Genomic analysis of canine pneumoviruses and canine respiratory coronavirus from New Zealand.

AIMS: To isolate canine respiratory coronavirus (CRCoV) and canine pneumovirus (CnPnV) in cell culture and to compare partial genomic sequences of CRCoV and CnPnV from New Zealand with those from other countries. METHODS: Oropharyngeal swab samples from dogs affected by canine infectious respiratory disease syndrome that were positive for CnPnV (n = 15) or CRCoV (n = 1) by virus-specific reverse transcriptase quantitative PCR (RT-qPCR) in a previous study comprised the starting material. Virus isolation was performed in HRT-18 cells for CRCoV and RAW 264.7 and Vero cells for CnPnV. The entire sequence of CnPnV G protein (1,266 nucleotides) and most (8,063/9,707 nucleotides) of the 3' region of CRCoV that codes for 10 structural and accessory proteins were amplified and sequenced. The sequences were analysed and compared with other sequences available in GenBank using standard molecular tools including phylogenetic analysis. RESULTS: Virus isolation was unsuccessful for both CRCoV and CnPnV. Pneumovirus G protein was amplified from 3/15 (20%) samples that were positive for CnPnV RNA by RT-qPCR. Two of these (NZ-048 and NZ-049) were 100% identical to each other, and 90.9% identical to the third one (NZ-007). Based on phylogenetic analysis of the G protein gene, CnPnV NZ-048 and NZ-049 clustered with sequences from the USA, Thailand and Italy in group A, and CnPnV NZ-007 clustered with sequences from the USA in group B. The characteristics of the predicted genes (length, position) and their putative protein products (size, predicted structure, presence of N- and O-glycosylation sites) of the New Zealand CRCoV sequence were consistent with those reported previously, except for the region located between open reading frame (ORF)3 (coding for S protein) and ORF6 (coding for E protein). The New Zealand virus was predicted to encode 5.9 kDa, 27 kDa and 12.7 kDa proteins, which differed from the putative coding capacity of this region reported for CRCoV from other countries. CONCLUSIONS: This report represents the first characterisation of partial genomic sequences of CRCoV and CnPnV from New Zealand. Our results suggest that the population of CnPnV circulating in New Zealand is not homogeneous, and that the viruses from two clades described overseas are also present here. Limited conclusions can be made based on only one CRCoV sequence, but the putative differences in the coding capacity of New Zealand CRCoV support the previously reported variability of this region. The reasons for such variability and its biological implications need to be further elucidated.

Quantifying replication slippage error in Cryptosporidium metabarcoding studies.

Genetic variation in Cryptosporidium, a common protozoan gut parasite in humans, is often based on marker genes containing trinucleotide repeats, which differentiate subtypes and track outbreaks. However, repeat regions have high replication slippage rates, making it difficult to discern biological diversity from error. Here, we synthesised Cryptosporidium DNA in clonal plasmid vectors, amplified them in different mock community ratios and sequenced them using next generation sequencing to determine the rate of replication slippage with dada2. Our results indicate that slippage rates increase with the length of the repeat region and can contribute to error rates of up to 20%.

The prevalence of Salmonella spp. in working farm dogs and their home-kill raw meat diets in Manawatu, New Zealand.

AIMS: To investigate the prevalence of Salmonella spp. in a convenience sample of working farm dogs and their home-kill raw meat diets in Manawatu, New Zealand. METHODS: Fifty farms in the Manawatu, with at least three working/herding dogs per farm that were fed raw home-killed meat at least fortnightly, were visited. One sample of dog faeces and one sample of food were collected per farm using convenience sampling. If a dog did not defecate, a sample was obtained by digital recovery. Basic descriptive data for all dogs, meat and farm characteristics were recorded. Stomached meat samples and swabs from faecal samples were pre-enriched in buffered peptone water followed by two selective enrichments with agar subculture. Isolates were confirmed to be Salmonella spp. by serology and biochemical characterisation. RESULTS: No Salmonella spp. were isolated from dog faeces or raw meat samples, giving an observed prevalence rate of 0 (95% CI = 0.0-7.1)%. CONCLUSIONS: In this study, there was no evidence that working farm dogs and their home-kill raw meat represent likely sources of infection with Salmonella spp. CLINICAL RELEVANCE: Although this study found no evidence suggesting that farmers should change their feeding practices, it is based on a small sample, from a single region of New Zealand and involved sampling on one occasion for Salmonella spp. only. Currently, although the prevalence of Salmonella spp. carriage appears to be low, feeding raw meat-based diets to working dogs remains a risk and due to the potential zoonotic implications for humans, hygienic measures should be maintained when in contact with dogs and raw meat.

