%) had involved margins. Nine of 19 (47%) had residual dysplasia 3 months postpartum, including 3 patients whose initial LOOP specimens were negative for dysplasia. Significant morbidity included 3 preterm births, 2 patients who required blood transfusion following LOOP, and 1 unexplained intrauterine fetal demise documented 4 weeks post-LOOP. The gestational age range of those patients who had significant morbidity was 27-34 weeks.

CONCLUSIONS

LOOP excision of the cervix during pregnancy does not consistently produce diagnostic specimens and is associated with a significant rate of residual disease. Morbidity appears similar to that of cone biopsy during pregnancy and occurs primarily when the procedure is performed in the third trimester. Until refinements in technique occur, LOOP excision during pregnancy should be reserved for limited indications.

Conformational photoswitching of a synthetic peptide foldamer bound within a phospholipid bilayer

The dynamic properties of foldamers, synthetic molecules that mimic folded biomolecules, have mainly been explored in free solution. We report on the design, synthesis, and conformational behavior of photoresponsive foldamers bound in a phospholipid bilayer akin to a biological membrane phase. These molecules contain a chromophore, which can be switched between two configurations by different wavelengths of light, attached to a helical synthetic peptide that both promotes membrane insertion and communicates conformational change along its length. Light-induced structural changes in the chromophore are translated into global conformational changes, which are detected by monitoring the solid-state (19)F nuclear magnetic resonance signals of a remote fluorine-containing residue located 1 to 2 nanometers away. The behavior of the foldamers in the membrane phase is similar to that of analogous compounds in organic solvents.

Cardiac avoidance in breast radiotherapy: a comparison of simple shielding techniques with intensity-modulated radiotherapy

BACKGROUND AND PURPOSE

Adjuvant breast radiotherapy (RT) is now part of the routine care of patients with early breast cancer. However, analysis of the Early Breast Cancer Trialists' Collaborative suggests that patients with the lowest risk of dying of breast cancer are at significant risk of cardiac mortality due to longer relapse-free survival. Patients with a significant amount of heart in the high-dose volume have been shown to be at risk of fatal cardiac events. This study was designed to assess whether conformal planning or intensity-modulated radiotherapy (IMRT) techniques allow reduced cardiac irradiation whilst maintaining full target coverage.

MATERIAL AND METHODS

Ten patients with early breast cancer were available for computed tomography (CT) planning. Each had at least 1 cm maximum heart depth within the posterior border of conventional tangents. For each patient, plans were generated and compared using dose volume histograms for planning target volume (PTV) and organs at risk. The plans included conventional tangents with and without shielding. The shielding was designed to either completely spare the heart or to shield as much heart as possible without compromising PTV coverage. IMRT plans were also prepared using two- and four-field tangential and six-field arc-like beam arrangements.

RESULTS

PTV homogeneity was better for the tangential IMRT techniques. For all patients, cardiac irradiation was reduced by the addition of partial cardiac shielding to conventional tangents, without compromise of PTV coverage. The two- and four-field IMRT techniques also reduced heart doses. The average percentage volume of heart receiving >60% of the prescription dose was 4.4% (range 1.0-7.1%) for conventional tangents, 1.5% (0.2-3.9%) for partial shielding, 2.3% (0.5-4.6%) for the two-field IMRT technique and 2.2% (0.4-5.6%) for the four-field IMRT technique. For patients with larger maximum heart depths the four-field IMRT plan achieved greater heart sparing than the partial shielding, although irradiation of the contralateral breast was increased. Full cardiac shielding resulted in the most complete heart sparing but with compromise of the PTV coverage; the mean volume receiving less than 95% of the prescription dose was 4% (range 1.5-8.7%).

CONCLUSION

All patients undergoing adjuvant tangential breast RT in whom the heart is seen to be in the high-dose volume should be considered for the addition of cardiac-sparing lead blocks. Three-dimensional CT planning and alternative beam arrangements with IMRT optimization enables more complete cardiac sparing without compromise of PTV coverage in certain patients.

Detection of 'Candidatus Liberibacter asiaticus' in Diaphorina citri and its importance in the management of citrus huanglongbing in Florida

Citrus huanglongbing (HLB or citrus greening), is a highly destructive disease that has been spreading in both Florida and Brazil. Its psyllid vector, Diaphorina citri Kuwayama, has spread to Texas and Mexico, thus threatening the future of citrus production elsewhere in mainland North America. Even though sensitive diagnostic methods have been developed for detection of the causal organisms, Candidatus Liberibacter spp., the pathogen cannot be detected consistently in plants until symptoms develop, presumably because of low titer and uneven distribution of the causal bacteria in nonsymptomatic tissues. In the present study, TaqMan based real-time quantitative polymerase chain reaction methodology was developed for detection of 'Ca. L. asiaticus' in D. citri. Over 1,200 samples of psyllid adults and nymphs, collected from various locations in Florida, from visually healthy and HLB symptomatic trees at different times of the year were analyzed to monitor the incidence and spread of HLB. The results showed that spread of 'Ca. L. asiaticus' in an area may be detected one to several years before the development of HLB symptoms in plants. The study suggests that discount garden centers and retail nurseries may have played a significant role in the widespread distribution of psyllids and plants carrying HLB pathogens in Florida.

