Effects of internal and external scatter on the build-up characteristics of Monte Carlo calculated absorbed dose for electron irradiation

The effects of internal and external scatter on surface, build-up and depth dose characteristics simulated by Monte Carlo code EGSnrc for varying field size and SSD for a 10 MeV monoenergetic electron beam with and without an accelerator model are extensively studied in this paper. In particular, sub-millimetre surface PDD was investigated. The percentage depth doses affected significantly by the external scatter show a larger build-up dose. A forward shifted Dmax depth and a sharper fall-off region compared to PDDs with only internal scatter considered. The surface dose with both internal and external scatter shows a marked decrease at 110 cm SSD, and then slight further changes with the increasing SSD since few external scattered particles from accelerator model can reach the phantom for large SSDs. The sharp PDD increase for the 5 cm x 5 cm field compared to other fields seen when only internal scatter is considered is significantly less when external scatter is also present. The effect of external scatter on surface PDD is more pronounced for large fields than small fields (5 cm x 5 cm field).

Effects of treatment distance and field size on build-up characteristics of Monte Carlo calculated absorbed dose for electron irradiation

Surface, build-up and depth dose characteristics of a monoenergetic electron point source simulated by Monte Carlo code MCNP4c for varying field size and SSD are extensively studied in this paper. MCNP4c (Monte Carlo N-Particle Transport Code System) has been extensively used in clinical dose simulation for its versatility and powerful geometrical coding tool. A sharp increase in PDD is seen with the Monte Carlo Modelling immediately at the surface within the first 0.2 mm. This effect cannot be easily measured by experimental instruments for electron contamination, and may lead to a clinical underdosing of the basal cell layer, which is one of the most radiation sensitive layers and the main target for skin carcinogenesis. A high percentage build-up dose for electron irradiation was shown. No significant effects in surface PDDs were modelled with different SSD values from 95 cm to 125 cm. Three depths were studied in detail, these being 0.05 mm, the lower depth of the basal cell layer; 0.95 mm, the lower depth of the dermal layer; and 0.95 cm, a position within the subcutaneous tissue. Results showed only small surface PDD differences were modelled for SSD variations from 95 cm to 125 cm and field sizes variation from the values between 5 cm and 10 cm squares to 25 cm. When the field side length is smaller than this, the surface dose shows an increasing trend by about 7% at 5 x 5 cm2.

Human granulocytic ehrlichiosis presenting as facial diplegia in a 42-year-old woman

Neurologic manifestations of human ehrlichiosis are unusual and have been described almost exclusively in human monocytic ehrlichiosis associated with Ehrlichia chaffeensis. We report here a case of a previously healthy 42-year-old woman who developed bilateral facial nerve palsies in association with infection by the agent of human granulocytic ehrlichiosis (aoHGE). The diagnosis was made by specific polymerase chain reaction amplification of aoHGE sequences from samples of the patient's blood and cerebrospinal fluid (CSF), as well as propagation of aoHGE in culture of HL60 cells inoculated with the patient's CSF. To our knowledge, this is the first report directly demonstrating the presence of aoHGE in CSF, and it underscores the importance of considering HGE in patients presenting with a nonspecific febrile illness and unexplained neurologic manifestations. HGE should also be considered in the differential diagnosis of bilateral facial palsy-a rare occurrence.

On-resin macrocyclization of peptides via intramolecular SnAr reactions

On-resin macrocyclization via an SNAr reaction is employed in the synthesis of tocinoic acid analogs. Specifically, an N-terminal nitrofluorobenzene is attacked by a nucleophilic C-terminal sidechain. The remaining nitro group can be reduced and acylated. NMR is used to compare the conformation of the new macrocyclic peptides to tocinoic acid.

Solid-phase total synthesis of polymyxin B1

The total solid-phase synthesis of polymyxin B1 (PMB1) has been achieved in 20% yield using the orthogonal protecting group N-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl-(Dde). This report demonstrates that a complex peptide macrocycle can be synthesized in high yields using solid-phase synthesis. According to MS and HPLC, the synthetic peptide was identical to the naturally occurring antibiotic.

Identification of a cyclin-cdk2 recognition motif present in substrates and p21-like cyclin-dependent kinase inhibitors

Understanding how cyclin-cdk complexes recognize their substrates is a central problem in cell cycle biology. We identified an E2F1-derived eight-residue peptide which blocked the binding of cyclin A and E-cdk2 complexes to E2F1 and p21. Short peptides spanning similar sequences in p107, p130, and p21-like cdk inhibitors likewise bound to cyclin A-cdk2 and cyclin E-cdk2. In addition, these peptides promoted formation of stable cyclin A-cdk2 complexes in vitro but inhibited the phosphorylation of the retinoblastoma protein by cyclin A- but not cyclin B-associated kinases. Mutation of the cyclin-cdk2 binding motifs in p107 and E2F1 likewise prevented their phosphorylation by cyclin A-associated kinases in vitro. The cdk inhibitor p21 was found to contain two functional copies of this recognition motif, as determined by in vitro kinase binding/inhibition assays and in vivo growth suppression assays. Thus, these studies have identified a cyclin A- and E-cdk2 substrate recognition motif. Furthermore, these data suggest that p21-like cdk inhibitors function, at least in part, by blocking the interaction of substrates with cyclin-cdk2 complexes.