A molecular survey of canine respiratory viruses in New Zealand.

AIMS: The aim of this study was to identify viruses associated with canine infectious respiratory disease syndrome (CIRDS) among a population of New Zealand dogs. METHODS: Convenience samples of oropharyngeal swabs were collected from 116 dogs, including 56 CIRDS-affected and 60 healthy dogs from various locations in New Zealand between March 2014 and February 2016. Pooled samples from CIRDS-affected (n = 50) and from healthy (n = 50) dogs were tested for the presence of canine respiratory viruses using next generation sequencing (NGS). Individual samples (n = 116) were then tested by quantitative PCR (qPCR) and reverse transcriptase qPCR (RT-qPCR) for specific viruses. Groups were compared using Fisher's exact or χ2 tests. The effect of explanatory variables (age, sex, type of household, presence of viral infection) on the response variable (CIRDS-affected or not) was tested using RR. RESULTS: Canine pneumovirus (CnPnV), canine respiratory coronavirus (CRCoV), canine herpesvirus-1 (CHV-1), canine picornavirus and influenza C virus sequences were identified by NGS in the pooled sample from CIRDS-affected but not healthy dogs. At least one virus was detected by qPCR/RT-qPCR in 20/56 (36%) samples from CIRDS dogs and in 23/60 (38%) samples from healthy dogs (p = 0.84). CIRDS-affected dogs were most commonly positive for CnPnV (14/56, 25%) followed by canine adenovirus-2 (CAdV-2, 5/56, 9%), canine parainfluenza virus (CpiV) and CHV-1 (2/56, 4% each), and CRCoV (1/56, 2%). Only CnPnV (17/60, 28%) and CAdV-2 (14/60, 23%) were identified in samples from healthy dogs, and CAdV-2 was more likely to be detected healthy than diseased dogs (RR 0.38; 95% CI = 0.15-0.99; p = 0.045). CONCLUSIONS: The frequency of detection of viruses traditionally linked to CIRDS (CAdV-2 and CPiV) among diseased dogs was low. This suggests that other pathogens are likely to have contributed to development of CIRDS among sampled dogs. Our data represent the first detection of CnPnV in New Zealand, but the role of this virus in CIRDS remains unclear. On-going monitoring of canine respiratory pathogens by NGS would be beneficial, as it allows rapid detection of novel viruses that may be introduced to the New Zealand canine population in the future. Such monitoring could be done using pooled samples to minimise costs. CLINICAL RELEVANCE: Testing for novel respiratory viruses such as CnPnV and CRCoV should be considered in all routine laboratory investigations of CIRDS cases, particularly in dogs vaccinated with currently available kennel cough vaccines.

Microbial contamination in drinking water at public outdoor recreation facilities in New Zealand.

AIM: The aim of our study was to assess the presence and risk of waterborne pathogens in the drinking water of outdoor facilities in New Zealand and track potential sources of microbial contamination in water sources. METHODS AND RESULTS: A serial cross-sectional study with a risk-based sample collection strategy was conducted at 15 public campgrounds over two summer seasons (2011-2012 and 2012-2013). Drinking water supplied to these campgrounds was not compliant with national standards, based on Escherichia coli as an indicator organism, in more than half of the sampling occasions. Campylobacter contamination of drinking water at the campgrounds was likely to be of wild bird origin. Faecal samples from rails (pukeko and weka) were 35 times more likely to return a Campylobacter-positive result compared to passerines. Water treatment using ultraviolet (UV) irradiation or a combination of filtration and UV irradiation or chemicals was more likely to result in water that was compliant with the national standards than water from a tap without any treatment. The use of filters alone was not associated with the likelihood of compliance. CONCLUSIONS: Providing microbiologically safe drinking water at outdoor recreational facilities is imperative to avoid gastroenteritis outbreaks. This requires an in-depth understanding of potential sources of contamination in drinking water sources and the installation of adequate water treatment facilities. SIGNIFICANCE AND IMPACT OF THE STUDY: Our study provides evidence that drinking water without treatment or filter-only treatment in public campgrounds is unlikely to comply with national standards for human consumption and extra water treatment measures such as UV irradiation or chemical treatment are needed.