Optimization by simulated annealing of three-dimensional conformal treatment planning for radiation fields defined by a multileaf collimator

Three-dimensional conformal radiotherapy may be achieved by using a combination of geometrically shaped radiation fields from different orientations around the patient. A convenient method to shape the fields is to use a multileaf collimator. These fields are shaped to the beam's-eye-view of the target volume, at each orientation of the collimator, and may also encompass sensitive structure, i.e. organs at risk, if the target region has concavities in its outline within which such structure may reside. The term 'conformal therapy' is used in this paper to mean tailoring the high dose volume to the target volume whilst minimising dose to other normal structures (organs at risk) which may be irradiated by the treatment fields, shaped by a multileaf collimator. The question then arises of the optimum distribution of beam weights to apply to the fields to minimise dose to organs at risk whilst aiming towards a uniform dose distribution in the target volume. This paper provides a method of optimizing the choice of beamweights to achieve this. The method is based on the well known optimization technique of simulated annealing. Either an optimal set of beamweights, one weight per field, is generated or the intensity may be spatially modulated across the field at each orientation (two weights per field) depending on whether there is just target volume or both target volume and volume containing organs at risk in the line of sight. It is shown that the dose matrix resulting from the latter optimization is closer to the dose prescription than that obtained by using either an optimal set of single weights per field or uniform beamweights.

Heterosynaptic metaplasticity in the hippocampus in vivo: a BCM-like modifiable threshold for LTP

The homeostatic maintenance of the "modification threshold" for inducing long-term potentiation (LTP) is a fundamental feature of the Bienenstock, Cooper, and Munro (BCM) model of synaptic plasticity. In the present study, two key features of the modification threshold, its heterosynaptic expression and its regulation by postsynaptic neural activity, were tested experimentally in the dentate gyrus of awake, freely moving rats. Conditioning stimulation ranging from 10 to 1,440 brief 400-Hz trains, when applied to medial perforant path afferents, raised the threshold for LTP induction heterosynaptically in the neighboring lateral perforant path synapses. This effect recovered slowly over a 7- to 35-day period. The same conditioning paradigms, however, did not affect the reversal of long-term depression. The inhibition of LTP by medial-path conditioning stimulation was N-methyl-D-aspartate (NMDA) receptor-dependent, but antidromic stimulation of the granule cells could also inhibit lateral path LTP induction, independently of NMDA receptor activation. Increased calcium buffering is a potential mechanism underlying the altered LTP threshold, but the levels of two important calcium-binding proteins did not increase after conditioning stimulation, nor was de novo protein synthesis required for generating the threshold shift. These data confirm, in an in vivo model, two key postulates of the BCM model regarding the LTP threshold. They also provide further evidence for the broad sensitivity of synaptic plasticity mechanisms to the history of prior activity, i.e., metaplasticity.

Optimization by simulated annealing of three-dimensional, conformal treatment planning for radiation fields defined by a multileaf collimator: II. Inclusion of two-dimensional modulation of the x-ray intensity

Interest is rapidly growing in using multiple x-radiation fields defined by a multileaf collimator to achieve conformal radiotherapy. Three-dimensional treatment planning in such situations is in its infancy and most 3D planning systems provide no tools for optimizing therapy. A previous paper addressed how to calculate optimum beamweights when both the target volume and all or some parts of organs at risk were in the fields-of-view. This work allowed a maximum of two weights per field. The present paper extends this technique to allow each radiation port to be spatially modulated across the geometrically shaped field. An optimization method based on simulated annealing is presented. It is shown that including spatial modulation leads to a wider separation between the dose-volume histograms of the target volume and organs at risk. The improvement is quantified in terms of the tumour control probability at constant normal tissue complication probability. Possible limitations of the a posteriori applied biological model are discussed in detail.

A cone-beam megavoltage CT scanner for treatment verification in conformal radiotherapy

PURPOSE

A prototype scanner for large-volume megavoltage computed tomography (MVCT) in a clinical set-up is described. The ultimate aim is to improve treatment accuracy in conformal radiotherapy through patient set-up error reduction and transit dosimetry.

MATERIALS AND METHODS

The scanner consists of a custom-built 2D CsI(Tl) crystal array viewed by a lens and a CCD camera. Image acquisition is synchronized with radiation pulses. The 2D projections resulting from a single continuous 360 degrees gantry rotation are reconstructed using a cone-beam tomography algorithm. Prior to reconstruction, the raw projections are calibrated and corrected for centre of rotation movement and accelerator output fluctuation. The performance of the system has been evaluated by reconstructing projections of open fields, test objects and a humanoid phantom.

