Calculation of correction factors for ionization chamber measurements with small fields in low-density media.

The quantity of interest for high-energy photon beam therapy recommended by most dosimetric protocols is the absorbed dose to water. Thus, ionization chambers are calibrated in absorbed dose to water, which is the same quantity as what is calculated by most treatment planning systems (TPS). However, when measurements are performed in a low-density medium, the presence of the ionization chamber generates a perturbation at the level of the secondary particle range. Therefore, the measured quantity is close to the absorbed dose to a volume of water equivalent to the chamber volume. This quantity is not equivalent to the dose calculated by a TPS, which is the absorbed dose to an infinitesimally small volume of water. This phenomenon can lead to an overestimation of the absorbed dose measured with an ionization chamber of up to 40% in extreme cases. In this paper, we propose a method to calculate correction factors based on the Monte Carlo simulations. These correction factors are obtained by the ratio of the absorbed dose to water in a low-density medium □D(w,Q,V1)(low) averaged over a scoring volume V₁ for a geometry where V₁ is filled with the low-density medium and the absorbed dose to water □D(w,QV2)(low) averaged over a volume V₂ for a geometry where V₂ is filled with water. In the Monte Carlo simulations, □D(w,QV2)(low) is obtained by replacing the volume of the ionization chamber by an equivalent volume of water, according to the definition of the absorbed dose to water. The method is validated in two different configurations which allowed us to study the behavior of this correction factor as a function of depth in phantom, photon beam energy, phantom density and field size.

Sanitary quality of edible bivalve mollusks in Southeastern Brazil using an UV based depuration system

The increase in seafood production, especially in mariculture worldwide, has brought out the need of continued monitoring of shellfish production areas in order to ensure safety to human consumption. The purpose of this research was to evaluate pathogenic protozoa, viruses and bacteria contamination in oysters before and after UV depuration procedure, in brackish waters at all stages of cultivation and treatment steps and to enumerate microbiological indicators of fecal contamination from production site up to depuration site in an oyster cooperative located at the Southeastern estuarine area of Brazil. Oysters and brackish water were collected monthly from September 2009 to November 2010. Four sampling sites were selected for enteropathogens analysis: site 1- oyster growth, site 2- catchment water (before UV depuration procedure), site 3 - filtration stage of water treatment (only for protozoa analysis) and site 4- oyster's depuration tank. Three microbiological indicators ! were examined at sites 1, 2 and 4. The following pathogenic microorganisms were searched: Giardia cysts, Cryptosporidium oocysts, Human Adenovirus (HAdV), Hepatitis A virus (HAV), Human Norovirus (HnoV) (genogroups I and II), JC strain Polyomavirus (JCPyV) and Salmonella sp. Analysis consisted of molecular detection (qPCR) for viruses (oysters and water samples); immunomagnetic separation followed by direct immunofluorescence assay for Cryptosporidium oocysts and Giardia cysts and also molecular detection (PCR) for the latter (oysters and water samples); commercial kit (Reveal-Neogee (R)) for Salmonella analysis (oysters). Giardia was the most prevalent pathogen in all sites where it was detected: 36.3%, 18.1%, 36.3% and 27.2% of water from sites 1, 2, 3 and 4 respectively; 36.3% of oysters from site 1 and 54.5% of depurated oysters were harboring Giardia cysts. The huge majority of contaminated samples were classified as Giardia duodenalis. HAdv was detected in water and o! ysters from growth site and HnoV GI in two batches of oysters ! (site 1) in huge concentrations (2.11 x 10(13), 3.10 x 10(12) gc/g). In depuration tank site, Salmonella sp., HAV (4.84 x 10(3)) and HnoV GII (7.97 x 10(14)) were detected once in different batches of oysters. Cryptosporidium spp. oocysts were present in 9.0% of water samples from site four. These results reflect the contamination of oysters even when UV depuration procedures are employed in this shellfish treatment plant. Moreover, the molecular comprehension of the sources of contamination is necessary to develop an efficient management strategy allied to shellfish treatment improvement to prevent foodborne illnesses. (C) 2011 Elsevier Ltd. All rights reserved.