Insights into the Geobacillus stearothermophilus species based on phylogenomic principles.

BACKGROUND: The genus Geobacillus comprises bacteria that are Gram positive, thermophilic spore-formers, which are found in a variety of environments from hot-springs, cool soils, to food manufacturing plants, including dairy manufacturing plants. Despite considerable interest in the use of Geobacillus spp. for biotechnological applications, the taxonomy of this genus is unclear, in part because of differences in DNA-DNA hybridization (DDH) similarity values between studies. In addition, it is also difficult to use phenotypic characteristics to define a bacterial species. For example, G. stearothermophilus was traditionally defined as a species that does not utilise lactose, but the ability of dairy strains of G. stearothermophilus to use lactose has now been well established. RESULTS: This study compared the genome sequences of 63 Geobacillus isolates and showed that based on two different genomic approaches (core genome comparisons and average nucleotide identity) the Geobacillus genus could be divided into sixteen taxa for those Geobacillus strains that have genome sequences available thus far. In addition, using Geobacillus stearothermophilus as an example, we show that inclusion of the accessory genome, as well as phenotypic characteristics, is not suitable for defining this species. For example, this is the first study to provide evidence of dairy adaptation in G. stearothermophilus - a phenotypic feature not typically considered standard in this species - by identifying the presence of a putative lac operon in four dairy strains. CONCLUSIONS: The traditional polyphasic approach of combining both genotypic and phenotypic characteristics to define a bacterial species could not be used for G. stearothermophilus where many phenotypic characteristics vary within this taxon. Further evidence of this discordant use of phenotypic traits was provided by analysis of the accessory genome, where the dairy strains contained a putative lac operon. Based on the findings from this study, we recommend that novel bacterial species should be defined using a core genome approach.

Isolation of Campylobacter spp. from Client-Owned Dogs and Cats, and Retail Raw Meat Pet Food in the Manawatu, New Zealand.

Campylobacter causes acute gastroenteritis in people worldwide and is frequently isolated from food, animals and the environment. The disease is predominately food-borne but many routes of transmission and sources of infection have been described, including contact with pets. The prevalence of Campylobacter spp. in dogs and cats varies widely, and data on New Zealand pets are limited. This study aimed to investigate the prevalence of Campylobacter spp. in dogs, cats and retail raw meat pet food products in New Zealand and to characterize Campylobacter jejuni isolates using multilocus sequence typing (MLST). Ninety dogs and 110 cats examined at the Massey University Veterinary Teaching Hospital for elective procedures, and fifty locally purchased retail raw meat pet diets were sampled. Two culture protocols combining Bolton broth enrichment and mCCDA and CAT agars in a microaerobic atmosphere at 42°C and 37°C with species identification using PCR were performed. The prevalence of Campylobacter spp., C. jejuni, Campylobacter upsaliensis and Campylobacter helveticus was 36%, 13%, 23% and 1% in dogs and 16%, 5%, 5% and 7% in cats, respectively. One dog had Campylobacter lari confirmed, and three dogs and one cat had multiple Campylobacter spp. detected. Significantly more animals tested positive using CAT than mCCDA agar (P < 0.001). Being neutered, vaccinated for Bordetella bronchiseptica, fed dry diets and brought in for neutering were protective factors for dogs, whereas attendance for dental treatment was a risk factor for cats. Campylobacter spp. were isolated from 28%, C. jejuni 22%, C. lari 6% and Campylobacter coli 6% of food samples. Six isolates positive by Campylobacter genus PCR were identified as Arcobacter butzleri. Poultry meat was more likely to be positive than non-poultry meat (P = 0.006). Of the 13 C. jejuni pet isolates with full MLST profiles, eight were of different sequence types (ST) and all nine food isolates were of different STs.

Seasonality of Campylobacter jejuni isolates associated with human campylobacteriosis in the Manawatu region, New Zealand.

A 9-year time-series of genotyped human campylobacteriosis cases from the Manawatu region of New Zealand was used to investigate strain-type seasonality. The data were collected from 2005 to 2013 and the samples were multi-locus sequence-typed (MLST). The four most prevalent clonal complexes (CCs), consisting of 1215 isolates, were CC48, CC21, CC45 and CC61. Seasonal decomposition and Poisson regression with autocorrelated errors, were used to display and test for seasonality of the most prevalent CCs. Of the four examined CCs, only CC45 showed a marked seasonal (summer) peak. The association of CC45 with summer peaks has been observed in other temperate countries, but has previously not been identified in New Zealand. This is the first in-depth study over a long time period employing MLST data to examine strain-type-associated seasonal patterns of C. jejuni infection in New Zealand.