RESULTS

Hundreds of 2D projections can be acquired with a clinically-acceptable data collection time (about 2 min) and dose (approximately 40 cGy, with a possible four-fold reduction). A maximum density resolution of about 2% is achieved offering some soft tissue discrimination without using image enhancement tools. A spatial resolution of 2.5 mm is obtained. The reconstructed image intensity is linear with electron density over the range of interest. Coronal or sagittal slices through the 3D reconstruction of the humanoid phantom show a better delineation of structures than the corresponding portal images taken at the same orientation.

CONCLUSIONS

A similar image quality to our current single-slice MVCT scanner is achieved with the advantage of providing tens of tomographic slices for a single gantry rotation. This work demonstrates the feasibility of clinical cone-beam MVCT and indicates how this prototype can be improved.

Intensity modulated radiation therapy: a clinical review

Intensity modulated radiotherapy represents a significant advance in conformal radiotherapy. In particular, it allows the delivery of dose distributions with concave isodose profiles such that radiosensitive normal tissue close to, or even within a concavity of, a tumour may be spared from radiation injury. This article reviews the clinical application of this technique to date, and discusses the practical issues of treatment planning and delivery from the clinician's perspective.

Factors influencing the idiopathic development of pulmonary hypertension in the fawn hooded rat

Fawn hooded rats (FHR), a strain of rat with a hereditary bleeding tendency due to a genetic defect in platelet aggregation, have recently been found to develop pulmonary hypertension. However, whether the pulmonary hypertension in FHR has a genetic basis or simply reflects the influence of extrinsic factors known to increase pulmonary artery pressure in other rat strains has not been fully evaluated. To further examine the structural and hemodynamic changes of pulmonary hypertension in FHR, and to investigate the extent to which alveolar hypoxia may have promoted these abnormalities, hemodynamic and morphometric measurements were made in FHR (4 to 24 wk) and compared with age-matched Sprague-Dawley (SDR) control rats. Increases in mean pulmonary artery pressure, total pulmonary resistance, and right ventricular enlargement were present in both male and female FHR and were evident at an early age (4 wk). Morphometric analysis of barium gelatin-infused lungs revealed marked pulmonary vascular remodelling in FHR characterized as extension of muscle into more peripheral pulmonary vessels, medial hypertrophy of proximal vessels, and reduced number of barium-filled arteries. The increases in pulmonary artery pressure in FHR were not due to the influence of more severe hypoxia, hypoventilation, or polycythemia, as blood gas tension and hematocrit were similar in FHR and SDR. Moreover, we found that pulmonary hypertension could be transmitted to backcross and second filial generation offspring arising from selective matings between FHR and control Wistar Kyoto rats, confirming the heritable basis for pulmonary hypertension in the FHR.

Atrial development in the human heart: an immunohistochemical study with emphasis on the role of mesenchymal tissues

The development of the atrial chambers in the human heart was investigated immunohistochemically using a set of previously described antibodies. This set included the monoclonal antibody 249-9G9, which enabled us to discriminate the endocardial cushion-derived mesenchymal tissues from those derived from extracardiac splanchnic mesoderm, and a monoclonal antibody recognizing the B isoform of creatine kinase, which allowed us to distinguish the right atrial myocardium from the left. The expression patterns obtained with these antibodies, combined with additional histological information derived from the serial sections, permitted us to describe in detail the morphogenetic events involved in the development of the primary atrial septum (septum primum) and the pulmonary vein in human embryos from Carnegie stage 14 onward. The level of expression of creatine kinase B (CK-B) was found to be consistently higher in the left atrial myocardium than in the right, with a sharp boundary between high and low expression located between the primary septum and the left venous valve indicating that the primary septum is part of the left atrial gene-expression domain. This expression pattern of CK-B is reminiscent of that of the homeobox gene Pitx2, which has recently been shown to be important for atrial septation in the mouse. This study also demonstrates a poorly appreciated role of the dorsal mesocardium in cardiac development. From the earliest stage investigated onward, the mesenchyme of the dorsal mesocardium protrudes into the dorsal wall of the primary atrial segment. This dorsal mesenchymal protrusion is continuous with a mesenchymal cap on the leading edge of the primary atrial septum. Neither the mesenchymal tissues of the dorsal protrusion nor the mesenchymal cap on the edge of the primary septum expressed the endocardial tissue antigen recognized by 249-9G9 at any of the stages investigated. The developing pulmonary vein uses the dorsal mesocardium as a conduit to reach the primary atrial segment. Initially, the pulmonary pit, which will becomes the portal of entry for the pulmonary vein, is located along the midline, flanked by two myocardial ridges. As development progresses, tissue remodeling results in the incorporation of the portal of entry of the pulmonary vein in left atrial myocardium, which is recognized because of its high level of creatine. Closure of the primary atrial foramen by the primary atrial septum occurs as a consequence of the fusion of these mesenchymal structures.