Optimisation of the imaging and dosimetric characteristics of an electronic portal imaging device employing plastic scintillating fibres using Monte Carlo simulations.

A Monte Carlo model of a novel electronic portal imaging device (EPID) has been developed using Geant4 and its performance for imaging and dosimetry applications in radiotherapy has been characterised. The EPID geometry is based on a physical prototype under ongoing investigation and comprises an array of plastic scintillating fibres in place of the metal plate/phosphor screen in standard EPIDs. Geometrical and optical transport parameters were varied to investigate their impact on imaging and dosimetry performance. Detection efficiency was most sensitive to variations in fibre length, achieving a peak value of 36% at 50 mm using 400 keV x-rays for the lengths considered. Increases in efficiency for longer fibres were partially offset by reductions in sensitivity. Removing the extra-mural absorber surrounding individual fibres severely decreased the modulation transfer function (MTF), highlighting its importance in maximising spatial resolution. Field size response and relative dose profile simulations demonstrated a water-equivalent dose response and thus the prototype's suitability for dosimetry applications. Element-to-element mismatch between scintillating fibres and underlying photodiode pixels resulted in a reduced MTF for high spatial frequencies and quasi-periodic variations in dose profile response. This effect is eliminated when fibres are precisely matched to underlying pixels. Simulations strongly suggest that with further optimisation, this prototype EPID may be capable of simultaneous imaging and dosimetry in radiotherapy.

SU-E-I-109: Sensitivity Analysis of an Electronic Portal Imaging Device Monte Carlo Model to Variations in Optical Transport Parameters.

PURPOSE: To investigate the sensitivity of a Monte Carlo (MC) model of a standard clinical amorphous silicon (a-Si) electron portal imaging device (EPID) to variations in optical photon transport parameters. METHODS: The Geant4 MC toolkit was used to develop a comprehensive model of an indirect-detection a-Si EPID incorporating x-ray and optical photon transport. The EPID was modeled as a series of uniform layers with properties specified by the manufacturer (PerkinElmer, Santa Clara, CA) of a research EPID at our centre. Optical processes that were modeled include bulk absorption, Rayleigh scattering, and boundary processes (reflection and refraction). Model performance was evaluated by scoring optical photons absorbed by the a-Si photodiode as a function of radial distance from a point source of x-rays on an event-by-event basis (0.025 mm resolution). Primary x-ray energies were sampled from a clinical 6 MV photon spectrum. Simulations were performed by varying optical transport parameters and the resulting point spread functions (PSFs) were compared. The optical parameters investigated include: x-ray transport cutoff thresholds; absorption path length; optical energy spectrum; refractive indices; and the 'roughness' of boundaries within phosphor screen layers. RESULTS: The transport cutoffs and refractive indices studied were found to minimally affect resulting PSFs. A monoenergetic optical spectrum slightly broadened the PSF in comparison with the use of a polyenergetic spectrum. The absorption path length only significantly altered the PSF when decreased drastically. Variations in the treatment of boundaries noticeably broadened resulting PSFs. CONCLUSIONS: Variation in optical transport parameters was found to affect resulting PSF calculations. Current work is focusing on repeating this analysis with a coarser resolution more typical of a commercial a-Si EPID to observe if these effects continue to alter the EPID PSF. Experimental measurement of the EPID line spread function to validate these results is also underway. Cancer Institute NSW Research Equipment Grants 10/REG/1-20 and 10/REG/1-10 Cancer Council NSW Grant, ID RG 11-06 NHMRC Project Grant, ID569211 The University of Sydney Postgraduate Research Scholarship in Medical Physics SWSCS Radiation Oncology Student Scholarship, 2012.

[Complications in children with varicella in 4 hospitals in Santiago, Chile: clinical spectrum and estimation of direct costs]. / Complicaciones en niños con varicela en cuatro hospitales de Santiago-Chile: espectro clínico y estimación de costos directos.

BACKGROUND: The knowledge of varicella complications and their associated cost may help for a better evaluation of varicella immunization benefits. AIM: To determine frequency, type, outcome and affected population of varicella complications in children requiring hospitalization, and to estimate their direct costs. MATERIAL AND METHODS: Retrospective analysis of medical records of children admitted to four hospitals in Santiago, Chile, due to varicella complications between January 1997 and February 1999. Calculation of direct costs of hospitalizations in a sample of 30 patients. RESULTS: One hundred fifty four patients were identified, 74% were younger than 5 years old, only one was immunocompromised. Complications identified were skin and soft tissue infections in 63%, invasive infections in 25.3%, neurological in 7.1% and miscellaneous in 4.5%. Staphylococcus aureus and Group A beta-haemolytic Streptococcus (GABS) were predominantly isolated. S. aureus was the main agent identified in superficial infections and GABS in invasive infections (sterile sites). Two patients died due to invasive infections (streptococcal toxic shock and S. aureus septicaemia) and 11 required surgical procedures. The average cost per hospitalization was US$ 600 in public hospitals and US$ 1,800 in the private hospital. CONCLUSIONS: Varicella complications requiring hospitalization are due mainly to bacterial infections and they affect immunocompetent toddlers. These complications can be severe and even fatal.

Seroprevalence of Norwalk virus and Mexico virus in Chilean individuals: assessment of independent risk factors for antibody acquisition.