Quantification of historical livestock importation into New Zealand 1860-1979.

AIMS: To quantify the numbers of live cattle, sheep and poultry imported into New Zealand and, where possible, their country of origin from 1860 to 1979. METHODS: Information on the origin and number of live animal importations into New Zealand was collected for cattle, sheep and poultry for the period 1868-1979 from the annual reports compiled by the New Zealand Registrar General's Office, Government Statistician's Office, Census and Statistics Office, Census and Statistics Department, Customs Department and Department of Statistics. Census data from 1851 to 1871 were also used to estimate the livestock population during this period. The number of animals imported and the mean population for each species in a decade were determined, and the major countries of origin were identified. RESULTS: A large number of cattle (53,384) and sheep (604,525) were imported in the 1860s, and then there was a marked reduction in importations. Live poultry were imported in relatively small numbers (20,701) from 1880 to 1939, then 1,564,330 live poultry were imported between 1960 and 1979. Australia was the predominant country of origin for sheep between 1868 and 1959 (51,347/60,918; 84.3%) and of cattle between 1868 and 1979 (10,080/15,157; 66.5%). Only 6,712 (11.0%) sheep and 3,909 (25.8%) cattle were imported from the United Kingdom over the same periods, and even fewer from other countries. CONCLUSIONS: The collated data and historical reports show that from 1860 to 1979 Australia has been the main source of livestock introduced into New Zealand. The pattern of importation showed that large numbers of cattle and sheep were initially imported in the 1860s, probably in response to rapid agricultural expansion. Thereafter importations continued at much reduced numbers. In contrast, relatively small numbers of poultry were introduced until the 1960s when large numbers were imported as part of the development of a modern high-production industry. The overall pattern for both cattle and sheep was of a bottleneck event, as initially a relatively limited number of animals arrived from outside populations, followed by population expansion with ongoing but limited immigration (admixture). Investigation into the genetic population structure of New Zealand's cattle and sheep, as well as their host-associated microorganisms, could reflect the impact of these early historical events.

Extensive intra-host genetic diversity uncovered in Cryptosporidium parvum using Next Generation Sequencing.

The theory about the Cryptosporidium life cycle predicts genetic diversity of sporozoites within the host. Nevertheless, the Cryptosporidium intra-host genetic diversity is difficult to study using conventional Sanger sequencing or electrophoretic resolution of amplicons, due to the methods' inability to resolve mixtures of templates. We analysed the within-isolate genetic diversity of two Cryptosporidium parvum isolates sharing common descent, by combining the use of Next Generation Sequencing and cloning of PCR amplicons with database searches. The analysis focused on the single-copy 70 kDa heat shock protein (HSP70) and the 60kDa surface glycoprotein (gp60) genes, which allowed any diversity to be ascribed to the presence of a heterogeneous population of sporozoites. The results indicated an unprecedented intra-host genetic diversity, with two HSP70 and 10 gp60 alleles in these isolates, in spite of the initial resolution of one allele per locus using Sanger sequencing. At both loci, the predominant alleles were those initially identified by Sanger sequencing. A significant (p<0.01) overrepresentation of gp60 alleles previously reported in New Zealand was observed. These results further our understanding of the genetic structure of C. parvum populations, and expose the limitations of the use of non-axenic isolates as operational taxonomic units of genetic studies of cryptosporidiosis.

Identification of a novel nidovirus associated with a neurological disease of the Australian brushtail possum (Trichosurus vulpecula).

A novel, fatal neurological disease of the Australian brushtail possum (Trichosurus vulpecula) was first identified in 1995 in a research facility and subsequently in free-living possums in New Zealand and termed wobbly possum disease (WPD). The results of previous transmission studies suggested that the aetiological agent of WPD is most likely a virus. However, the identity of the presumed viral agent had not been elucidated. In the current report, we describe identification of a novel virus from tissues of WPD-affected possums using a combination of next generation sequencing and traditional molecular methods. The proportion of possums positive for the novel virus by PCR was significantly higher (p<0.0001) among animals with WPD than clinically healthy possums, strongly suggesting an aetiological involvement of the virus in WPD. Analysis of the partial genomic sequence of the putative WPD virus indicated that it is a novel nidovirus, most closely related to the current members of the family Arteriviridae.

