SU-E-T-235: Dosimetry Study within a Millimeter from Beam Exiting Air-Tissue Interface Using Film and Monte-Carlo Simulation on a Co-60 Beam

PURPOSE

Objective is to assess dose accuracy and uniformity to cells in a well-plate irradiated on Co-60 beam using film (EBT2) and Monte-Carlo simulation.

METHODS

Situations are encountered where dosimetry near beam-exiting air-interface is needed. Typical example is irradiation of cells in well-plate. Small amount of cell emulsion may amount to as small as 1 mm thickness which results in leaving large air space above the cells. Due to contamination concerns, air space above cells can not be avoided. Preferred geometry for irradiation is to place cell-plate on a build-up slab (water-equivalent), and point the beam upward. On Co-60 beam, film EBT2 was employed to measure doses 0.1, 0.4, 0.7, 1.2 mm upstream from air- interface. Film optical-density (OD) was measured with a pocket densitometer, and converted to dose using film-calibration specific to film-batch. To validate film data, Monte-Carlo code DOSXYZNRC was employed for simulation. Doses were calculated employing (i) calculation grid 0.1-1.0 mm along beam direction, and (ii) 15000 million incident particles to achieve < 0.2% precision.

RESULTS

Doses measured with film close to beam-exiting air-interface are ∼16% lower than the maximum doses 1.2 mm up-stream and beyond. Monte-Carlo calculations validate the doses measured with film within measurement uncertainty (2%). Dose variation is steep in the first 0.1 mm zone near air-interface, but it is within 10% further up-stream.

CONCLUSIONS

In cell irradiations, dose in-homogeneity in the top 1mm zone is significant. Layer close to air-interface receives ∼16% lower dose than the layer 1.2mm up-stream. Dose homogeneity with the stated geometry can be improved by having (i) larger amount of cells to provide emulsion thickness > 3mm, and (ii) non-homogeneous emulsion for cells to settle at the bottom. Since dose in-homogeneity is primarily due to air interface, results are expected to be qualitatively similar for other beams (Cs-137 or 6 MV.

New capabilities for Mycosphaerella graminicola research

Mycosphaerella graminicola is a major pathogen of wheat worldwide, causing Septoria leaf blotch disease. Targeted gene disruption in M. graminicola, by Agrobacterium tumefaciens-mediated transformation, has become an established functional genomics tool for M. graminicola research in recent years. However, in order to advance research into this economically important pathogen, further functional genomics tools need to be developed. Here, we report three new capabilities for M. graminicola research: (i) two selectable markers have been shown to work robustly in M. graminicola, namely G418 and the fungicide carboxin; (ii) the generation of a strain of M. graminicola in which the KU70 (MUS-51) homologue has been disrupted; in this strain, homologous recombination efficiencies increased to more than 95%, whilst maintaining wild-type growth in vitro and full pathogenicity on wheat leaves; (iii) the ability to efficiently target and generate precise mutations of specific genes in the genomic context in M. graminicola. In addition, the insertion of the E198A mutation into the beta-tubulin gene (MgTUB1), conferring resistance to the fungicide benomyl, suggests that this mutant allele may provide an additional selectable marker. The collective use of these tools will permit further advancements in our knowledge of the biology and pathogenicity of this important plant pathogen.

Isolated central nervous system involvement in adult T-cell lymphoma/leukaemia

Central nervous system (CNS) presentation of adult T-cell lymphoma/leukaemia is rare, and almost invariably associated with systemic disease. We report an unusual manifestation of adult T-cell lymphoma/leukaemia, with isolated CNS involvement and unusual imaging findings. We also describe objective response to antiviral therapy. To our knowledge, this is the first report of such presentation and response.

Identification and characterization of a novel Bacillus thuringiensis delta-endotoxin entomocidal to coleopteran and lepidopteran larvae

A new class of Bacillus thuringiensis delta-endotoxins, or insecticidal control proteins (ICPs), is defined by an apparently cryptic protein with a unique primary structure and novel entomocidal specificity for certain coleopteran and lepidopteran species. The discovery of a new group of ICPs will extend the use of this natural insecticide in integrated pest-management systems.

The genome of Bacillus subtilis phage SPP1: structure of an early promoter

The strongest of five 'early' promoters of Bacillus subtilis phage SPP1 was localized in a DNA restriction fragment by analysis of RNA polymerase binding and R-loop formation. The nucleotide sequence of the promoter region was established. The signal structures identified were similar to those recognized by the sigma 55 RNA polymerase of B. subtilis. The promoter precedes an open reading frame with 51 codons. A protein with the Mr predicted from the nucleotide sequence was identified in minicells.

Restriction and modification in Bacillus subtilis: nucleotide sequence, functional organization and product of the DNA methyltransferase gene of bacteriophage SPR

Bacillus subtilis phage SPR codes for a DNA methyltransferase (Mtase) which methylates the 5' cytosine in the sequence GGCC and both cytosines in the sequence CCGG. A 2126-bp fragment of SPR DNA containing the Mtase gene has been sequenced. This fragment has only one significant open reading frame of 1347 bp, which corresponds to the Mtase gene. Within the sequence the Mtase promoter has been defined by S1 mapping. The size of the SPR Mtase predicted from the deduced amino acid composition is 49.9 kDal. This is in agreement with both the Mr of the purified enzyme and with that of the SPR Mtase gene product identified here by minicell technique. Base changes leading to mutants affected in Mtase activity were localized within the Mtase gene.

Unusual behaviour of SPO1 DNA with respect to restriction and modification enzymes recognizing the sequence 5'-G-G-C-C

SPO1 DNA contains only 5 cleavage sites for restriction enzymes which recognize and cleave the sequence 5'-G-G-C-C (HaeIII or BsuR). Fragments of SPO1 DNA cloned in E. coli to substitute 5'-hydroxymethyluracil (HMU) by thymine (T) remain resistant to HaeIII indicating that this unexpectedly small number of cleavages by HaeIII is not correlated with the presence of HMU in the normal phage DNA. It was previously shown that SPO1 is neither subject to B. subtilis R restriction (Trautner et al., 1974) nor modification in vivo (Günthert et al., 1975). We now show that SPO1 DNA can however be restricted and modified in vitro.