Norwalk virus (NV) and Mexico (MX) virus represent distinct genetic clusters within the same genus of human caliciviruses (CVs), a major cause of diarrhea in adults. The magnitude and potential risk factors of human CV infection in populations from Santiago and Punta Arenas, Chile, were assessed. Individuals (n = 1,864) gave a blood sample and answered a questionnaire during a household survey. Sera were tested for antibody to NV and MX virus with use of recombinant capsid antigens. Overall, NV and MX virus seroprevalence rates were 83% and 91% in Santiago vs. 67% and 90% in Punta Arenas, respectively (P < .001 for NV virus). Lower socioeconomic status and increasing age were risk factors for infection with both viruses (P < .001). Consumption of seafood, consumption of vegetables, and child care center attendance were population risk factors for infection, but the association of a factor with a virus depended on the city. Prevention of human CV infections will require individual assessment in different communities.

Infección por helicobacter pylori y daño gástrico en niños de nivel socioeconómico bajo / Helicobacter pylori infection and gastric damage in children of low socioeconomical status

Objetivo: establecer relaciones entre los hallazgos histológicos y endoscópicos en la mucosa gástrica y la identificación de H. pylori en niños en que se realizó endoscopía digestiva por distintas razones. Pacientes y método: Se estudiaron retrospectivamente los antecedentes clínicos, el aspecto endoscópico de la mucosa gástrica y los informes del examen histológico de la misma en 100 niños (59 varones) de baja condición socioeconómica, de 2 a 17 (promedio 11) años de edad en que, por diferentes motivos, fue necesario realizar endoscopia digestiva alta para fines diagnósticos y se cotejaron con la presencia o ausencia de H. pylori registrada mediante prueba de la ureasa o en el examen histológico de los muestras obtenidas. Resultados: En 69 por ciento de los niños de la serie se identificó H. pylori, cuya prevalencia aumento con la edad (X2 11,97 p= 0,0025), llegando a 78 por ciento entre los 13 a 17 años. El agente se detectó en 30 por ciento de los niños sin lesiones histológicas y en 88 por ciento de los que tenían signos de gastritis. En 16 por ciento de los niños infectados no se registraban síntomas de la afección a pesar de estar contaminados y tener lesiones gástricas. Conclusión: estos resultados sugieren una fuerte asociación causa a efecto (X2 = 34,5, p= 0,00001) entre colonización de la mucosa gástrica con el agente y la existencia de signos endoscópicos e histológicos de gastritis

Cost-benefit analysis for the use of Haemophilus influenzae type b conjugate vaccine in Santiago, Chile.

Cost-benefit analyses can be integral to the evaluation of interventions in developing countries. The authors compare the potential benefits to the Chilean Ministry of Health, in terms of treatment costs averted, by prevention of Haemophilus influenzae type b (HIB) invasive disease, with the costs of adding HIB conjugate vaccine to the diphtheria-tetanus-pertussis (DTP) immunization routinely administered to infants. In their basecase model, over a 10-year period (1991-2000), vaccination against HIB will prevent 1,229 cases of HIB invasive disease, including 713 cases of meningitis, 107 of whom would suffer severe, long-term sequelae, and between 29 and 116 deaths. Assuming a cost of US$1 for a full three-dose regimen of vaccine, the benefit/cost ratio of 1.66, with a net discounted savings of over $403,225, illustrates that HIB vaccine can be cost-beneficial. Sensitivity analyses which alter each of the variables in the analysis indicate that if the true incidence of HIB disease is twice the published rate, then three doses of vaccine remains cost-beneficial at US#3.

PIP: Health practitioners reviewed the clinical records of all 6-60 month old children who were treated for meningitis caused by Haemophilus influenzae type b (HIB) in 1989-1990 at Roberto del Rio Children's Hospital in Santiago, Chile, to estimate costs for all phases of meningitis treatment (ambulatory visits, hospitalization, and follow-up). They also estimated annual HIB incidence. They determined the cost of adding HIB conjugate vaccine to the DTP vaccine. They assumed a cost of US$1 for a full 3-dose regimen of vaccine. They then conducted a cost benefit analysis of the use of HIB conjugate vaccine to prevent invasive HIB disease in Santiago. The National Health Service had to pay an average of US$1301/case of HIB meningitis and US$887/case of HIB invasive disease other than meningitis, including pre- and post-hospitalization costs and adjustment for frequency of sequelae. Several factors indicated that the estimates were actually underestimates. For example, the researchers did not take into account herd immunity and the fact that sequelae often do not appear until the children are older. The addition of the HIB conjugate vaccine to the immunization program would prevent at least 1229-3111 cases of HIB invasive disease, disabling sequelae, and deaths during a 10-year period. Further, it would save the National Health Service more than US$403,225. The benefit/cost ratio was 1.66. The researchers changed each of the variables in the cost benefit analysis. These sensitivity analyses revealed that if the true incidence of HIB disease were 2 times greater than the based on reported data, the 3 doses of HIB conjugate vaccine would still have a cost benefit of US$3. These results indicated that adding HIB conjugate vaccine would exert a considerable public health and cost benefit. Cost benefit analyses of vaccines would also prove useful to decision-makers in other developing countries.