Measurement of scatter factors for 4, 6, 10, and 23 MV x rays at scattering angles between 30 degrees and 135 degrees.

NCRP Report No. 49, published in 1976, describes how to calculate the shielding for the medical use of x rays and gamma rays for energies up to 10 MV, including primary, scattered, and leakage radiation. However, in that report, data for scattered radiation for linear accelerators exist only for 6 MV, and leakage radiation is assumed, incorrectly, to be equivalent to primary radiation. Since the publication of that report, linear accelerators with energies up to 25 MV have been widely used in the radiation therapy community. Thus, there is a need to measure additional data for all energies in the range 4-25 MV. In this study, measurements were made of the "a" factor for 4, 6, 10, and 23 MV x rays at scattering angles between 30 degrees and 135 degrees. The results show that the 6 and 10 MV "a" factor data are consistent with published data, and the 23 MV data are also consistent with recently published data at 18 and 25 MV. The data show that, in general, the "a" factor decreases with energy; the exception is that 23 MV data show a sharp increase at low scattering angles, much greater than at other energies.

BAC contig from a 3-cM region of mouse chromosome 11 surrounding Brca1.

Even with the completion of a draft version of the human genome sequence only a fraction of the genes identified from this sequence have known functions. Chromosomal engineering in mouse cells, in concert with gene replacement assays to prove the functional significance of a given genomic region or gene, represents a rapid and productive means for understanding the role of a given set of genes. Both techniques rely heavily on detailed maps of chromosomal regions, initially to understand the scope of the regions being modified and finally to provide the cloned resources necessary to allow both finished sequencing and large insert complementation. This report describes the creation of a BAC clone contig on mouse chromosome 11 in a region showing conservation of synteny with sequences on human chromosome 17. We have created a detailed map of an approximately 3-cM region containing at least 33 genes through the use of multiple BAC mapping strategies, including chromosome walking and multiplex oligonucleotide hybridization and gap filling. The region described is one of the targets of a large effort to create a series of mice with regional deletions on mouse chromosome 11 (33-80 cM) that can subsequently be subjected to further mutagenesis.

Linkage and LOH studies in 19 cylindromatosis families show no evidence of genetic heterogeneity and refine the CYLD locus on chromosome 16q12-q13.

Familial cylindromatosis is an autosomal dominant predisposition to multiple neoplasms of the skin appendages. The susceptibility gene has previously been mapped to chromosome 16q12-q13 and has features of a recessive oncogene/tumour suppressor gene. We have now evaluated 19 families with this disease by a combination of genetic linkage analysis and loss of heterozygosity in cylindromas from affected individuals. All 15 informative families show linkage to this locus, providing no evidence for genetic heterogeneity. Recombinant mapping has placed the gene in an interval of approximately 1 Mb. There is no evidence, between families, of haplotype sharing that might be indicative of common founder mutations.

Identification of the familial cylindromatosis tumour-suppressor gene.

Familial cylindromatosis is an autosomal dominant genetic predisposition to multiple tumours of the skin appendages. The susceptibility gene (CYLD) has previously been localized to chromosome 16q and has the genetic attributes of a tumour-suppressor gene (recessive oncogene). Here we have identified CYLD by detecting germline mutations in 21 cylindromatosis families and somatic mutations in 1 sporadic and 5 familial cylindromas. All mutations predict truncation or absence of the encoded protein. CYLD encodes three cytoskeletal-associated-protein-glycine-conserved (CAP-GLY) domains, which are found in proteins that coordinate the attachment of organelles to microtubules. CYLD also has sequence homology to the catalytic domain of ubiquitin carboxy-terminal hydrolases (UCH).

AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams.

A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between 60Co and 50 MV and electron beams with nominal energies between 4 and 50 MeV. The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, N(60Co)D,w which are traceable to national primary standards, and the equation D(Q)w = MkQN(60Co)D,w where Q is the beam quality of the clinical beam, D(Q)w is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, M is the fully corrected ion chamber reading, and kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a beam of quality Q. Values of kQ are presented as a function of Q for many ion chambers. The value of M is given by M = PionP(TP)PelecPpolMraw, where Mraw is the raw, uncorrected ion chamber reading and Pion corrects for ion recombination, P(TP) for temperature and pressure variations, Pelec for inaccuracy of the electrometer if calibrated separately, and Ppol for chamber polarity effects. Beam quality, Q, is specified (i) for photon beams, by %dd(10)x, the photon component of the percentage depth dose at 10 cm depth for a field size of 10x10 cm2 on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by R50, the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size > or =10x10 cm2 on the surface of the phantom (> or =20x20 cm2 for R50>8.5 cm) at an SSD of 100 cm. R50 is determined directly from the measured value of I50, the depth at which the ionization falls to 50% of its maximum value. All clinical reference dosimetry is performed in a water phantom. The reference depth for calibration purposes is 10 cm for photon beams and 0.6R50-0.1 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a 10x10 cm2 field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup. For electron beams clinical reference dosimetry is performed with a field size of > or =10x10 cm2 (> or =20x20 cm2 for R50>8.5 cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification compared to the AAPM's TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors. Worksheets for various situations are presented along with a list of equipment required.

Measurement of the collection efficiency of a large volume spherical ionization chamber in megavoltage therapy beams.

The collection efficiency of a 5.7 cm diameter spherical ionization chamber has been measured in 4 MV and 10 MV x-ray beams at various distances from the source. This chamber was found to have a substantial inefficiency due to its large volume and the high dose rate and pulsed nature of the therapy beams. It was also found that the efficiency depended on the dose rate of the machine because the inter-pulse separation time of the linac is significantly less than the ion transit-time for this chamber. Thus, ionization from more than one beam pulse is collected by the chamber at the same time. The efficiency was determined using three techniques (i) the two-voltage technique, (ii) the voltage extrapolation technique and (iii) a method originally devised for determining the collection efficiency of large volume ionization chambers in diagnostic radiology. The results show that methods (ii) and (iii) agree well, but that the two-voltage technique predicts a much lower efficiency. At about 4 m from the source, the collection efficiency for this chamber varied between 98% and 97% for dose rates between 50 and 250 cGy/min for 4 MV and between 97% and 90% for dose rates between 100 and 600 cGy/min for 10 MV. At isocenter, the comparable figures were 78% and 56% respectively for 4 MV and 65% and 38% respectively for 10 MV.

Distance-dose curve for a miniature x-ray tube for stereotactic radiosurgery using an optimized aperture with a parallel-plate ionization chamber.

A recently developed miniature x-ray tube operating at 40 kV has been used in a randomized trial for the treatment of small intracranial lesions. The diameter of these lesions ranges from 10 to 30 mm. A thin window parallel-plate ionization chamber was used to calibrate the output of the x-ray tube, modified by the addition of a thin platinum aperture to reduce the charge collecting area of the chamber. The effect of such an aperture on the measurement of dose versus distance from the x-ray tube in a phantom has been examined as a function of aperture diameter. Aperture diameters were varied between 0 and 5 mm and dose measurements were made for distances between the x-ray source and the front surface of the chamber of 5-30 mm in water. The ratio of doses measured with and without an aperture, when normalized to unity at a distance of 10 mm, differs significantly from unity, for distances between 7.5 and 15 mm, for aperture diameters <1.5 mm and differs from unity, but less significantly, for apertures > or =3 mm. For intermediate diameters, however, this dose dependence is minimized, indicating an aperture diameter that provides a similar distance-dose curve as the measurement taken without an aperture over this range of distances. This diameter was found to be between 2 and 2.5 mm with a dose variation of less than +/- 1%. For distances <7.5 mm, measurements made with a 1.5-mm-diam aperture agree better with those taken with a 1.7-mm-diam chamber compared with a 5.2-mm-diam chamber.

Evidence for a significant timer error on a linear accelerator: consequences for a special therapy application.

A significant timer error has been found on a linear accelerator used for intraoperative radiation therapy. Since typical treatment doses range up to 20 Gy, calibrations that do not account for this effect can incur a large dosimetry error. The effect is dose rate dependent, but energy independent. For a dose rate of 900 cGy min(-1), the error in dose calibration varies between 2 and 3% for an average of 2.5%. At dose rates of 600 and 300 cGy min(-1), the error was 1.7% and 0.8% respectively. The average timer error at 900 cGy min(-1) was found to be 1.2 +/- 0.1 monitor units. It is argued that even a small timer error has serious dosimetry consequences for the implementation of intensity modulated radiation therapy.